A exoplanet the size of Neptune was discovered around the young star AU Microscopii, thanks in part to the work of Jonathan Gagné, former postdoctoral researcher iREx Banting who is now a scientific advisor at the Rio Tinto Alcan Planetarium.
Astrophysicists have been looking for exoplanets in this system, a unique laboratory for studying planetary formation, for more than a decade. The breakthrough, announced today at Nature, was made possible in part by NASAthis is TESS and the Spitzer space telescopes.
Located about 32 light years from Earth, AU Microscopii, or AU Mic, is a young star aged 20 to 30 million years, which is about 180 times younger than our own Sun. In the 2000s, it turned out that he was still surrounded by a large disc of debris, a vestige of his training. Since then, astrophysicists have been actively searching for planets around AU Mic, as it is inside such disks of dust and gas that they are formed.
“AU Mic is a small star, with only about 50% of the mass of the sun,” said Gagné, who participated in the observations and data processing. These stars generally have very strong magnetic fields, which makes them very active. This partly explains why it took almost 15 years to detect the exoplanet, called AU Mic b. The numerous spots and eruptions on the surface of AU Mic hindered its detection, which was already complicated by the presence of the disc. “
A great challenge
Jonathan Gagné at the top 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 works in the infrared, where the team hoped to better see the signal from the planet, because the activity of the star is less intense in this type of light.
For his part, Gagné made numerous observation trips to the IRFT during his doctoral studies. It was then that he got involved in the project. “A few years after joining the team, we noticed a possible periodic variation in the radial velocity of AU Mic,” he recalls.
“We were thus made aware of the plausible presence of a planet around it.” When a planet orbits, its gravity pulls on its host star, which moves slightly in response. Sensitive spectrographs such as that of the IRTF can detect the radial speed of the star, its back and forth movement along our line of sight.
Space telescopes to the rescue
the precision data obtained in the field was unfortunately not sufficient to confirm without any doubt that the signal was due to an exoplanet. It was thanks to the transit method, 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 spectator, periodically hiding a small fraction of its light. Astronomers observed two transits of AU Mic b during the first mission of NASA’s exoplanets in transit (TESS) satellite in summer 2018. They then observed two others with the space telescope NASA Spitzer in 2019.
Since the amount of blocked light depends on the size of the exoplanet and its distance from its star, these observations allowed scientists to determine that AU Mic b is roughly the size of Neptune and that it passes in front of its star every 8.5 days.
Thanks to previous observations on the ground, the team also has a partial constraint on the mass of AU Mic b. Combining IRTF observations with data from the European Southern Observatory in Chile and the WM Keck Observatory in Hawai’i, they concluded that its mass was less than about 3.4 times the mass of Neptune (or 58 times that of Earth).
A unique laboratory
AU Mic provides a unique laboratory to determine how exoplanets and their atmospheres are formed, and how they interact with the disk of debris and gases from which they originate.
Scientists are delighted with their latest 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 since the planetary formation period of a system is relatively short compared to the life of a star.
The AU Mic system is close to Earth and therefore appears brighter, allowing astrophysicists to observe it with a range 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 of the planets,” explained É Etienne Artigau, project scientist at the University of Montreal. “This will allow us to accurately determine the mass of AU Mic b and whether this exoplanet is more like a large Earth or a Neptune twin.”
Other iREx astronomers are enthusiastic about detecting the planet’s atmosphere and seeing the effect of the active star on it. These observations can also be made with SPIRou.
AU Mic is part of an association of young stars who trained at about the same time in the same place. Beta Pictoris, the star which gives its name to this association, also has a disc and two known planets.
The star and the planets are however considerably more massive (1.75 times the mass of the Sun and 11 and nine times the mass of Jupiter, respectively). They do not seem to have evolved in the same way as AU Mic and its planet. By studying these two systems, which have many characteristics in common, scientists can compare two very different scenarios of planetary formation.
Many surprises are undoubtedly hiding in the AU Mic system, according to iREX researchers. Will new observations of the system with TESS confirm the existence of other planets? Is the atmosphere of the planet evolving because of the strong stellar activity? How does this system compare to others of the same age? These are all questions for future study.
Reference: “A planet in the debris disc around the main pre-sequence star AU Microscopii” by Peter Plavchan, Thomas Barclay, Jonathan Gagné, Peter Gao, Bryson Cale, William Matzko, Diana Dragomir, Sam Quinn, Dax Feliz, Keivan Stassun, Ian JM Crossfield, David A. Berardo, David W. Latham, Ben Tieu, Guillem Anglada-Escudé, George Ricker, Roland Vanderspek, Sara Seager, Joshua N. Winn, Jon M. Jenkins, Stephen Rinehart, Akshata Krishnamurthy, Scott Dynes, John Doty, Fred Adams, Dennis A. Afanasev, Chas Beichman, Mike Bottom, Brendan P. Bowler, Carolyn Brinkworth, Carolyn J. Brown, Andrew Cancino, David R. Ciardi, Mark Clampin, Jake T. Clark, Karen Collins, Cassy Davison, Daniel Foreman-Mackey, Elise Furlan, Eric J. Gaidos, Claire Geneser, Frank Giddens, Emily Gilbert, Ryan Hall, Coel Hellier, Todd Henry, Jonathan Horner, Andrew W. Howard, Chelsea Huang, Joseph Huber, Stephen R. Kane, Matthew Kenworthy, John Kielkopf, David Kipping, Chris Klenke, Ethan Kruse, Natasha Latouf, Patrick Low rance, Bertrand Mennesson, Matthew Mengel, Sean M. Mills, Tim Morton, Norio Narita, Elisabeth Newton, America Nishimoto, Jack Okumura, Enric Palle, Joshua Pepper, Elisa V. Quintana, Aki Roberge, Veronica Roccatagliata, Joshua E. Schlieder, Angelle Tanner, Johanna Teske, CG Tinney, Andrew Vanderburg, Kaspar von Braun, Bernie Walp, Jason Wang, Sharon Xuesong Wang, Denise Weigand , Russel White, Robert A. Wittenmyer, Duncan J. Wright, Allison Youngblood, Hui Zhang and Perri Zilberman, June 24, 2020, Nature.
DOI: 10.1038 / s41586-020-2400-z
About the study
“A planet in the debris disc around the main pre-sequence star AU Microscopii” was published on June 25, 2020 in Nature. In addition to Jonathan Gagné (iREx, Université de Montréal, Espace pour la vie), the research team includes the first author Peter Plavchan from George Mason University; second author Thomas Barclay, associate researcher at the University of Maryland, Baltimore County and associate researcher for TESS at NASA’s Goddard Space Flight Center in Greenbelt, Maryland; and 82 other co-authors, including former iREx member David Berardo, now a doctoral student at WITH.