Exoplanets & Exomoons

Formation Of Super-Earths In Icy Dead Zones Around Low-mass Stars

By Keith Cowing
Press Release
November 16, 2022
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Formation Of Super-Earths In Icy Dead Zones Around Low-mass Stars
Shaded regions show the icy regions of a disc with a dead zone for different steady state accretion rate around a star with mass 𝑀 = 1 M (left) and 𝑀 = 0.1 M (right). The green solid line shows where the fully turbulent disc model has 𝑇 = 𝑇crit = 800 K (see Section 2.1). The red dashed lines show where the irradiation temperature equals the snow line temperature for the flared disc model (see Section 2.2). The blue dashed lines show the snow line radius in a self-gravitating disc and the magenta lines show where 𝑇 = 𝑇crit (see Section 2.3). Between the magenta and green lines there are no steady solutions. The outer extent of the inner icy region can vary in time between the magenta and green lines. Since the dead zone region is not in a steady state, the accretion rate on to the star is lower than the steady rate shown. — astro-ph.EP

While giant planet occurrence rates increase with stellar mass, occurrence rates of close-in super-Earths decrease.

This is in contradiction to the expectation that the total mass of the planets in a system scale with the protoplanetary disc mass and hence the stellar mass. Since the snow line plays an important role in the planet formation process we examine differences in the temperature structure of protoplanetary gas discs around stars of different mass.

Protoplanetary discs likely contain a dead zone at the midplane that is sufficiently cold and dense for the magneto-rotational instability to be suppressed. As material builds up, the outer parts of the dead zone may be heated by self-gravity. The temperature in the disc can be below the snow line temperature far from the star and in the inner parts of a dead zone.

The inner icy region has a larger radial extent around smaller mass stars. The increased mass of solid icy material may allow for the in situ formation of larger and more numerous planets close to a low-mass star. Super-Earths that form in the inner icy region may have a composition that includes a significant fraction of volatiles.

David Vallet, Anna C. Childs, Rebecca G. Martin, Mario Livio, Stephen Lepp

Comments: Accepted for publication in MNRAS Letters
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2211.07759 [astro-ph.EP] (or arXiv:2211.07759v1 [astro-ph.EP] for this version)
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Submission history
From: Rebecca Martin
[v1] Mon, 14 Nov 2022 21:25:11 UTC (72 KB)

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) πŸ––πŸ»