A planet has been found within a disc of dust and gas around a young star for the first time, a new study reports. The research confirms predictions that planets can coalesce within 10 million years, but it does not settle the debate over just how that formation proceeds.
Planets are thought to grow as dust collides and sticks together in so-called protoplanetary discs around young stars. Observations have shown that these discs tend to dissipate within the first 10 million years of a star鈥檚 life, suggesting that planets must take shape before that time.
But until now, none of the 270 known extrasolar planets had been found around stars young enough to still have protoplanetary discs. That鈥檚 because most planet searches have excluded young stars, whose pulsations can mimic the telltale wobbles caused by potential planets.
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Now, a team led by Johny Setiawan of the Max Planck Institute for Astronomy in Heidelberg, Germany, has found the first planet around a star young enough to boast a protoplanetary disc.
Using spectroscopic measurements taken with a 2.2-metre telescope at La Silla Observatory in Chile, the researchers carefully observed the star, called TW Hydrae (TW Hya). They found a disturbance in the star鈥檚 spectra every 3.56 days that did not correspond to signs of the star鈥檚 activity, suggesting the repeating signal was indeed caused by an orbiting planet.
Tight orbit
The planet, dubbed TW Hya b, appears to be quite massive, weighing in at nearly 10 Jupiters. It also orbits extremely close to the star 鈥� 10 times closer than Mercury鈥檚 distance from the Sun, within the inner rim of the star鈥檚 dusty disc.
The researchers believe the planet formed farther away from the star 鈥� between the orbits of Earth and Jupiter in our solar system, where there was more material from which it could grow. Then, as it lost energy through gravitational interactions with gas in its natal disc, it moved to its present location over the course of 100,000 years or so, stopping when it reached a region cleared of gas.
The fact that the planet was found around a star estimated to be 10 million years old 鈥済ives a real upper limit to the timescales of planet formation and migration鈥�, the researchers write in their study.
Scott Kenyon, who studies planet formation at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, US, agrees.
鈥淭he planet is pretty much where all of the other close-in Jupiters are. So I draw two main conclusions: you need to make Jupiters within 10 million years, and you need to migrate them in close to the star within 10 million years,鈥� he told New 杏吧原创. That lines up with what scientists had previously thought, he says, 鈥渂ut it is nice to see our ideas confirmed by a specific case鈥�.
Competing models
Still younger planets may be needed to determine which of two competing theories accounts for exactly how planets take shape.
Most scientists believe it takes millions of years for planets to build up, starting out as tiny particles of dust that clump together and then 鈥榮nowball鈥� over time into a rocky core.
Other scientists, however, argue that giant planets can form in just a few thousand years, collapsing quickly from a clump of gas and dust in the disc. This is known as the gravitational instability model.
鈥淚t is still not clear whether a planet as massive as 9.8 Jupiters could have formed through core accretion or whether gravitational instabilities in the disc must have been involved,鈥� the researchers write. Nonetheless, they say the new discovery is the 鈥渋deal system鈥� to test computer simulations of the various models.
Says Kenyon: 鈥淚 think the challenge for either model is to connect the properties of the planet with the properties of the disc.鈥�
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