Scientists spot rare Neptune-like planet

Washington: Scientists have discovered a planet about as large as Neptune that circles a nearby star which is still surrounded by a disk of debris left over from its formation.

Using data from NASA’s Transiting Exoplanet Survey Satellite (TESS) and retired Spitzer Space Telescope, the discovery, published in the journal Nature, makes the system a touchstone for understanding how stars and planets form and evolve.

The planet circles the young star, AU Microscopii, in just over a week.

The system, known as AU Mic for short, provides a one-of-a-kind laboratory for studying how planets and their atmospheres form, evolve and interact with their stars.

“AU Mic is a young, nearby M dwarf star. It’s surrounded by a vast debris disk in which moving clumps of dust have been tracked, and now, thanks to TESS and Spitzer, it has a planet with a direct size measurement,” said study co-author, Bryson Cale, a doctoral student at George Mason University in Fairfax, Virginia, US.

“There is no other known system that checks all of these important boxes.”

AU Mic is a cool red dwarf star with an age estimated at 20 million to 30 million years, making it a stellar infant compared to our Sun, which is at least 150 times older.

The star is so young that it primarily shines from the heat generated as its own gravity pulls it inward and compresses it.

Less than 10 per cent of the star’s energy comes from the fusion of hydrogen into helium in its core, the process that powers stars like our Sun.

The system is located 31.9 light-years away in the southern constellation Microscopium.

It is part of a nearby collection of stars called the Beta Pictoris Moving Group, which takes its name from a bigger, hotter A-type star that harbors two planets and is likewise surrounded by a debris disk.

Although the systems have the same age, their planets are markedly different. The planet AU Mic b almost hugs its star, completing an orbit every 8.5 days.

It weighs less than 58 times Earth’s mass, placing it in the category of Neptune-like worlds.

Beta Pictoris b and c, however, are both at least 50 times more massive than AU Mic b and take 21 and 3.3 years, respectively, to orbit their star.

“We think AU Mic b formed far from the star and migrated inward to its current orbit, something that can happen as planets interact gravitationally with a gas disk or with other planets,” said co-author Thomas Barclay, an associate research scientist at the University of Maryland, Baltimore County.

“By contrast, Beta Pictoris b’s orbit doesn’t appear to have migrated much at all. The differences between these similarly aged systems can tell us a lot about how planets form and migrate,” said Barclay who is also an associate project scientist for TESS at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

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