Earth-like Exomoons Proven to be Detectable

For immediate release

Cambridge — March 20, 2015 — For years, scientists have theorized as to what kinds of exomoons - moons outside the Solar System - could be detected with modern telescopes. Now, for the first time, speculation gives way to hard data as the
Hunt for Exomoons with Kepler (HEK) project delivers an answer. In a paper appearing today online, a group of researchers led by Dr. David Kipping of Harvard University report that after a detailed analysis of approximately 60 planets, that whilst the first detection remains elusive, current technology is definitively sensitive to exomoons which are akin to those found in the Solar System and even potentially habitable moons.

One of the biggest questions facing astronomers today is whether worlds orbiting other stars are able to support life. Whilst astronomers are starting to find rocky planets in the habitable-zones of their parent suns, gaseous planets are also frequently found at such locations - which may mean that the habitable exomoons so frequently depicted in science fiction are indeed common. Until now, astronomers have only been able to guess how small an exomoon is detectable with current telescopes by using computer simulations and theoretical arguments, which are forced to make various simplifying and potentially erroneous assumptions. After several years of searching for exomoons, the HEK project is now finally able to provide the real answer after a detailed and painstaking survey of nearly 60 planets.

After surveying nearly 60 exoplanets for moons, the HEK team have derived empirical limits for each world, demonstrating an ability to detect even the smallest moons capable of sustaining an Earth-like atmosphere (“Mini-Earths”) for 1 in 4 cases studied. Whilst a confirmed discovery remains elusive, the painstaking survey of 60 planets spanning several years reveals what is possible with current technology. Image credit: The Hunt for Exomoons with Kepler (HEK) Project.

“For each planet where we don’t discover an exomoon, we are able to say how massive a moon is excluded by the current data, telling us about our sensitivity,” said Dr. David Kipping of the Harvard-Smithsonian Center for Astrophysics and first author of the paper. Using data from NASA’s Kepler space telescope, the authors look at each exoplanet for a variety of signatures betraying the presence of an exomoon, such as the shadow of the moon in front of the host star and the gravitational influence it imparts on the parent planet. Critically, even when these effects are not seen, the absence of them tells astronomers what kinds of moons can be ruled out.

“We most directly constrain the mass-ratio between the target planet and a potential moon. For example, the Moon is about 1% the mass of the Earth and whilst such a tiny signal is very challenging, we are sensitive to this level for about 1 in 8 planets studied,” explains Kipping. One of the largest mass-ratios found in the Solar System is 11.6% for Pluto and her moon Charon, for which the
New Horizons spacecraft will visit later this year. The HEK team report a Pluto-Charon ratio is detectable for about 40% of the planets surveyed, making it a relative cinch for the team.

The team then go on to convert these mass-ratios into actual physical masses too. For example, the team find that an exomoon with the same mass as the Earth is detectable for around 1 in 3 planets surveyed. Even the smallest moon thought to be capable of supporting an Earth-like atmosphere is detectable for 1 in 4 cases. With a proven sensitivity to exomoons then but no discoveries, should we be worried? “Here we report on our null results and the first estimate of empirical sensitivities. Ultimately, we would like to actually discover a clear signal and are pursuing some interesting candidates we hope will pan out. Either way though, I like to recall what the Nobel Prize winning American physicist Richard Feynman said about science: ‘Nature is there and she’s going to come out the way she is, and therefore when we go to investigate it we shouldn’t pre-decide what it is we’re trying to do except to find out more about it’.”

The Moon has about 1% the mass of the Earth posing a challenge for the HEK team, since such configurations are detectable for 1 in 8 planets surveyed. The much larger Pluto-Charon mass-ratio of 11.6% is much more detectable.

This research was funded by the National Science Foundation and NASA. The paper, "The Hunt for Exomoons with Kepler (HEK): V. A Survey of 41 Planetary Candidates for Exomoons" by Kipping and the HEK team is submitted to the Astrophysical Journal and is available
online here.