All About Space: Could you tell us what first got you interested in searching for exomoons?
René Heller: What I find interesting about moons is that they are so much more abundant than planets, at least in the Solar System. They are so diverse! Some of them have atmospheres, some have volcanoes, some have internal magnetic fields, some have subsurface oceans – one of them [Titan] even has clouds and rain. This suggests an enormous population of moons around the known extrasolar planets, some of which might fundamentally challenge our picture of planet and moon formation. I consider it almost natural that some of these strange moons have habitable environments.
All About Space: You’re mainly researching the properties of moons around gas giants?
René Heller: Yes, so far I’ve mostly been interested in big moons around Jupiter-like planets. Theories suggest that the biggest planets have the biggest moons – the king of the moons in our own Solar System, Ganymede, orbits Jupiter and some extrasolar planets are even more-massive than Jupiter, so they might have moons as massive as Mars. If such a system orbited a Sun-like star at an Earth-like distance, the moon – though not the gas giant planet – could be habitable. Isn’t that an absolutely fascinating possibility?
All About Space: These kinds of moon might be more common than worlds like our own Moon, but aren’t they much more difficult to detect than the planets they orbit?
René Heller: Using the standard dynamical strategies for moon- detection, moons around very massive planets are indeed hard to find – but if we consider the direct transit signature of the moon itself, then the moons around giant planets are actually very promising targets. A moon’s own transit signal depends only on its diameter relative to the star – the larger it is, the larger the dip in the transit light curve. With the biggest planets hosting the largest moons, the search for a moon’s own transit dip works actually best for moons around giant planets.
All About Space: So what factors would influence the habitability of an exomoon?
René Heller: Talking about the possibility of liquid water on the surface, rather than under a frozen ice sheet, a moon needs to be at least as massive as Mars in order to have a gravitational field that is strong enough to retain a substantial atmosphere for billions of years. An atmosphere is needed to enable moderate temperature variations between night and day, as well as to sustain some kind weathering and global water transport. Since atmospheres of moons cannot possibly be detected directly in the near future, the size and mass of a moon are fundamental to assess its habitability to begin with. Intriguingly, all the available exomoon detection methods work best for bigger moons, so they are actually biased towards potentially habitable worlds.
All About Space: Are there any other constraints to the areas you have to search within?
René Heller: Beyond mass and size, stellar illumination plays a key role. Too close to the star and the moon might be in a runaway greenhouse state, similar to Venus, or may lose its atmosphere entirely due to thermal escape. Too far away and the moon’s atmosphere will freeze out, similar to what has happened on Mars. So the stellar distance of the planet-moon system must be within a certain range, maybe in the so-called stellar habitable zone. But on top of that, a moon receives illumination from the planet – reflected starlight as well as the planet’s own thermal emissions. What makes things even more interesting is the internal heat reservoir generated in the moon by tidal dissipation. If a moon isn’t on a perfectly circular orbit, maybe because there are other moons causing gravitational perturbations, then the varying gravitational pull of the planet on the moon will knead the moon, heating it up through internal friction. The closer a moon to its planet, the stronger this tidal heating.
All About Space: So just as in our Solar System, that could create a potentially habitable environment outside of the normal habitable zone. What chance do you think we have of proving such worlds exist?
René Heller: If an exomoon can be detected, then its mass, its distance to the planet and maybe even its radius would be known by means of the methods that we use to search for them. A moon’s mass and radius combined would give us an idea about its average density, which might enable us to classify it as a rocky, a water-rich, or an icy moon. So depending on the data quality, the basic moon characteristics, as well as the basic physical values of the planet and the star should come alongside the detection. These values alone should give us a reasonable evaluation of the possibility of a moon being habitable – they might be [found] in data from Kepler.
But answering the question of [whether or not] a moon is actually inhabited by some form of life would certainly be a task for the next generation of astronomers and astrobiologists, who would need to measure whether the moon has an atmosphere and liquid surface water. Currently available instruments wouldn’t permit such studies, but they’re possible in principle. Humanity just needs to decide to perform such investigations and then we need a bunch of very capable engineers.
You can learn more about the hunt for alien moons and their habitability in issue 29 of All About Space