29/08/2011
For at least 20 years, the textbook view has been that it takes a long time to cook a planet. Little by little — says the old recipe — clumps of solid mass form within the gas, dust and ice that whirl like pizza dough around a young star. Gravity gathers bits of dust, which merge into boulder-sized chunks, which in turn coalesce into bigger rocks.
In about a million years — according to this recipe — these rocks become planets, and over the next few million years, gas from the disk settles around some of these young planets to grow Jupiter-size gas giants, which have most of their mass in a gaseous envelope surrounding the solid core. According to this view, called the “core-accretion model,” it would take as much as ten million years for a gas giant planet to form.
The core-accretion model first came up for rethinking, says Quinn, in the early 90s, when the Hubble Space Telescope was able to see these gaseous protoplanetary disks (or proplyds). Astronomers realized that these disks can’t last for a long time, roughly a million years, because the gas is rapidly cooked away by radiation from nearby stars.
Still, core accretion held up as a theory, because — so the thinking went — massive planets like Jupiter and Saturn are relatively rare. But the Marcy and Butler team carefully surveyed about 1,000 stars and showed that roughly one out of 10 has a planet, probably more, with most of them gas giants, from roughly the size of Jupiter to ten times larger.
“Now,” says Quinn, “we see that planet formation is not particularly special, and this doesn’t jibe with the standard model. If a gas giant planet can’t form quickly, it probably won’t form at all.”
