The Atacama Large Millimeter/submillimeter Array (ALMA), an array of 64 radio dishes in Chile, has become famous for producing stunning images of protoplanetary disks, the flattened clouds of dust and gas from which planets form around young stars. The images show bright rings separated by dark gaps, tantalizing astronomers with the idea that the gravity of nascent planets has swept the gaps clean of material.
But ALMA doesn’t have the resolution to see planets themselves, so researchers have been left guessing exactly how and where they are born. Now, two teams have detected three planets, all around the same star, by detecting the swirl of gas around them as they move.
Both teams focused on carbon monoxide, a gas that emits a distinctive signal at millimeter wavelength, perfect for the ALMA’s sensitivity. By looking at very tiny shifts in the signal’s wavelength, caused by the Doppler effect, they could detect places in the disk where the gas is moving toward or away from Earth. That enabled them to sense gas moving as a planet went past, like water in a stream flowing around a stone. (The image above shows the moving gas around HD 163296. The kink at the top of the curve is caused by a hidden planet, with its likely location shown by the arrow.)
One team detected two planets circling the star HD 163296, a young star about 330 light-years from Earth and twice the mass of the sun (but only a 1000th of its age, at some 4 million years old). The planets are roughly Jupiter-size, the researchers estimate, but their orbits are much farther out, at 12 billion and 21 billion kilometers. That’s farther than all the sun’s planets and even the Kuiper belt.
The other team found a planet they think is twice as big and more than twice as far from the star, at 39 billion kilometers. The papers, both published in The Astrophysical Journal Letters on 13 June, mark the first planets detected using the ALMA and perhaps the youngest exoplanets ever detected. Researchers are hoping this technique, applied to other disks, will reveal more planets information and help us understand how gravity molds primordial clouds into planetary systems, including our own.
*Correction, 15 June, 9:55 a.m.: An earlier version of this story misstated the size of HD 163296.