Disc gaps hint at planet formation
Young stars are surrounded by flat discs of gas and dust. Over time, the material in these discs clumps together to form planets. But the formation of planets is not well understood. Thanks to its high sensitivity and sharp vision, ALMA has now revealed more details about the process.
By looking at the heat radiation of dust particles, astronomers had already discovered that some discs have a large empty region in the center: relatively close to the star, there’s almost no dust. However, no one was sure how such a central gap could form.
There are two possibilities. Maybe the dust particles in the central part of the disc are evaporated away by the radiation of the new-born star. Or maybe the material in the central part of the disc has already clumped together into planets, which are very hard to detect.
Using ALMA, astronomers have now studied four of those discs-with-gaps. ALMA can map the distribution of dust, but it can also study the distribution of gas. It turned out that the empty regions in the discs are not really empty. Although they don’t contain dust particles, there’s still gas around in those central gaps. Only in the very center, close to the star, the gas is absent, too.
If the empty gaps are caused by the radiation of the central stars, they would be really empty: no dust and no gas. Therefore, the ALMA observations rule out the first possible explanation for the formation of the ‘dust gaps’.
Instead, the astronomers think that massive planets have already formed around the stars observed by ALMA. The planets would be a few times as massive as the planet Jupiter in our own solar system. They have swept up the gas in the central parts of the discs, while their gravitational perturbations prevent dust particles to come very close.
Surprisingly, the observations were carried out while ALMA was still under construction – the observatory hadn’t yet reached its full potential. In the future, ALMA will observe many more protoplanetary discs, and many even directly detect young giant planets around nearby stars.
ALMA has studied the protoplanetary discs of four young stars: SR 21, HD 135344B, DoAr 44 and Oph IRS 48. At one wavelength, ALMA mapped the distribution of dust particles in the discs; at another wavelength, the observations revealed the distribution of gas.
The observations were carried out by an international team of astronomers, led by Nienke van der Marel of Leiden University in the Netherlands. Nienke worked together with her thesis advisor Ewine van Dishoeck, two other Leiden astronomers, and colleagues from Germany, the USA and China. They have published their results in the professional journal Astronomy & Astrophysics.