Habitable Zones & Global Climate

The Demographics Of Kepler’s Earths And super-Earths Into The Habitable Zone

By Keith Cowing
Press Release
September 12, 2022
Filed under , , , , , ,
The Demographics Of Kepler’s Earths And super-Earths Into The Habitable Zone
Comparison of habitable zone occurrence rates for Earth-sized planets. Our result for the combined [0.56, 1.63] M sample is plotted at full opacity, and lower-opacity points denote results from individual fits to five stellar mass bins with boundaries spanning {0.56, 0.81, 0.91, 1.01, 1.16, 1.63} M (increasing in mass from top to bottom). Circles indicate values assuming the occurrence follows a functional form (our work, Bryson et al. 2021, 2020b; Neil & Rogers 2020; Pascucci et al. 2019; Zink et al. 2019; Garrett et al. 2018, and Mulders et al. 2018), and squares indicate values from grid-based occurrence rates (Kunimoto & Matthews 2020; Hsu et al. 2019). Left-pointing arrows indicate upper limits. The blue region represents works directly considering an underlying population of intrinsically rocky planets, the yellow region represents works considering reliability, and our work (shown in green) implements both considerations. The vertical grey line indicates the occurrence values from ExoPAG SAG13 via Kopparapu et al. (2018) used in the final reports of the LUVOIR and HabEx missions. A similar plot for Γ⊕ estimates prior to Bryson et al. (2021) appears in Kunimoto & Matthews (2020) (their Figure 14).

Understanding the occurrence of Earth-sized planets in the habitable zone of Sun-like stars is essential to the search for Earth analogues. Yet a lack of reliable Kepler detections for such planets has forced many estimates to be derived from the close-in (2<Porb<100 days) population, whose radii may have evolved differently under the effect of atmospheric mass loss mechanisms.

In this work, we compute the intrinsic occurrence rates of close-in super-Earths (∼1−2R⊕) and sub-Neptunes (∼2−3.5R⊕) for FGK stars (0.56−1.63M⊙) as a function of orbital period and find evidence of two regimes: where super-Earths are more abundant at short orbital periods, and where sub-Neptunes are more abundant at longer orbital periods. We fit a parametric model in five equally populated stellar mass bins and find that the orbital period of transition between these two regimes scales with stellar mass, like Ptrans∝M1.7±0.2∗.

These results suggest a population of former sub-Neptunes contaminating the population of Gyr-old close-in super-Earths, indicative of a population shaped by atmospheric loss. Using our model to constrain the long-period population of intrinsically rocky planets, we estimate an occurrence rate of Γ⊕=15+6−4% for Earth-sized habitable zone planets, and predict that sub-Neptunes may be ∼twice as common as super-Earths in the habitable zone (when normalized over the natural log orbital period and radius range used). Finally, we discuss our results in the context of future missions searching for habitable zone planets.

Galen J. Bergsten, Ilaria Pascucci, Gijs D. Mulders, Rachel B. Fernandes, Tommi T. Koskinen

Comments: 27 pages, 12 figures, 3 tables; Accepted for publication in AJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2209.04047 [astro-ph.EP] (or arXiv:2209.04047v1 [astro-ph.EP] for this version)
Submission history
From: Galen Bergsten
[v1] Thu, 8 Sep 2022 22:07:54 UTC (6,043 KB)

SpaceRef co-founder, Explorers Club Fellow, ex-NASA, Away Teams, Journalist, Space & Astrobiology, Lapsed climber.