Home About us Products Services Contact us Bookmark
:: wikimiki.org ::
Interstellar Planet

Interstellar planet

An interstellar planet is a hypothetical type of rogue planet that has been ejected from its solar system by a proto-gas giant to become an outcast, drifting in interstellar space.

Theoretical ideas on the atmospheres of interstellar exoplanets

In 1998, David J. Stevenson authored a paper entitled "Possibility of Life Sustaining Planets in Interstellar Space" [http://www.gps.caltech.edu/faculty/stevenson/pdfs/interstellar_planets.pdf]. In this paper, Stevenson theorizes that wandering planets which drift in the vast expanses of cold interstellar space could possibly sustain a thick atmosphere which would not freeze out due to radiative heat loss. The mechanism he proposes which preserves atmosphere formation in these bodies is due to the pressure-induced far infrared radiation opacity of a thick hydrogen-containing atmosphere. It is thought that during planetary system formation, several small protoplanetary bodies may be ejected from the forming system. With the reduced ultraviolet light associated with its increasing distance from the parent star, the planet's predominantly hydrogen and helium containing atmosphere would be easily confined even by an Earth-sized body's gravity. It is calculated that for an Earth-sized planet at a kilobar hydrogen atmospheric pressures in which a convective gas adiabat has formed, geothermal energy from residual core radioisotope decay will be sufficient to heat the surface to temperatures above the melting point of water. Thus, it is proposed that interstellar planetary bodies with extensive liquid water oceans may exist. It is further suggested that the bodies are likely to remain geologically active for long periods of time, providing a geodynamo-created protective magnetosphere and possible sea floor volcanism which could provide an energy source for life. The author admits these bodies will be difficult to detect due to the intrinsically weak thermal microwave radiation emissions emanating from the lower reaches of the atmosphere.

Reference


- Stevenson, D. "Life-Sustaining Planets in Interstellar Space?" Nature 400, 32, 1999.
- [http://www.gps.caltech.edu/faculty/stevenson/pdfs/interstellar_planets.pdf Article by Stevenson similar to the Nature article but containing more information, titled: "Possibility of Life Sustaining Planets in Interstellar Space"] Category:Planets

Rogue planet

A rogue planet is a planet that either has an extremely elongated orbit around its star so that it is not on the same orbital plane as the other planets in the system, or it is an interstellar planet, a planet that drifts freely through space and doesn't orbit a star at all. If the orbit is extremely elongated or sharply inclined the planet could take hundreds or thousands of years to make a complete orbit around the system. The strange orbit of a rogue planet may make it undetectable and unknown to an observer on any of the other normally aligned planets in a solar system. There are many theories on how rogue worlds are created; they could be massive brown dwarfs, or be terrestrial planets that for some reason break free of their solar system orbits and are flung away into space. Some planets orbiting a star may have been captured rogue worlds. Since most definitions of "planet" describe in some fashion, "an object which orbits a star", the idea of rogue planets casts doubt on the very nature of what a planet is. Some definition concerning origin, such as "an object that formed from a star's accretion disk" would be required to include rogue planets in the same class as the planets in our solar system. The theory of rogue planets could also account for some of the so called dark matter detected in space. The planet Nibiru, the "12th planet" in the writings of Zecharia Sitchin, based on his extra-terrestrial interpretation of Sumerian mythology, could be considered a rogue planet, since it supposedly possesses a highly elliptical, 3600-year orbit. However there is no scientific evidence for such a planet and his claims are largely considered pseudoscience. Category:Planets

Solar system

The solar system comprises our Sun and the retinue of celestial objects gravitationally bound to it. Traditionally, this is said to consist of the Sun, nine planets and their 158 currently known moons; however, a large number of other objects, including asteroids, meteoroids, planetoids, comets, and interplanetary dust, orbit the Sun as well. Although the term "solar system" is frequently applied to other star systems and the planetary systems which may comprise them, it should strictly refer to our system specifically: the word "solar" is derived from the Sun's Latin name, Sol (and the term sometimes appears as Solar System). When talking about another stellar system (or planetary system), including the star(s) and bodies associated with them through gravity, it is usual to shorten it to "the system" (e.g. "the Alpha Centauri system" or "the 51 Pegasi system").

Structure and layout of the solar system

The Sun (astronomical symbol ☉) is a main sequence G2 star that contains 99.86% of the system's known mass. Its two largest orbiting bodies, Jupiter and Saturn, account for 91% of the remainder (The Oort Cloud might hold a substantial percentage, but as yet its existence is unconfirmed). In broad terms, the charted regions of our solar system consist of the Sun and its planetary system: the eight bodies in relatively unique orbits (commonly called planets or major planets) and two belts of smaller objects (which can be called minor planets, planetoids, meteoroids, planetesimals or, in the case of Pluto, planets). Objects in orbit round the Sun all lie within the same shallow plane, called the ecliptic, and all orbit in the same direction. Many are in turn orbited by moons, and the largest are encircled by planetary rings of dust and other particles. The major planets are, in order, Mercury (☿), Venus (♀), Earth (♁), Mars (♂), Jupiter (♃), Saturn (♄), Uranus (♅/10px), Neptune (♆), and Pluto (♇), though Pluto's status has been thrown into question by the discovery of (see below). Eight of the nine planets are named after or derived from gods and goddesses from Greco-Roman mythology; Earth, a Germanic word, is known in many Romance languages as Terra, the Roman goddess of the Earth. Distances within the solar system are measured most often in astronomical units, or AU. 1 AU is the distance between the Earth and the Sun, or 149 598 000 kilometers. Pluto is roughly 38 AU from the Sun, while Jupiter lies at roughly 5.2 AU. For very large distances within the solar system, such as regions beyond Pluto or the orbital circumferences of planets, the terameter (Tm, one milliard kilometers) is sometimes used. Despite the fact that many diagrams (like the image at the top of this article), for practicality's sake, represent the solar system as having each orbit the same distance apart, in actuality the orbits are largely arranged geometrically, that is, each is roughly double the distance from the Sun as the one before it. Venus’s distance from the Sun is roughly double that of Mercury, Earth’s distance is roughly double that of Venus, Mars’s double that of Earth and so on. This relationship is roughly expressed in the Titius-Bode law, a mathematical formula for predicting the semi-major axes of planets in AU. In its simplest form, it is written : a= 0.4 + 0.3\times k where k=0,1,2,4,8,16,32,64,128. By this formulation, we would expect Mercury's orbit (k=0) to be 0.4 AU, and Mars's orbit (k=4) to be at 1.6 AU. In fact their orbits are 0.38 and 1.52 AU.Ceres, the largest asteroid, lies at k=8. This law is only a rough guide, and doesn't fit all of the planets (Neptune is far closer than predicted, though Pluto lies at Neptune's predicted orbit). As of now, there is no scientific explanation for why this law "works," and many claim it is merely a coincidence. Pluto

Origin and evolution of the solar system

The current hypothesis of solar system formation is the nebular hypothesis, first proposed in 1755 by Immanuel Kant. It states the solar system was formed from a gaseous cloud called the solar nebula. It had a diameter of 100 AU and was 2-3 times the mass of the Sun. Over time, the nebula began to collapse, possiby due to disturbance by a nearby supernova. This explosion sent shock waves into space, which squeezed the nebula, pushing more and more matter inward until gravitational forces overcame its internal gas pressure and it also began to collapse. As the nebula collapsed, it decreased in size, which in turn caused it to spin faster to conserve angular momentum. And as the competing forces associated with gravity, gas pressure, magnetic fields, and rotation acted on it, the contracting nebula began to flatten into a spinning pancake shape with a bulge at the center. When the nebula further condensed, a protostar was formed in the middle. This system was heated by the friction of the rocks colliding into each other. Lighter elements such as hydrogen and helium evaporated out of the centre and migrated to the edges of the disc, thus concentrating the heavier elements to form dust and rocks in the centre. These heavier elements clumped together to form planetesimals and protoplanets. In the outer regions of this solar nebula, ice and volatile gases were able to survive, and as a result, the inner planets are rocky and the outer planets were massive enough to capture large amounts of lighter gases, such as hydrogen and helium. After 100 million years, the pressures and densities of hydrogen in the centre of the collapsed nebula became great enough for the protosun to sustain thermonuclear fusion reactions. As a result of this, hydrogen was converted to helium, and a great amount of heat was released. 4×1H → 4He + neutrinos + photons During that time, the protostar turned into the Sun and the protoplanets and planetesimals were transformed into planets. All of the planets formed in a relatively short time of a few million years.

Regions of the solar system

protostar's rotating magnetic field on the plasma in the interplanetary medium (Solar Wind) [http://quake.stanford.edu/~wso/gifs/HCS.html]. (click to enlarge) ]] According to their location, the objects in the solar system are divided into three zones: Zone I or the inner solar system, including terrestrial planets and the Main belt of asteroids; Zone II, including the giant planets, their satellites and the centaurs, and Zone III, or the outer solar system, comprising the area of the Trans-Neptunian objects including the Kuiper Belt, the Oort cloud, and the vast region in between.

Interplanetary medium

The environment in which the solar system resides is called the interplanetary medium. The Sun radiates a continuous stream of charged particles, a plasma known as solar wind, which forms a very tenuous "atmosphere" (the heliosphere), permeating the interplanetary medium in all directions for at least ten billion (10) miles (16 Tm or 16 km) into space. Small quantities of dust are also present in the interplanetary medium and are responsible for the phenomenon of zodiacal light. Some of the dust is likely interstellar dust from outside the solar system. The influence of the Sun's rotating magnetic field on the interplanetary medium creates the largest structure in the Solar System, the heliospheric current sheet.

The inner planets

The four inner or terrestrial planets are characterised by their dense, rocky makeup. They formed in the hotter regions close to the Sun, where lighter and more volatile materials evaporated, leaving only those with high melting points, such as silicates, which form the planets' solid crusts and semi-liquid mantles, and iron, which forms their cores. All have impact craters and many possess tectonic surface features, such as rift valleys and volcanoes. The four inner planets are: volcanoes
- Mercury (0.39 AU from the Sun): The closest planet to the Sun is also the smallest and most atypical of the inner planets, having no atmosphere and, to date, no observed geological activity save that produced by impacts. Its relatively large iron core suggests that it was once a much larger world whose outer mantle was sheared off in early formation by the Sun’s gravity.
- Venus (0.72 AU): The first truly terrestrial planet, Venus, like the Earth, possesses a thick silicate mantle around an iron core, as well as a substantial atmosphere and evidence of one-time internal geological activity, such as volcanoes. It is much drier than Earth, and its atmosphere is 90 times as dense as Earth’s, however, and composed overwhelmingly of carbon dioxide with traces of sulfuric acid.
- Earth/Moon (1 AU): The largest of the inner planets, Earth is also the only one to demonstrate unequivocal evidence of ongoing geological activity. Its liquid hydrosphere, unique among the terrestrials, is probably the reason why Earth is also the only planet where multi-plate tectonics has been observed, since water acts as a lubricant for subduction. Its atmosphere is radically different from the other terrestrials, having been altered by the presence of life to contain 21 percent free oxygen. Its satellite, the Moon, is sometimes considered a terrestrial planet in a co-orbit with its partner, since its orbit around the Sun never actually loops back on itself when observed from above. The Moon possesses many of the features in common with other terrestrial planets, though it lacks an iron core.
- Mars (1.5 AU): Smaller than the Earth or Venus, Mars possesses a tenuous atmosphere of carbon dioxide. Its surface, peppered with vast volcanoes and rift valleys such as Valles Marineris, shows that it was once geologically active and [http://www.universetoday.com/am/publish/mars_volcanoes_active.html recent evidence] suggests it may have continued to be so until very recently. Mars possesses two tiny moons thought to be captured asteroids.

The asteroid belt

Asteroids are objects smaller than planets that mostly occupy the orbit between Mars and Jupiter, between 2.3 and 3.3 AU from the Sun, and are composed in significant part of non-volatile minerals. The main belt contains tens of thousands (possibly millions) over 1 km across, though they can be as small as dust. Despite their large numbers, the total mass of the main asteroid belt is unlikely to be more than a thousandth that of the Earth. Asteroids with a diameter of less than 50 m are called meteoroids. The largest asteroid, Ceres, has a diameter of roughly 1000 km; large enough to be spherical, which would make it a planet by some definitions of the word. The asteroids are thought to be the remnants of a small terrestrial planet that failed to coalesce due to the gravitational interference of Jupiter. They are subdivided into asteroid groups and families based on their specific orbital characteristics. Asteroid moons are asteroids that orbit larger asteroids. They are not as clearly distinguished as planetary moons, sometimes being almost as large as their partners. Trojan asteroids are located in either of Jupiter's L4 or L5 points, though the term is also sometimes used for asteroids in any other planetary Lagrange point as well. The inner solar system is dusted with rogue asteroids, many of which cross the orbits of the inner planets.

The outer planets

The four outer planets, or gas giants, (sometimes called Jovian planets) are so large they collectively make up 99 percent of the mass known to orbit the Sun. Their large sizes and distance from the Sun meant they could hold on to much of the hydrogen and helium too light for the smaller and hotter terrestrial planets to retain.
- Jupiter (5.2 AU), at 318 Earth masses, is 2.5 times the mass of all the other planets put together. Its composition of largely hydrogen and helium is not very different from that of the Sun. Three of its 63 satellites, Ganymede, Io and Europa, share elements in common with the terrestrial planets, such as volcanism and internal heating. Jupiter has a faint, smoky ring.
- Saturn (9.5 AU), famous for its extensive ring system, shares many qualities in common with Jupiter, including its atmospheric composition, though it is far less massive, being only 95 Earth masses. Two of its 49 moons, Titan and Enceladus, show signs of geological activity, though they are largely made of ice. Titan is the only satellite in the solar system with a substantial atmosphere.
- Uranus (19.6 AU) and Neptune (30 AU), while having many characteristics in common with the other gas giants, are nonetheless more similar to each other than they are to Jupiter or Saturn. They are both substantially smaller, being only 14 and 17 Earth masses, respectively. Their atmospheres contain a smaller percentage of hydrogen and helium, and a higher percentage of “ices”, such as water, ammonia and methane. For this reason some astronomers suggested that they belong in their own category, “Uranian planets,” or “ice giants.” Both planets possess dark, insubstantial ring systems. Neptune’s largest moon Triton is geologically active. Centaurs are icy comet-like bodies that have less-eccentric orbits so that they remain in the region between Jupiter and Neptune. The first centaur to be discovered, 2060 Chiron, has been called a comet since it has been shown to develop a tail, or coma, just as comets do when they approach the sun.

The trans-Neptunian region

The area beyond Neptune, often referred to as the outer solar system or simply the "trans-Neptunian region", is still largely unexplored.

The Kuiper belt

This region's first formation, which actually begins inside the orbit of Neptune, is the Kuiper belt, a great ring of debris, similar to the asteroid belt but composed mainly of ice and far greater in extent, which lies between 30 to 50 AU from the Sun. This region is thought to be the place of origin for short-period comets, such as Halley's comet. Though there are estimated to be over 70,000 Kuiper belt objects with a diameter greater than 100 km, the total mass of the Kuiper belt is relatively low, perhaps equalling or just exceeding the mass of the Earth. Many Kuiper belt objects have orbits that take them outside the plane of the ecliptic.
- Pluto, the solar system's smallest planet, is considered to be part of the Kuiper Belt population. Like others in the belt, it has a relatively eccentric orbit inclined 17 degrees to the ecliptic and ranging from 29.7 AU from the Sun at perihelion to 49.5 AU at aphelion. It has a large moon (the largest in the solar system relative to its own size), called Charon, and, new observations suggest, two other, much smaller moons. Like the Earth/Moon, Pluto and Charon are often considered a double planet. A member of the traditional nine planets, Pluto's tiny mass (less than 1% of Earth's) and diameter have called this status into question. Kuiper belt objects with Pluto-like orbits are called Plutinos. Other Kuiper belt objects have resonant orbits and are grouped accordingly. The remaining Kuiper belt objects, in more "classical" orbits, are classified as Cubewanos. The Kuiper Belt has a very sharply defined edge. At around 49 AU, a sharp dropoff occurs in the number of objects observed. This dropoff is known as the "Kuiper Cliff", and as yet its cause is unknown. Some speculate that something must exist beyond the belt large enough to sweep up the remaining debris, perhaps as large as Earth or Mars. This view is still controversial, however.

The scattered disc

Overlapping the Kuiper belt but extending much further outwards is the scattered disc. Scattered disc objects are believed to have been originally native to the Kuiper belt, but were ejected into erratic orbits in the outer fringes. One particular scattered disc object, originally found in 2003 but confirmed two years later by Mike Brown, has renewed the old debate about what constitutes a planet since, though its size is not yet known, it is almost certainly larger than Pluto. It currently has no name, but has been given the provisional designation , and has been nicknamed "Xena" by its discoverers, after the television character. It has many similarities with Pluto: its orbit is highly eccentric, with a perihelion of 38.2 AU (roughly Pluto's distance from the Sun) and an aphelion of 97.6 AU, and is steeply inclined to the ecliptic plane, indeed, at 44 degrees, more so than any known object in the solar system. Like Pluto, it is believed to consist largely of rock and ice, and has a [http://www.gps.caltech.edu/%7Embrown/planetlila/moon/index.html moon]. Whether it and the largest Kuiper belt objects should be considered planets or whether instead Pluto should be reclassified as a minor planet has not yet been resolved.

A new region?

Sedna, the newly discovered Pluto-like object with a gigantic, highly elliptical 10,500-year orbit that takes it from about 76 to 928 AU, has too distant a perihelion to be a scattered member of the Kuiper Belt and could be the first in an entirely new population. is also believed to be a member of this population.

Comets

Comets are composed largely of volatile ices and have highly eccentric orbits, generally having a perihelion within the orbit of the inner planets and an aphelion far beyond Pluto. Short-period comets exist with apoapses closer than this, however, and old comets that have had most of their volatiles driven out by solar warming are often categorized as asteroids. Long period comets have orbits lasting thousands of years. Some comets with hyperbolic orbits may originate outside the solar system.

And beyond

The point at which the solar system ends and interstellar space begins is not precisely defined, since its outer boundaries are delineated by two separate forces: the solar wind and the Sun's gravity. gravity The heliosphere expands outward in a great bubble to about 95 AU, or three times the orbit of Pluto. The edge of this bubble is known as the termination shock; the point at which the solar wind collides with the opposing winds of the interstellar medium. Here the wind slows, condenses and becomes more turbulent, forming a great oval structure known as the heliosheath that looks and behaves very much like a comet's tail; extending outward for a further 40 AU at its stellar-windward side, but tailing many times that distance in the opposite direction. The outer boundary of the sheath, the heliopause, is the point at which the solar wind finally terminates, and one enters the environment of interstellar space. Beyond the heliopause, at around 230 AU, lies the bow shock, a plasma "wake" left by the Sun as it travels through the Milky Way. But even at this point, we could not be said to have left the solar system, for the Sun's gravity will still hold sway even up to the Oort cloud, the great mass of icy objects, currently hypothetical, believed to be the source for all long-period comets and to surround our solar system like a shell from 50,000 to 100,000 AU beyond the Sun, or almost halfway to the next star system. The vast majority of the solar system, therefore, is completely unknown.

Age of the solar system

Scientists estimate that the solar system is 4.6 billion years old. To calculate this figure, they examine an unstable element, which is subject to radioactive decay. By observing how much this element has decayed, they can calculate how old this element is. The oldest rocks on earth are approximately 3.9 billion years old, however it is hard to find these rocks as the earth has been thoroughly resurfaced. To estimate the age of the solar system, scientists must find rocks from space, such as meteorites – which are formed during the early condensation of the solar nebula. The oldest meteorite was found to have an age of 4.6 billion years, hence the solar system must be around 4.6 billion years old.

Galactic orbit of the solar system

The solar system is part of the Milky Way galaxy, a spiral galaxy with a diameter of about 100,000 light years containing approximately 200 billion stars, of which our Sun is rather large and bright. (The vast majority of stars are red dwarfs; our Sun is placed near the middle of the Hertzsprung-Russell diagram, but stars larger and hotter than it are rare, whereas stars dimmer and cooler than it are very common, although we can observe only those few other red dwarfs that are very near our Sun in space). Estimates place the solar system at between 25,000 and 28,000 light years from the galactic center in the Orion Arm. Its speed is about 220 kilometres per second, and it completes one revolution every 226 million years. At the galactic location of the solar system, the escape velocity with regard to the gravity of the Milky Way is about 1000 km/s. The solar system appears to have a very unusual orbit. It is both extremely close to being circular, and at nearly the exact distance at which the orbital speed matches the speed of the compression waves that form the spiral arms. The solar system appears to have remained between spiral arms for most of the existence of life on Earth. The radiation from supernovae in spiral arms could theoretically sterilize planetary surfaces, preventing the formation of large animal life on land. By remaining out of the spiral arms, Earth may be unusually free to form large animal life on its surface.

Planetary system formation

For many years, our solar system had the only planetary system known, and so theories of planetary formation only had to explain one system to be plausible. The discovery in recent years of many extrasolar planets has uncovered systems very different to our own, and theories have had to be revised accordingly. Exoplanets have not been seen by astronomers yet, however we know they exist because of the gravitational tug the planets induce on the star, and hence making the star ‘wobble’. Astronomers can calculate how massive the planets are by observing how much the star wobbles. Exoplanets can also be observed more directly by their occultation of the stars' discs, which dims them slightly. In October, 1995, astronomers Michel Mayor and Didier Queloz announced the discovery of a massive planet orbiting 51 Pegasi – a Sun-like star in the constellation Pegasus. This planet is about half as massive as Jupiter, and had an orbital period of 4.2 Earth days, due to its closeness to the star (0.05 AU). Since then, over 160 more planets have been identified. Many extrasolar planetary systems contain such a “hot Jupiter”: a planet comparable to or larger than Jupiter orbiting very close to the parent star, perhaps orbiting it in a matter of days. It has been hypothesised that while the giant planets in these systems formed in the same place as the gas giants in our system did, some sort of migration took place which resulted in the giant planet spiralling in towards the parent star. Any terrestrial planets which had previously existed would presumably either be destroyed or ejected from the system. There has also been some photographic evidence to suggest that regions in the Orion Nebula, which is 1500 light years from Earth, have star systems forming.

Discovery of the solar system

The planets out to Saturn were known to ancient astronomers, who observed the wandering of these objects against the apparently fixed pattern of stars. Venus and Mercury were each identified as single objects despite the difficulty of connecting "evening" and "morning stars". It was also identified that the two non-pointlike objects, the sun and the Moon, moved across the same fixed background. However knowledge of the nature of these celestial drifters was entirely speculative and largely incorrect. The nature and structure of the solar system were long misperceived, for at least two reasons:
- The Earth was considered stationary, and the motion of objects in the sky was therefore taken at face value: the sun was thought to orbit the Earth, for example (This conception of the universe, in which the Earth is at the center, is called the Geocentric model; geos means "Earth" in Greek).
- Many solar system objects and phenomena cannot be perceived at all without technical aid. Over the last several hundred years, conceptual and technological advances have helped us understand the solar system much better. The first and most fundamental of the conceptual advances was the Copernican Revolution, which proposed that the planets orbit the sun—models of the solar system with the sun in the center are called heliocentric (helios meaning "Sun" in Greek). Despite the name, the most striking (and then-controversial) Copernican realization was not that the sun was central but that the Earth was peripheral, orbital: planets had been considered merely points in the sky, but if the Earth itself was a planet, perhaps the other planets were, like Earth, huge solid spheres. Philosophically, there were a number of objections to heliocentrism:
- If the Earth is moving, what force keeps the air from flying off into space?
- The Earth is made of heavy rock. Heavy rock moves down. Down in a sphere means the centre. The planets are ephemeral and light, so they are above. How can Earth be a planet?
- If the Earth is mobile, then why do we not observe parallax in the stars (the stars appearing to shift in relation to further objects due to the change in position)? The subsequent invention of the telescope gave the principal technological advance on discovering the solar system, with Galileo's improved version of the telescope rapidly giving benefit in terms of discovering satellites of other planets, especially Jupiter's four major satellites. This showed that all objects in the universe did not orbit the Earth. However, perhaps Galileo's most important discovery was that the planet Venus has phases like the Moon, proving that it must orbit the Sun. Then, in 1687, Isaac Newton devised his law of universal gravitation which explained the force that both kept the Earth moving through the heavens and also kept the air from flying away. Finally, in 1838, astronomer Friedrich Wilhelm Bessel successfully measured the parallax of the star 61 Cygni, proving conclusively that the Earth was in motion.

Exploration of the solar system

Since the start of the space age, a great deal of exploration has been performed by unmanned space missions that have been organized and executed by various space agencies. The first probe to land on another solar system body was the Soviet Union's Luna 2 probe, which impacted on the Moon in 1959. Since then, increasingly distant planets have been reached, with probes landing on Venus in 1965, Mars in 1976, the asteroid 433 Eros in 2001, and Saturn's moon Titan in 2005. Spacecraft have also made close approaches to other planets: Mariner 10 passed Mercury in 1973. The first probe to explore the outer planets was Pioneer 10, which flew by Jupiter in 1973. Pioneer 11 was the first to visit Saturn, in 1979. The Voyager probes performed a grand tour of the outer planets following their launch in 1977, with both probes passing Jupiter in 1979 and Saturn in 1980–1981. Voyager 2 then went on to make close approaches to Uranus in 1986 and Neptune in 1989. The Voyager probes are now far beyond Pluto's orbit, and astronomers anticipate that they will encounter the heliopause which defines the outer edge of the solar system in the next few years. Pluto remains the only planet not having been visited by a man-made spacecraft, though that will change with the launching of New Horizons by NASA in January 2006. It is scheduled to fly by Pluto in July 2015 and then make an extensive study of as many Kuiper Belt objects as it can. Through these unmanned missions, we have been able to get close-up photographs of most of the planets and, in the case of landers, perform tests of their soils and atmospheres. Manned exploration, meanwhile, has only taken human beings as far as the Moon, in the Apollo program. The last manned landing on the Moon took place in 1972, but the recent discovery of ice in deep craters in the polar regions of the Moon has prompted speculation that mankind may return to the Moon in the next decade or so. Manned missions to Mars have been eagerly anticipated by generations of space enthusiasts, and it was hoped that the first manned interplanetary flights would take place in the 1980s, after the successful Apollo program. Europe (ESA and EU) now plans manned Lunar and Mars missions as part of Aurora Exploration Programme endorsed in 2001. United States followed with similar programme called Vision for Space Exploration in 2004.

Attributes of major planets

All attributes below are measured relative to the Earth: Of the other objects, Ganymede has the largest mass (0.02). Note: Although is a minor planet, it is being considered as possibly being a major planet (the tenth in the solar system). See Planet (Table) for a more comprehensive table.

Attributes of the largest minor planets

The largest minor planets are smoothly rounded, like planets, because their gravity overcomes material strength that keeps smaller bodies in non-spherical shapes. Before the discovery of 2060 Chiron and the trans-Neptunian objects, the term "minor planet" was a synonym for asteroid, but many people now prefer to restrict the use of "asteroid" to refer to rocky bodies of the inner solar system. Most trans-Neptunian objects are icy, like comets, although those we can detect at that distance are much larger than comets. Several asteroids, in the strict sense, are large enough to be spherical. The largest known trans-Neptunian objects are much larger than the large asteroids. (Natural satellites of major planets also range smoothly from small non-spherical objects to large spherical ones, and the largest are larger than 1 Ceres, the largest asteroid). All attributes below are measured relative to the Earth:

Other facts

The total surface area of the solar system's objects that have solid surfaces and a diameter greater than 1 km is ~1.7 km2 —about 11 times the area of the Earth's land masses. It has been suggested that the Sun may be part of a binary star system, with a distant companion named Nemesis. Nemesis was proposed to explain some timing regularities of the great extinctions of life on Earth. The hypothesis says that Nemesis creates periodical perturbations in the Oort cloud of comets surrounding the solar system, causing a "comet shower". Some of them hit Earth, causing destruction of life. This hypothesis is no longer taken seriously by most scientists, mostly because infrared surveys failed to spot any such object, which should have been very conspicuous at those wavelengths. The concept of the tenth planet has frequently been explored in science fiction works and conspiracy theories (see also Planet X, and hypothetical planet).

The solar system in small scales

Scaling down the size of the solar system makes it easier for students to grasp the relative distances. The enormous ratio of interplanetary distances to planetary diameters makes constructing a scale model of the solar system a challenging task. (For example, the distance between the Earth and the Sun is almost 12,000 times the diameter of the Earth.) Several places have built such models.

The solar system in astrology

See also


- Astronomical symbols
- Definition of planet
- Geological features of the Solar System
- Laws of Kepler
- :Category:Lists of Solar system objects
- Minor planet
- Numerical model of solar system
- Origin of life
- Planetary system
- Planetary nomenclature
- Solar system by size
- Stellar system
- Table of planetary attributes
- Timeline of solar system astronomy
- Titius-Bode law
- Zodiacal light

External links


- [http://solarsystem.nasa.gov/index.cfm NASA's Solar System Exploration site]
- [http://space.jpl.nasa.gov NASA's Solar System Simulator]
- [http://www.jpl.nasa.gov/solar_system NASA/JPL Solar System main page]
- [http://members.aol.com/astroequation/ Astronomical Enigma] Mathematical Order in the orbits of the solar system.
- [http://www.solarviews.com Solarviews]
- [http://celestia.sourceforge.net Celestia] Free 3D realtime space-simulation (OpenGL)
- [http://www.nineplanets.org/ The Nine Planets] Comprehensive solar system site by Bill Arnett
- [http://www.krysstal.com/solarsys_planets.html Planetary data]
- [http://www.solstation.com/habitable.htm Stars and Habitable Planets]
- [http://www.michaelschultz.de/index_en.html Solar System] An interactive planets animation (145 zoom steps and time effects)
- [http://my.execpc.com/~culp/space/timeline.html Timeline of solar system exploration]
- [http://www.anzwers.org/free/universe/index.html An Atlas of the Universe]
- mirror matter [http://uk.arxiv.org/abs/astro-ph/0104251 planets] and other [http://uk.arxiv.org/abs/astro-ph/0110161 mirror objects] in the solar system?
- [http://www.solarsystem.org.uk/ The Virtual Solar System, including a scale model of the system]
- ko:태양계 ms:Sistem suria ja:太陽系 simple:Solar system th:ระบบสุริยะ zh-min-nan:Thài-iông-hē

Gas giant

:This article refers to a astronomical phenomenon. For the rock band, see Gas Giants A gas giant is a large planet that is not composed mostly of rock or other solid matter. Gas giants may still have a rocky or metallic core—in fact, it is expected that such a core is probably required for a gas giant to form—but the majority of its mass is in the form of gas (or gas compressed into a liquid state). Unlike rocky planets, gas giants do not have a well-defined surface. Terms like diameter, surface area, volume, surface temperature and surface density may refer to the outermost layer visible from outside, e.g. from the Earth. gas There are four gas giants in our solar system: Jupiter, Saturn, Uranus, and Neptune. These are also known as the Jovian planets. Uranus and Neptune have been referred by scientists in the past as a separate subclass of giant planets, ice giants, or Uranian planets due to their structure made mostly of ice and rock and gas, which differs from the "traditional" gas giant such as Jupiter or Saturn. This is because their proportion in hydrogen and helium is much lower than the latter's, mostly because of their greater distance from the Sun.

Common structure

The four solar system gas giants share a number of features. All have atmospheres that are mostly hydrogen and helium, and that blend into the liquid interior at pressures greater than the critical pressure, so that there is no clear boundary between atmosphere and body. They have very hot interiors, ranging from about 5000 K for Neptune to over 20,000 K for Jupiter. This great heat means that, beneath their atmospheres, the planets are most likely entirely liquid. When discussions refer to a "rocky core", one should not picture a ball of solid granite, or even, at 20,000 K, liquid granite. Rather, what is meant is a region in which the concentration of heavier elements such as iron and silicon is greater than that in the rest of the planet. All four planets rotate relatively rapidly, which causes wind patterns to break up into east-west bands or stripes. These bands are prominent in Jupiter, muted in Saturn and Neptune, and barely detectable at all in Uranus. Finally, all four are accompanied by elaborate systems of rings and moons. Saturn's rings are the most spectacular, and the only ones known before the 1970s. As of 2004, Jupiter was thought to have the most moons, with more than sixty found.

Jupiter and Saturn

Jupiter and Saturn consist almost entirely of hydrogen and helium, and they are so large that this is true even though both are thought to have several Earth masses of heavier elements. Their deep interiors consist of liquid metallic hydrogen, a form of hydrogen distinguished by the fact that it conducts electricity. Both planets have magnetic fields oriented fairly close to their axes of rotation.

Uranus and Neptune

Uranus and Neptune have distinctly different interior compositions, with the bulk of their interiors thought to consist of a mixture (or layered assortment) of rock, water, methane, and ammonia. Both have magnetic fields that are sharply inclined to their axes of rotation.

Terminology

The term was coined by the science fiction writer James Blish. Arguably it is a misnomer, since all of these planets are primarily liquid and not gaseous. In fact, for Neptune and Uranus, the gaseous atmospheres are quite thin compared to the planetary radii -- only extending perhaps one percent of the way to the center. However, at least for Jupiter and Saturn, the name is defensible because their compositions are dominated by hydrogen and helium, which are gases in the outer solar system when not under pressure. Planetary scientists often use 'rock', 'gas', and 'ice' as shorthands for classes of elements and compounds commonly found as planetary constituents, irrespective of what phase they appear in. In the outer solar system, hydrogen and helium are "gases"; water, methane, and ammonia are "ices"; and silicates are rock. When deep planetary interiors are considered, it may not be far off to say that, by "ice" astronomers mean oxygen and carbon, by "rock" they mean silicon, and by "gas" they mean hydrogen and helium. With this terminology in mind, some astronomers are starting to refer to Uranus and Neptune as "ice giants", to indicate the apparent predominance of the "ices" (in liquid form) in their interior composition.

Extrasolar gas giants

Because of the techniques currently available to detect extrasolar planets, all of those found to date have been of a scale associated, in the Solar system, with gas giants. The smallest found as of September 2004 is comparable in mass to Neptune, and many have masses several times that of Jupiter. Many of the extrasolar planets are much closer to their parent stars and hence much hotter than gas giants in the solar system, making it possible that some of those planets are a type not observed in our solar system. Considering the relative abundances of the elements in the universe (approximately 90% hydrogen), it would be surprising to find a predominantly rocky planet more massive than Jupiter. On the other hand, previous models of planetary system formation suggested that gas giants would be inhibited from forming as close to their stars as have many of the new planets that have been observed. extrasolar planet The upper mass limit of a gas giant planet is approximately 70 times that of Jupiter (around 0.08 times the mass of the Sun). Above this point, the intense heat and pressure at the planet's core begin to induce nuclear fusion and the planet ignites to become a red dwarf. Interestingly there appears to be a mass gap between the heaviest gas giant planets detected (about 10 times the mass of Jupiter) and the lightest red dwarfs. This has led to suggestions that the formation process for planets and binary stars may be fundamentally different.

See also


- jovian planet
- brown dwarf
- terrestrial planets
- solar system
- planetary system
- floating cities Category:Planets ms:Gergasi gas

Interstellar space

In astronomy, the interstellar medium (or ISM) is the matter and energy content that exists between the stars (or their immediate circumstellar environment) within a galaxy. The ISM plays a crucial role in astrophysics precisely because of its intermediate role between stellar and galactic scales. Stars themselves form within cold regions of the ISM, and replenish the ISM with matter and energy through stellar winds and supernovae. In turn, this interplay between stars and the ISM sets the rate at which a galaxy depletes its gaseous content and therefore determines its lifespan of active star formation. The ISM consists of an extremely dilute (by terrestrial standards) plasma, consisting of a mixture of atoms, molecules, dust, electromagnetic radiation, cosmic rays, and the magnetic field. The matter normally consists of about 99% gas particles and usually 1% dust. It fills interstellar space. This mixture is usually extremely tenuous, with typical gas densities ranging from a few single to a few hundred particles per cubic centimeter. As a result of primordial nucleosynthesis, the gas is roughly 90% hydrogen and 10% helium, with additional elements ("metals" in astronomical parlance) present in trace amounts. The medium is also responsible for extinction, namely the decreasing light intensity of a star as the light travels through the medium. This extinction is caused by refraction and absorption of photons in certain wavelengths. For example, a typical absorption wavelength of atomic hydrogen lies at about 121.5 nanometers, the Lyman-alpha transition. Therefore, it is nearly impossible to see light emitted at that wavelength from a star, because most of it is absorbed during the trip to Earth by Lyman-alpha absorption. The interstellar medium is usually divided into three phases, depending on the temperature of the gas: hot (millions of kelvins), warm (thousands of kelvins), and cold (tens of kelvins). This "three-phase" model of the ISM was initially developed by McKee and Ostriker in a 1977 paper, which has formed the basis for further study over the past quarter-century. The relative proportions of the phases are still a matter of considerable contention in scientific circles. Features prominent in the study of the interstellar medium include molecular clouds, interstellar clouds, supernova remnants, planetary nebulae, and similar diffuse structures.

History

Originally, astronomers thought that space was an empty vacuum. In 1913, Norwegian explorer and physicist Kristian Birkeland may have been the first to predict that space is not only a plasma, but also contains "dark matter". He wrote: "It seems to be a natural consequence of our points of view to assume that the whole of space is filled with electrons and flying electric ions of all kinds. We have assumed that each stellar system in evolutions throws off electric corpuscles into space. It does not seem unreasonable therefore to think that the greater part of the material masses in the universe is found, not in the solar systems or nebulae, but in "empty" space. (See "Polar Magnetic Phenomena and Terrella Experiments", in The Norwegian Aurora Polaris Expedition 1902-1903 (publ. 1913, p.720).

See also


- Cosmic dust
- Diffuse interstellar band
- Heliosphere
- Intergalactic space
- List of molecules in interstellar space
- Outer space
- Solar system
- Stellar system
- Timeline of knowledge about the interstellar and intergalactic medium Category:Astronomical objects Category:Stars ko:성간매질 ja:星間物質

1998

1998 (MCMXCVIII) is a common year starting on Thursday of the Gregorian calendar, and was designated the International Year of the Ocean.

Events

January


- January 1998 - A massive ice storm, caused by El Niño, strikes New England, southern Ontario and Quebec, resulting in widespread power failures, severe damage to forests, and a number of deaths.
- January 1 - Smoking is banned in all California bars and restaurants.
- January 2 - Russia begins to circulate new rubles to stem inflation and promote confidence.
- January 2 - Gunman shoots Antario Teodoro Filho, Brazilian politician and radio presenter, in a middle of his broadcast.
- January 4 - Wilaya of Relizane massacres of 4 January 1998 in Algeria; over 170 killed in three remote villages.
- January 6 - The Lunar Prospector spacecraft is launched into orbit around the Moon and later found evidence for frozen water in soil in permanently shadowed craters near the Moon's poles.
- January 8 - Ramzi Yousef is sentenced to life in prison for planning the World Trade Center bombing.
- January 8 - Cosmologists announce that the expansion rate of the universe is increasing.
- January 11 - Sidi-Hamed massacre in Algeria; over 100 people killed.
- January 12 - 19 European nations agree to forbid human cloning.
- January 13 - A tourist visiting the White House sprays paint on to marble busts of Giuseppe Ceracchi
- January 14 - Researchers in Dallas, Texas present findings about an enzyme that slows aging and cell death (apoptosis).
- January 15 - The stalker of Howard Stern, Lance Carvin, is sentenced to 2 1/2 years for threatening to kill Stern and his family.
- January 16 - NASA announces that John Glenn will return to space when Space Shuttle Discovery blasts off in October 1998.
- January 17 - Paula Jones accuses President Bill Clinton of sexual harassment.
- January 20 - Nepalese police intercepts a shipment of 272 human skulls in Kathmandu
- January 22 - Suspected "Unabomber" Theodore Kaczynski pleads guilty and accepts a sentence of life without the possibility of parole.
- January 26 - Lewinsky scandal: On American television, Bill Clinton denies he had "sexual relations" with former White House intern Monica Lewinsky.
- January 26 - Compaq buys Digital Equipment Corporation.
- January 26 - Monkeys attack people in Ito, Japan
- January 27 - American First Lady Hillary Rodham Clinton appears on the Today show calling the attacks against her husband part of a "vast right-wing conspiracy."
- January 28 - Ford Motor Company announces the buyout of Volvo Cars for $6.45 billion.
- January 28 - Gunmen hold at least 400 children and teachers hostage for several hours at an elementary school in Manila, Philippines.
- January 29 - In Birmingham, Alabama a bomb explodes at an abortion clinic killing one and severely wounding another. Serial bomber Eric Rudolph is suspected as the culprit.

February


- February - Iraq disarmament crisis: The United States Senate passes resolution 71, which urged President Bill Clinton to "take all necessary and appropriate actions to respond to the threat posed by Iraq's refusal to end its weapons of mass destruction programs."
- February 3 - Cavalese cable-car disaster: a United States Military pilot causes the death of 20 people near Trento, Italy when his low-flying plane severs the cable of a cable-car.
- February 3 - Karla Faye Tucker is executed in Texas becoming the first woman executed in the United States since 1984.
- February 4 - An earthquake measuring 6.1 on the Richter Scale in northeast Afghanistan kills more than 5,000.
- February 6 - Washington National Airport is renamed Ronald Reagan Washington National Airport.
- February 6 - The French prefect Claude Erignac is assassinated in the streets of Ajaccio (Corse) by a commando of Corsican insurgents, among them Yvan Colonna (trial june 2).
- February 7 - Roger Nicholas Angleton committed suicide in a prison cell in Houston, Texas by cutting himself with razor blades. He admitted to murdering socialite Doris Angleton in her River Oaks home in his suicide note.
- February 10 - A college dropout becomes the first person to be convicted of a hate crime committed in cyberspace.
- February 10 - Voters in Maine repeal a gay rights law passed in 1997 becoming the first U.S. state to abandon such a law.
- February 12 - The presidential line-item veto is declared unconstitutional by a United States federal judge.
- February 14 - Authorities in the United States announce that Eric Rudolph is a suspect in an Alabama abortion clinic bombing.
- February 15 - Dale Earnhardt wins the Daytona 500 in his 20th try after many unsucsessful attempts.
- February 16 - China Airlines Flight 676 crashed into a residential area near by Chiang Kai-shek International Airport, killing 202 people, included all 196 on board and six on the ground.
- February 18 - Two white separatists were arrested in Nevada and accused of plotting a biological attack on New York City subways.
- February 19 - 66-day blackout begins in Auckland, New Zealand.
- February 19 - Larry Wayne Harris of the Aryan Nations and William Leavitt are arrested in Henderson, New York for possession of military grade anthrax
- February 20 - Iraq disarmament crisis: Iraqi President Saddam Hussein negotiates a deal with U.N. secretary-general Kofi Annan, allowing weapons inspectors to return to Baghdad, preventing military action by the U.S. and Britain.
- February 22 - Collapse of one third of the Tower block "Palace II" in Rio de Janeiro, Rio de Janeiro, Brazil.
- February 23 - Tornadoes in central Florida destroy or damage 2,600 structures and kill 42 (see Florida El Niño Outbreak).
- February 23 - Osama bin Laden publishes fatwa declaring jihad against all Jews and Crusaders.
- February 24 - Hustler publisher Larry Flynt is acquitted of charges of defamation of Jerry Falwell.
- February 24 - A man tries to hijack Turkish Airlines passenger plane claiming that he has a bomb in his teddy bear. Passengers disapprove and apprehend him
- February 28 - Serbian police begin to wipe out so-called "terrorist gangs" in Kosovo.

March


- March 1 - Attack Submarine USS Sea Devil (now ex-Sea Devil (SSN-664)) starts to be deactivated
- March 2 - Data sent from the Galileo probe indicates that Jupiter's moon Europa has a liquid ocean under a thick crust of ice
- March 4 - Gay rights: The Supreme Court of the United States rules that federal laws banning on-the-job sexual harassment also apply when both parties are the same sex.
- March 5 - NASA announced that the Clementine probe orbiting the Moon had found enough water in polar craters to support a human colony and rocket fueling station
- March 5 - NASA announces the choice of United States Air Force Lt. Col. Eileen Collins as commander of a future Space Shuttle Columbia mission to launch an X-ray telescope making Collins the first woman commander of a space shuttle mission.
- March 6 - Closure of the South Crofty tin mine
- March 6 - The Imperial Wizard of the Ku Klux Klan is fined for burning a cross in his garden and infringing air regulations in California
- March 10 - American troops stationed in the Persian Gulf begin to receive the first vaccinations against anthrax.
- March 11 - Danish parliamentary election held, unexpectedly returning Prime Minister Poul Nyrup Rasmussen to power.
- March 14 - An earthquake measuring 6.9 on the Richter scale hits southeastern Iran
- March 23 - At the Academy Awards ceremony Titanic wins 11 Oscars
- March 24 - In Jonesboro, Arkansas, two young boys (aged 11 and 13 years) fire upon students at Westside Middle School while hidden in woodlands near the school. Four students and one teacher are killed and 10 injured
- March 26 - Oued Bouaicha massacre in Algeria; 52 people killed with axes and knives, 32 of them babies under the age of 2.
- March 27 - The FDA approves Viagra for use as a treatment for male impotence, becoming the first pill to be approved to treat this condition in the United States.

April


- April 1 - Ukrainian serial killer Anatoly Onoprienko is sentenced to death for 52 murders
- April 5 - In Japan, the Akashi-Kaikyo Bridge linking Shikoku with Honshu and costing cost about US$3.8 billion, opens to traffic, becoming the largest suspension bridge in the world.
- April 6 - Pakistan tests medium-range missiles capable of hitting India
- April 7 - Citicorp and Travelers Group announce plans to merge creating the largest financial-services conglomerate in the world, Citigroup
- April 8 - Iraq disarmament crisis: UNSCOM reports to the UN Security Council that Iraq's declaration on its biological weapons program is incomplete and inadequate.
- April 10 - Good Friday: 18 hours after the end of talks deadline the Belfast Agreement is signed between the Irish and British governments and most Northern Ireland political parties, with the notable exception of the Democratic Unionist Party.
- April 16 - A massive tornado occurred in Nashville, Tennessee. It is the first tornado in 11 years to make a direct hit on a major city. (see Nashville Tornado of 1998)
- April 25 - A waste reservoir at Los Frailes mine in Andalusia, Spain, ruptures, discharging heavy metal waste into the Guadiamar River. The pollution threatens the sensitive ecosystem and endangered species of Doñana National Park, Spain's largest nature reserve, but is diverted into the Guadalquivir River. Up to 100 km² of farmland are ruined by the spill. [http://edition.cnn.com/EARTH/9804/25/spain.disaster.reut/]

May


- May 2 - Japanese rock star hide (Hideto Matsumoto) mysteriously dies of asphyxiation.
- May 7 - Apple Computer unveils the iMac.
- May 9 - Dana International, a transexual singer from Israel, wins the 1998 Eurovision Song Contest in Birmingham,UK.
- May 11 - Nuclear testing: In the Rajasthan Desert, India conducts its second series of underground nuclear tests (the first were in 1974) and inflaming its rival neighbor Pakistan (who already has nuclear weapons).
- May 13 - Following India's second round of nuclear tests the United States and Japan impose economic sanctions on the nation.
- May 15 - Iraq disarmament crisis: UNSCOM learns that an Iraqi delegation has travelled to Bucharest to meet with scientists who can provide the country with missile guidance systems.
- May 18 - United States v. Microsoft: The United States Department of Justice and 20 U.S. states file an antitrust case against Microsoft
- May 21 - School shooting: At Thurston High School in Springfield, Oregon, Kipland Kinkel (who was suspended for bringing a gun to school) shoots a semi-automatic rifle into a room filled with students killing 2 wounding 25 others after killing his parents at home
- May 21 - Reproductive rights: In Miami, Florida, five abortion clinics are hit by a butyric acid attacker
- May 21 - Suharto resigns, after 32 years as Indonesian President and 7th consecutive re-election by the Indonesian Parliament (MPR). Suharto's hand-picked Vice President, B. J. Habibie, became Indonesia's third president.
- May 21 to September 30 - Expo '98 is held in Lisbon, Portugal, with the title "Oceans, an Heritage for the Future". UNESCO had previously declared 1998 to be the International Year of the Oceans due to the Expo. 12 million people attend the world fair
- May 22 - Lewinsky scandal: A federal judge rules that United States Secret Service agents can be compelled to testify before a grand jury concerning the scandal
- May 27 - Oklahoma City bombing: Michael Fortier is sentenced to 12 years in prison and fined $200,000 for failing to warn authorities about the terrorist plot.
- May 28 - Nuclear testing: In response to a series of Indian nuclear tests, Pakistan explodes six nuclear devices of its own in the Chaghai hills of Baluchistan, prompting the United States, Japan and other nations to impose economic sanctions.
- May 28 - Wife of US comedian Phil Hartman kills him and commits suicide afterwards
- May 30 - Nuclear testing: Pakistan conducts two more nuclear explosions following its first test.
- May 30 - A 6.6 magnitude earthquake hits northern Afghanistan killing up to 5,000.
- May 31 - Geri Halliwell, better known as "Ginger Spice", announced her departure from the biggest selling girl group of all time, the Spice Girls

June


- June 2 - The CIH virus is discovered in Taiwan.
- June 2 - Voters in California approved California Proposition 227, abolishing that state's bilingual education program.
- June 3 - Eschede train disaster: an ICE high speed train derails, causing 101 deaths.
- June 4 - Terry Nichols is sentenced to life in prison for his role in the Oklahoma City bombing
- July 5 - Japan launches a probe to Mars, and thus joins the United States and Russia as a space exploring nation
- June 5 - A strike begins at the General Motors parts factory in Flint, Michigan that quickly spreads to five other assembly plants (the strike lasted seven weeks)
- June 8 - Charlton Heston assumes the presidency of the National Rifle Association.
- June 8 - President Sani Abacha of Nigeria dies of apparent heart failure
- June 12 - A jury in Hattiesburg, Mississippi, convicts 17-year-old Luke Woodham of killing two students and wounding seven others at Pearl High School [http://www.cnn.com/US/9806/12/school.shooting.verdict/]
- June 12 - 13-year old Christina Marie Williams was kidnapped in Seaside, California while taking her dog for a walk.
- June 14 - The Chicago Bulls win their sixth NBA title in 8 years when they beat the Utah Jazz, 87-86 in Game Six. This is also Michael Jordan's last game as a Bull.
- June 16 - The Detroit Red Wings sweep the Washington Capitals in 4 games in the 1998 Stanley Cup Finals.
- June 25 - In Clinton v. City of New York, the United States Supreme Court decides that the Line Item Veto Act of 1996 is unconstitutional.

July


- July 6 - The new Hong Kong International Airport at Chek Lap Kok opens.
- July 10 - The DNA-identified remains of United States Air Force 1st Lt. Michael Joseph Blassie arrive home to his family in St. Louis, Missouri after being in the Tomb of the Unknowns since 1984
- July 10 - Catholic priests' sex abuse scandal: The Diocese of Dallas agrees to pay $23.4 million to nine former altar boys who claimed they were sexually abused by former priest Rudolph Kos
- July 12 - France defeats Brazil 3-0 to win the Football World Cup 1998
- July 17 - In St. Petersburg, Nicholas II of Russia and his family are buried in St. Catherine Chapel 80 years after he and his family were killed by Bolsheviks
- July 17 - A tsunami triggered by an undersea earthquake destroys 10 villages in Papua New Guinea killing an estimated 1,500, leaving 2,000 more unaccounted for and thousands more homeless
- July 17 - Biologists report in the journal Science how they sequenced the genome of the bacterium that causes syphilis, Treponema pallidum
- July 24 - Russel Eugene Weston Jr. bursts into the United States Capitol and opens fire killing two police officers. He is later ruled to be incompetent to stand trial
- July 25 - The United States Navy commissions the aircraft carrier USS Harry S. Truman and puts her into service
- July 25 - Wakayama Arsenic poison case - 63 poisoned and 4 dead by arsenic in a festival in the town in Wakayama Prefecture in Japan - Masumi Hayashi is arrested for murder
- July 28 - Monica Lewinsky scandal: Ex-White House intern, Monica Lewinsky receives transactional immunity in exchange for her grand jury testimony concerning her relationship with US President Bill Clinton.
- July 31 - UK import ban on landmines

August

landmines
- August 7 - Yangtze River Floods: In China the Yangtze River breaks through the main bank, before this from August 1-5 periphery levees collapsed consecutively in Jiayu County Baizhou Bay. The death toll was more than 12,000 injuring many thousands more.
- August 5 - Iraq disarmament crisis: Iraq officially suspends all cooperation with UNSCOM teams
- August 7 - 1998 U.S. embassy bombings: Bombing of the United States embassies in Dar es Salaam, Tanzania, and Nairobi, Kenya kills 224 people and injures over 4,500. The bombings were linked to Osama Bin Laden.
- August 15 - The Real IRA detonate a car bomb in Omagh, County Tyrone, Ireland, killing 29 and injuring over 200 - the greatest loss of life in a single incident of The Troubles.
- August 16 - Silk-Miller police murders: Australian police officers murdered in Moorabbin, Victoria.
- August 17
  - Monica Lewinsky scandal: US President Bill Clinton admits in taped testimony that he had an "improper physical relationship" with White House intern Monica Lewinsky. On the same day he admits before the nation that he "misled people" about his relationship
  - Russian financial crisis: Devaluation of the rouble. The ruble lost 70% of its value against US dollar in 6 months following August 1998. Several largest Russians banks collapsed, and millions of people lost their savings.
- August 20 - The Supreme Court of Canada states Quebec can not legally secede from Canada without the federal government's approval
- August 20 - 1998 U.S. embassy bombings: The United States military launches cruise missile attacks against alleged Al-Qaeda camps in Afghanistan and a suspected chemical plant in Sudan in retaliation for the August 7 bombings of American embassies in Kenya and Tanzania. The al-Shifa pharmaceutical factory in Khartoum is destroyed in the attack
- August 26 - Iraq disarmament crisis: Scott Ritter resigns from UNSCOM, sharply criticized the Clinton administration and the U.N. Security Council for not being vigorous enough about insisting that Iraq's weapons of mass destruction be destroyed. Ritter told reporters that "Iraq is not disarming," "Iraq retains the capability to launch a chemical strike."
- August 31 - North Korea reportedly launches Kwangmyongsong, their first satellite. Although North Korea reports that it reached stable orbit, NORAD was never able to confirm this assertion

September


- September 2 - In Canada, pilots for Air Canada launch the first strike in company's history
- September 2 - A McDonnell Douglas MD-11 airliner carrying Swissair flight 111 crashes near Peggys Cove, Nova Scotia after taking off from New York City en-route to Geneva. All 229 people on board are killed
- September 2 - A United Nations court finds Jean-Paul Akayesu, the former mayor of a small town in Rwanda, guilty of nine counts of genocide, marking the first time that the 1948 law banning genocide is enforced
- September 3 - In Somalia, the southern port of Kismayo is declared the capital of independent Jubaland under Muhamed Said Hersi
- September 7 - Google Inc. is founded.
- September 8 - St. Louis Cardinals first baseman Mark McGwire breaks baseball's single season homerun record, formerly held by Roger Maris. McGwire hits #62 at Busch Stadium in the fourth inning off of Chicago Cubs pitcher Steve Trachsel.
- September 9 - The United Nations General Assembly elects Didier Opertiri of Uruguay as president for its 53rd session
- September 14 - GSPC formed in Algeria, splitting off from the GIA over its policy of massacring civilians.
- September 15 - Telecommunications companies MCI Communications and WorldCom complete their $37 billion merger to form MCI WorldCom.
- September 25 - 28 September -- Major creditors of Long-Term Capital Management, a Greenwich, Connecticut based hedge fund, after days of tough bargaining and some informal mediation by officials of the Federal Reserve agree on terms of a re-capitalization -- i.e. they create a consortium that takes over the fund's failing portfolio.
- September 26 - The Adelaide Crows do what the critics said was impossible, win their 2nd AFL (Australian Football League) Premiership to make it Back2Back.
- September 29 - Iraq disarmament crisis: The U.S. Congress passes the "Iraq Liberation Act", which states that the United States wants to remove Saddam Hussein from power and replace the government with a democratic institution.

October


- October 3 — In Australia, John Howard's coalition government was re-elected for a second term.
- October 4 - Leafie Mason is murdered in her Hughes Springs, Texas house by Angel Maturino Resendiz. She was his second victim in his second incident.
- October 6 - Matthew Shepard, a Wyoming college student, is found tied to a fence, the victim of a gay-bashing. He dies on Monday, October 12, becoming a symbol of victims of gay-bashing and sparking public reflection on homophobia.
- October 7 - Oslo