Inhibition of giant-planet formation by rapid gas depletion around young stars

Abstract

ALTHOUGH stars form from clouds of gas and dust, there are insig-nificant amounts of gas around ordinary (Sun-like) stars. This suggests that hydrogen and helium, the primary constituents of planets such as Jupiter and Saturn, are not easily retained in orbit as a star matures. The gas-giant planets in the Solar System must therefore have formed rapidly. Models of their formation generally suggest that a solid core formed in 106 yr, followed by the accretion of the massive gaseous envelope in ∼107 yr (refs 1–5). But how and when the gas of the solar nebula dissipated, and how this compares with the predicted timescale of gas-giant formation, remains unclear6,7, in part because direct observations of circumstellar gas have been made only for stars either younger or older than the critical range of 106–107 yr (refs 8–15). Here we report observations of the molecular gas surrounding 20 stars whose ages are likely to be in this range. The gas dissipates rapidly; after a few million years the mass remaining is typically much less than the mass of Jupiter. Thus, if gas-giant planets are common in the Galaxy, they must form even more quickly than present models suggest.