H2 Substantial reservoirs of molecular gas in the debris disks around young stars.
W.F. Thi, G.A. Blake, E.F. van Dishoeck et al., 2001, Nature, January 4

ISO finds the 'missing ingredient' to make Jupiter-like planets

Astronomers have detected so far about 50 planets orbiting other stars. They are all giant, Jupiter-like planets, made mostly of gas, and their formation process is still unclear. ESA's Infrared Space Observatory, ISO, sheds now some light on this problem. Observing with ISO, a Dutch-US team of astronomers has detected in the faint disks of matter that surround three nearby stars a key ingredient for planet making: the gas molecular hydrogen. The discovery, published in the January 4th issue of Nature, is relevant because current theories about the formation of giant planets were built on the assumption that the gas was 'not' present in the kind of disks observed by ISO. These models will now have to be reviewed. They said, for instance, that Jupiter-like planets had to form in just a few million years, but the ISO result implies that the process can take up to 20 million years.

Planetary systems form very early in the star's life, actually during the star-birth itself. In the first steps of star formation, when the star is merely a swirling sphere of gas, a thick circumstellar disk of gas and dust forms around the star's equator, and planets form from the material in that disk - called 'protoplanetary disk'. While the whole process of star and planet formation is taking place the system remains covered by opaque dust and therefore optical telescopes cannot see it. When, after the dispersion of the dust, the star becomes visible, much of the material in its protoplanetary disk is already 'locked' in the planets, and only left-over dust grains remain in the now very thin, feeble, circumstellar ring of debris. The gas, a key component of giant, Jupiter-like planets, is all gone already. Or so it seemed.

The team led by Wing Fai Thi (Leiden University) and Geoffrey Blake (California Institute of Technology) observed with ESA's ISO three rings of the 'thin' kind --that is, made of left-over debris material after planets have been made-- around the nearby stars Beta Pictoris, 49 Ceti and HD135344. Contrary to what the scenario described above says, Thi, Blake et al. detected the presence of molecular hydrogen in the disks.

This dicovery implies that planet formation might still be going on there. In one of the disks there is gas enough to make up to ten Jupiters, while in Beta Pictoris the detected gas could only be used to make a small Saturn.

"The present result suggests that Jovian planet formation can occur on time scales up to 20 million years", authors say in the Nature paper. The time scale accepted so far to make a Jupiter-like planet was a few million years, an estimation based on the ages of the youngest stars with --apparently-- gas-free disks.

This sheds new light on the theories describing how giant gas-rich planets form, as Ewine van Dishoeck (Leiden University) explains: "There are basically two models describing the formation of the giant planets. According to the first one, there are 'instabilities' due to piling up of material that trigger the collapse of part of the disk and the quick building up of the planets. The other says that a small 'Earth-like' core is formed first, and then the lighter material in the disk, the gas, is attracted by gravity. This second model needs more time than a few million years. Our results imply that it cannot be ruled out. You don't need to make planets that quickly".

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