Exoplanets & Exomoons

A Formation Pathway For Terrestrial Planets With Moderate Water Content Involving Atmospheric-volatile Recycling

By Keith Cowing
Status Report
June 14, 2024
Filed under , , , , , , ,
A Formation Pathway For Terrestrial Planets With Moderate Water Content Involving Atmospheric-volatile Recycling
Cartoon of the planetary envelope in different scenarios, each featuring one volatile species (in blue) and one refractory species (in brown). The infalling pebbles are shown on the left side of the envelope. Initially, they consist of a refractory core (brown dot) and an icy volatile layer (blue ring). On the right side, we show the recycling flows (in magenta) and the dust grains (blue dots) that they carry away. The envelope temperature increases overall from A to D. A: Cold envelope in which the volatile sublimation temperature is not reached. B: The volatile species evaporates in the envelope (indicated by the blue arrows), but the recycling flows do not reach deep enough to prevent the accretion of the volatile. Latent heat is absorbed during the evaporation of the volatile ice (indicated by the red arrows). C: The volatile species evaporates in the envelope and the recycling flows transport the vapor back beyond the volatile evaporation front, where the volatile recondenses onto the dust grains. The dust grains covered in ice are then recycled back into the disk. There is no net latent heat absorption due to the recondensation subsequent to the evaporation. D: Same as C, but the envelope is hot enough to vaporize the refractory species, causing the absorption of latent heat in the process. The refractory vapor is not recycled. — astro-ph.EP

Of the many recently discovered terrestrial exoplanets, some are expected to harbor moderate water mass fractions of a few percent. The formation pathways that can produce planets with these water mass fractions are not fully understood.

Here, we use the code chemcomp, which consists of a semi-analytical 1D protoplanetary disk model harboring a migrating and accreting planet, to model the growth and composition of planets with moderate water mass fractions by pebble accretion in a protoplanetary disk around a TRAPPIST-1 analog star.

This star is accompanied by seven terrestrial planets, of which the outer four planets likely contain water mass fractions of between 1% and 10%. We adopt a published model that considers the evaporation of pebbles in the planetary envelope, from where recycling flows can transport the volatile vapor back into the disk. We find that with this model, the planetary water content depends on the influx rate of pebbles onto the planet.

A decreasing pebble influx with time reduces the envelope temperature and consequently allows the formation of planets with moderate water mass fractions as inferred for the outer TRAPPIST-1 planets for a number of different simulation configurations. This is further evidence that the recycling of vapor is an important component of planet formation needed to explain the vast and diverse population of exoplanets.

Final planetary water mass fraction versus initial local disk temperature at the position of the planet as a proxy for the distance from the star. Planets that are not shown in the plot have lower water mass fractions than 10−5 . The columns correspond to different values of the α-viscosity, the rows show different solid fragmentation velocity values. Each marker corresponds to one planet starting at a specific disk temperature. The color of the markers indicates the final mass of the planet (see color-bars on the right). The shape of the markers specifies the initial mass of the disk in which the planet grows (see legend). The hatched regions in the background correspond to the 1σ-regions of the water mass fractions of the TRAPPIST-1 planets estimated by Raymond et al. (2022) using their model ii and are color-coded according to the observed masses of the TRAPPIST-1 planets using the same color-bar. The vertical lines indicate the location of the water-evaporation front. We use our full model, which consists of both the Brouwers opacity model (vlim = 30.0 m s−1 , F = 0.0) and the Birnstiel pebble drift, while applying the planetary envelope model described above. In addition, we account for the planetary migration and the evaporation of pebbles in the disk as they drift inwards. — astro-ph.EP

Jonas Müller, Bertram Bitsch, Aaron David Schneider

Comments: Accepted by A&A, 19 pages, 8 figures
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2406.09186 [astro-ph.EP] (or arXiv:2406.09186v1 [astro-ph.EP] for this version)
Submission history
From: Jonas Müller
[v1] Thu, 13 Jun 2024 14:48:10 UTC (373 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) 🖖🏻