Page last updated at 17:31 GMT, Tuesday, 6 January 2009

Baby Jupiter's huge weight gain

Jupiter (Nasa)
Jupiter would have grown fast in its infancy

The planet Jupiter must have gained mass fast during its infancy, according to astronomers.

It had to, because the material from which the planet formed disappeared in just a few million years.

After studying other stars, the US team came to the conclusion that gas giants like Jupiter must grow rapidly.

Details of the group's work were outlined at the 213th meeting of the American Astronomical Society (AAS), held in Long Beach, California.

Astronomers examined the five million-year-old star cluster NGC 2362 with Nasa's Spitzer Space Telescope.

Spitzer can detect the signatures of actively forming planets in infrared light.

The research team found that all stars with the mass of the Sun or greater have lost their "proto-planetary", or planet-forming, discs.

Only a few stars less massive than the Sun retain these discs of dust and gas, which provide the raw material for gas giants which are in the process of forming.

Therefore, the astronomers concluded, gas giants have to form in less than five million years or they probably will not form at all.

"Even though astronomers have detected hundreds of Jupiter-mass planets around other stars, our results suggest that such planets must form extremely fast," said the study's lead scientist Thayne Currie of the Harvard-Smithsonian Center for Astrophysics.

"Whatever process is responsible for forming 'Jupiters' has to be incredibly efficient."

Even though nearly all gas giant-forming discs in NGC 2362 have disappeared, several stars in the cluster retain their "debris disks".

This indicates that smaller rocky or icy bodies such as Earth, Mars, or Pluto may still be forming there.

"The Earth got going sooner, but Jupiter finished first, thanks to a big growth spurt," explained co-author Scott Kenyon, also from the Harvard-Smithsonian Center for Astrophysics.

Dr Kenyon added that while Earth took about 20 to 30 million years to reach its final mass, Jupiter was fully grown in only two to three million years.

Previous studies had indicated that proto-planetary discs disappear within 10 million years.

The new findings place even tighter constraints on the time available to create gas giant planets around stars of various masses.



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