By G. Purushoath

YOUNG STAR HD STAR New Planet being formed

ABOUT THE PLANET This artist’s impression shows the disk of gas and cosmic dust around the young star HD Astronomers using the Atacama Large Millimetre / sub-millimeter 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.

ALMA T ELESCOPE 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.

S TUDY O UTCOME The international team studied the young star HD , 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.

S TUDY O UTCOME Observations made with the ALMA telescope of the disk of gas and cosmic dust around the young star HD , 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.

T HEORY P ROVED 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!”