Solar system: Encyclopedia II - Solar system - Planetary system formation

Solar system - 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(exo) 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.

Adapted from the Wikipedia article "Planetary system formation", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki