Discovering an exoplanet size of Neptune

An exoplanet the size of Neptune has been discovered around the young star AU Microscopii, thanks in part to the work of Jonathan Gagné, a former iREx Banting postdoctoral researcher who is now a scientific advisor at the Rio Tinto Alcan Planetarium.

Astrophysicists have been searching for exoplanets in this system, a unique laboratory for studying planetary formation, for more than a decade. The breakthrough, announced today i n Nature, was made possible in part by NASA’s TESS and Spitzer space telescopes.

Located about 32 lightyears from Earth, AU Microscopii, or AU Mic, is a young star between 20 and 30 million years old, which is about 180 times younger than our own Sun. In the 2000s, it was found to still be surrounded by a large disc of debris, a remnant of its formation. Since then, astrophysicists have been actively searching for planets around AU Mic, since it is within such discs of dust and gas that they form.

“AU Mic is a small star, with only about 50 per cent of the Sun’s mass,” said Gagné, who participated in the observations and data processing. 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.”

A big challenge

Jonathan Gagné at the summit of Mauna Kea, where astrophysicists have been searching for a planet around AU Mic since 2010. Credit : Jonathan Gagné. In 2010, a team led by Peter Plavchan, now an assistant professor at George Mason University, began observing AU Mic from the ground using NASA’s Infrared Telescope Facility (IRTF).

The telescope operates in the infrared, where the team hoped to see the signal of the planet better, since the star’s activity is less intense in this type of light.

For his part, Gagné made numerous observational trips to the IRFT during his doctoral studies. That is when he became involved in the project. “A few years after I joined the team, we noticed a possible periodic variation in the radial velocity of AU Mic,” he recalled.

“We were thus made aware of the plausible presence of a planet around it.” As a planet orbits, its gravity tugs on its host star, which moves slightly in response. Sensitive spectrographs such as the one on the IRTF can detect the star’s radial velocity, its motion to-and-fro along our line of sight.

Space telescopes to the rescue

The accuracy of the data obtained on the ground was unfortunately not sufficient to confirm without a doubt that the signal was due to an exoplanet. It’s thanks to the transit m ethod, a different detection technique, that the team was finally able to confirm the presence of AU Mic b.

A transit occurs when a planet passes directly between its host star and the viewer, periodically hiding a small fraction of its light. Astronomers observed two transits of AU Mic b during NASA’s Transiting Exoplanet Survey Satellite (TESS)

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