An exoplanet the size of Neptune has been discovered orbiting a young star which, astronomers say, is relatively close to Earth.

AU Mic b is around 32 light years away and orbits its host star, AU Microscopii (AU Mic), in the constellation of Microscopium.

Astronomers say AU Mic is 20 or 30 million years old, 150 times younger than the Sun.

Scientists believe that studying this relatively new star system is a “unique laboratory for studying planetary formation” and could help shed light on how our own Solar System formed.

Dr Thomas Barclay, an associate research scientist at the University of Maryland Baltimore County, US, and one of the study authors, said: “This star probably hasn’t had time to form small, rocky planets yet.

“It gives us a chance to get a picture of what might have happened before our own terrestrial planets like Earth and Venus formed.”

AU Mic is a small star, with only about 50% of the Sun’s mass. These stars generally have very strong magnetic fields, which make them very active

Since first identifying the flaring behaviour of the star in the 1970s, scientists have wondered whether AU Mic was host to planets.

In the 2000s, astronomers observed a large disc of dust and gas surrounding the star, a remnant of its formation.

Then, in 2018, a research team led by Peter Plavchan, now an assistant professor at George Mason University, detected the first “signals” of a planet orbiting AU Mic using Nasa’s planet-hunting spacecraft Tess (Transiting Exoplanet Survey Satellite).

These observations were confirmed by Nasa’s Spitzer Space Telescope in 2019.

Jonathan Gagne, a scientific adviser at the Rio Tinto Alcan Planetarium in Montreal, Canada, who is also an author on the study, said: “AU Mic is a small star, with only about 50% of the Sun’s mass.

“These stars generally have very strong magnetic fields, which make them very active.

“That explains in part why it took nearly 15 years to detect the exoplanet, called AU Mic b.

“The numerous spots and eruptions on the surface of AU Mic hampered its detection, which was already complicated by the presence of the disc.”

By analysing the amount of light blocked by AU Mic b as it passed in front of its host star, scientists were able to calculate its size and orbit period.

Dr Barclay, who is also an associate project scientist for Tess, said: “Dips in brightness tell you about the size of the planet, and measuring how regularly spaced the transits are tells us how long it takes the planet to go around the star.”

The team determined that AU Mic b is the size of Neptune, has a mass of no more than 58 Earths, and completes an orbit of AU Mic every 8.5 days.

As part of the next steps, the researchers want to learn more about the atmosphere of AU Mic b.

– The research is published in the journal Nature.

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