Solar System - astronomy.
Publié le 11/05/2013
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dwarf planets according to the IAU because they have rounded shapes from their own gravity but have not cleared their neighborhoods in space of other objects—bothorbit through the Kuiper Belt, a region beyond Neptune containing thousands of small icy bodies.
Pluto and Eris are composed of layers of ice around a rocky core.Ceres qualifies as a dwarf planet because it is spherical but is found in the asteroid belt, a zone between the orbits of Mars and Jupiter that contains thousands of smallrocky bodies.
Ceres is likely made up of a rocky core surrounded by a mantle containing a mix of rock and ice.
Like asteroids, dwarf planets are listed by the IAU asminor planets with numbers and names.
The asteroids are small, rocky bodies that move in orbits primarily between the orbits of Mars and Jupiter.
Numbering in the thousands, asteroids range in size fromaround 530 km (329 mi)—about half the size of the dwarf planet Ceres—to microscopic grains.
Some asteroids are perturbed, or pulled by forces other than theirattraction to the Sun, into eccentric orbits that can bring them closer to the Sun.
If the orbits of such bodies intersect that of Earth, they are called meteoroids.
Whenthey appear in the night sky as streaks of light, they are known as meteors, and recovered fragments are termed meteorites.
Laboratory studies of meteorites haverevealed much information about primitive conditions in our solar system.
The surfaces of Mercury, Mars, and several satellites of the planets (including Earth’s moon)show the effects of an intense bombardment by asteroidal and cometary objects early in the history of the solar system.
On Earth that record has eroded away, exceptfor a few recently found impact craters.
Some meteors and interplanetary dust may also come from comets, which are basically aggregates of dust and frozen gases typically 5 to 10 km (about 3 to 6 mi) indiameter.
Comets orbit the Sun at distances so great that they can be perturbed by stars into orbits that bring them into the inner solar system.
As comets approachthe Sun, they release their dust and gases to form a spectacular coma and tail.
Under the influence of Jupiter’s strong gravitational field, comets can sometimes adoptmuch smaller orbits.
The most famous of these is Halley’s Comet, which returns to the inner solar system at 75-year periods.
Its most recent return was in 1986.
In July1994 fragments of Comet Shoemaker-Levy 9 bombarded Jupiter’s dense atmosphere at speeds of about 210,000 km/h (130,000 mph).
Upon impact, the tremendouskinetic energy of the fragments was released through massive explosions, some resulting in fireballs larger than Earth.
Comets circle the Sun in two main groups, within the Kuiper Belt or within the Oort cloud.
The Kuiper Belt is a disk of icy debris that orbits the Sun beyond the planetNeptune.
The population of the Kuiper Belt is made up of Kuiper Belt Objects (KBOs).
KBOs range in size from clumps of ice mixed with rock dust (“dirty snowballs”) upto dwarf planets such as Pluto and Eris.
Any of these icy objects could be considered a comet nucleus that would give off gas and dust to produce a coma and a tail if itsorbit were to bring it close enough to the Sun.
Most of the comets with periods of less than 500 years come from the Kuiper Belt.
The Oort cloud is a hypothetical region about halfway between the Sun and the heliopause.
Astronomers believe that the existence of the Oort cloud, named for Dutchastronomer Jan Oort, explains why some comets have very long periods.
A chunk of dust and ice may stay in the Oort cloud for thousands of years.
Nearby starssometimes pass close enough to the solar system that their gravitational force will push an object in the Oort cloud into an orbit that takes it close to the Sun.
The first detection of the long-hypothesized Oort cloud came in March 2004 when astronomers reported the discovery of a planetoid about 1,700 km (about 1,000 mi)in diameter.
They named it Sedna, after a sea goddess in Inuit mythology.
Sedna was found about 13 billion km (about 8 billion mi) from the Sun.
At its farthest pointfrom the Sun, Sedna is the most distant object in the solar system and is about 130 billion km (about 84 billion mi) from the Sun.
Many of the objects that do not fall into the asteroid belts, the Kuiper Belt, or the Oort cloud may be comets that will never make it back to the Sun.
The surfaces of theicy satellites of the outer planets are scarred by impacts from such bodies.
The asteroid-like object Chiron, with an orbit between Saturn and Uranus, may itself be anextremely large inactive comet.
Similarly, some of the asteroids that cross the path of Earth’s orbit may be the rocky remains of burned-out comets.
Chiron and similarobjects called the Centaurs probably escaped from the Kuiper Belt and were drawn into their irregular orbits by the gravitational pull of the giant outer planets, Jupiter,Saturn, Neptune, and Uranus.
The Sun was also found to be encircled by rings of interplanetary dust.
One of them, between Jupiter and Mars, has long been known as the cause of zodiacal light, afaint glow that appears in the east before dawn and in the west after dusk.
Another ring, lying only two solar widths away from the Sun, was discovered in 1983.
V MOVEMENTS OF THE PLANETS AND THEIR SATELLITES
If one could look down on the solar system from far above the North Pole of Earth, the planets would appear to move around the Sun in a counterclockwise direction.
Allof the planets except Venus and Uranus, and the dwarf planet Pluto, rotate on their axes in this same direction.
The entire system is remarkably flat—only Mercuryamong the major planets has an obviously inclined orbit.
However, the dwarf planets Pluto and Eris have orbits that are strongly tilted out of the main plane of the solarsystem, Pluto at 17.2° and Eris at 44°.
Both objects also have highly elliptical orbits.
Pluto’s orbit sometimes takes it closer than Neptune to the Sun.
At its nearest pointto the Sun, Eris passes inside the orbit of Pluto, though well beyond the orbit of Neptune.
The satellite systems mimic the behavior of their parent planets and move in a counterclockwise direction, but many exceptions are found.
Jupiter, Saturn, Uranus, andNeptune each have a number of satellites that move around the planet in a retrograde orbit (clockwise instead of counterclockwise), and several satellite orbits arehighly elliptical.
Uranus has some satellites that follow its clockwise direction and others that move in counterclockwise orbits.
Jupiter, moreover, has trapped twoclusters of planetesimals or small rocky bodies (the so-called Trojan asteroids) leading and following the planet by 60° in its orbit around the Sun.
Neptune also hasgroups of planetesimals that share its orbit.
(Some satellites of Saturn have done the same with smaller bodies that occupy different parts of the same orbits as thesatellites.) The long-period comets exhibit a roughly spherical distribution of orbits around the Sun, while most of the short-period comets appear to originate from thedisklike distribution of Kuiper Belt Objects.
Within this maze of motions, some remarkable patterns exist: Mercury rotates on its axis three times for every two revolutions about the Sun; no asteroids exist withperiods 1/2, 1/3, … 1/ n (where n is an integer) the period of Jupiter; the three inner Galilean satellites of Jupiter have periods in the ratio 4:2:1.
Some Kuiper Belt Objects (including Pluto) orbit the Sun in a 2:3 ratio to Neptune’s orbit.
These and other examples demonstrate the subtle balance of forces that is established in agravitational system composed of many bodies.
VI THEORIES OF ORIGIN
Despite their differences, the members of the solar system probably form a common family.
They seem to have originated at the same time; few indications exist ofbodies joining the solar system, captured later from other stars or interstellar space.
Early attempts to explain the origin of this system include the nebular hypothesis of the German philosopher Immanuel Kant and the French astronomer andmathematician Pierre Simon de Laplace, according to which a cloud of gas broke into rings that condensed to form planets.
Doubts about the stability of such rings ledsome scientists to consider various catastrophic hypotheses, such as a close encounter of the Sun with another star.
Such encounters are extremely rare, and the hot,tidally disrupted gases would dissipate rather than condense to form planets.
Current theories connect the formation of the solar system with the formation of the Sun itself, about 4.6 billion years ago.
The fragmentation and gravitational collapseof an interstellar cloud of gas and dust, triggered perhaps by nearby supernova explosions, may have led to the formation of a primordial solar nebula.
The Sun wouldthen form in the densest, central region.
It is so hot close to the Sun that even silicates, which are relatively dense, have difficulty forming there.
This phenomenon may.
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