Searching for life on exomoons

So-called exomoons have yet to be found outside our solar system, and a detection could be a decade away — or more. So, while some scientists search for habitable planets outside our solar system, other researchers are doing the same, but for the moons of these planets. Some scientists suggest a Mars-sized exomoon of a gas giant planet and ask whether or not liquid water could be found on its surface. The closest exomoon in our solar system is Jupiter’s Ganymede, the biggest moon in the solar system that is about five-sixth times bigger than Mars.

Rene Heller, an astrophysicist at the Max Planck Institute for Solar System Research in Germany.

The picture of Mars-sized exomoon is murky. The scientists considered energy sources such as stellar radiation, the stellar reflected light from a Jupiter-sized planet on the moon, the planet’s own thermal emission on the moon, and the tidal heating inside the moon that is generated due to the changing gravitational pull of the planet. “We investigate, for the first time, the interplay of all the possible exomoon heat sources as a function of various distances from the host star,” says Rene Heller, an astrophysicist at the Max Planck Institute for Solar System Research in Germany. “Actually, we even consider two possible types of host stars: a sun-like star, and a red dwarf star.”

The authors found that any moon around a gas giant beyond three astronomical units, or three Earth-sun distances, would have a high enough energy flux to stop the tidal thermostat effect from happening. Heller claims that this situation is dangerous for organisms. “They might have lots of liquid surface water, but their surfaces could at the same time be blotched with devastating volcanoes,” he wrote. “Nevertheless, we illustrate that they could be habitable given the right amount of tidal heating, and we show at which distances to their planets these moons would need to be.”

Although, there are several new planet-hunting telescopes joining astronomy in the nearest future, Heller said they are not targeted at exomoons, because searching for exomoons is financially risky and the payoff highly uncertain. This means it is likely to remain a low priority for the astronomical community. “I know that some people in the CHEOPS Science Team are actively compiling strategies to go and search for moons around transiting planets in wider orbits,” Heller said. But he added the “ultimate weapon” would likely be PLATO (PLAnetary Transits and Oscillations of stars), which will launch around 2024. It will conduct dedicated planet searches like the Kepler space telescope, but around more bright stars.

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