![]() ![]() There are many definitions for the "Goldilocks zone" or Habitable zone. Why the Circumstellar Habitable Zone is defined as it is, if life could be possible outside of it? The entire orbits of the Moon, Mars, and numerous asteroids also lie within various estimates of the habitable zone. The aphelion of Venus, for example, touches the inner edge of the zone and while atmospheric pressure at the surface is sufficient for liquid water, a strong greenhouse effect raises surface temperatures to 462 ☌ (864 ☏) at which water can only exist as vapour. However their atmospheric conditions vary substantially. Numerous planetary mass objects orbit within, or close to, this range and as such receive sufficient sunlight to raise temperatures above the freezing point of water. doi:10.From the Wikipedia article Circumstellar habitable zone, which is just another name for the Goldilocks zone:Įstimates for the habitable zone within the Solar System range from 0.38 to 10.0 astronomical units, though arriving at these estimates has been challenging for a variety of reasons. “On the Habitable Lifetime of Terrestrial Worlds with High Radionuclide Abundances,” Manasvi Lingam and Abraham Loeb 2020 ApJL 889 L20. 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 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. These higher concentrations may be enough to generate the heat needed to sustain liquid on the planets’ surfaces. ![]() ![]() 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. Long-lived ethane oceans are easier to achieve, requiring only 100 times Earth’s radioisotope abundances.Īrtist’s impression of the collision and merger of two neutron stars. 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. 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. 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. 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? 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. External heating from starlight is not the only way to keep a planet warm enough for surface liquids, Lingam and Loeb argue. The blue, green, and brown horizontal lines bound the temperature range in which liquid water, ammonia, and ethane can exist, respectively. 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. ![]()
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