This illustration shows a rocky exomoon orbiting a distant gas giant. Researchers have detected 121 of these giant planets, which are likely orbited by terrestrial exomoons, in the habitable zones of their stars.
We often look at rocky exoplanets when searching for habitability beyond our solar system, but what about gas giants? Although their gaseous atmospheres and non-terrestrial surfaces aren’t ideal for housing life, their rocky moons could be. Recently, data from NASA’s Kepler spacecraft unearthed over 100 gas giants within the habitable zones of stars, giving researchers targets for potentially habitable exomoons.
To fall within the habitable zone, a planet must sit far enough from its host star to not be scalding hot, but close enough for its surface liquid to not freeze — ideal conditions for harboring life. These factors, along with a rocky, terrestrial surface, are what Kepler looked for during its 9-year hunt for habitable exoplanets.
But rather than looking at terrestrial exoplanets themselves, researchers from the University of California, Riverside and the University of Southern Queensland used Kepler’s data to look for gas giants that could have rocky moons, with similar atmospheres and surfaces to Earth’s, orbiting them. In total, they were able to track down 121 gas giants that lie within their star’s habitable zone. Their research will be published in an upcoming issue of the Astrophysical Journal.
“There are currently 175 known moons orbiting the eight planets in our solar system. While most of these moons orbit Saturn and Jupiter, which are outside the Sun’s habitable zone, that may not be the case in other solar systems,” said Stephen Kane, an associate professor of planetary astrophysics at the University of California, Riverside, in a press release. “Including rocky exomoons in our search for life in space will greatly expand the places we can look.”
Looking at these exomoons won’t be an easy task, though. These gas giants have radii of at least three times that of Earth, making them relatively easy to spot with the right equipment, but their moons are suspected to be much smaller and more difficult to find. No exomoons have been directly identified so far, but Kepler’s data gives researchers a solid starting point.
“Now that we have created a database of the known giant planets in the habitable zone of their star, observations of the best candidates for hosting potential exomoons will be made to help refine the expected exomoon properties. Our follow-up studies will help inform future telescope design so that we can detect these moons, study their properties, and look for signs of life,” said University of Southern Queensland undergraduate student, Michelle Hill.
And according to scientists, the conditions for life on exomoons have the potential to be more favorable than those on Earth. Here, we’re able to harbor energy that radiates from the Sun, but an exomoon could benefit from the energy of both its host star and its host planet.
While the hunt for Earth-like exoplanets is by no means called off, embracing the possibility of Earth-like exomoons certainly increases our chances of finding life in the great beyond.
