The search for sub-planetary scale features in other solar systems continues, with encouraging news from the Hunt for Exomoons with Kepler project. A moon around a distant exoplanet is a prize catch, but as we’ve also seen recently, scientists are weighing the possibilities in detecting exoplanetary ring systems (see Searching for Exoplanet Rings). Confirming either would be a major observational step, but exomoons carry the cachet of astrobiology. After all, a large moon around a gas giant in the habitable zone might well be a living world.
David Kipping (Harvard University) and colleagues at HEK have released a new study that tackles the question of how detectable exomoons really are. Published online today by the Astrophysical Journal, the paper examines 41 Kepler Objects of Interest, bringing the total number of KOIs surveyed by HEK thus far up to 57. The paper demonstrates that the process is beginning to move out of the realm of computer simulations and assumption-laden theory to actual data from Kepler. The paper’s goal is to determine how small a moon could be detected in each case given the kind of signatures that flag an exomoon’s presence.
Image: 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. Credit: The Hunt for Exomoons with Kepler (HEK) Project.
An examination of an exoplanet that does not turn up an exomoon thus leads to a statement of how massive a moon has been excluded by the current data, which means the team is learning much about the sensitivity of its methods. From the paper:
… based on empirical sensitivity limits, we show for the first time that the HEK project is sensitive to even the smallest moons capable of being Earthlike for 1 in 4 cases (after accounting for false-positives). In terms of planet-mass ratios, we find even that the Earth-Moon mass-ratio is detectable for 1 in 8 of cases, posing a challenge but not an insurmountable barrier. Mass ratios of ∼ 10−4, such as that of the Galilean satellites, have never been achieved. However, if Galilean-like satellites reside around lower-mass planets than Jupiter, of order ∼ 20 M⊕, then we do find sensitivity, as demonstrated by the limit of 1.7 Ganymede masses achieved for Kepler-10c.
This is encouraging news, for the team can now make statements about the actual mass of a detectable exomoon. In 1 of 3 planets surveyed, an exomoon with Earth’s mass is detectable. Kipping believes that we can move down to the smallest moon thought capable of supporting an Earth-like atmosphere and still detect it in 1 of 4 of the cases studied. No exomoons have yet been detected but we are learning just what our capabilities are. Says Kipping:
“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’.”
Image: 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. Credit: Hunt for Exomoons with Kepler Project.
No exomoons turn up in the 41 KOIs surveyed in the study, with four, KOI-0092.01, KOI-0458.01, KOI-0722.01 and KOI-1808.01, showing up as false positives for an exomoon. Stellar activity is a likely cause, as the paper comments:
When dealing with a handful of transits, quasi-periodic distortions to the transit profile, such as those due to spots… can be well fitted by the flexible exomoon model. However, since an exomoon is not the underlying cause, this model lacks any predictive power and thus should fail F2a [a follow-up test described in the paper]. We therefore suggest that stellar activity is likely responsible for these four instances.
KOI-1808.01, in fact, passes the basic criteria for an exomoon detection, but the paper explains that the observed transit signal is distorted by the effects of star spots. Transit timing variations observed at KOI-0072.01 (Kepler-10c) seem to point to an additional planet in the system rather than an exomoon.
Thirteen of the KOIs produce some kind of spurious detection, assigned by the paper to effects like perturbations from unseen bodies, stellar activity or instrumental artifacts. Through the range of KOIs the project has studied thus far (57), 46 null detections are found from which upper limits on an exomoon’s mass can be derived. The paper reminds us that “…exomoons live in the regime where correlated noise is present and one must employ methods to guard against it when seeking such signals.”
The declared purpose of the Hunt for Exomoons with Kepler project is to ‘determine the occurrence rate of large moons around viable planet hosts,’ a task with implications for the abundance of life in the universe, for if habitable moons are common, there could be more of them than habitable planets, and conceivably more than one orbiting a single planet. An additional benefit of studying exomoons is that they can teach us about solar systems formation by showing us planet/moon systems in a variety of configurations.
The paper is Kipping et al., “The Hunt for Exomoons with Kepler (HEK): V. A Survey of 41 Planetary Candidates for Exomoons,” submitted to the Astrophysical Journal (preprint).