2023 February: Small planets orbiting red dwarf stars detected by SPIRou and TESS

Since the discovery in 1995 of a planet in orbit around a star other than the Sun, research in exoplanetology has revolutionized our knowledge of planetary systems. Among the thousands of systems known today, many have planets that have no equivalent in our Solar System. This is the case of the sub-Neptune and super-Earth type exoplanets recently discovered by an international team around red dwarf stars close to the Sun, and which provide us with information on the astonishing diversity of exoplanets. These planets were detected with the TESS satellite and the SPIRou instrument, a spectropolarimeter installed at the Canada-France-Hawaii Telescope.

For a long time, only the planets of the Solar System were known and observed: four giant planets far from the Sun, and four telluric planets (including Earth) closer to our star. If it seemed very likely for several centuries that many other stars, if not all, also harbored planets, these remained however inaccessible to our means of observation. It was therefore not known whether these possible exoplanets really existed, were similar to those of the Solar System, or had different properties.  The situation changed in 1995 with the first detection of an exoplanet, made at the Haute-Provence Observatory by astronomers Michel Mayor and Didier Queloz, later awarded the Nobel Prize in Physics. New instruments have since been developed and enabled the detection and characterization of thousands of exoplanets, revolutionizing our knowledge of planetary systems, in the aim of unveiling their formation and evolution.

One of these new instruments is SPIRou, designed and constructed by an international consortium managed by a French team, and installed at the Canada-France-Hawaii Telescope on the Big Island of Hawaii. It is a spectropolarimeter operating in the infrared range, that makes it possible to search for planets around stars less massive, smaller and cooler than the Sun, the red dwarfs (whose temperature ranges between 2500° and 4000°, while the Sun is at 5500°).  It also allows one to study the magnetic activity of these small dwarf stars, which happen to be the most numerous stars in our galaxy.  SPIRou is used in particular to characterize candidate planets (objects thought to be planets) already identified around red dwarf stars by NASA’s TESS satellite, but that require complementary observations from the ground to firmly establish their planetary nature.  Thanks to the extreme stability of SPIRou, it’s possible to detect the tiny reflex motion of a star induced by an orbiting planet, and thereby to measure the mass of the planet – a tour de force for this cryogenic instrument cooled to the temperature of liquid nitrogen (-200°).

A team led by French astronomer Flavier Kiefer, involving astronomers of the SPIRou science consortium from several CNRS-INSU laboratories in particular, carried out a study which led to the discovery and characterization of a new planet called TOI-1695b. Barely twice as big and six times more massive than Earth, TOI-1695b circles its red dwarf star in only three days. Slightly smaller than Neptune (hence its designation of sub-Neptune), this new planet has a density slightly lower than that of Earth, and a temperature higher by a few hundred degrees. Its atmosphere most likely contains large amounts of hydrogen, helium and water vapor. This discovery will help scientists to better understand how such planets, absent from our solar system, can become gaseous planets for some, or rocky planets for others.

A few months ago, another team from the SPIRou science consortium announced the discovery and characterization of super-Earth TOI-1452b and sub-Neptune TOI-1759b, also orbiting red dwarf stars. The size of these new planets ranges between 1.7 and 3.1 times that of Earth, and their masses between 5 and 7 times that of Earth. They are therefore intermediate planets between Earth and Neptune, but much closer to their host stars. These discoveries confirm that such planets, although not present in the Solar System, are very abundant in our galaxy.

The discoveries of the planets TOI-1695b, TOI-1452b and TOI-1759b are presented in the following scientific publications:

– A sub-Neptune planet around TOI-1695 discovered and characterized with SPIRou and TESS, par Kiefer, Hébrard, Martioli, Artigau, Doyon, Donati, Cadieux, Carmona, Ciardi, Cristofari et al. 2022 (A&A, sous presse, https://arxiv.org/abs/2211.06205) ;
– TOI-1452b: SPIRou and TESS reveal a super-Earth in a temperate orbit transiting an M4 dwarf, par Cadieux, Doyon, Plotnykov, Hébrard, Jahandar, Artigau, Valencia, Cook, Martioli, Vandal et al. 2022 (AJ 164, 96, https://iopscience.iop.org/article/10.3847/1538-3881/ac7cea) ;
– TOI-1759b: A transiting sub-Neptune around a low mass star characterized with SPIRou and TESS, par Martioli, Hébrard, Fouqué, Artigau, Donati, Cadieux, Bellotti, Lecavelier des Étangs, Doyon, do Nascimento et al. 2022 (A&A 660, A86, https://www.aanda.org/articles/aa/full_html/2022/04/aa42540-21/aa42540-21.html)