Water/Hycean Worlds & Oceanography

Preparing for the Early eVolution Explorer: Detecting the Primordial, Transiting Exoplanet Population

By Keith Cowing
Status Report
astro-ph.EP
June 8, 2026
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Preparing for the Early eVolution Explorer: Detecting the Primordial, Transiting Exoplanet Population
The small planet population can be explained by competing hypotheses. Planets may form with substantial, low mean-molecular weight atmospheres (gas-dwarfs). Depending on when the gas envelope was accreted, these planets may undergo rapid radial evolution during the first hundred million years due to the combined atmospheric escape effects of boil-off, contraction, and photo-evaporation. Planets may also form with volatile rich, higher mean molecular weight envelopes (water-worlds). Such planets undergo far less radial evolution within this time frame. The densities and radii of two populations are degenerate by the mature ages sampled by the Kepler and TESS missions. Targeting planets around known star forming regions and young associations can statistically differentiate between these two scenarios. — astro-ph.EP

The close-in small planet population may be formed either with hydrogen/helium dominated envelopes or with water-rich interiors. Both scenarios reproduce the present day planet population in mass, radius, and periods, and are difficult to differentiate with the mature planet demographic.

Hydrogen/Helium ‘gas-dwarfs’ have low mean molecular weight atmospheres, while ‘water-worlds’ have envelopes that are significantly heavier, and as such these two scenarios have different evolution tracks that diverge in the first ~50 Myr of their evolution.

We show that a low Earth orbit multi-band photometric survey mission, within the scope of the NASA Small Explorers Program (SMEX), can determine the frequency of young close-in planets at the 5% level and definitively differentiate between the competing ‘gas-dwarf’ and ‘water-world’ hypotheses.

We simulate a 2.5 year mission capable of simultaneous multi-band near-ultraviolet (NUV), optical, and near infrared (NIR) wide field photometry. Such a mission would perform a photometric survey of 30 different stare-fields selected to probe the young star population. The mission will yield ~100 transiting planets in young star clusters and associations with ages <50 Myr. In comparison, only 20 such planets are known from K2 and TESS today.

George Zhou, James G. Rogers, Jennifer A. Burt, Eve J. Lee, Sydney Vach, Ann Marie Cody, Mark Swain, Neal J. Turner, Andrew W. Mann, Madyson G. Barber, Eric Gaidos, Ward Howard, Laura Venuti, Damon F. Landau, Valerie Scott, Alan Didion, David Makowski, Jamie Nastal, Evgenya L. Shkolnik, Meredith A. MacGregor

Comments: Accepted for publication in AJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2606.04283 [astro-ph.EP] (or arXiv:2606.04283v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2606.04283
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Submission history
From: George Zhou
[v1] Tue, 2 Jun 2026 23:18:04 UTC (3,260 KB)
https://arxiv.org/abs/2606.04283

Astrobiology,

Keith Cowing

Biologist, Explorers Club Fellow, ex-NASA Space Biologist and Payload integrator, Editor of NASAWatch.com and Astrobiology.com, Lapsed climber, Explorer, Synaesthete, Former Challenger Center board member 🖖🏻

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