Life Beyond the Habitable Zone

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Whether or not a planet lies in its star’s habitable zone is commonly used to gauge its ability to host life. But what about non-habitable-zone planets that have sources of heat besides starlight?

Warming Up in the Zone

A star’s traditional habitable zone marks the range of distances at which an orbiting planet receives enough heat from its star to host liquid water on its surface. Since water (or another liquid) is generally considered a necessary ingredient for life to arise and survive, stellar habitable zones represent convenient boundaries within which to search for life beyond our solar system.

Habitable zones

Schematic showing how the traditional habitable zone’s location and width changes around different types of stars. [NASA]

But we already know that habitable zones don’t tell the whole story. Plenty of planets that lie within their stars’ habitable zones aren’t livable — perhaps because they’re inhospitable gas giants, or because they have the wrong type of atmosphere, or because they’re routinely blasted by energetic stellar flares from their host.

Could the opposite be true as well, however? Could planets outside of a star’s habitable zone be capable of supporting life? A new study by scientists Manasvi Lingam (Harvard University; Florida Institute of Technology) and Abraham Loeb (Harvard University) explores this possibility. 

Stars Aren’t Everything

surface temperatures over time

Surface temperature as a function of age in Myr for a world with radioisotope abundances 1,000x that of Earth, for three different planet masses. The blue, green, and brown horizontal lines bound the temperature range in which liquid water, ammonia, and ethane can exist, respectively. [Lingam & Loeb 2020]

External heating from starlight is not the only way to keep a planet warm enough for surface liquids, Lingam and Loeb argue. There are additional processes that can instead heat a planet’s surface from the inside — in particular, radioactive decay and primordial heat from the planet’s formation. How powerful would these processes need to be for a planet to maintain liquid on its surface long enough for life to arise and evolve, even without the added heat from starlight?

To be inclusive of life forms that may be different from Earth’s, Lingam and Loeb choose to explore three different liquids in their models: water, ammonia, and ethane. The authors investigate the radioactive heat flux from both long-lived and short-lived isotopes, as well as the typical heat flux released as a world cools after its formation.

Radioactive Worlds

Lingam and Loeb find that a rocky super-Earth with a tenuous atmosphere would need radioactive isotope abundances roughly 1,000 times higher than that of Earth to host long-lived water oceans without the help of starlight. Long-lived ethane oceans are easier to achieve, requiring only 100 times Earth’s radioisotope abundances.

neutron star merger

Artist’s impression of the collision and merger of two neutron stars. [NSF/LIGO/Sonoma State University/A. Simonnet]

Are these high concentrations feasible? Worlds in the dense inner regions of the galactic bulge (where radioisotope-producing neutron-star mergers are more common) or in gas-poor environments are expected to exhibit higher radioisotope abundances. These higher concentrations may be enough to generate the heat needed to sustain liquid on the planets’ surfaces.

Since the number of planets outside of stellar habitable zones is likely orders of magnitude larger than the number inside them, the chance for life on non-habitable-zone worlds opens a wealth of possibilities. Keep an eye out in the future — the James Webb Space Telescope may be able to detect the infrared signatures of some of these internally heated worlds!

Citation

“On the Habitable Lifetime of Terrestrial Worlds with High Radionuclide Abundances,” Manasvi Lingam and Abraham Loeb 2020 ApJL 889 L20. doi:10.3847/2041-8213/ab68e5