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Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) telescope have seen a key stage in the birth of giant planets for the first time. Vast streams of gas are flowing across a gap in the disk of material around a young star. These are the first direct observations of these streams, which are expected to be created by giant planets guzzling gas as they grow. 

Artist’s impression of the disc and gas streams around HD 142527

This artist’s impression shows the disk of gas and cosmic dust around the young star HD 142527. Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) telescope have seen vast streams of gas flowing across the gap in the disk. These are the first direct observations of these streams, which are expected to be created by giant planets guzzling gas as they grow, and which are a key stage in the birth of giant planets. Image Credit: ALMA (ESO/NAOJ/NRAO)/M. Kornmesser (ESO)

The international team studied the young star HD 142527, over 450 light-years from Earth, which is surrounded by a disk of gas and dust — the remains of the cloud from which the star formed. The dusty disk is divided into an inner and an outer part by a gap, which is thought to have been carved by newly forming giant gas planets clearing out their orbits as they circle the star. The inner disk reaches from the star out to the equivalent of the orbit of Saturn in the Solar System, while the outer disk begins about 14 times further out. The outer disk does not reach all the way round the star; instead, it has a horseshoe shape, probably caused by the gravitational effect of the orbiting giant planets.

According to theory, the giant planets grow by capturing gas from the outer disk, in streams that form bridges across the gap in the disk.

Astronomers have been predicting that these streams must exist, but this is the first time we’ve been able to see them directly,”says Simon Casassus (Universidad de Chile, Chile), who led the new study. Thanks to the new ALMA telescope, we’ve been able to get direct observations to illuminate current theories of how planets are formed!”

Casassus and his team used ALMA to look at the gas and cosmic dust around the star, seeing finer details, and closer to the star, than could be seen with previous such telescopes. ALMA’s observations, at submillimeter wavelengths, are also impervious to the glare from the star that affects infrared or visible-light telescopes. The gap in the dusty disk was already known, but they also discovered diffuse gas remaining in the gap, and two denser streams of gas flowing from the outer disk, across the gap, to the inner disk.

We think that there is a giant planet hidden within, and causing, each of these streams. The planets grow by capturing some of the gas from the outer disk, but they are really messy eaters: the rest of it overshoots and feeds into the inner disk around the star says Sebastián Pérez, a member of the team, who is also at Universidad de Chile.

ALMA observations of the disc and gas streams around HD 142527

Observations made with the ALMA telescope of the disk of gas and cosmic dust around the young star HD 142527, showing vast streams of gas flowing across the gap in the disk. The dust in the outer disk is shown in red. Dense gas in the streams flowing across the gap, as well as in the outer disk, is shown in green. Diffuse gas in the central gap is shown in blue. The gas filaments can be seen at the three o’clock and ten o’clock positions, flowing from the outer disc towards the centre. The dense gas observed is HCO+, and the diffuse gas is CO. The outer disk is roughly two light-days across. If this were our own Solar System, the Voyager 1 probe — the most distant manmade object from Earth — would be at approximately the inner edge of the outer disk. Image Credit: ALMA (ESO/NAOJ/NRAO), S. Casassus et al.

The observations answer another question about the disk around HD 142527. As the central star is still forming, by capturing material from the inner disk, the inner disk would have already been devoured, if it was not somehow topped up. The team found that the rate at which leftover gas streams onto the inner disk is just right to keep the inner disk replenished, and to feed the growing star.

Another first is the detection of the diffuse gas in the gap. Astronomers have been looking for this gas for a long time, but so far we only had indirect evidence for it. Now, with ALMA, we can see it directly,” explains Gerrit van der Plas, another team member at Universidad de Chile.

This residual gas is more evidence that the streams are caused by giant planets, rather than even larger objects such as a companion star. A second star would have cleared out the gap more, leaving no residual gas. By studying the amount of gas left, we may be able to pin down the masses of the objects doing the clearing.” adds Pérez.

What about the planets themselves? Casassus explains that, although the team did not detect them directly, he is not surprised. We searched for the planets themselves with state-of-the-art infrared instruments on other telescopes. However, we expect that these forming planets are still deeply embedded in the streams of gas, which are almost opaque. Therefore, there may be little chance of spotting the planets directly.

Nevertheless, the astronomers aim to find out more about the suspected planets by studying the gas streams as well as the diffuse gas. The ALMA telescope is still under construction, and has not yet reached its full capabilities. When it is complete, its vision will be even sharper, and new observations of the streams may allow the team to determine properties of the planets, including their masses.

This research was presented in a paper, “Observations of gas flows inside a protoplanetary gap”, in the journal Nature on 2 January 2013.

Source: The Atacama Large Millimetre/submillimeter Array (ALMA)

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