Search for habitable planets

Keywords

Exoplanets in the habitable zone, detection and caracterisation, F-G-K-M stars, stellar variability, host star characterisation

Motivations

Our team has a long history in discovering and studying planets around very-low-mass stars, in particular with both Doppler spectroscopy and transit photometry methods. Over the years our surveys have become sensitive to ever lower-mass, smaller planets and did detect several exo-Earths in the so called “habitable zones” of their host stars. These planets are prized targets for further characterization with JWST and ELTs, which may soon detect and probe the composition of their atmospheres. Meanwhile, however, whether these worlds are truly habitable (and inhabited!) remains highly speculative. The motivation remains therefore strong to push the detection sensitivity toward more massive stars that are more like our Sun. Reaching sensitivity to Earth-like planets orbiting Sun-like stars does require detail understandings of the host stars and in particular of their variability. Moreover, we also have the motivation to understand the whole architecture of planetary systems hosting habitable exo-Earths, combining astrometry to Doppler spectroscopy and transit photometry.

Objectives

  • Make the census of nearby temperate exo-Earths, both around M dwarfs with current surveys and FGK dwarfs with future surveys
  • Measure the bulk properties and probe the atmospheres of those amenable to detail characterization
  • Characterize and quantify the impact of stellar variability on the detectability and characterization of planets
  • Understand stellar variability on different time scales, in particular the relationship between chromospheric indices, so as to better handle their effect on Doppler spectroscopy and transit photometry

Methods

  • Search for exoplanets with radial velocity surveys, and in particular with SOPHIE(-Red), HARPS+NIRPS, SPIRou, and ESPRESSO (Fig. 2)
  • Search, confirmation, and precise properties of transiting planets with transit photometry, and in particular with ExTrA, CHEOPS and PLATO
  • Measure the mass of transiting planets (with either velocimeters or with transit timing variations)
  • Development of codes and algorithms to detect planets and model their orbits
  • Involvement in the THEIA astrometric mission (relative astrometry with an accuracy of 0.1µas)
  • Simulations of stellar activity time series, blind tests on large samples (Fig. 3)
  • Exploitation of GAIA data to better characterise the host star (mass, age, metallicity, variabilities) and the systems (multiplicity, dynamical masses) (Fig. 1)
  • Define habitability conditions and biomarkers that can be implemented

Contacts

Carine Babusiaux, Xavier Bonfils, Xavier Delfosse, Thierry Forveille, Fabien Malbet, Nadège Meunier, José-Manuel Almenara Villa, Andres Carmona, Marion Cointepas, Pierre Larue

Illustrations

Fig 1. gauche : Mouvement astrométrique apparent d’une étoile seule (haut) et du photocentre d’un système binaire (bas). Plus la masse du compagnon est faible, plus le signal astrométrique est faible. droite : Diagramme H-R des solutions astrométriques Gaia DR3 avec une fonction de masse astrométrique faible (en vert, le fond gris étant le diagramme H-R global DR3). La faible fonction de masse peut être due soit à un compagnon de faible masse, soit à des binaires ayant un rapport de masse similaire à leur rapport de flux, ce qui conduit à deux branches principales pour la séquence principale dans ce diagramme (Gaia Collaboration, Arenou et al. 2023) // left: Apparent astrometric motion of a single star (top) and a binary system photocenter (bottom). The lower the mass of the companion, the smaller the astrometric signal. right: H-R diagram of the Gaia DR3 astrometric solutions with a low astrometric mass function (in green, the grey background being the global DR3 low extinction H-R diagram). The low mass function can be due either to a low-mass companion or to binaries with a mass ratio similar to their flux ratio, which leads to two main branches for the main-sequence in this diagram (Gaia Collaboration, Arenou et al. 2023).



Fig. 2 Détection par la méthode des vitesses radiales de la planètes Gl411b. A gauche : l’attraction gravitationnelle de la planète crée un léger mouvement réflexe de l’étoile qui se détecte ici en mesurant sa vitesse dans notre direction. Le fait que la vitesse change indique la présence d’une planète. A droite : cette planète est la seconde plus proche détectée, après la planète orbitant autour de Proxima du Centaure. Cette caractéristique en fait une des planètes phares pour des observations de caractérisations à venir avec la futur génération de télescope géant. Elle possède une masse d’environ 3 fois celle de la Terre et orbite en seulement 9.9 jours autour de son étoile. Nous l’avons découverte en utilisant le vélocimètre SOPHIE monté sur les télescope de 1.93m de l’Observatoire de Haute Provence (OHP). Ces conclusions sont publiées dans Diaz, Delfosse et al. (2019), A&A 625, A17. // Detection of the planet Gl411b using the radial velocity method. Left: the gravitational attraction of the planet creates a slight reflex movement of the star, which is detected here by measuring its velocity in our direction. The fact that the velocity changes indicates the presence of a planet. Right: this planet is the second closest detected, after the planet orbiting Proxima Centauri. This makes it one of the key planets for future characterisation observations with the next generation of giant telescopes. Gl411b has a mass around 3 times that of the Earth and orbits its star in just 9.9 days. We discovered it using the SOPHIE velocimeter mounted on the 1.93m telescope of the Observatoire de Haute Provence (OHP). These conclusions are published in Diaz, Delfosse et al (2019), A&A 625, A17.



Fig 3. Reconstruction du signal en vitesses radiales du Soleil dû au contraste des taches et des plages observées (en noir), et en incluant l’inhibition du blueshift convectif dans ces plages (en rouge), d’après Meunier et al. (2010). Le signal d’une planète Terre est représenté en bleu. L’image du Soleil en haut à gauche illustre les taches et les plages (BASS2000), et l’image en bas à gauche représente le champ magnétique mesuré par MDI/SOHO, dont ont été extraites les plages pour cette reconstruction. Ces mêmes processus ont par la suite été modélisés à partir de structures synthétiques, dans le cas solaire (Borgniet et al. 2015), puis pour des étoiles de type spectral et niveau d’activité différents (Meunier et al. 2019) // Reconstruction of the radial velocity signal of the Sun due to the contrast of observed spots and plages (in black), and including the inhibition of the convective blueshift in plages (in red), adapted from Meunier et al. (2010). The signal of an Earth planet is represented in blue. The image of the Sun (top left panel) illustrates the solar spots and plages (BASS2000), and the lower left panel represents the solar magnetic field measured by MDI/SOHO, from which plages have been extracted for this reconstruction. These processes have then been modelled from time series of synthetic structures in the solar case (Borgniet et al. 2015), and then for stars of different spectral type and activity levels (Meunier et al. 2019).