Around which planets can we find moons that may contain life? A team of astronomers compiled a ranking of the best contenders.

In the meantime there are more than 5,000 exoplanets discovered† planets orbiting a star other than our sun. But we’re still waiting for the first confirmed discovery of an exomaán—a moon orbiting an exoplanet. Yet astronomers Vera Dobos (University of Groningen) and colleagues already tried to find out which exoplanets have the greatest chance of having a habitable moon.

### Outside the habitable zone

If you want to know whether life is possible on a certain planet or moon – at least, life as we know it here on Earth – you have to see if there could be liquid water. With planets you then have to rely on the so-called habitable zone: the ring-shaped area around a star where it is neither too cold nor too warm. An exomoon orbiting a planet in the habitable zone could therefore harbor liquid water – and thus life.

But a planet outside the habitable zone can also have a habitable moon. “If a moon gets close to its planet and has an elliptical orbit, it will experience tidal forces,” says Dobos. “That leads to friction in the interior of such a moon, which releases heat.” Heat that could also make liquid water possible.

### 100,000 moons per planet

To determine which planets are most likely to have such a habitable moon, Dobos and her team first looked at the full list of known exoplanets. They deleted the planets associated with stars that are too light to generate energy through nuclear fusion, the planets that are too massive, and the planets we know too little about to make much sense about them. After that they had 4140 left.

For each of those planets, they then simulated 100,000 fictional moons with random properties. Then, based on the amount of energy they received, they looked at how many of those 100,000 moons could have liquid water. They called the ratio between the number of habitable moons and the total moon habitability probability, or the lunar habitability probability. In other words, if 10,000 of the 100,000 simulated moons orbiting a planet could harbor liquid water, according to the simulation, that planet’s lunar habitability probability was 10 percent.

### Not from top to bottom

Based on that habitability probability for any moons, the researchers then drew up a ranking, which can be found at the end of their article. At the top is the planet Kepler-459 b, with a habitability probability of no less than 70 percent. In second place is Kepler-456 b (shown in the illustration above), with 69 percent. It is followed by HD 7199 b, with 60 percent. You can find the whole list in this pdf, on page 7

So does this mean that astronomers who want to find habitable exomoons have to work through the list from top to bottom? No, says Dobos; she gladly leaves that choice to the scientists who read the article. “They have to determine for themselves which targets are most suitable for them, based on the observation techniques they use.”

### Moons as big as the Earth

And what makes exomoons so interesting that we are already talking about their habitability, even before the first specimen is found? “Anyway, it’s super interesting to find a habitable world, be it a planet or a moon,” says Dobos. “And if you also include moons, you greatly increase the number of places where life can occur. The most likely scenario is that there are more moons than planets. Moreover, there is no reason to believe that those moons cannot be as large as the Earth.”