Formation of Solar Systems

Last: 5. Giant Planets: Hydrogen and Helium Next: 7. The Face of the Sun

After centuries of speculation, we now know that other stars have planetary systems. However, most are quite different from our solar system. This is forcing us to revise theories on the way our solar system formed.



  8.2 Kuiper-Belt Objects   p. 190-191
  8.3 Comets   p. 191-194
  8.5 Asteroids   p. 203-204
  8.5a General Properties of Asteroids   p. 204-206
  9.1 The Formation of the Solar System   p. 211
  9.1a Collapse of a Cloud   p. 211-212
  5.8 Clues to the Formation of Our Solar System   p. 105-107
  9.1b Models of Planet Formation   p. 213-214
  9.3 Planetary Systems in Formation   p. 221-222
  9.2 Extra-solar Planets (Exoplanets)   p. 214
  9.2a Discovering Exoplanets   p. 214-216
  9.2b The Nature of Exoplanet Systems   p. 219

Overview of the Solar System

In addition to the terrestrial and giant planets, the solar system contains two belts and an extended cloud of small objects.

The inner solar system   The outer solar system
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.

The Asteroid Belt

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.   Sizes and colors of asteroids

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.

Types of Asteroids

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.

  Asteroids Mathilde and Eros

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.

The Kirkwood Gaps

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!

  Distribution of asteroid semi-major axes
Kirkwood gap [Wikipedia]

Trojan `Asteroids'

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.

  Positions of Trojan asteroids
The Trojan Page [IfA]

The Kuiper Belt

Kuiper belt objects compared with the Moon

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.

Kuiper Belt Objects: Orbits

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.

  • Classical: roughly circular, outside Neptune's orbit
  • Scattered: highly elliptical, possibly due to past encounters with Neptune
  • Resonant: avoid Neptune in the same way Pluto does
  Orbits of KBOs
Plan View of the Solar System [IfA]

Pluto's Orbit

Neptune and Pluto are in a 3:2 resonance; Neptune makes 3 orbits in the same time that Pluto makes 2 orbits. Pluto is at aphelion (furthest from the Sun) when Neptune passes it, and crosses Neptune's orbit only when that planet is safely out of the way.   Animation of Pluto's 3:2 resonance


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.

  Orbits of KBOs
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.   The two tails of Hale-Bopp

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.

Orbit and Development of a Comet

Interactive Comet Animation [UCAR]

Notice that the tail always points away from the Sun; it trails behind the comet on the way in, and in front on the way out.

Comet Nuclei

Nucleus of Halley
Halley's Nucleus: An Orbiting Iceberg [NASA]
  Nucleus of Wild 2
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.

Deep Impact: the Nucleus of Tempel 1

Water ice on Tempel 1
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 1 after impact
Tempel Alive With Light [NASA]

Lives of Comets

All comets now observed are `young' compared to the ~4.6×109 yr age of the solar system.

In the Kuiper Belt, collisions between KBOs may send fragments plunging inward. Some of these are deflected by the gravity of giant planets towards the inner solar system. These comets stay fairly close to the ecliptic.

In the Oort cloud, comets are deflected by the gravitational field of the galaxy as a whole or by passing stars. As a result, a small fraction pass close to the Sun and become visible. These comets seem to arrive from all directions.

Comets lose mass each time they pass near the Sun, and no comet can survive more than a few thousand passages before all its ices evaporate. In addition, comets may be deflected by planets, or even collide with them.

Cloud Collapse & Protoplanetary Disks

  Collapse of cloud
The Cartoon History of the Universe

Conservation of Angular Momentum

As noted in this week's homework, angular momentum is proportional to speed of rotation and radius, so the product

(speed of rotation) × (radius)

can't change (unless an external force is present). If the radius decreases then the speed of rotation must increase.
  Spinning figure skater
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.

Cloud Collapse Simulations

Simulation of cloud collapse - face-on view   Simulation of cloud collapse - edge-on view

Protoplanetary Disks. I

Protoplanetary disk around beta Pic

Protoplanetary Disks. II

Protoplanetary disk around AU Mic

Formation of Terrestrial & Giant Planets

  Planet formation
The Cartoon History of the Universe

Dust to Planets

Formation of solar system

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.

Inner vs. Outer Solar System

The makeup of these planetesimals depended on the temperature. The inner disk was too warm for water to freeze, so planetesimals contained sillicates and metals, but no ice. In the outer disk, beyond the `snow line', frozen water was included in planetesimals (as were other ices); as a result, there was more to work with, and planets could grow to about 15 times the mass of the Earth.

In regions of the protoplanetary disk where a lot of hydrogen and helium was available, a planet 10 to 15 times the mass of the Earth could capture gas directly from its surroundings. Two planets did so, and they became Jupiter and Saturn.

Formation of the Moon

  Animation of Moon Formation

The Age of the Solar System

Radioactive 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.

Radioactive dating
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.

Other Planetary Systems

Radial velocity data for GJ876   Transit method of planet detection

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.

Extrasolar Planets: Masses

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.   Masses of extrasolar planets

Extrasolar Planets: Orbits

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!

  Orbits of extrasolar planets

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 (
Last modified: September 28, 2006
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