A young star in the constellation Sagittarius (the Archer) is orbited by two infant planets. The planets were found by ALMA. They’re about twice as large as the planet Saturn in our own solar system. Planets orbiting other stars have been found before. But in this case, the planets are still forming. They’re baby planets.
The two baby planets have not been seen directly. Instead, ALMA has detected their ‘footprints’, so to say. The young star (called HD 163296) is surrounded by a flat, rotating disk of gas and dust. Planets clump together from material in this disk. And newborn planets create empty gaps in the disk – they sweep up all the gas and dust close to their orbit.

In the past, ALMA has studied other ‘protoplanetary disks’, as they are called. In two cases, concentric empty gaps have been found, around the stars HL Tauri and TW Hydrae. But in both earlier cases, ALMA only studied the distribution of dust particles. The astronomers didn’t know for sure if the gaps in the dust were really due to planets. To be absolutely certain, it had to be shown that the gaps were also free of gas.

With HD 163296, this has now indeed been confirmed. ALMA discovered three concentric gaps in the dust, at 900 million, 1.5 billion and 2.4 billion kilometers from the central star. At other millimeter wavelengths, ALMA also studied the distribution of carbon monoxide gas. It turned out that the outer two of the three gaps did not contain any gas. The gaps are empty. And there’s just one way to create such empty gaps: planets.

Large, massive planets create wider gaps than small planets. From the width of the gaps in the disk of HD 163296, the astronomers could calculate the mass of the two planets: about twice the mass of Saturn.

And what about the innermost gap? That one turned out not to be completely empty. It still contains a lot of gas. The lack of dust particles in this innermost gap may be due to some other process.

HD 163296 is a young star. It was born only five million years ago or so. Our own sun is almost a thousand times as old. The star is about two times more massive than the sun. It is located in the constellation Sagittarius, the Archer, at a distance of some 400 light-years – much too far to be visible with the naked eye. Like other young stars, HD 163206 is surrounded by a flat, rotating disk of gas and dust. Such disks are called ‘protoplanetary disks’: they contain the material from which planets clump together.

The ALMA observations of the disk of HD 163296 were carried out by Andrea Isella and his colleagues. Andrea is an astronomer at Rice University in Houston, Texas. In this research project, he worked together with colleagues from the United States, Italy, Germany, the Netherlands, Ireland, and Chile. Their discovery of the empty gaps in the disk – strong evidence for the existence of two planets – has been published in a professional science magazine called Physical Review Letters.