Planets With Weird Orbits And White Dwarfs Can Be Well Suited For Life

Improved astrophysics and climate modeling, and better telescopes, are allowing us to better understand where there are conditions best suited to life in our galaxy.

It appears that planets with high levels of axial title that have highly elongated orbits may actually be better suited to creating habitable conditions for life than planets with little axial tilt and near circular orbits. 

Obliquity means having an axil tilt relative to the plane in which a planet is rotating.

Eccentricity is the extent to which an elliptical orbit of a planet deviations from being circular.

Planetary obliquity and eccentricity influence climate by shaping the spatial and temporal patterns of stellar energy incident at a planet's surface, affecting both the annual mean climate and magnitude of seasonal variability. 

Previous work has demonstrated the importance of both planetary obliquity and eccentricity for climate and habitability, but most studies have not explicitly modeled the response of life to these parameters. While exaggerated seasons may be stressful to some types of life, a recent study found an increase in marine biological activity for moderately high obliquities <45∘ assuming an Earth-like eccentricity. However, it is unclear how life might respond to obliquities >45∘, eccentricities much larger than Earth's, or the combination of both. 

To address this gap, we use cGENIE-PlaSim, a 3-D marine biogeochemical model coupled to an atmospheric general circulation model, to investigate the response of Earth-like marine life to a large range of obliquities (0-90∘) and eccentricities (0-0.4). 

We find that marine biological activity increases with both increasing obliquity and eccentricity across the parameter space we considered, including the combination of high obliquity and high eccentricity. We discuss these results in the context of remote biosignatures, and we argue that planets with high obliquity and/or eccentricity may be superhabitable worlds that are particularly favorable for exoplanet life detection.

Jonathan Jernigan, Émilie Laflèche, Angela Burke, Stephanie Olson, "Superhabitability of High-Obliquity and High-Eccentricity Planets" arXiv:2303.02188 (March 3, 2023) (published at 944 ApJ 205). https://doi.org/10.3847/1538-4357/acb81c

Another potentially attractive place for life outside Earth is on planets with short period orbits around white dwarf stars - these planets are heated by both the sunlight from its star and by the tidal tug and release on the planet itself (presumably giving rise to geothermal heating) caused by the gravitational interactions of the star and the planet with each other. 

In recent years, there have been a growing number of observations indicating the presence of rocky material in short-period orbits around white dwarfs. 

In this Letter, we revisit the prospects for habitability around these post-main-sequence star systems. In addition to the typically considered radiative input luminosity, potentially habitable planets around white dwarfs are also subjected to significant tidal heating. The combination of these two heating sources can, for a narrow range of planetary properties and orbital parameters, continuously maintain surface temperatures amenable for habitability for planets around white dwarfs over time scales up to 10 Gyr. We show that for a specific locus of orbital parameter space, tidal heating can substantially extend the timescale of continuous habitability for a planet around a white dwarf.

Juliette Becker, Darryl Z. Seligman, Fred C. Adams, Marshall J. Styczinski, "The Influence of Tidal Heating on the Habitability of Planets Orbiting White Dwarfs" arXiv:2303.02217 (March 3, 2023) (Accepted to ApJL).

One factors that makes the existence of life beyond Earth plausible is our detection of complex organic molecules, such as CH₃OH, C₂H₃CN, CH₃OCHO, CH₃COCH₃, aGg'-(CH_2LOH)₂, ¹³CO, CO(2-1), and SiO(5-4), in the matter spewing out of distant galaxies.

This would give the development of life a huge head start if some of those molecules ended up in a place that was otherwise habitable.