profc.jpg (13600 bytes)Formation of the Solar System

Key points: Steps in forming a planetary system; the Late Heavy Bombardment; evidence for other planet systems - Doppler recoil, transit, debris disk



Planetary systems form around new stars. As an example, consider the solar system. We believe the early solar system was a cold cloud of interstellar gas that had a fairly fast rotation so not all of the gas could fall into the star forming at the center.

"This world was once a fluid haze of light,

Till toward the center set the starry tides,

And eddied into suns, that wheeling cast

The planets."

-- Tennyson, "The Princess"

As with similar stars, the very young Sun was therefore surrounded by gas and dust leftover from its formation. Through a series of collisions between the gas molecules and dust particles, this material became organized in the form of a circumstellar disk. The circumstellar disk is where the planets formed. en00500_1.jpg (18578 bytes)(animation from  L. Close, solarsysform.gif (1353338 bytes)

This movie shows the forming system of planets. We rocket through a molecular cloud, penetrating the cold cloud core where the Sun is being born. As we approach, we see the disk of material orbiting the protostar, the end point of the animation just above. It begins to glow bright red as energy is released by its contraction under gravity. Gas clouds come and go above the disk and then a wind starts from the young Sun and clears excess gas from the disk, leaving the young planetary system. Not shown, eventually the star blows away the excess gas and some of the dust to become visible. buttonbook.jpg (10323 bytes)(adapted by G. Rieke, from JPL, M. Roessler,

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Artist concept of very young sun

At an early stage (less than a million years old), when the young Sun was still surrounded by the dense disk of both gas and dust, the gas giant planets Jupiter and Saturn formed. The simulation below illustrates the growth of instabilities in a disk and the eventual formation of planets. (from Ken Rice, ( (10323 bytes)

accretion.jpg (21367 bytes) Once the gas had been ejected from the system, the possibilities for giant planets forming were over.  "Terrestrial" planets (like the earth) can take longer to form. Planet embryos formed in the disk within a few million years and continued to grow through multiple violent collisions for millions of years after the gas had left the system. (from Chris Butler, Animation below from G. J. Taylor,

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Forming solar system, with early planets As this process continued, young terrestrial planets formed in the disk but still collided frequently, and comets fell into the young Sun at a high rate. (top and bottom pictures from Don Dixon)  
Until very recently the process of planet formation just described had never actually been seen. However, here is a very high resolution image of a young star, and we can see the outer ring, the inner clearing, and within it two young, massive planets (objects b and c)! The star itself has been eliminated from the image, but its position is marked by the cross. (from Stephanie Sallum et al., 2015, Nature 527, 342)

However, the excitement was not over for the Solar System. Around 700 million years after the formation of the system, Jupiter and Saturn got into orbits where the period of Saturn was exactly twice that of Jupiter. This "orbital resonance" caused strong interactions because of the continuously repeating gravitational effects from these two massive planets.* The outer planets were shifted into their current orbits as a result. At the same time, the overall gravitational field of the system kept changing in ways that resulted in the ejection of most of the smaller bodies that had not yet been captured into planets. As these bodies shifted orbits, there was a period of high impact rates, called the Late Heavy Bombardment. In this computer simulation of the process, the Sun is in the center, the circles are the orbits of Jupiter, Saturn, Uranus, and Neptune, and the green dots are the smaller bodies. The system appears to be stable as Saturn and Jupiter migrate slowly toward the 2:1 resonance, but when they reach it Uranus and Neptune are scattered into their current orbits and most of the smaller bodies are very quickly thrown out of the system - it is a cataclysm! (The event occurs a little late compared with the solar system in this particular simulation.)


* Now that they are no longer in resonance, their gravitational tugs tend to cancel out.

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Starburst Candy

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Mechanical model of the solar system, or orrery, made in 1712 for the Earl of Orrery (hence the name).

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hypertext copyright.jpg (1684 bytes) G. H. Rieke

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