Exoplanets, -moons, -comets

Determining the Host Stars of Planets in Binary Star Systems with Asterodensity Profiling: Investigating the Canonical Radius Gap

By Keith Cowing

Status Report

astro-ph.EP

April 9, 2026

Filed under astro-ph.EP, circumprimary planet, circumsecondary planet, exoplanet, gas giant, radius gap, rocky exoplanet, Stellar Cartography

Determining the Host Stars of Planets in Binary Star Systems with Asterodensity Profiling: Investigating the Canonical Radius Gap — The transit fitting results for KOI-1300.01. The upper right panel shows the phase-folded and contamination-corrected transit lightcurve for the planet’s circumprimary case. The gray circles are the normalized and contamination corrected flux values. The blue squares are the normalized flux values obtained from binning 800 data points at a time. The red line is the model fit to the data. The lower right panel shows the same information for the circumsecondary case. The upper left panels shows the corner plots from the MCMC sampling in the circumprimary case. The lower left shows the corner plots for the circumsecondary case. — astro-ph.EP

Over the past 30 years, thousands of exoplanets have been discovered, revealing detailed demographics of planets outside the Solar System.

One of the most dramatic features of the planet radius distribution is the radius gap, a lack of planets between ∼1.8-2 R_⊕. The radius gap is thought to mark the distinction between rocky and gas planets. Recent research has found that the radius gap may not be present in binary star systems.

In past studies of planets in binary star systems, the common assumption has been that all of the planets are hosted by the primary star. In many cases, the radius of the planet would be significantly larger if it were orbiting the companion star, which could potentially affect the true radius distribution. It is possible to identify the host stars of planets through stellar density estimates obtained from transit fitting.

Using this method, we made probabilistic estimates for the host stars of a sample of 15 transiting exoplanets across 10 binary star systems hosting either 1 or 2 planets, at least one of which would reside in the canonical radius gap if it was circumprimary. We found that 5 of the planets are highly likely to be circumprimary, while the remainder have ambiguous host stars.

The lack of unambiguously circumsecondary planets is caused by physical and observational biases that favor circumprimary planets. Nonetheless, the summed posterior probabilities suggest that the canonical radius gap appears less vacant for planets in binaries.

Nathanael Burns-Watson, Kendall Sullivan, Adam Kraus

Comments: 15 pages, 9 figures
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2604.06388 [astro-ph.EP] (or arXiv:2604.06388v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2604.06388
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Submission history
From: Nathanael Burns-Watson
[v1] Tue, 7 Apr 2026 19:19:28 UTC (1,265 KB)
https://arxiv.org/abs/2604.06388
Astrobiology, exoplanet,

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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