Posted: 16 December, 2008
Instead of looking for potentially habitable planets around stars outside our Solar System, University College London astronomer David Kipping is searching for moons.
Kipping’s calculations would allow astronomers to not only confirm if a planet has a satellite but to calculate its mass and distance from its host planet – factors that would determine the likely habitability of the moon. However, out of the 333 exoplanets discovered to date, there has been no evidence of any of them harbouring moons. But, says Kipping, this is probably unsurprising, since the field of exoplanet detection is only a little over a decade old. “When we had discovered only a handful of exoplanets, astronomers were much more interested in finding more planets rather than thinking about moons,” he says. “Now that our catalogue of planets is beginning to flesh out, it is quite natural for us to start thinking about progressing the field towards characterising these worlds, which includes looking for moons, planetary atmospheres and temperature measurement.”
Artist impression of a gas giant exoplanet with a habitable moon. Image: Andy McLatchie.
Until now astronomers have only looked at the changes in the position of a planet as it orbits its star, which has made it difficult to confirm the presence of a moon since these changes can be caused by other phenomena, such as a smaller planet. But by adapting this method and looking at variations in a planet’s position and velocity each time it transits in front of its star, Kipping says we can gain far more reliable information, and even have the potential to detect an Earth-mass moon around a Neptune-mass gas planet. “The method relies on looking at transiting planets,” he tells Astronomy Now. “These are planets which have just the right orbital inclination so that once every orbital period the planet passes in front of the star causing a tell-tale dip in starlight.”
The appearance of wobbles in a planet’s position and velocity are caused by the planet and its moon orbiting a common centre of gravity. “If a planet has a moon, it causes the planet to wobble about during its orbit,” says Kipping. “Because of this wobble, two things are changing: the planet's position and the planet's velocity. If the position of the planet changes, it means the transit occurs slightly earlier or slightly later than we expected it to. If the velocity of the planet changes, it means the duration of the transit will also change. The combination of these two effects exhibits a unique signature – it's like seeing the fingerprint of a moon. So by using both effects, we are able to say whether what we are seeing is a moon or something else. As an added bonus, the method allows us to calculate both the mass of the moon and its orbital distance from the planet. These factors are very important in the possible habitability of such objects.”
In September 2008 astronomers revealed this image of an eight-Jupiter mass planet around a normal Sun-like star. Perhaps it won't be long until we capture Earthlike moons orbiting exoplanets, too. Image: Gemini Observatory.
Around 30 known exoplanets reside in the habitable zone of their host star – the region of space where conditions are favourable for life as we know it on Earth – but all of these planets are uninhabitable gas giants. The search for moons in orbit around these planets is important in the search for life as they too will be in the habitable zone, and are much more likely to be rocky and Earthlike.
“If a moon did go around a hot-Jupiter, it would probably be a very hot, dry and barren rock – not a good place for life,” says Kipping. “But every year we are discovering more and more planets and we are no longer limited to hot-Jupiters. “For example, out the known exoplanets, almost 30 orbit in the so-called habitable zone of their star. Unfortunately, all of these planets are gas giants, but if just one of these had a moon then this would be a great place for life to start. Sadly, none of these 30 planets transit their host star [on which the detection method relies] but it will not be long before we find one that does and then we can look for moons.”
While finding habitable moons around exoplanets may seem like searching for a needle in a haystack, some supporting evidence for such a scenario may lie much closer to home. Taking our own Solar System as an example, Jupiter’s moon Europa and Saturn’s moon Enceladus have both been highlighted by the science community as possibly harbouring conditions favourable to life. Their interiors are kept warm by the huge forces of tidal heating exerted by their parent gas planets, which could maintain a warm liquid ocean beneath an icy shell. Even though these moons reside outside the conventional habitable zone of the Sun, in this case it illustrates the point that being a moon may be more beneficial than being a planet in the same location.
“Almost everything we have discovered about exoplanets is in complete opposition to the conventional thinking of 20 years ago,” comments Kipping. “Ask an astronomer 20 years ago whether it was possible to get a supermassive Jupiter-like planet orbiting a star in a two day orbit and they would probably think you were joking. The only way to really say whether moons can live in such places is to look for them!”
The hunt is on. Kipping describes his proposed method for exo-moon detection in the 11th December edition of the Monthly Notices of the Royal Astronomical Society.