As the search for new exoplanets (and possible signs of life on them) has been progressing, exomoons have started to become the target of a lot of interest. And now new research is suggesting that, at the very least, exomoons are just as likely as exoplanets to harbor life.
So far, well over “850 extrasolar planets — planets outside the solar system — are known, and most of them are sterile gas giants, similar to Jupiter. Only a few have a solid surface and orbit their host stars in the habitable zone, the circumstellar belt at the right distance to potentially allow liquid surface water and a benign environment.”
This is thought to be caused by the fact that larger planets are much easier to spot, not because there are less Earth-sized planets. And while no exomoons have been discovered so far, it’s very likely that they are present in quantities at least as high as in our solar system.
Researchers René Heller and Rory Barnes, from the Leibniz Institute for Astrophysics Potsdam and the University of Washington/NASA Astrobiology Institute, decided to investigate some of the possibilities that these exomoons bring with them. Could they host life? How would the moon’s cycles affect that theoretical life? Is there a ‘habitable zone’ for moons?
Some of their more interesting findings focus on the climatic conditions of moons, especially how they are affected by the cycles of their planet and it’s Sun.
“The climatic conditions expected on extrasolar moons will likely differ from those on extrasolar planets because moons are typically tidally locked to their planet. Thus, similar to Earth’s moon, one hemisphere permanently faces the planet. Beyond that moons have two sources of light — that from the star and the planet they orbit — and are subject to eclipses that could significantly alter their climates, reducing stellar illumination.”
“An observer standing on the surface of such an exomoon would experience day and night in a totally different way than we do on Earth.” said Heller. “For instance stellar eclipses could lead to sudden total darkness at noon.”
The researchers also “identified tidal heating as a criterion for exomoon habitability. This additional energy source is triggered by a moon’s distance to its host planet; the closer the moon, the stronger tidal heating. Moons that orbit their planet too closely will undergo strong tidal heating and thus a catastrophic runaway greenhouse effect that would boil away surface water and leave them forever uninhabitable.”
“They also devised a theoretical model to estimate the minimum distance a moon could be from its host planet and still allow habitability, which they call the ‘habitable edge.’ This concept will allow future astronomers to evaluate the habitability of extrasolar moons.”
“There is a habitable zone for exomoons, it’s just a little different than the habitable zone for exoplanets,” Barnes said.
“The exquisite photometric precision of NASA’s Kepler space telescope now makes the detection of a Mars- to Earth-sized extrasolar moon possible, indeed imminent. Launched in 2009, the telescope enabled scientists to reveal thousands of new extrasolar planet candidates. Since 2012 the first dedicated ‘Hunt for Exomoons with Kepler’ is under way.”
The new research is being published in the January issue of the journal Astrobiology.
Image Credits: R. Heller, AIP