Exoplanetology: Exoplanets & Exomoons

Diversity Of Exoplanets

By Keith Cowing

Status Report

astro-ph.SR

May 16, 2025

Filed under astro-ph.SR, astrogeology, astronomy, atmosphere, exoplanet, gas giant, habotability, ice giant, icy world, rocky planet, water world

Diversity Of Exoplanets — Exoplanets with masses error and radii error less than 25%. Symbol size is proportional to mass. Colors correspond to equilibrium temperatures calculated with bond albedos of 0.3 and redistribution factors of 4. Colors are loosely binned according to physical processes: planets with icy surfaces should they have any water (0-273 K), planets that allow for liquid water (273-373 K), planets that are in runaway greenhouse below the critical point of water (373-670K), hot planets that cannot have water clouds (670-1200K) and magnetic effects on their atmospheres are negligible, hot planets where magnetic effects like ohmic dissipation are important, very hot planets where magnetic effects are less important (due to full ionization and freezing to magnetic field). Solar system planets are labeled and shown as black symbols for reference. — astro-ph.EP

This review article delves into the study of low-mass exoplanets: super-Earths, mini-Neptunes and the new categories within and between that we are starting to discover.

We provide an overview of current exoplanet observational capabilities, their limitations, and what they are allowing us to learn about low-mass planets. We briefly summarize the most important aspects of planet formation, with an emphasis on processes that may be testable with small exoplanets, in particular those that affect their composition.

We also describe the observed compositional diversity of low-mass exoplanets and what it teaches us about planet formation pathways.

We finish this review summarizing the study of the composition of small exoplanets during the very last stage of stellar evolution, by studying white dwarfs. This review is written as the JWST is making its first contributions to small planet characterization, rapidly opening new lines of inquiry.

Top Figure: Primary transit of K2-18b obtained with the NIRspec (Near-Infrared Spectrograph) instrument on JWST (light curve from Madhusudhan et al. (2023)). Bottom Figure: Transmission spectrum of K2-18 b obtained JWST instruments NIRISS (Near-Infrared Imager and Slitless Spectrograph) and NIRSpec. It shows methane, carbon dioxide, and a possible detection of dimethyl sulfide (DMS) consistent with K2-18b being a Hycean world (possessing a liquid water ocean underneath a hydrogen-rich atmosphere). The mass, radius and equilibrium temperature of H2-18b are estimated to be ∼ 8.6 ME, 2.61 RE, and ∼250- 300 K, respectively, for an albedo between 0-0.3 (Benneke et al., 2019). Illustration credits: NASA, ESA, CSA, Ralf Crawford (STScI), Joseph Olmsted (STScI). Science credits: Nikku Madhusudhan (IoA). — astro-ph.EP

Top Figure: 55 Cnc-e secondary eclipse taken with MIRI camera on JWST. Its mass and radius are close to 8ME and 1.9RE. The eclipse depth yields a brightness temperature of 1800, which is lower than the temperature corresponding to the case of zero heat redistribution, indicating the presence of an atmosphere. Bottom Figure: Thermal Emission Spectrum of 55 Cnc-e taken with NIRCAM on JWST consistent with an atmosphere composed of CO-CO2 (Hu et al., 2024b). Top figure: Illustration credits: NASA, ESA, CSA, Joseph Olmsted (STScI). Science credits: Aaron Bello-Arufe (NASA-JPL). Bottom figure: Illustration credits: NASA, ESA, CSA, Joseph Olmsted (STScI). Science credits: Renyu Hu (NASA-JPL), Aaron Bello-Arufe (NASA-JPL), Michael Zhang (University of Chicago), Mantas Zilinskas (SRON). — astro-ph.EP

Top figure: Schematic representation of the difference in gaseous water content in the case of a compact protoplanetary disk without gaps (left) and a more extended protoplanetary disk with gaps (right). When disk gaps are present, the drifting in of icy pebbles from regions beyond the iceline is impeded because pebbles get trapped in the rings; this results in a decreased water vapor enrichment in the inner disk region (1–10 AU) where rocky planets might be forming, compared to the case without gaps where a higher flux of icy pebbles cross the iceline and get their volatiles vaporized (Banzatti et al., 2023). Bottom figure: Spectra from the JWST Mid-Infrared Instrument showing water vapor emission in a a compact protoplanetary disk without gaps (purple line) and in an extended disk with gaps (white line), showing excess cool water in the former, as expected if the water enrichment is the result of pebble drift. Top figure illustration credits: NASA, ESA, CSA, Joseph Olmsted (STScI). Bottom figure illustration credits: NASA, ESA, CSA, Leah Hustak (STScI). Science credits: Andrea Banzatti (Texas State University). — astro-ph.EP

Diana Valencia, Amaya Moro-Martin, Johanna Teske

Comments: 50 pages, 9 figures
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2505.09754 [astro-ph.EP] (or arXiv:2505.09754v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2505.09754
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Journal reference: In: Weis, D and Anbar, A. (eds.) Treatise on Geochemistry, 3e. Vol. 7, pp. 19-49.UK: Elsevier (2025)
Related DOI:
https://doi.org/10.1016/B978-0-323-99762-1.00139-X
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Submission history
From: Amaya Moro-Martin
[v1] Wed, 14 May 2025 19:38:07 UTC (40,854 KB)
https://arxiv.org/abs/2505.09754
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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