An international team led by the Institut d'astrophysique de Paris recently published studies of the young planetary system AU Microscopii. Using the SPIRou instrument installed at the Canada-France-Hawaï Telescope, they measured the intense magnetic activity of its star and the obliquity of a first planet; and with NASA’s TESS satellite, they detected and characterized a second transiting planet. These results contribute to a better understanding of the formation and evolution processes of planetary systems.

Artist’s view of the young star AU Mic and one of its two planets. Figure 1: Artist’s view of the young star AU Mic and one of its two planets. The red streaks suggest the magnetic field of the star. Eruptions are also stylized on the surface of this active star. Credits: NASA-JPL/Caltech.

The first planet to orbit a star other than the Sun, that is to say the first exoplanet, was discovered in 1995 by Swiss astronomers Michel Mayor and Didier Queloz at the Observatoire de Haute-Provence, France. The Nobel Prize in Physics that they received in 2019 for this discovery highlights the revolution that it created for astrophysics. Hundreds of astronomers now work in exoplanetology around the world. The more than 4,500 exoplanets known today reveal the great abundance and diversity of planets in our galaxy and allow researchers to better understand the processes for formation and evolution of planetary systems.

The study of planets around different types of stars allows one to establish the possible relationships between the properties of the stars and those of the planets they host. Although most of known planets orbit around stars having an age similar to that of the Sun (a few billion years), some orbit much younger stars. The study of the latter allow researchers to explore the properties of planetary systems that have been formed recently, and thus to better understand the physical processes driving their evolution.

In this context, the planetary system AU Microscopii (AU Mic) is particularly interesting. It is hosted by a star half as massive as the Sun, only 22 million years old, and surrounded by a disc formed of dust and gas. It could be the residual of the “protoplanetary” disc (in which the planets would have formed). The youth of this star is characterised by its high rotation speed and strong magnetic activity (eruptions, intense magnetic field, star spots similar to sunspots but much more numerous; see Figure 1).

Observing this star in 2018, NASA's TESS satellite (Transiting Exoplanet Survey Satellite) revealed a planet about four times larger than the Earth orbiting this star and passing in front of it every 8.5 days; thus planetary transits are observed. This planet is at a distance of 0.065 astronomical unit from its star (the astronomical unit corresponds to the distance between the Earth and the Sun). The discovery of a planet around such a young star has provoked great interest among exoplanetologists, and many follow-up studies of this system have been undertaken.

As part of an international collaboration, a team from the Institut d'astrophysique de Paris led by post-doctoral fellow Eder Martioli undertook, in 2019, observations of this planet with the SPIRou spectropolarimeter, an instrument conducting its scientific programs since 2019 at the Canada-France-Hawaï Telescope (CFHT). The observations allowed them to measure the magnetism of the star, to better understand its effects on spectroscopic measurements, and to deduce the obliquity of the system (see Figure 2). The obliquity is the angle between the orbit plane of the planet around the star and the plane perpendicular to the stellar rotation axis(“equatorial” plane). The measurement revealed a zero obliquity: the planet orbits in the equatorial plane of its star, and in the same direction as the rotation of the star on itself. This is in accordance with the models considering that the planets are formed in a disk also perpendicular to the stellar rotation axis. Several exoplanets orbiting older stars have non-aligned orbits: the results of SPIRou suggest that these non-alignments are not present at the birth of planetary systems, but occur later in their evolution.

A second study of the AU Mic system was carried out by the same team at the Institut d'astrophysique de Paris. It is based on TESS' observations, using the 2018 data supplemented by the new observations made in 2020. The fine analysis of these data confirmed that the star rotates rapidly (in a little less than five days) and had several eruptions per day on its surface (see Figure 3), similar to the solar eruptions seen sometimes on the surface of our star, but much more intense. The modelling and correction of these effects have allowed new refined measurements of the parameters of the known planet in this system. Morevoer, they have allowed the detection and characterization of an additional planet, also transiting in front of the star. The second planet in the AU Mic system is a little farther and smaller than the first one: it orbits its star in 18.9 days and is three times larger than the Earth. It is located at 0.11 astronomical unit from its star.

The configuration of this system is dynamically stable: the gravitational interactions between the two planets will not lead to the ejection of one of them, nor to collisions between them. However, these interactions are significant and cause small variations in their orbits over time: their transits are expected to occur a few tens of seconds ahead or behind of what is expected for an exact periodic orbit. Future measurements should confirm this prediction. AU Mic is a key system that will allow many studies concerning young planets, their atmospheres, planet-planet and planet-disk interactions. This will allow researchers to better understand the early phases in the evolution of planetary systems.

Schematic view of the obliquity measurement of the AU Mic system

Figure 2: Schematic view of the obliquity measurement of the AU Mic system. The planet orbits in the same direction as the star rotates, and its orbital plane is aligned with the equatorial plane of the star. Credits: R Cardoso Reis, IA/UPorto

Detection of the second planet in the AU Mic system

Figure 3: Detection of the second planet in the AU Mic system. The amount of light measured by the TESS satellite as a function of time is shown in blue. The eruptions of the star are modelled (in yellow) as well as the transit of the planet in front of the star (in red), i.e. the moment when the planet passes in front of the star and hides part of its light. Credits: Martioli et al. (2021), IAP.

This work was conducted by the Institut d'astrophysique de Paris (CNRS, Sorbonne Université) through international collaborations. They are partly funded by the Île-de-France Region and the Agence nationale de la recherche.


puce Astronomy & Astrophysics article: Martioli, Hébrard, Moutou, Donati, Artigau et al. (2020), “Spin-orbit alignment and magnetic activity in the young planetary system AU Mic” (public version)

puce Astronomy & Astrophysics article (in press): Martioli, Hébrard, Correia, Laskar, Lecavelier des Étangs (2021), “New constraints on the planetary system around the young active star AU Mic - Two transiting warm Neptunes near mean-motion resonance” (public version)

puce Press release from the Institut de Recherche en Astrophysique et Planétologie (IRAP) regarding AU Mic observations with SPIRou: “SPIRou stares at a young rebel: the AU Mic planetary system”

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April 2021

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