Many Jupiters

Gas-giant planets such as Jupiter and Saturn consist mainly of the light gases hydrogen and helium. These gases are drawn from the disk of debris around young stars, and so the gas giants must coalesce before the available material is either absorbed into the star or blown away.

Estimates based on carbon monoxide, which is easy to detect, of the amount of gas in the debris around young stars had suggested that light gases are present for a relatively short time — giving gas giants a window of just a few million years to reach full size. Yet astronomers' simulations predict that more time is needed for these planets to evolve. Although it is possible for Jupiter-like planets to form in two million years, four to eight million years is a much more likely figure. (Rocky planets such as Earth are thought to form over 20-100 million years.)

So either the Sun and other nearby stars are particularly fortunate to have so many jovian planets, or the calculations are wrong somewhere. Happily, Thi et al., writing in Nature this week, seem to have solved this conundrum. Using observations from the Infrared Space Observatory, they have calculated the amount of hydrogen in stellar disks directly for three nearby stars that are between 8 million and 30 million years old. Their results show a thousand times more hydrogen than observations of carbon monoxide had suggested. This indicates that stellar-disk hydrogen is much more persistent than was previously supposed, which would give ample time for gas-giants to form.

So why did carbon monoxide give such a wrong answer? The researchers suggest that there are two processes at work. First, carbon monoxide in the disk may be destroyed by ultraviolet light from the central star and other nearby stars. Second, the molecules in the cooler centre may condense onto dusty particles. Molecular hydrogen is both more resistant to destruction by ultraviolet light and unlikely to condense.

Currently, the sample size is very small, owing to the difficulty of detecting hydrogen so close to a star, but the process of gas-giant formation will certainly need to be re-examined in the light of these surprising results. The group looks to NASA's Space Infrared Telescope Facility, to be launched in 2002, to provide new data and reveal just what life is like within the planetary nurseries of young stars.

Cover caption: How and over what time scales are planets assembled? Broadband imaging of young stars such as Vega and beta Pictoris has revealed signatures of planetary system dynamics - dusty 'debris' disks thought to be largely free of the molecular hydrogen (H₂) needed to create jovian planets. Direct H₂ measurements have only recently become possible, and have revealed nearly interstellar gas/dust ratios in several debris disk systems. This opens a new door to the observational examination of disk dissipation and planetary growth processes around Sun-like stars.