Neptune-Sized Exoplanet Discovered Around Nearby Young Star

Artists representation of AU Mic b and its star. NASA Goddard Space Flight Centre.

Slightly less than 32 light-years away from Earth there is a star we call AU Microscopii, located in the southern constellation of the Microscope. At less than 30 million years of age, it is among the youngest stars near us. Now, researchers have announced the discovery of a Neptune-size planet forming in its surrounding disk of material.

As reported in Nature, the planet has a radius slightly larger than Neptune, and is at most three times as heavy, which is about 58 Earths. Like Neptune, it's a gas planet, but it's unlike the gas giants that orbit far out in our Solar System. AU Mic b, as it's known, orbits its star every 8.46 days, making it closer even than Mercury is to the Sun.

The planet was discovered with the transit method, that detects planets as they pass in front of the star and temporarily dim it, using NASA's planet-hunting Transiting Exoplanet Survey Satellite (TESS) mission, with independent confirmation from the retired Spitzer observatory and ground-based telescopes in Hawaii and Chile.

Animated GIF illustrating one interpretation of planet AU Mic b and its host star. NASA's Goddard Space Flight Center/Chris Smith (USRA)

"One of the things we want to understand is, 'When do planets form, and what do they do in their early days?'" said co-author Thomas Barclay, from the University Of Maryland Baltimore County, said in a statement.

"My role is to take the brightness data for the star and use that to understand what the size and other properties of the planet are," he explained. "Dips in brightness tell you about the size of the planet, and measuring how regularly spaced the transits tells us how long it takes the planet to go around the star."

Animated GIF illustrating the AU Mic system. Credit: NASA's Goddard Space Flight Center/Chris Smith (USRA)

The youth and closeness of the system are the most exciting characteristics for the researchers. We are getting a close-up shot of how a planetary system might form, almost from the very beginning. Studying this system will provide clues on the formative years of planets, and possibly gives us more information on how our own Solar System began.

"This star probably hasn't had time to form small, rocky planets yet," Barclay added. "It gives us a chance to get a picture of what might have happened before our own terrestrial planets like Earth and Venus formed."

The team is interested in the planet’s atmosphere for a few reasons. It must have formed relatively recently and, given the planet’s proximity to the star, it is likely being thrown into space. Its composition is also important. Certain chemicals can only form at certain distances from the star. Their presence would indicate where the planet formed and if it has moved since its formation.

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