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Our nearest star system may have a planet with a colossal set of rings

Proxima Centauri

Proxima Centauri is the fainter red star in the lower right

Digitized Sky Survey 2; Acknowledgement: Davide De Martin and Mahdi Zamani

We may have the first picture of the nearby exoplanet Proxima c, and it looks strange. It’s far brighter than astronomers expected, which may mean that it is surrounded by a huge disk of dust or even a shining system of rings.

Proxima Centauri is the closest star to the solar system, at just 4.25 light years away, and we know that it has a planet roughly the same size as Earth called Proxima b. But in 2019, astronomers spotted an extra wiggle in the star’s motion that may be caused by the gravitational pull of a second, larger planet called Proxima c.

Raffaele Gratton at the Astronomical Observatory of Padova in Italy and his colleagues used SPHERE, an instrument attached to the Very Large Telescope in Chile, to search for light reflected off Proxima c in the hope of taking the first image of it.

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“At the beginning, I thought that there was little hope of seeing it because we expected that the planet would be too faint to be observed,” says Gratton. “But there seems to be some signal there, even if we are not completely sure that it is really a planet.”

The observations had some noise in them, partially because of bad weather near the observatory, so the researchers had to stack several images together to get their final image. Nevertheless, the point of light that they found, which appears to be Proxima c, is brighter than expected.

A planet alone wouldn’t be big enough to be so bright, so if the planet exists it is probably surrounded by a disk of debris or a huge set of rings that reflects Proxima Centauri’s light. “It would look like Saturn, but exaggerated, with a bigger ring system and a smaller planet,” says Gratton.

To reflect enough light to explain the brightness of the images, that disk or set of rings would have to be more than twice as wide as the bright part of Saturn’s ring system. “It needs to take up a lot of real estate to be bright enough,” says Mark Marley at NASA’s Ames Research Center in California.

If Proxima c is there, its size and orbit suggest that it is a relatively old planet, and we aren’t sure how such an old planet would retain rings for a long time – in contrast, Saturn’s rings are probably relatively young at just 100 million years old.

“I don’t know of any theoretical reason why it couldn’t exist, but this would be a beast – we don’t have anything like it in our solar system,” says Luke Dones at the Southwest Research Institute in Colorado. “If it turns out to be true, it’ll be a really interesting puzzle to try to explain this.”

It will require more observations with some of our best telescopes to figure out whether Proxima c and its enormous disk really exist, but these SPHERE observations make that easier by telling us exactly where to look.

“Proxima c is in theory completely observable at this moment, but the telescopes are closed due to the coronavirus,” says Gratton. “Maybe we will be able to observe it next year.”

Reference: arxiv.org/abs/2004.06685

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