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Rocky, radioactive exoplanets beyond the habitable zone of host stars could support "long-lived" water oceans without the help of starlight, according to a recent study published in the journal The Astrophysical Journal Letters.
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Radioactive exoplanets beyond the habitable zone
The new study — authored by scientists Manasvi Lingam of Harvard University and Florida Institute of Technology, and Abraham Loeb, also of Harvard, explores the possibility of planets supporting life outside of the habitable zone.
Usually, rocky exoplanets need to be within the habitable zone of a host star to form the oceans of water necessary for life to begin. They need to be close enough to the star to melt ice, yet not so close that a runaway greenhouse effect happens, similar to what happened to Venus.
But external heat from starlight isn't the only way to warm planets enough to retain surface liquids, argue Lingam and Loeb. Additional processes abound that can heat a planet's surface from the inside.
Radioactive decay and primordial heat from a planet's formation can also heat the surface.
To support lifeforms different than ones found on Earth, Lingam and Loeb decided to approach the idea for three different liquids: water, ethane, and ammonia.
Of course, different isotopes of radioctive material have varying durations of active, heat-emitting periods (called "heat flux"), as a world cools post-formation.
Warm oceans without sunlight
The scientists found that a rocky super-Earth with a thin atmosphere would need 1,000 times the radioactive isotope abundance than that found on Earth to host long-lived water oceans without starlight. Long-living ethane oceans form more easily, requiring only 100 times the radioisotope abundance found on Earth.
Either scenario requires worlds with a density far exceeding that of Earth.
And we could find planets like these in the dense inner regions of the galactic bulge (near the center of the galaxy, where radioisotope-producing neutron-star mergers are common). They might also be found in gas-poor sectors of the galaxy. Both are expected to exhibit higher radioisotope abundances, and could be enough to generate the heat necessary to sustain liquid on planet surfaces.
The number of planets beyond their respective stellar habitable zones is thought to be orders of magnitude higher than the number of planets within them. This means the probability of life on planets beyond habitable zones of stars could be very high.
Of course, this is still theory — but once the James Webb Space Telescope is up and functioning, we may detect exoplanets with liquid oceans that exist beyond not only the light of stars, but also our wildest dreams of life in the galaxy.