Evidence discovered for two distinct giant planet populations
2017 July 05

In a paper¹ highlighted today by Astronomy & Astrophysics journal, a team² of researchers from the Instituto de Astrofísica e Ciências do Espaço (IA³) discovered observational evidence for the existence of two distinct populations of giant planets.

So far, more than 3500 planets have been detected orbiting solar type stars. Although recent results suggest that most planets in our Galaxy are rocky like Earth, a large population of giant planets, with masses that can go up to 10 or 20 times the mass of Jupiter (itself 320 times the mass of the Earth), was also discovered.

A large amount of the information about how these planets are formed is coming from the analysis of the connection between the planets and their host star. Initial findings have shown, for example, that there is a tight connection between the metallicity⁴ of the star and the planet occurrence or frequency. Stellar mass has also been suggested to influence planet formation efficiency.

State-of the art models of planet formation suggest that two main avenues exist for the formation of gas giants. The so called core-accretion process says that first you form a rocky/icy core, and then this core draws gas around it, giving origin to a giant planet. The other suggests that instabilities in the protoplanetary disk⁵ can lead to the formation of gas clumps, which then contract to form a giant planet.

Vardan Adibekyan (IA & Universidade do Porto) comments: “Our team, using public exoplanet data, obtained an interesting observational evidence that giant planets such us Jupiter and its larger mass cousins, several thousand times more massive than the Earth (of which we do not have an example in the Solar System) form in different environments, and make two distinct populations.”

While objects smaller than about 4 Jupiter masses show a strong preference for metal-rich stars, in the 4 to 20 Jupiter mass range, host stars tend to be more metal-poor and more massive, suggesting that these massive giant planets form with a different mechanism than their lower mass brothers. Nuno Cardoso Santos  (IA & Faculdade de Ciências da Universidade do Porto) adds: “The result now published suggests that both mechanisms may be at play, the first forming the lower mass planets, and the other one responsible for the formation of the higher mass ones.”

On one side, the lower mass giant planets (mass below 4 Jupiter masses) seem to be formed by the core-accretion process, around more metal-rich stars, while more massive planets seem to be formed mainly through gravitational instability. But Adibekyan adds that: “Although this discovery was a large and important step towards a complete understanding of planet formation, it is not the last and final one. Our team will enthusiastically continue addressing many still open questions.”

Observations with GAIA (ESA), whose sensitivity will allow the detection of thousands of giant exoplanets in long period orbits around stars of different masses, may shed some light into this. In the near future, missions like ESA’s CHEOPS  and PLATO, or NASA’s TESS, which will allow for the study of mass-radius relation, along with studies of their atmospheric composition using instruments such as ESO’s ESPRESSO at the VLT and HIRES at the ELT, or the James Webb Space Telescope (JWST), may also bring new constraints about the processes of planet formation.