Big and mysterious

The current theory of planet formation has just been proved wrong. It is time astrophysicists got back to the drawing- board

Published: Tuesday 29 February 2000

the largest planet in the solar system, Jupiter, is somewhat of an oddball. It is large -- its radius is 11 times that of the Earth -- and massive, about 15 times as heavy as the Earth. In some sense, it is almost a star. Till recently, astronomers thought they understood how Jupiter and the other giant planets (Neptune, Saturn and Uranus) were formed. Now, a group of astrophysicists at the University of Hawaii in Honolulu, usa , has revealed the embarrassing fact that we may not know anything at all of how these planets were formed.

The widely accepted theory is that planets were formed from gases of the early solar system that condensed around rocky cores about five billion years ago. Each planet did not simply get the elements from the gas in the region in which it was formed. Rather, it acquired various elements from different sources. It is well known that the solar nebula (the gaseous substance from which the Sun and the solar system are formed) basically comprises hydrogen and helium. However, the giant planets contain a lot of carbon and sulphur. The accepted theory was that these heavy elements were trapped in icy bodies called planetesimals, which formed in the colder outer regions of the solar system and were then catapulted back into the inner solar system to collide with Jupiter and Saturn, thereby 'enriching' them. Some of them went towards the outer regions to get trapped as comets in what is known as the Oort cloud.

However, there is a problem. The region where these icy bodies formed would not have been cold enough to trap certain heavier elements -- nitrogen and the inert gases argon, krypton and xenon -- within the ice. So, if this model is right, Jupiter should contain no more of these elements than does the Sun.

Now, Tobias Owen of the University of Hawaii, after analysing Jupiter's composition from the Galileo space mission, says that this may not be the case. He says that Jupiter is as rich in these inert gases as in other heavy elements. This implies that the planetesimals condensed under much colder conditions, or they were formed much further out in the solar nebula ( Nature , Vol 402, No 6759).

There is another dramatic possibility that Jupiter may have been formed further out in the solar system (where temperatures were lower) and then somehow wandered to its present position. At the moment it is not clear which theory is correct. On the other hand, what is clear is that the accepted theory of how the giant planets came into existence is wrong.

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