Astronomers spot possible moon-forming region for the first time

It's in a 10-million-year-old star system 370 light years from Earth.

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This image, taken with the Atacama Large Millimeter/submillimeter Array (ALMA), in which ESO is a partner, shows wide (left) and close-up (right) views of the moon-forming disc surrounding PDS 70c, a young Jupiter-like planet nearly 400 light-years away. The close-up view shows PDS 70c and its circumplanetary disc centre-front, with the larger circumstellar ring-like disc taking up most of the right-hand side of the image. The star PDS 70 is at the centre of the wide-view image on the left. Two planets have been found in the system, PDS 70c and PDS 70b, the latter not being visible in this image. They have carved a cavity in the circumstellar disc as they gobbled up material from the disc itself, growing in size. In this process, PDS 70c acquired its own circumplanetary disc, which contributes to the growth of the planet and where moons can form. This circumplanetary disc is as large as the Sun-Earth distance and has enough mass to form up to three satellites the size of the Moon.
ALMA (ESO/NAOJ/NRAO)/Benisty et al

While scientists have found plenty of exoplanets over the years, they've yet to spot to moons orbiting those worlds outside our solar system. Now, a group of astronomers has discovered (PDF) what's believed to be a region with exomoons-in-the-making for the first time. Myriam Benisty and team from the University of Grenoble found the disk of dust — the moon-forming region — around a young exoplanet in a star system dubbed PDS 70 located 370 light years from Earth. 

The team found the first protoplanet (PDS 70b) in the system back in 2018 using European Southern Observatory's Very Large Telescope in Chile. A year later, they found another young gas giant (PDS 70c) using the same equipment. The astronomers believe based on the data they have that the star system is only 10 million years old and that both gas giants are several times bigger than Jupiter. To know more about the system, they focused all other possible instruments on it, including the Atacama Large Millimeter/submillimeter Array. ALMA is made up of 66 short-wavelength radio dishes, and its observations made it possible to spot the dust around PDS 70c.

The disk of dust spans a distance slightly wider than that between Earth and the Sun, and there's enough mass in there for three moons the same size as ours. Benisty says the moons may have already formed, but there's no conclusive proof yet because they can't be seen with ALMA. According to Science, the Extremely Large Telescope, which will be the world's largest optical telescope when it's built, may have the power to see if the moons have already formed around the protoplanet. The telescope is still under construction, though, and scientific operations won't start until 2027 at the earliest. 

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