Periodic Reporting for period 4 - PLANETESYS (The next-generation planet formation model)
Reporting period: 2022-02-01 to 2023-09-30
The overarching goal of the PLANETESYS project was to understand the formation of planets from a theoretical perspective. Today, we know not only of the eight planets in the Solar System, but also of more than 5,000 exoplanets (extrasolar planets) orbiting around stars other than the Sun. Understanding how planets form is ultimately related to the question of how the Earth formed 4.5 billion years ago and how life arose on our planet. Therefore research on exoplanets and the formation of the planets in the Solar System enjoy a broad interest from the public and the media. PLANETESYS specifically focuses on the newly developed pebble accretion theory for planet formation. Young stars are surrounded by an orbiting protoplanetary disc of gas and dust. These dust grains collide and stick together to form pebbles of millimeter-centimeter sizes. Such pebbles are accreted very efficiently by growing protoplanets. Therefore pebble accretion can explain how planets can form within a few million years while there is still dust and gas orbiting the young star. More specifically, PLANETESYS considers the growth of pebbles, the heating of these pebbles to form little spheres called chondrules that are found in meteorites, the growth of planetary systems by pebble accretion and the chemical composition of the forming planets. Combined, this approach gave major new insight to how planetary systems form.
In the first half of the PLANETESYS project, my group studied how the radial pebble flux through a protoplanetary disc determines whether a planetary system forms super-Earths or terrestrial planets (Lambrechts et al., 2019). Regarding larger planets, I developed during a short sabbatical at Tokyo University of Technology a mathematical model that demonstrates how growth by pebble accretion outcompetes the tendency for planets to migrate towards the star (Johansen et al., 2019). These results were also demonstrated in actual N-body simulations of how a multitude of protoplanets grow to form systems of a few giants planets (Bitsch et al., 2019). We have shown that the classical planet formation theory (planetesimal accretion) fails at forming gas giants (Johansen & Bitsch, 2019). We have also shown that super-Earths that orbit around other stars have a characteristic mass that scales linearly with the mass of the host star and that this scaling arises naturally from pebble accretion simulations (Liu et al., 2019). This was the first evidence that the super-Earths that are so common around other stars form by pebble accretion. The gas giants Jupiter and Saturn in our Solar System are orbited by systems of smaller moons. In Ronnet & Johansen (2020) we showed that such moon systems could also form by pebble accretion and that the solid material can be supplied to discs orbiting around young planets by thermal ablation of planetesimals from the main protoplanetary disc. Meteorites are fragments of asteroids and they contain abundant chondrules of millimeter sizes. Those chondrules could be the pebbles from the solar protoplanetary disc, but the chondrules appear to have been melted and recrystallised as solid spherules. I developed a new model where chondrules are melted by the heat from lightning in protoplanetary discs (Johansen & Okuzumi, 2018). This was a major milestone for PLANETESYS and the results have been presented at several interdisciplinary meetings on meteorites and planet formation. In the latter half of the PLANETESYS project, we focused on applying the pebble accretion model to the formation of Earth and other rocky planets. in Johansen et al. (2021) I demonstrated that the orbits and masses of Venus, Earth and Mars are consistent with rapid formation by pebble accretion and that the inventories of volatiles essential for life (water, carbon and nitrogen) could have been delivered to the young planets as rims of ice and organics on the accreted pebbles. I further explored in three papers in 2023 (Johansen et al., 2023abc) how these volatiles distribute between the core, mantle and atmosphere of rocky planets and how the rapid formation of Earth is consistent with constraints on the core formation time-scale from radioactive decay of 182Hf to 182W. In Onyett et al. (2023) in Nature, we reported together with meteorite researchers at the University of Copenhagen that Earth's composition of silicon isotopes is also in agreement with the pebble accretion model. Silicon is a major element in the Earth's mantle and its isotopes appear as a combination of neutron-poor material from the inner Solar System and neutron-rich material from the outer Solar System. This is in agreement with the radial drift of pebbles from the outer regions of the protoplanetary disc and hence the silicon isotope data support a significant contribution of pebble accretion during the formation of our planet.
The papers published in the PLANETESYS project have focused on further developing the pebble accretion theory for planet formation. This is a novel model that was first proposed a decade ago by Ormel & Klahr (2010) and Lambrechts & Johansen (2012). We have in the past 5 years demonstrated how pebble accretion can form terrestrial planets, super-Earths, ice giants, gas giants and moon systems around gas giants. This way we are pushing the pebble accretion theory to be the new de facto standard model for planet formation.