ISO finds the 'missing ingredient' to make Jupiter-like planets
3 January 2001Astronomers have so far detected 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, now sheds some light on this problem. Observing with ISO, a Dutch-US team of astronomers has detected a key ingredient for planet making in the faint disks of matter that surround three nearby stars: 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 a 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 - called a 'protoplanetary disk' - forms around the star's equator, and planets form from the material in that disk. While the whole process of star and planet formation is taking place the system remains covered by opaque dust and can therefore not be seen using an optical telescope. When, after the dispersion of the dust, the star becomes visible, much of the material in its protoplanetary disk is already 'locked' into 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, has all gone. Or so it seemed.
Using ESA's ISO, the team led by Wing Fai Thi (Leiden University) and Geoffrey Blake (California Institute of Technology), observed three rings of the 'thin' kind - that is, a ring made from left-over debris material after planets have been formed - 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 discovery implies that planet formation might still be going on there. In one of the disks there is enough gas to make up to ten Jupiters, while in Beta Pictoris the detected gas could only be used to make a small Saturn.
"The present results suggest that Jovian planet formation can occur on time scales up to 20 million years", the authors say in the Nature paper. Until now, the accepted time scale 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 model there are 'instabilities' due to the piling up of material, triggering the collapse of part of the disk and the quick building up of the planets. The second model says that a small 'Earth-like' core is formed first, and then the lighter material in the disk, the gas, is attracted by gravity. In this second model planet formation takes longer than just a few million years. Our results imply that it cannot be ruled out. You don't need to make planets that quickly".
The authors warn that they have detected only the relatively warm gas in the disk, at about 80 degrees Kelvin - minus 190 degrees Centigrade -; a much colder gas could also be present. They also stress the need to observe more circumstellar disks with future infrared space telescopes, which are the only telescopes able to detect the emission of molecular hydrogen.
In fact, the presence of molecular hydrogen in the disks had never been ruled out on the basis of direct measurements. Until ESA's ISO - the first infrared space observatory able to search for this signature - astronomers had to infer the presence, or absence, of molecular hydrogen by using another molecule as a 'tracer', namely carbon monoxide. But, as it has been discovered recently, carbon monoxide is not always a good tracer for molecular hydrogen. So only with ISO have astronomers been able to search for the missing hydrogen directly.
The European Space Agency's infrared space telescope, ISO, operated from November 1995 till May 1998, almost a year longer than expected. As an unprecedented observatory for infrared astronomy, able to examine cool and hidden places in the Universe, ISO successfully made nearly 30 000 scientific observations.
For more information, please contact:
-Ewine van Dishoeck (Leiden University, The Netherlands)
-Wing-Fai Thi (Leiden University, The Netherlands)
-Geoffrey A. Blake (Caltech, US)
-Martin F. Kessler, ISO Project Scientist (ESA, Villafranca Satellite Tracking Station, Spain)
-ESA Science Communication Service