Giant planets like Jupiter need a large reservoir of gas to grow to full size. New observations indicate that such planetary nurseries last twice as long as previously thought.
Until now, astronomers have believed that giant gas planets, such as Jupiter and Saturn, must form in a few million years or less — quite fast in astronomical terms. On page 60 of this issue, however, Thi et al.1 report evidence to the contrary. They have detected substantial amounts of molecular hydrogen (H2) gas around three youngish nearby stars in spectroscopic measurements taken by the Infrared Space Observatory spacecraft.
These stars were already known to possess dust disks, so why is it surprising that they are also surrounded by molecular hydrogen, the most common of all the elements, and why should anyone not specializing in stellar environments care? We care because hydrogen is the main ingredient needed to make the giant planets in our own Solar System, and its discovery around other stars means that giant-planet formation may be more common than previously thought. The result is surprising to astronomers because very little carbon monoxide gas orbits these stars. Although it is far less abundant, carbon monoxide is much easier to observe than hydrogen, and has even been detected around a very distant quasar, as described by Papadopoulos et al.2 on page 58 of this issue.
Models of planet formation have been developed primarily to explain the existence of planets and smaller bodies within our Solar System3. The major planets have almost circular orbits in roughly the same plane, suggesting that they formed from a disk of material orbiting the Sun. The inner planets, moons and asteroids have a rocky composition, whereas most moons and small bodies beyond the asteroid belt are rich in ice. All of these bodies grew by condensing the material around them. The variation in composition implies that the temperature of the disk material decreased away from the centre of the Solar System.
The four jovian planets (Jupiter, Saturn, Uranus and Neptune) have large masses but low densities, so they must mainly be composed of light materials, such as hydrogen and helium. The most popular models for the formation of giant planets begin with the accumulation of a core of rock and ice roughly ten times the mass of Earth. The core grows into a giant planet by gravitationally accumulating hydrogen and helium gas from the surrounding protoplanetary disk. These models generally require a few million years to form a Jupiter-like planet (Fig. 1; blue curve). Although this is rapid compared to the 20–100 million years believed necessary to form Earth-like planets3,4, giant planets require a substantial gas reservoir to complete their growth, whereas rocky planets can keep growing in a gas-free environment like that of our Solar System today. So according to current models of planet formation, these giant planets must accumulate gas fairly rapidly.
Do giant planets form only in unusually long-lived protoplanetary disks, or can they accumulate around most stars? The measurements of Thi et al .1 suggest that gas remains in many protoplanetary disks long enough to form Jupiter-like planets. Massive dust disks are known to occur around most stars less than 1 million years old, but few stars older than 5 million years possess such disks. Thi et al. have now measured the amount of H2 gas in the smallish dust disks found around three stars between 8 million and 30 million years old. Previous observations of these stars, based on measurements of carbon monoxide gas, suggested that there is surprisingly little gas left around the star, giving a much lower gas-to-dust ratio than found in the interstellar medium (which is mostly hydrogen). But Thi et al.'s hydrogen measurements show that these stars have around one Jupiter mass of H2 in their dust disks, with gas-to-dust ratios similar to that in the interstellar medium. One of the three disks may still have enough gas for the growth of Jupiter-mass planets. The presence of H 2 but not carbon monoxide may be explained by carbon monoxide condensing onto dust grains in icy conditions, or being destroyed by ultraviolet starlight.
More generally, Thi et al.'s results suggest that dust is a good indicator of hydrogen in disks around young stars. The discovery of larger amounts of gas in the disks of older systems suggests that jovian planets can form on timescales of up to 10–20 million years within some disks (Fig. 1; purple curve). This means that the formation of giant planets is likely to be fairly common, at least around isolated Sun-like stars. The situation will be different in a star-forming cloud that is producing luminous massive stars, whose bright ultraviolet radiation could destroy nearby protoplanetary disks5.
What else can be learned from observations of this kind? Since 1995, giant planets have been discovered around 50 other stars. Although the masses of these planets are all Jupiter-like, their orbits are not. Jupiter is more than five times the Earth–Sun distance from our star, whereas most known extrasolar planets are less than two Earth–Sun distances from theirs. Extrasolar planets at Jupiter's distance are difficult to detect, so they may be quite abundant. Nonetheless, the observed orbits are hard to explain because it is unlikely that such large planets formed so close to their star.
Further measurements of hydrogen on dust disks around a wide range of stars (both young and old) may be the key to new insights in this area. For example, the manner in which gas is removed from a protoplanetary disk could have as much influence on the ultimate configuration of the planetary system as does the lifetime of the disk. A planet gravitationally tugs surrounding disk material, and this interaction can alter planetary orbits substantially. Although the possibility of significant planetary migration was predicted more than two decades ago6, this type of interaction was largely ignored, because theory suggested that a planet would move faster as it approached a star. Planets capable of migrating a significant distance were therefore expected to spiral inwards to a hot death.
The discovery of the first Jupiter-mass planet orbiting at only one-twentieth of the Earth–Sun distance from its star7 (with an orbital period of 4.2 days) led to the suggestion that planetary migration could be stopped very close to the star. This could happen either because the planet enters a gas-free orbit, cleared by magnetic processes close to the star, or because the planet experiences counterbalancing forces resulting from tidal motion on the star that is induced by the planet8. But these models do not account for the many giant planets subsequently discovered with intermediate orbital periods ranging from a few weeks to a few years9. Such planets are still too close to their star to have grown in situ10, yet too far away for the proposed stopping mechanisms to operate. Might these giant planets have been migrating inwards only to be stranded as the star cleared away disk material from the inside outwards? The Space Infrared Telescope Facility11, to be launched by NASA in 2002, will be able to observe disks containing hydrogen at much higher resolution, so hundreds of nurseries will soon be available to test this and other ideas of giant-planet formation.