Issue |
A&A
Volume 685, May 2024
|
|
---|---|---|
Article Number | A22 | |
Number of page(s) | 21 | |
Section | Planets and planetary systems | |
DOI | https://doi.org/10.1051/0004-6361/202349039 | |
Published online | 30 April 2024 |
Accretion of primordial H–He atmospheres in mini-Neptunes: The importance of envelope enrichment
1
Institut für Astrophysik, Universität Zürich,
Winterthurerstr. 190,
8057
Zürich,
Switzerland
e-mail: marit@ics.uzh.ch
2
Weltraumforschung und Planetologie, Physikalisches Institut, Universität Bern,
Gesellschaftsstrasse 6,
3012
Bern,
Switzerland
Received:
20
December
2023
Accepted:
15
February
2024
Context. Out of the more than 5000 detected exoplanets, a considerable number belong to a category called “mini-Neptunes”. Interior models of these planets suggest that they have primordial H–He-dominated atmospheres. As this type of planet is not found in the Solar System, understanding their formation is a key challenge in planet formation theory. Unfortunately, quantifying how much H–He planets have, based on their observed mass and radius, is impossible due to the degeneracy of interior models.
Aims. Another approach to estimating the range of possible primordial envelope masses is to use formation theory. As different assumptions in planet formation can heavily influence the nebular gas accretion rate of small planets, it is unclear how large the envelope of a protoplanet should be. We explore the effects that different assumptions regarding planet formation have on the nebular gas accretion rate, particularly by exploring the way in which solid material interacts with the envelope. This allows us to estimate the range of possible post-formation primordial envelopes. Thereby, we demonstrate the impact of envelope enrichment on the initial primordial envelope, which can be used in evolution models.
Methods. We applied formation models that include different solid accretion rate prescriptions. Our assumption is that mini-Neptunes form beyond the ice line and migrate inward after formation; thus, we formed planets in situ at 3 and 5 au. We considered that the envelope can be enriched by the accreted solids in the form of water. We studied how different assumptions and parameters influence the ratio between the planet’s total mass and the fraction of primordial gas.
Results. The primordial envelope fractions for low- and intermediate-mass planets (total mass below 15 M⊕) can range from 0.1% to 50%. Envelope enrichment can lead to higher primordial mass fractions. We find that the solid accretion rate timescale has the largest influence on the primordial envelope size.
Conclusions. Rates of primordial gas accretion onto small planets can span many orders of magnitude. Planet formation models need to use a self-consistent gas accretion prescription.
Key words: planets and satellites: atmospheres / planets and satellites: composition / planets and satellites: formation
© The Authors 2024
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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