Sen— Moons orbiting planets in other star systems could be as likely to support life as exoplanets, according to new research.
The search for habitable worlds beyond our Solar System has discovered hundreds of exoplanets, with about 850 confirmed so far and thousands of planet candidates being observed further. Assuming the exoplanets have moons, the environmental conditions of the moons are likely to be different from the exoplanets they orbit, say the researchers. These extrasolar moons - also called exomoons - could have the right conditions for life to exist.
The research, which is published in the January issue of Astrobiology, was conducted by René Heller of Germany's Leibniz Institute for Astrophysics Potsdam and Rory Barnes of the Univeristy of Washington and the NASA Astrobiology Institute. Their theory is that the climatic conditions of a moon are likely to be different from those of their parent planet primarily because most moons are tidally locked with their planet, like our own Moon, meaning one hemisphere permanently faces the planet.
Exomoons would also have two sources of light, one from the star and another from their parent planet which would result in eclipses that could alter the exomoon's climate significantly. "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,” explained Heller. “For instance stellar eclipses could lead to sudden total darkness at noon.”
The distance of the moon to the planet would also play a role in determining habitability, because the closer the moon to the planet the stronger the tidal heating would be. Too close and the moon would be likely to suffer from a catastrophic runaway greenhouse effect that would boil away surface water and leave it uninhabitable. The new research has constructed a model to calculate the minimum distance a moon could be from its host planet to be habitable, and this has been named the "habitable edge". If an exomoon orbited their planet closer than the habitable edge it would experience tidal heating too strong for the world to support life as we know it. "There is a habitable zone for exomoons, it's just a little different than the habitable zone for exoplanets," explained Barnes.
Although an exomoon has not yet been discovered, scientists have considered how they might be detected, for example measuring whether an exoplanet's orbit wobbles as a result of gravitational tugging from one or more exomoons.
NASA's Kepler space telescope and the European Southern Observatory's HARPS (High Accuracy Radial velocity Planet Searcher) instrument are being used to search for planets in other star systems. Some, such as Kepler-22b, orbit their parent star in the habitable zone where the temperature is just right for liquid water to exist on the surface. NASA's Kepler mission was extended last year until 2016, and in the last few days it was announced that another 461 planet candidates had been found, taking the total number of candidate planets being considered by Kepler to 2,740.
Whilst the number of confirmed planets is still under one thousand, a recent scientific study suggests there are at least 100 billion planets in our Milky Way galaxy - which is likely to mean there are also billions of exomoons.
Within our own Solar System scientists believe the moons orbiting the gas giants could be the best place to search for microbial life which might exist in the underground oceans believed to exist on some of the moons orbiting Jupiter and Saturn. The European Space Agency's Jupiter icy moons explorer (Juice) mission, which is aiming for a launch in 2022, aims to study the Galilean moons of Ganymede, Europa and Callisto which are believed to have sub-surface oceans that could support life. Around Saturn, planetary scientist Dr Carolyn Porco has declared that the best place in the Solar System for a big mission to search for life is the tiny moon of Enceladus.