A planet around the young star AU Microscopii

Jonathan Gagné, collaborator of the Center for Research in Astrophysics of Quebec (CRAQ), former iREx Banting postdoctoral researcher and now Science Advisor at Planétarium Rio Tinto Alcan, was involved in the discovery of a planet the size of Neptune around the young star AU Microscopii. Astrophysicists have been searching for planets in this system, which is a unique laboratory for studying planetary formation, for more than a decade. The breakthrough, announced today in Nature, was made possible in part by NASA’s TESS and Spitzer space telescopes.

Artistic rendition of AU Mic b, a planet similar in size to Neptune, but possibly more massive (at most 3.4 times the size of Neptune, according to ground observations).  Credit: NASA Goddard Space Flight Centre.
Located about 32 lightyears from Earth, AU Microscopii, or AU Mic, is a young star only 22 million years old which is about 150 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.

Gagné, who participated in the observations and data processing, explains why it took nearly 15 years to detect the planet: “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. The numerous spots and eruptions on the surface of AU Mic hamper the detection of planets, which is already difficult due to the presence of a disc.”

A big challenge

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). They chose this telescope, which operates in the infrared, because the star’s activity is less intense in this type of light. 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. In their observations, the astrophysicists noticed a possible periodic variation in the radial velocity of AU Mic and were thus made aware of the plausible presence of a planet around it.

Space telescopes to the rescue!

The accuracy of the data obtained on the ground was unfortunately not good enough to confirm that the signal was due to an exoplanet without a doubt. However, thanks to the transit method, a different detection technique, the team was 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 the planet during NASA’s Transiting Exoplanet Survey Satellite (TESS) first mission, in the summer of 2018. They then observed two more with NASA’s Spitzer Space Telescope in 2019. Since the amount of light blocked depends on the size of the planet and its distance from its star, these observations allowed scientists to determine that AU Mic b is about the size of Neptune, and that it passes in front of its star every 8.5 days.

Thanks to previous ground-based observations, the team also has a partial constraint on the mass of the planet. Combining IRTF’s observations with data obtained at the European Southern Observatory in Chile and the W. M. Keck Observatory in Hawai’i, they concluded that the mass is less than about 3.4 times the mass of Neptune (or 58 times that of Earth).

A unique laboratory for understanding planetary formation

AU Mic provides a unique laboratory to determine how planets and their atmospheres form, and how they interact with the disc of debris and gas from which they are born. Astrophysicists are excited about this discovery as very few systems like AU Mic are known. Not only is the detection of exoplanets difficult in these systems, but they are also very rare because a system’s period of planetary formation is relatively short compared to the life of a star.

The system is close to us and therefore appears brighter, allowing us to observe it with a range of instruments, such as the SPIRou spectrograph. Étienne Artigau, a Project Scientist at the Université de Montréal, explains: ” 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. This will allow us to determine the mass of AU Mic b accurately and to know if this planet is more like a super-Earth or a Neptune twin.” Other iREx astronomers are enthusiastic about studying the planet’s atmosphere, which can also be accomplished with SPIRou.

AU Mic is part of an association of young stars that formed at about the same time in the same place. Beta Pictoris, the star that gives its name to this association, also has a disc and two known planets. Both the star and the planets are however considerably more massive (1.75 times the mass of the Sun and 11 and 9 times the mass of Jupiter, respectively) and do not appear to have evolved in the same way. The study of these two systems, which have many characteristics in common, allow to compare two very different scenarios of planetary formation.

Many surprises undoubtedly still hide within AU Mic’s system. Will further observations of the system with TESS confirm the existence of other planets? Is the atmosphere of the planet outgassing because of the strong stellar activity? How does this system compare to others of the same age? iREx researchers are eager to get answers to these questions!

About this study

The article “A planet within the debris disk around the pre-main-sequence star AU Microscopii” was published on June 24, 2020 in Nature. In addition to Jonathan Gagné (CRAQ, Université de Montréal, Space for Life), the research team includes first author Peter Plavchan from George Mason University, second author Thomas Barclay, an Associate Research Scientist at the University of Maryland, Baltimore County and an Associate Project Scientist for TESS at NASA’s Goddard Space Flight Center in Greenbelt, Maryland and 82 other coauthors, including former CRAQ MSc student David Berardo, now a PhD student at MIT.

Paper’s URL : https://www.nature.com/articles/s41586-020-2400-z

Media contact:
Marie-Eve Naud
Scientific and EPO coordinator
Institute for research on exoplanets, Université de Montréal, Montréal, Canada

Nathalie Ouellette
Institute for Research on Exoplanets, Université de Montréal, Montréal, Canada

Scientific contact:
Jonathan Gagné
Scientific Advisor
Planétarium Rio Tinto Alcan | Espace pour la vie, Montréal, Canada