The Atacama Large Millimeter/submillimeter Array (ALMA) has found strong evidence that massive planets have recently formed around four very young stars, called Oph IRS 48, HD 135344B (also known as SAO 206462), DoAr 44, and SR 21.
Artist’s impression of a transitional disc around a young star. Image credit: ALMA / ESO / NAOJ / NRAO / M. Kornmesser.
These new exoplanets, each presumably several times more massive than Jupiter, were inferred by the telltale structures they produced in the disks of gas and dust that surround the stars.
Though planets appear remarkably plentiful in our Milky Way Galaxy, scientists still don’t fully understand how and under what conditions they form. To help answer these questions, they study the structure and composition of the rotating disks of dust and gas around young stars.
A special class of discs, called transitional disks, have a surprising absence of dust at their centers, in the region around the star.
Two main ideas have been put forward to explain these strange gaps. Firstly, stellar winds and intense radiation could have blown away or destroyed the encircling material. Alternatively, massive exoplanets in the process of formation could have cleared the material as they orbit the star.
ALMA has now allowed astronomers to map the distribution of gas and dust in four of these transitional discs better than ever before.
The images show that there are significant amounts of gas within the dust gaps. But the gas also possessed a gap, up to 3 times smaller than that of the dust.
This could only be explained by the scenario in which massive planets have cleared the gas as they traveled around their orbits, but trapped the dust particles further out.
This image from the Atacama Large Millimeter/submillimeter Array combines a view of the dust around the young star HD 135344B (orange) with a view of the gaseous material (blue). The smaller hole in the inner gas is a sign of the presence of a young exoplanet clearing the disc. The bar at the bottom of the image indicates the diameter of the orbit of Neptune in the Solar System. Image credit: ALMA / ESO / NAOJ / NRAO.
“Previous observations already hinted at the presence of gas inside the dust gaps. But as ALMA can image the material in the entire disc in much greater detail than other facilities, we could rule out the alternative scenario,” said Dr Nienke van der Marel of Leiden Observatory, lead author of a paper published this week in the journal Astronomy & Astrophysics (arXiv.org preprint).
“The deep gap points clearly to the presence of planets with several times the mass of Jupiter, creating these caverns as they sweep through the disc.”
“All the transitional discs studied so far that have large dust cavities also have gas cavities. So, with ALMA, we can now find out where and when giant planets are being born in these discs, and compare these results with planet formation models,” said co-author Dr Ewine van Dishoeck, of the Max Planck Institute for Extraterrestrial Physics and Leiden Observatory.
“Direct planetary detection is just within reach of current instruments, and the next generation telescopes currently under construction, such as the European Extremely Large Telescope, will be able to go much further. ALMA is pointing out where they will need to look,” he said.
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N. van der Marel et al. 2015. Resolved gas cavities in transitional disks inferred from CO isotopologs with ALMA. A&A 585, A58; doi: 10.1051/0004-6361/201526988
