Although many more planets are being discovered outside the solar system, none of them looks anything like our own planets. And it is possible that they formed in a completely different way, making our planetary system rather unique.
In the traditional model of planet formation, the dust in a disc of gas around a star gradually clumps together into rocks, which eventually merge to make planetary cores. The cores then accumulate gaseous atmospheres. In this model, gas giants such as Jupiter form in the relatively cooler outskirts of the system.
But this model does not fully explain the formation of the 110 or so extrasolar planets that have been discovered in the past decade. Typically, these planets are much heavier than Jupiter, and most are so-called "hot Jupiters" that orbit closer to their star than does the Earth, or even Mercury. ...
There are other differences between the newly discovered planets and those in our solar system. The orbits of the extrasolar planets are much more elliptical than the relatively circular orbits of planets such as Earth or Jupiter. ...
Astronomers have reported a few alien planets with nearly circular orbits (New Scientist print edition, 12 January 2002), but according to Martin Beer,
an astrophysicist at the University of Leicester in the UK, even these orbits are slightly more elliptical than Jupiter's and the planets lie fairly close to their stars.
Beer and his colleagues are now arguing that the alien systems might not have formed in the same way as our solar system. It is possible that the hot-Jupiter systems might have come about when the dusty discs around stars became unstable and suddenly fragmented, with the individual fragments collapsing under their own weight to form planets. This process naturally creates more elliptical orbits and would be unlikely to form Earth-like planets, they say.
It will take a few years to resolve this debate. ...
I can't believe it!
"It will be another five years or so before we know whether the solar system is truly different," says Beer. "But if it is, we may have to revise our theories of planet formation, since the existing theories are largely based on information gathered in the solar system." There might be more than one way to make a world.