Venus May Have Supported Life Too
Conditions for life may be possible on planets circling in the inner edge of their parent star's so-called habitable zone.
With surface temperatures approaching 900 degrees Fahrenheit and an atmosphere 90 times thicker than Earth's, Venus is a hellish place, hot enough to melt lead. But earlier in its history, the second planet from the sun may have had a liquid water ocean and temperatures suitable for life, a new study shows.
Assuming that Earth and Venus started off with roughly the same ingredients, NASA climate models suggest that Venus was able to hold on to its water for about 2 billion years, despite its orbital perch roughly one-third closer to the sun than Earth.
Even with 46 to 70 percent more solar radiation, if Venus rotated more slowly than about 16 Earth days, it could have had moderate temperatures, climate scientist Michael Way, with the NASA Goddard Institute for Space Studies in New York, and colleagues write in a paper published in Geophysical Research Letters.
Venus today is the solar system's slowest spinner, rotating once every 243 Earth days. Even at that rate, Venus' climate could have remained habitable until at least 715 million years ago, Way and colleagues concluded.
Still unknown is if Venus could have maintained its warm climate long enough for life to evolve.
"The rotation rate and topography of Venus play crucial roles in its surface temperature and moisture," the study said.
Nevertheless, the study has implications for looking for life beyond Earth, the authors noted.
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"Since Venus is similar to Earth in size and adjacent to it in our solar system, it is of particular interest for the characterization of seemingly similar exoplanets that could have followed different evolutionary paths," Way wrote.
The results suggest that warm rocky planets that retain significant water after formation and that rotate slowly may be habitable even if they orbit closer in to their parent stars than planets in more life-friendly orbits like Earth.
The scientists also noted that a planet with a modest amount of surface liquid water is more conducive to habitability over a wide range of stellar heating scenarios than a planet that is completely or mostly covered in water.
"The inner edge (of a star's habitable zone) should therefore be considered a transition region in which the probability of habitability gradually decreases inward rather than a strict boundary separating completely different regimes," Way and colleagues concluded.