In addition to the terrestrial and giant planets, the solar system contains two belts and an extended cloud of small objects.
 |
 |
|
| Inner Solar System: Mercury, Venus, Earth, Mars and asteroids. | Outer Solar System. Left: Jupiter, Saturn, Uranus, Neptune, Pluto, a comet, and the Kuiper belt. Right: The Oort cloud. |
| A large number of asteroids orbit the inner solar system, mostly between the orbits of Mars and Jupiter, and this region is known as the asteroid belt. Ceres, the first asteroid discovered, is 950 km in diameter; smaller asteroids are far more common. We expect to find about 100,000 asteroids with automated telescopes. |
|
Although there are many asteroids, the amount of material they contain is small -- about 5% the mass of the Moon. For this reason alone, it's probably incorrect to speak of the asteroids as a `failed' planet -- but they may be considered as `leftovers' from planet formation.
| Asteroids come in various types. Some are
`rubble piles' held together by gravity (eg., Mathilde, in `A'
at right). Others are solid objects (eg., Eros, in `B' at
right).
Mathilde is made of carbon-rich compounds and appears quite dark. Eros, in contrast, is made of silicon-rich rock. |
|
About three-fourths of all asteroids are dark and carbon-rich like Mathilde, while most of the rest are brighter and silicon-rich like Eros. There are also a significant number of metallic asteroids composed of iron and nickel. This indicates that some asteroids are fragments of larger objects which differentiated after forming and before being broken up by collisions.
| Jupiter's powerful gravity creates structure in
the asteroid belt: very few asteroids are found on orbits
which are resonant with Jupiter's period.
Collisions scatter asteroids into resonant orbits, and such orbits will grow more and more elliptical until they tangle with inner planets like Mars and Earth. When this happens -- LOOK OUT! |
Kirkwood gap [Wikipedia] |
| Orbits in a 1:1 resonance with Jupiter
are allowed as long as they stay about 60° ahead or
behind Jupiter. These are the orbits of Trojan asteroids, now
thought to be more like Kuiper-Belt objects in their makeup
and general properties.
The origin of the Trojans is a major puzzle -- how did so many objects get trapped into this resonance? A recent theory resolves this puzzle in a dramatic way. |
The Trojan Page [IfA] |
Pluto, discovered in 1930, was the first Kuiper Belt Object (KBO) found; the next was found in 1992, and over a thousand are now known. These objects take their name from Gerald Kuiper, who that suggested a belt of icy objects orbiting beyond Neptune might be the source of some comets.
Pluto has a density about twice that of water; this implies it's made of a mixture of ice and rock. We know very little about the makeup of other Kuiper Belt objects, although crystalline ice has been found on Quaoar. Since this form of ice is destroyed by radiation in space, it must be replenished from Quaoar's interior.
| The orbits of KBOs form a thick belt largely
outside the orbit of Neptune. Like Pluto's, most are tilted by
up to 20° with respect to the ecliptic, and a few even more.
Three types of KBO orbits can be defined.
|
Plan View of the Solar System [IfA] |
| A significant number of Kuiper Belt Objects
have now been found in the same 3:2 resonance with Neptune that
Pluto occupies. These objects avoid Neptune by approaching
its orbit only when Neptune is somewhere else.
In this diagram, objects above the line labeled `q = 30 AU' actually pass within Neptune's orbit (just like Pluto). Notice that all KBOs which approach Neptune are in resonant orbits. |
The Plutinos [IfA] |
| Comets are small, icy objects which find their way into the inner solar system. Once warmed by the Sun, they begin to vaporize and develop long tails of dust and gas like the ones shown here. Since most comets are on extremely elliptical orbits, they spend only a small fraction of their time in the active state; the rest of the time they are nearly inert lumps of ice and dust. |
|
Comets are interesting because they may represent samples of the early solar system, preserved in their original condition in the deep freeze of outer space.
Halley's Nucleus: An Orbiting Iceberg [NASA] |
Comet Wild 2's Nucleus from Stardust [NASA] |
A comet nucleus is irregularly-shaped, and usually covered with a crust of dark material. When heated, jets of vaporized ices escape through cracks in the crust.
Evidence of Cometary Ice [NASA] In 2005, NASA slammed a copper slug into comet Tempel 1 to investigate the comet's interior. Analysis of the debris suggests that Tempel 1 formed at about the same distance as Uranus and Neptune. |
Tempel Alive With Light [NASA] |
Cloud Collapse & Protoplanetary Disks |
The Cartoon History of the Universe |
|
As noted in this week's homework, angular momentum is
proportional to speed of rotation and radius, so the
product can't change (unless an external force is present). If the radius decreases then the speed of rotation must increase. |
The Cartoon Guide to Physics |
The organized rotation of the solar system is due to angular momentum conservation. At first, the cloud's internal motions would have been turbulent and random. But by the time it had collapsed to about 0.1% of its initial size, whatever random angular momentum it had initially would have become quite important.
Formation of Terrestrial & Giant Planets |
The Cartoon History of the Universe |
Within the Sun's protoplanetary disk, dust grains collected into larger planetesimals with diameters of several kilometers. As these grew they attracted each other via gravity, forming planet-sized objects.
The Age of the Solar SystemRadioactive elements decay according to a definite time-table. For example, uranium-238 decays to lead-206 with a half-life of 4.6 billion years; if you start with a sample of pure uranium-238, and wait 4.6 billion years, half the atoms will have turned into lead-206. |
The Cartoon History of the Universe |
Meteorites -- fragments from the asteroid belt which fall to Earth -- have ages of 4.6±0.1 billion years. The oldest Moon rocks are only slightly younger: 4.4 billion years. An entirely separate chain of reasoning puts the Sun's age at 4.6 billion years. The evidence points to 4.6 billion years for the age of the entire solar system.
|
|
Over 100 stars are now known to have planetary systems. Two methods of detection are shown here. Planets orbiting a star causes the star to `wobble' slightly; by measuring this wobble the number, mass, and orbits of the planets can be deduced. Planets may also pass directly between us and their star, causing regular (albeit small) dips in the star's brightness.
| Massive planets produce bigger wobbles, so it's not too surprising that all planets found to date are relatively massive -- like Jupiter, or even larger. The smallest planet detected to date is about 8 times the Earth's mass. |
|
|
Close-orbiting planets also produce bigger wobbles, but it's
still very surprising that so many have such small
orbits. Roughly half of all known extrasolar planets orbit
closer than 1 AU to their primary star -- and many
of these planets are more massive than Jupiter!
Another surprise was the large number of planets with highly elliptical orbits -- much more elliptical than planetary orbits in our solar system! |
|
At present we have no idea how giant planets could form so close to their parent stars. Attention is focusing on the idea that planets may migrate -- outward, or more likely inward -- from their places of birth in protoplanetary disks.
| Last: 5. Giant Planets: Hydrogen and Helium | Next: 7. The Face of the Sun |
|
Joshua E. Barnes
(barnes@ifa.hawaii.edu)
Last modified: September 28, 2006 http://www.ifa.hawaii.edu/~barnes/ast110_06/foss.html |
|