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The hunt is on for exomoons
Posted: 30 March 2012

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The Hunt for Exomoons with Kepler project has identified around 100 potential light curves that could reveal moons orbiting alien worlds already found by the Kepler Space Telescope.

Planets with moons are a common feature in our Solar System, and with an abundance of planets found around other stars it is only natural to assume that many of these planets also have moons. NASA’s Kepler mission is tasked with hunting for Earth-like planets that orbit in the habitable zones (HZs) of their parent stars, the area around a star where the temperature is just right for water to be present in a liquid state. Discovering planets within the HZ does not necessarily make them habitable though, especially if they are gas giants. However, if a moon orbited a gas giant within the HZ, then the moon would be a possible location for life to flourish.

An artist’s impression of an Earth-like moon orbiting a gas giant planet. Image: David A. Aguilar, CfA.

Detecting exomoons is no easy task, but it is thought that signals of these moons could be hidden somewhere within the Kepler database of transiting exoplanet light curves, and the hunt is now on to find these signals. The Hunt for Exomoons with Kepler (HEK) was initiated six months ago, and David Kipping from the Harvard-Smithsonian Center for Astrophysics discussed the details at the National Astronomy Meeting in Manchester this week.

He explained that HEK is using data from the Kepler public archive, and the team hope not only to detect exomoons, but to determine how common they are. Kipping is confident that Kepler should be able to find Earth-sized moons, since the telescope is already capable of detecting Earth-sized planets. However, smaller moons will be beyond the technological limits of Kepler. “It’s possible that with the next generation telescopes we can detect moons around Earth-sized planets, but Kepler is really limited to gas giants in the habitable zone, and maybe Earth-sized things around that gas giant,” he says.

The HEK team have already compiled a list of around 100 potential exomoon light curves, which they found by using both automated and visual inspection methods. These light curves harbour unusual features compared to the normal dip of an exoplanet light curve, and identifying if these anomalies are caused by exomoons is a challenging task. Kipping and his colleagues will need to compare the Kepler data to different computer models; one which solely simulates a planet, and one that includes both a planet and a moon.

Starspots can also “contaminate” exomoon light curves by creating a bump on the dip of a planet light curve as the planet passes in front of the star’s dark spot and thus blocks less light. “If we see sometimes a moon and a planet and a star all line up at the same moment in time we get an increase in flux right at the bottom of the transit, which looks a lot like a starspot,” Kipping told Astronomy Now. “So in order to tell the difference between that we could measure the transit in different colours from the ground. Kepler only looks in one colour unfortunately, but if we use a different telescope from the ground, we measure in different colours. An exomoon, of course, should be the same in all colours, whereas a starspot has big changes; it appears very different in different colours so we can tell the difference between them in that way.”

Even if exomoons are detected, it will still be quite some time before we can search for signatures of life. “They’re going to be small, they’re going to be Earth-sized,” explained Kipping. “The limitation is the same as doing it for Earth-sized planets really, and for that it’s thought that the James Webb Space Telescope would be necessary, and it would need a lot of time on JWST in order to do that.”

Detecting exomoons also has added importance as it will help in understanding the formation of planets and moons.