Multiple Habitable Phases on Outer Exosolar Worlds

• Published in: The Astrophysical journal Vol. 962; no. 1; pp. 83 - 96
• Main Authors: Sparrman, Viktor, Bladh, Sara, Way, M. J.
• Format: Journal Article
• Language: English
• Published: Goddard Space Flight Center The American Astronomical Society 01-02-2024; American Astronomical Society; IOP Publishing
• Subjects: Astrobiology; Circumstellar habitable zone; Evolved stars; Exoplanet evolution; Exoplanets; Extraterrestrial life; Habitable planets; Habitable zone; Horizontal branch stars; Incident light; Luminosity; Lunar and Planetary Science and Exploration; Natural satellites (Extrasolar); Outer solar system; Red giant branch; Saturn; Solar analogs; Solar evolution; Stars; Stellar evolution; Water; Water loss; Solar Evolution; Natural Satellites (extrasolar); Red Giant Branch; Evolved Stars; Horizontal Branch Stars; Astrobiology; Solar Analogs; Exoplanet Evolution; Habitable Zone; Stellar Evolution; Exoplanets; Habitable Planets
• DOI: 10.3847/1538-4357/ad1685
• EISSN: 1538-4357
• ISSN: 0004-637X; 1538-4357

As stars evolve to higher luminosities during first ascension of the giant branch, previously frozen terrestrial worlds may thaw and host liquid water on their surfaces. Eventually these outer worlds again become uninhabitable due to receiving too much incident light and their water inventory evaporating. Solar-mass stars experience a sudden decrease in luminosity entering the horizontal branch, which could result in a secondary habitable phase for their outer worlds. The outer worlds' time with habitable surface climates is key in evaluating the possibility of extraterrestrial life arising. The times inside the habitable zone (TIHZ) are calculated for outer worlds orbiting between 5 and 45 au around a Sun-like star. By comparing the TIHZ to time estimates for life to arise on Earth, we evaluate whether such outer worlds are promising candidates in the search for extraterrestrial life. We use two different solar evolution models (PARSEC and Dartmouth) and both optimistic and conservative habitable zone (HZ) definitions. Multiple habitable phases are found for each outer world. Outer worlds with orbits as large as Saturn are found to have a secondary habitable phase which exceeds the first in duration. Generally, the time inside the HZ is found to decrease almost monotonically with orbiting distance. Water loss is calculated after the first habitable phase to determine whether a secondary habitable phase is possible. For all orbiting distances the water loss is insufficient to deplete a water inventory equivalent to that of many moons in the outer solar system.