Exoplanetology: Exoplanets & Exomoons

From Super-Earths to Sub-Neptunes: Observational Constraints and Connections to Theoretical Models

By Keith Cowing

Press Release

June 7, 2024

Filed under Earth-like Planet, exoplanet, FG-dwarfs, Habitable Zone, K-dwarfs, M-dwarfs, PlanetS catalog, radius valley, sub-Neptune, super-Earth, water worlds

From Super-Earths to Sub-Neptunes: Observational Constraints and Connections to Theoretical Models — Mass-Radius Diagram comparing the data from the PlanetS catalog and the data from NASA Exoplanet Archive to highlight the effects of our selection criteria and our detailed study of references. The grey dots represented the default parameters from NASA, the light blue triangles the same but filtered with our criteria on uncertainties and the blue dots correspond to the PlanetS catalog. The resulting M-R relationships from Sect.2.2 are in solid red lines with the confidence intervals represented by the shaded red areas. The transitions between the three populations are represented by the composition line of 20% water at 650 K from Luo et al. (2024) in dashed orange and the dotted black line at 138 M_⊕. — astro-ph.EP

We update the PlanetS catalog of transiting planets with precise and robust mass and radius measurements and use this comprehensive catalog to explore Mass-Radius (M-R) diagrams.

On one hand, we propose new M-R relationships to separate exoplanets into three populations. On the other hand, we explore the transition in radius and density between Super-Earths and Sub-Neptunes around M-dwarfs and compare it with those orbiting K- and FG-dwarfs.

Using Kernel Density Estimation method with a re-sampling technique, we estimate the normalized density and radius distributions, revealing connections between observations and theories on composition, internal structure, formation, and evolution of these exoplanets orbiting different spectral types.

Firstly, the substantial 30% increase in the number of well-characterized exoplanets orbiting M-dwarfs compared with previous studies shows us that there is no clear gap in either composition or radius between Super-Earths and Sub-Neptunes.

The “water-worlds” around M-dwarfs cannot correspond to a distinct population, their bulk density and equilibrium temperature can be interpreted by several different internal structures and compositions. The continuity in the fraction of volatiles in these planets suggests a formation scenario involving planetesimal or hybrid pebble-planetesimal accretion.

Moreover, we find that the transition between Super-Earths and Sub-Neptunes appears to happen at different mass (and radii) depending on the spectral type of the star. The maximum mass of Super-Earths seems to be close to 10 M_⊕ for all spectral types, but the minimum mass of Sub-Neptunes increases with the star’s mass.

This effect also contributes to the fading of the radius valley for M-planets compared to FGK-planets. While Sub-Neptunes are less common around M-dwarfs, smaller ones exhibit lower density than their equivalents around FGK-dwarfs.

M-R diagram of small planets around M-dwarfs, K-dwarfs and FG-dwarfs from the PlanetS catalog. The planets are colorcoded by their equilibrium temperature calculate within the catalog with a bond albedo of 0 and a total heat redistribution. The composition lines of pure-silicates (brown) from Zeng et al. (2016), Earth-like planets (yellow), and 50% water (blue) from Zeng et al. (2019) are displayed. The vertical dotted lines correspond to the minimum mass of the Sub-Neptunes across spectral types (at 1.9, 3.4 and 4.3 M_⊕ for M-, K- and FG-dwarfs) discussed in Sect. 4.1 and 4.4. — astro-ph.EP

Léna Parc, François Bouchy, Julia Venturini, Caroline Dorn, Ravit Helled

Comments: 19 pages, 11 figures. Submitted to A&A on 2024 March 9, re-submitted after referee’s minor comments being incorporated on 2024 May 28
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2406.04311 [astro-ph.EP] (or arXiv:2406.04311v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2406.04311
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Submission history
From: Léna Parc
[v1] Thu, 6 Jun 2024 17:55:58 UTC (44,007 KB)
https://arxiv.org/abs/2406.04311
Astrobiology

Keith Cowing

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) 🖖🏻

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