The sky is full of stars, and for the first time in human history, we know what’s orbiting some of them. There are more than 4,000 confirmed exoplanets, but just a few have been directly imaged. A young gas giant called PDS 70c is one of them, and now astronomers using the Atacama Large Millimetre/submillimeter Array (ALMA) have spotted something new: a moon-forming disc around the exoplanet.
Most of the known exoplanets were discovered via the transit method, which is the basis for NASA’s (Transiting Exoplanet Survey Satellite) TESS instrument and the dearly departed Kepler Space Telescope. This way of spotting exoplanets watches for dips in luminance as the planets transit in front of the star. The PDS 70 system is not positioned in such a way that we can use the transit method, but that’s rather lucky. Its orientation allowed the European Southern Observatory’s Very Large Telescope (VLT) to look down on the planetary disk and pick out PDS 70c and 70b with a high level of detail.
Both of the exoplanets in this solar system are very young. The star itself is only a few million years old. The opportunity to watch these gas giants form is extremely valuable to science, which is why ALMA was scanning it in the first place. There were hints of a moon-forming region around PDS 70c, but we didn’t have confirmation until now.
As you can see in the image above, the bright point inside the protoplanetary disc (zoomed on the right) is PDS 70c. That haze around it is material that has become caught in its gravity, of which it has plenty. PDS 70c is still forming and it’s already about four times as massive as Jupiter.
We don’t know how much of this disc will eventually fall into the planet, but the ALMA data shows there’s a great deal of matter floating around. The disc has a diameter of about 1 AU (the distance between Earth and the sun), and there’s enough mass to form three satellites the size of Earth’s moon. Our understanding of how planets and moons form is still rudimentary, but further observations of the PDS 70 system could answer a lot of questions.
PDS 70 is only 370 light-years away, which is right next door in galactic terms. This makes it an ideal observational target for upcoming instruments like the ESO’s Extremely Large Telescope (ELT), which is being built in Chile near ALMA. NASA also hopes to have the James Webb Space Telescope launched by the end of the year. Both these instruments will have the ability to look deeper into the infrared to probe the moon-forming ring of dust.
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