Properties of exoplanets

• Numerous exoplanets, varied families and troubled histories
  • The vast majority of stars, whether young, old, or even dead such as white dwarfs ^? , would appear to be accompanied by planets. The interstellar medium ^?  also appears to contain free-floating planetary mass objects (should they be called planets?).
  • Exoplanets do not resemble the planets of the Solar System. There are exo-Jupiters, exo-Saturns, but no exo-Neptune or exo-Earth. This does not mean that icy giants or Earth-like planets do not exist, simply that they are beyond the range of our existing detection methods.
  • Based on current observations, it would appear that there are fewer gas giant planets than small solid planets, but that most of the exoplanets discovered are sub-Neptunes: at least two terrestrial radii and several tens of terrestrial masses, these sub-Neptunes are less dense than the solid planets and probably icy.
  • These sub-Neptunes appear to be fewer in number close to stars, probably because of the evaporation of the atmosphere caused by the star’s UV radiation.
  • The habitable zone (HZ) around a star is the region in which the physical conditions on the surface of a planet make it possible for liquid water to exist. We estimate that 20% of the stars possess a solid planet in their HZ. About fifty planets have been detected in the HZ of their star in the Galaxy ^? , for example, the star closest to the Sun, Proxima Centauri, has a planet in its HZ.
  • Exoplanets have a wide variety of orbits:
    • The discoveries of 51 Peg b and its companions the hot Jupiters, giant planets orbiting very close to their star, are a mystery as yet unsolved: either these planets formed further away and migrated towards the star, or they were formed where they are now by means of a mechanism yet to be identified. The hot Jupiters are often isolated planets, so a formation mechanism involving no intervention by other planets must be imagined.
    • There are highly eccentric orbits, in particular for isolated planets. In systems with several planets, with the eccentric orbits having led to catastrophic encounters, the remaining planets follow more circular orbits.
    • There are also exoplanets with heavily inclined, polar or even retrograde orbits. As in principle the planets form in the circumstellar disc rotating in the same direction as the star, these significant inclinations are another sign that the orbits of the planets change after their formation.
    • In systems with several terrestrial planets ^? , their sizes, masses and distances from each other show that these systems interacted until they reached a stable situation in which they are aligned in an orderly arrangement.
    • Several tens of exoplanets have been found in orbit around binary stars and several hundred around stars which are part of multiple star systems (it should be noted that most stars are in multiple systems).
    • Planetary mass objects have also been detected in the interstellar medium ^? . Given the troubled history that numerous planetary systems appear to have undergone, these objects could be planets that have been ejected from their system. However, they could also be micro-stars, formed in the interstellar clouds, but not massive enough to trigger thermonuclear reactions, which would have made them luminous.

We believe that there are several planet formation methods, by accretion ^? in a disc like the planets of the Solar System, by gravitational instability in clouds, like stars, by instability in massive circumstellar discs, etc. This would explain the considerable diversity of exoplanetary systems.
Several hundred protoplanetary discs ^? and several planets under formation within these discs have been observed, such as PDS 70.

• Strange worlds
  We are starting to be able to estimate the properties, temperature, density and composition of certain exoplanets.
  • Planets are so close to the star that they lose a part of their atmosphere, like the dust tail of comets ^? .
  • Ocean planets most probably exist: a planet formed far from its star could have a large ice core. If it migrates towards its star, it loses its atmosphere, the ice melts and the ice core is covered by an ocean of water across the entire surface of the planet.
  • Planets are covered with molten lava. For example, 55 Cancri e is on average 60 times closer to its star than Earth is to the Sun and its temperature reaches 2,500°C in places.
  • Free-floating planets have been discovered far from any star. They may have been ejected after formation around a star, or formed as stars in the interstellar medium ^? .
  • Some exoplanets are 10 times less dense than Saturn (itself less dense than water), while others are as dense as lead. This is perhaps the most intriguing property of the exoplanets.
  • The habitable zone (HZ) around a star is the region in which the physical conditions on the surface of a planet make the existence of liquid water possible. We can estimate that 20% of the stars have a solid planet in their HZ. About fifty planets have been detected in the HZ of their star in our Galaxy ^? . For example, the star closest to the Sun, Proxima Centauri, has a planet in its HZ.
  • The planets of M stars must adapt to stars with variable brightness and for which the HZ thus moves.
  • Hycean planets with immense oceans under a hydrogen atmosphere are promising candidates in the search for life.
  • With its two stars, Tatouine appears familiar to us.

• Improving observation methods
  • There are now more and better detection methods. They give access to planets smaller than Earth and to new parameters, notably concerning the composition of the atmosphere of exoplanets.
  • The observation of transits is particularly revealing: primary transit, when the planet passes in front of the star’s disc, gives a measurement of the size of the planet. For some planets, it is possible to detect the secondary transit, i.e. the planet’s passage behind the star. Sometimes we can see the light from the star reflected by the planet, emission from the planet itself, or the deformation of the star owing to tidal forces. In addition to the size of the planet, observing these phenomena gives information about its temperature, the composition of its atmosphere, or even the presence of other planets, satellites ^? , or planetary rings. All of these measurements are extremely difficult, at the limit of the capacity of our instruments. They are only possible for a small number of nearby systems and rely on models. For example, to measure the radius of a planet with the transit method, we have to know the radius of the star, which is generally identified by comparison with star development models.
  • The sophistication of the radial velocity ^? method gives access to new parameters, for instance the inclination of the planet’s orbital plane with respect to the axis of rotation of the star.
  • Imaging of young giant planets provides important information: this technique has led to the discovery of exoplanets in the outer part of exoplanetary systems and to the study of the light emitted by these exoplanets. These data help us understand how planets and their atmospheres are formed and evolve.

• Conclusions
  Exoplanetology is in its infancy and a number of instruments will soon be able to discover and explore thousands of exoplanets allowing astronomers to map the wide diversity of exoplanetary systems, but also systems increasingly similar to our own Solar System: Thanks to a satellite ^? lifespan of more than 10 years, the 2026 and 2030 versions of the GAIA catalogs will enrich our knowledge of thousands of exoplanets observed by transit and astrometry ^? .NASA’s Nancy Grace Roman and ESA’s PLATO missions will continue to use the transits method, following on from Corot and Kepler, to explore solar type systems. ESA’s ARIEL mission will be the first to study exoplanet atmospheres to access the history of the formation of exoplanets, from the gas giants to the curious sub-Neptunes.