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Our solar system is home to a magnificent menagerie of moons, from frozen moons filled with turbulent oceans to volcanics adorned with pits of raging hellfire. To date, astronomers have discovered 4,438 worlds orbiting other stars, and there is no doubt that various moons dance around most of these exoplanets. But astronomers have yet to conclusively find one – these exomoons have turned out to be too small and too far away to spot.
Today, after years of observing a pair of Jupiter-like exoplanets nearly 400 light years from Earth, astronomers have found the next best thing: a disc of debris orbiting the Earth. one of these worlds, a ring of rock and gas gradually merging under its own gravity. In other words, astronomers overheard a circumplanetary foundry making moons.
This is the first time that such a feature has been unambiguously detected. And unlike many extrasolar discoveries, this object was not found by indirect methods – the subtle flicker of a star revealing the presence of an orbiting planet, for example. This disc was actually photographed. This is a real image of a small planet surrounded by its own moon forge.
Astronomers are delighted unreservedly, and a little lost for words. “I don’t have coherent scientific thoughts. I just look at the image and say ‘wow’ every time I see it, ”said Bruce Macintosh, a Stanford University astronomer who is not involved in the study.
The discovery – reported Thursday in The Astrophysical Journal Letters – will help scientists answer one of the most puzzling questions in astronomy: How are planets and their moons formed? The possible answers invoke a myriad of world-building methods, from titanic impacts to the gluing of space hail, where the force of conglomerate gravity can counteract the disturbances caused by the magnetic vortices that the nascent worlds are supposed to encounter.
“We have all of these theories which are beautiful, but if you can’t test them they could be completely wrong,” said Myriam Benisty, astronomer at the University of Grenoble and lead author of the study. This extraordinary system that she and her colleagues have identified is a perfect place to revisit these ideas.
The solar system, 4.6 billion years old, is somewhat of middle age. This distant star system, on the other hand, is in its infancy. Its star, PDS 70, was born barely six million years ago. The disk of gas and dust surrounding the stars that made its two Jupiter-like planets, PDS 70b and PDS 70c, is still present. As new planets are slowly being reconstituted, the two young people who already exist continue to siphon the debris from this disc and grow stronger from it.
The Very Large Telescope in the Atacama Desert in Chile found the two exoplanets, with the discovery of PDS 70b announced in 2018 and PDS 70c in 2019. A month later, scientists using the Chilean Atacama Large Millimeter / submillimeter network Array (ALMA) reported that the radio waves emitted by the fine dust emanated from around PDS 70c – promising evidence that a disc of lunar debris surrounded it.
The signal, however, was weak. Dr Benisty and his colleagues used ALMA to make follow-up observations, no doubt demonstrating that the PDS 70c has its own debris disk, large enough to be three moons the size of Earth’s satellite.
It won’t be long before such discoveries become routine. In the months and years to come, more and more powerful space telescopes and observatories will become operational. Soon the first exomoons themselves will be captured on camera. Maybe someday we’ll capture an exo-Earth on camera – a second pale blue dot. “This is our dream,” said Kate Follette, an astronomer at Amherst College not involved in the study.
Astronomers can’t wait to see it all. But their appetite for discovery was temporarily sated by the remarkable sight of alien moons preparing to make their debut. “It’s rare, especially in our field, that you see something so beautiful,” said Dr. Follette.
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