Planet AU Mic b is about the size of Neptune. Credit: NASA Goddard Space Flight Center / Chris Smith (USRA)
A exoplanet the size of Neptune It has been discovered around the young star AU Microscopii, thanks in part to the work of Jonathan Gagné, a former post-doctoral researcher at iREx Banting 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 preview, announced today in Naturewas made possible in part by POT‘s TESS and Spitzer space telescopes.
Located about 32 light years from Earth, AU Microscopii, or AU Mic, is a young star between 20 and 30 million years old, which is approximately 180 times younger than our own Sun. In the 2000s, it was discovered it was still surrounded by a large disk of debris, a remnant of its formation. Since then, astrophysicists have been actively searching for planets around AU Mic, as it lies within such dust and gas disks that form.
“AU Mic is a small star, with only about 50 percent of the Sun’s mass,” said Gagné, who participated in the observations and data processing. These stars generally have very strong magnetic fields, making them very active. That partly explains why it took almost 15 years to detect the exoplanet, called AU Mic b. The numerous points and eruptions on the surface of AU Mic made it difficult to detect, which was already complicated by the presence of the disc ”.
Illustration of AU Mic b orbiting its mother star, AU Mic. Credit: NASA Goddard Space Flight Center / Chris Smith (USRA)
A great challenge
Jonathan Gagné at the Mauna Kea summit, 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 expected to see the planet’s signal better, since the star’s activity is less intense in this type of light.
For his part, Gagné made numerous observation trips to IRFT during his doctoral studies. It is then when he got involved in the project. “A few years after joining the team, we noticed a possible periodic variation in AU Mic’s radial velocity,” he recalled.
“This is how we realized the possible presence of a planet around it.” When a planet orbits, its gravity pulls on its host star, which moves slightly in response. Sensitive spectrographs like the IRTF can detect the star’s radial velocity, its movement back and forth along our line of sight.
Au Mic is a “red dwarf”, the most numerous type of star in the galaxy. Credit: NASA Goddard Space Flight Center / Chris Smith (USRA)
Space telescopes to the rescue
the accuracy Unfortunately, the data obtained in the field was not sufficient to confirm without a doubt that the signal was due to an exoplanet. Thanks to the transit method, a different detection technique, 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 the first NASA Transit Exoplanet Inspection Satellite (TESS) mission in the summer of 2018. They then observed two more with NASA’s Spitzer Space Telescope 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 approximately the size of Neptune, and that it passes in front of its star every 8.5 days.
Thanks to previous ground observations, the team also has a partial restriction on the mass of AU Mic b. Combining IRTF observations with data obtained from the European Southern Observatory in Chile and the WM Keck Observatory in Hawaii, they concluded that its mass is 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 for determining how exoplanets and their atmospheres form, and how they interact with the debris and gas disk from which they originate.
Scientists are excited about their latest discovery, as very few systems are known as AU Mic. Detection of exoplanets is not only difficult in these systems, but they are also very rare because 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 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 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 excited about trying to detect the planet’s atmosphere and see the effect of the active star on it. These observations can also be achieved with SPIRou.
AU Mic is part of an association of young stars that formed at approximately the same time in the same location. Beta Pictoris, the star that gives its name to this association, also has a disk and two known planets.
However, both the star and the planets are considerably more massive (1.75 times the mass of the Sun, and 11 and nine times the mass of Jupiter, respectively). They do not appear to have evolved in the same way as AU Mic and her planet. By studying these two systems, which have many characteristics in common, scientists can compare two very different planetary formation scenarios.
Undoubtedly, many surprises are still hidden within the AU Mic system, iREX researchers believe. Will new observations of the system with TESS confirm the existence of other planets? Is the planet’s atmosphere degassing due to strong stellar activity? How does this system compare to others of the same age? Those are all questions for future study.
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Reference: “A planet within the debris disk around the star in the sequence prior to the AU microscope” 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, América 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 within the debris disk around the star of the pre-main sequence AU Microscopii” was published on June 25, 2020 in Nature. In addition to Jonathan Gagné (iREx, Université de Montréal, Space for Life), the research team includes first author Peter Plavchan of George Mason University; Second Author Thomas Barclay, Research Associate Scientist at the University of Maryland, Baltimore County and Associate Project Scientist 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 PhD student at MIT.
