Linking the Habitable Zone to Solar System and Exoplanetary Habitability

Although observations of exoplanetary observations remain rather limited, our inner solar system continues to be a useful springboard for understanding exoplanetary habitability. Indeed, the habitable zone (HZ), which is a navigational tool for finding potentially habitable exoplanets, was partially designed from our knowledge of Earth, Mars, and Venus. Here, I discuss how these solar system links have updated the HZ and continued to improve our understanding of planetary habitability both inside and outside our solar system. The notion that early Mars may have been a habitable planet with a dense CO₂-H₂ atmosphere has led to the development of a wider volcanic hydrogen HZ with more potentially habitable planets (e.g. TRAPPIST-1h)¹. Also, spectral class greatly influences habitability. For instance, high amounts of methane in dense CO₂-CH₄ atmospheres near the outer edge of hotter ( A - mid-K) stars may suggest inhabitance², which is consistent with recent studies of the early Earth³. I then assess whether abiotic processes (e.g. serpentinization, impacts, volcanism) can also produce similar CO₂-CH₄ atmospheres and whether we can distinguish such false positives. In contrast, M-star HZ outer edge planets with such CO₂-CH₄ atmospheres are likely to be frozen worlds instead. Plus, the possibility that both Venus and Mars may have been habitable illustrates why the pre-main-sequence HZ is crucial for estimating planetary water inventories and inferring whether planets located in the HZ today are truly habitable or not⁴. Finally, I briefly discuss how rotation rate impacts the habitability of ocean worlds with water inventories much larger than Earth's⁵.

REFERENCES:

1. Ramirez, R.M., Kaltenegger, L., 2017. A volcanic hydrogen habitable zone. The Astrophysical Journal Letters, 837, 1

2. Ramirez, R.M. and Kaltenegger, L. 2018. A methane extension to the classical habitable zone. ApJ 858, 2

3. Krissansen-Totton, Joshua, Stephanie Olson, and David C. Catling. "Disequilibrium biosignatures over Earth history and implications for detecting exoplanet life." Science advances 4.1 (2018): eaao5747.

4. Ramirez, R.M., Kaltenegger, L., 2014. Habitable Zones of Pre-Main-Sequence Stars. ApJL, 797, 2, L25

5. Ramirez, R.M. and Levi, A. 2018. The ice cap zone: a unique habitable zone for ocean worlds. MNRAS, 477, 4, 4627- 4640