‘Moon-forming’ disk found in distant system

US astronomers have reported the first-ever observations of a circumplanetary disk, the belt of dust and gas that, it is theorised, controls the formation of planets and gives rise to an entire system of moons, like those found around Jupiter.

It was observed, using the Atacama Large Millimeter/submillimeter Array (ALMA), around one of the planets in PDS 70, a young star located approximately 370 light-years from Earth. 

Recently, astronomers confirmed the presence of two massive, Jupiter-like planets there. The European Southern Observatory’s Very Large Telescope (VLT) detected the warm glow naturally emitted by hydrogen gas accreting onto the planets. 

The new ALMA observations instead image the faint radio waves given off by the tiny (about one tenth of a millimetre across) particles of dust around the star.

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ALMA image of the dust in PDS 70. Credit: ALMA (ESO/NAOJ/NRAO); A Isella

Combined with the earlier optical and infrared VLT observations, these provide compelling evidence that a dusty disk capable of forming multiple moons surrounds the outermost known planet in the system, the astronomers write in an article to be published in the journal Astrophysical Journal, Letters and currently available on the pre-print server arXiv.

“For the first time, we can conclusively see the tell-tale signs of a circumplanetary disk, which helps to support many of the current theories of planet formation,” says lead author Andrea Isella, from Rice University in Houston. 

“By comparing our observations to the high-resolution infrared and optical images, we can clearly see that an otherwise enigmatic concentration of tiny dust particles is actually a planet-girding disk of dust, the first such feature ever conclusively observed.” 

Unlike the icy rings of Saturn, which likely formed by the crashing together of comets and rocky bodies relatively recently in the history of our solar system, a circumplanetary disk is the lingering remains of the planet-formation process.

Optical studies of planetary systems are notoriously challenging, the astronomers say, because the star is so much brighter than the planets that it is difficult to filter out the glare.

ALMA observations don’t have that limitation, however, since stars emit comparatively little light at millimetre and submillimetre wavelengths.

“This means we’ll be able to come back to this system at different time periods and more easily map the orbit of the planets and the concentration of dust in the system,” says Isella. 

“This will give us unique insights into the orbital properties of solar systems in their very earliest stages of development.”

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