Irradiated ocean planets bridge super-Earth and sub-Neptune populations. (arXiv:2002.05243v1 [astro-ph.EP])

Irradiated ocean planets bridge super-Earth and sub-Neptune populations. (arXiv:2002.05243v1 [astro-ph.EP])
<a href="http://arxiv.org/find/astro-ph/1/au:+Mousis_O/0/1/0/all/0/1">Olivier Mousis</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Deleuil_M/0/1/0/all/0/1">Magali Deleuil</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Aguichine_A/0/1/0/all/0/1">Artyom Aguichine</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Marcq_E/0/1/0/all/0/1">Emmanuel Marcq</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Naar_J/0/1/0/all/0/1">Joseph Naar</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Aguirre_L/0/1/0/all/0/1">Lorena Acu&#xf1;a Aguirre</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Brugger_B/0/1/0/all/0/1">Bastien Brugger</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Goncalves_T/0/1/0/all/0/1">Thomas Goncalves</a>

With radii ranging between those of the Earth (1 Rearth) and Neptune (~3.9
Rearth), small planets constitute more than half of the inventory of the
4000-plus exoplanets discovered so far. This population follows a bimodal
distribution peaking at ~1.3 Rearth (super-Earths) and 2.4 Rearth
(sub-Neptunes), with few planets in between. Smaller planets are sufficiently
dense to be rocky, but those with radii larger than ~1.6 Rearth are thought to
display large amounts of volatiles, including in many cases hydrogen/helium
gaseous envelopes up to ~30% of the planetary mass. With orbital periods less
than 100 days, these low-mass planets are highly irradiated and their origin,
evolution, and possible links are still debated. Here we show that close-in
ocean planets affected by greenhouse effect display hydrospheres in
supercritical state, which generate inflated atmospheres without invoking the
presence of large H/He gaseous envelopes. We derive a new set of mass-radius
relationships for ocean planets with different compositions and different
equilibrium temperatures, well adapted to low-density sub-Neptune planets. Our
model suggests that super-Earths and sub-Neptunes could belong to the same
family of planets. The differences between their interiors could simply result
from the variation of the water content in those planets. Close-in sub-Neptunes
would have grown from water-rich building blocks compared to super-Earths, and
not concurrently from gas coming from the protoplanetary disk. This implies
that small planets should present similar formation conditions, which resemble
those known for the terrestrial and dwarf planets in the solar system.

With radii ranging between those of the Earth (1 Rearth) and Neptune (~3.9
Rearth), small planets constitute more than half of the inventory of the
4000-plus exoplanets discovered so far. This population follows a bimodal
distribution peaking at ~1.3 Rearth (super-Earths) and 2.4 Rearth
(sub-Neptunes), with few planets in between. Smaller planets are sufficiently
dense to be rocky, but those with radii larger than ~1.6 Rearth are thought to
display large amounts of volatiles, including in many cases hydrogen/helium
gaseous envelopes up to ~30% of the planetary mass. With orbital periods less
than 100 days, these low-mass planets are highly irradiated and their origin,
evolution, and possible links are still debated. Here we show that close-in
ocean planets affected by greenhouse effect display hydrospheres in
supercritical state, which generate inflated atmospheres without invoking the
presence of large H/He gaseous envelopes. We derive a new set of mass-radius
relationships for ocean planets with different compositions and different
equilibrium temperatures, well adapted to low-density sub-Neptune planets. Our
model suggests that super-Earths and sub-Neptunes could belong to the same
family of planets. The differences between their interiors could simply result
from the variation of the water content in those planets. Close-in sub-Neptunes
would have grown from water-rich building blocks compared to super-Earths, and
not concurrently from gas coming from the protoplanetary disk. This implies
that small planets should present similar formation conditions, which resemble
those known for the terrestrial and dwarf planets in the solar system.

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