January 2, 2013
New Observations Help Shed Light On Planet-Forming Gas Streams
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Astronomers who made observations with the Atacama Large Millimeter/submillimeter Array (ALMA) telescope are helping to shed light on planet-forming gas streams.
The international team of astronomers studied the young star HD 142527, seeing a key stage in the birth of giant planets for the first time.
HD 142527 sits about 450 light-years away from Earth, and is surrounded by a disc of gas and cosmic dust. The dusty disc is divided into an inner and an outer part by a gap, which is thought to have been carved by newly forming gas giant planets clearing out their orbits as they circle the star.
The inner disc reaches from the star out to the equivalent of the orbit of Saturn in the solar system, while the outer disc begins about 14 times further out.
Astronomers believe that giant planets grow by capturing gas from the outer disc, in streams that form bridges across the gap in the disc.
“Astronomers have been predicting that these streams must exist, but this is the first time we´ve been able to see them directly,” said Simon Casassus, a professor at Universidad de 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!”
The team used ALMA to look at the gas and cosmic dust around the star, helping to pick out finer details than what could be seen with previous telescopes. ALMA's observations help look through the glare from the star that affects infrared or visible-light telescopes.
The gap in the dusty disc was already known, but they also discovered diffuse gas remaining in the gap, and two denser streams of gas flowing form the outer disc, to the inner disc.
“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 disc, but they are really messy eaters: the rest of it overshoots and feeds into the inner disc around the star” said SebastiÃ¡n PÃ©rez, a member of the team, who is also at Universidad de Chile.
The team found that the rate at which leftover gas streams onto the inner disc is just right to keep the inner disc replenished, and to feed the star. They also were able to become the first astronomers to detect 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," said Gerrit van der Plas, another team member at Universidad de Chile.
The gas is more evidence that the streams are caused by giant planets, rather than even larger objects like 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.” PÃ©rez said.
The team wrote about their findings in a paper entitled "Flows of gas through a protoplanetary gap”, which was published in the journal Nature.
Image 2 (below): ALMA observations of the disc and gas streams around HD 142527. Credit: ALMA (ESO/NAOJ/NRAO), S. Casassus et al.