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Astronomers find Neptune-sized star orbiting young, nearby star

An artistic rendering imagines the newly discovered Neptune-sized planet in the protoplanetary disk of the nearby star AU Mic. Photo by NASA's Goddard Space Flight Center<br>
An artistic rendering imagines the newly discovered Neptune-sized planet in the protoplanetary disk of the nearby star AU Mic. Photo by NASA's Goddard Space Flight Center

June 24 (UPI) -- After looking for more than a decade, astronomers have finally found an exoplanet in the young AU Microscopii star system.

With the help of NASA's TESS and Spitzer space telescopes, scientists identified a Neptune-sized world circling the young star, positioned just 32 light-years from our solar system.

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Researchers announced the discovery on Wednesday in the journal Nature.

In addition to being so close by, AU Mic is estimated to be just 20 to 30 million years old, making it 180 times younger than our sun -- offering scientists an opportunity to study the early evolution of a planetary system.

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"AU Mic is a small star, with only about 50 percent of the sun's mass," Jonathan Gagné, a former postdoctoral researcher at the University of Montreal who is now a scientific advisor at the Rio Tinto Alcan Planetarium, said in a press release.

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"These stars generally have very strong magnetic fields, which make them very active," Gagné said. "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."

For the past several years, astronomers have been using ground telescopes at NASA's Infrared Telescope Facility to monitor the star system in infrared. The star's fluctuating emissions are less sporadic in the infrared.

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"A few years after I joined the team, we noticed a possible periodic variation in the radial velocity of AU Mic," Gagné said. "We were thus made aware of the plausible presence of a planet around it."

As the exoplanet completes its orbit, its gravity creates slight perturbations in the star's radial velocity. But while the ground telescopes were able to pick up on this periodic dance of infrared light, the observations weren't precise enough to confirm the presence of an exoplanet.

However, researchers were able to observe the Neptune-sized exoplanet using a different technique. When exoplanets pass in front of their host stars, they cause a slight dimming. Using TESS and Spitzer, astronomers measured a total of four transits of AU Mic b -- two with each space observatory.

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The TESS and Spitzer data confirmed the exoplanet orbits AU Mic every 8.5 days. By combining the transit data with the radial velocity data collected by NASA's IRFT telescopes, researchers were able to estimate the exoplanet's mass.

Finding a young planet in a young stellar system is rare, researchers say. And observing the phenomena in a stellar system so close to Earth is even more rare. The close proximity of AU Mic and AU Mic b, however, allowed astronomers to study the system using a variety of instruments -- like the SPIRou spectrograph.

"This instrument, with its polarimetric capabilities, will allow us to better distinguish the effects of stellar activity, which are often confused with the signal from the planets," said É tienne Artigau, a project scientist at University of Montréal. "This will allow us to determine the mass of AU Mic b accurately and to know if this exoplanet is more like a large Earth or a Neptune twin."

In the future, scientists hope to use SPIRou to study the effect of AU Mic's stellar activity on the atmosphere of AU Mic b.

Scientists also plan to compare the peculiarities the AU Mic system with nearby stellar systems, such as Beta Pictoris -- one of several stars that formed at the same time and in the same place.

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