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The next-generation planet formation model

Periodic Reporting for period 2 - PLANETESYS (The next-generation planet formation model)

Reporting period: 2019-01-01 to 2020-07-31

The overarching goal of the PLANETESYS project is 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 4,000 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 and 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 will give major new insight to how planetary systems form.
During the first half of the PLANETESYS project the team and our collaborators have obtained and published major results on planet formation. We have studied how the radial pebble flux through a protoplanetary disc determines whether a system forms super-Earths or terrestrial planets (Lambrechts et al., 2019). Regarding larger planets, we have developed 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 formed 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 is 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, in print) 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.

An important goal of the PLANETESYS project is the development of the PLANETESYS code to simulate the growth from dust grains to planetary systems. The framework for the code is now in place and has been tested on a supercomputer in Lund. The first results on the growth from dust to pebbles were published in 2020 (Eriksson et al., 2020, in print). We have also developed an algorithm for the formation of planetesimals and the dynamics of planetesimals - this paper is submitted to Astronomy & Astrophysics (Lorek & Johansen, 2020, submitted).

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. We have 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.
The papers published so far 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 2.5 years demonstrated how pebble accretion forms 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.

Regarding dust growth and planetesimal formation, we have developed a novel model for converting the dust aggregate pebbles in protoplanetary discs into solid chondrules found in meteorites. Together with Satoshi Okuzumi the PI demonstrated how pebbles will melt because of lightning in protoplanetary discs. We are also progressing the formation of planetesimal formation by the streaming instability, demonstrating that the streaming instability can for planetesimals even when there is background turbulence present in the protoplanetary disc. The streaming instability model for planetesimal formation recently gained strong support from the analysis of the Kuiper belt object Arrokoth by the New Horizons mission. The New Horizons team concluded in three papers published in Science in 2020 that the properties of Arrokoth are consistent with formation by the streaming instability. This was a major science news story and the PI of PLANETESYS was interviewed by several media, including BBC and Washington Post.