From CfA: “HEK – Hunt for Exomoons with Kepler

Smithsonian Astrophysical Observatory
Smithsonian Astrophysical Observatory


Undated, but 2013
No Writer Credit

NASA Kepler Telescope


The Hunt for Exomoons with Kepler, or HEK, is an astronomy project designed to search for observational evidence of exomoons (extrasolar moons). The “exo” part of the word simply means that the moon lies outside of our own solar system. Because the nearest star to us is several light years away, the stars which we look at in our hunt are in the range of 10’s to 1000’s of light years away. So far, no-one has ever found an exomoon but there has never been a systematic search for their existence before. HEK will therefore test the hypothesis that moons exist in other solar systems aside from our own. Our primary mission is to determine the occurrence rate of large moons around viable planets hosts, which we denote with the symbol η☾.


Why should we care about exomoons? Perhaps, the most fundamental reason is life. Science fiction writers and film makers have long toyed with the idea of moons teaming with life, such as the moon “Pandora” in the recent film Avatar (pictured). But this is not just science fiction- astrobiologists believe that Europa, Titan and Enceladus (the moons of Jupiter and Saturn) are potentially viable homes for some form of primitive biology. Sadly though, there are no moons in our own solar system which offer truly Earth-like conditions such as that depicted in Avatar.

But could there be a vast population of habitable exomoons out there just waiting for us to find them? If such habitable moons are possible, then there could even be more habitable moons than habitable planets. Planet-based life could even be a rarity in the Galaxy! HEK cannot tell us whether life inhabits exomoons or not, but the first step is to establish whether moons big enough to support a biosphere exist or not. HEK will hopefully answer this question.


Another important implication of moons is that aside from being habitable themselves, they also may affect the habitability of any planets they orbit. For example, the Moon (pictured) is thought to stabilize the axial tilt of the Earth which is beneficial to the climate and habitability of our planet. If the Moon wasn’t there, would our planet still have complex life (like us) on it? With just one known example, the Earth-Moon system, it is difficult to make this determination. But HEK will seek evidence of exomoons around habitable-zone planets in order to say whether planets in the habitable-zone of their host star frequently have large moons or not.


Finally, the third important reason to try and find exomoons is that they can teach us a lot about how planets, moons and solar systems form and evolve. The solar system is billions of years old and so we have only been observing it for a very short fraction of that time – therefore a major challenge in planetary astronomy is work out how our own solar system formed and evolved over all that time we weren’t looking. A significant piece of this puzzle is how moons form and evolve. Taking the example of the Earth-Moon system once again, the Moon is thought to have formed through a giant collision between the primordial Earth and a Mars-sized planet which drifted too close (pictured). This enormous collision broke up the smaller planet into a disc of vaporized rock which slowly coalesced into what we now call the Moon. This extraordinary tale is our best guess for how the Moon formed but we have no idea as to whether this happens all the time in other solar systems or whether the Earth-Moon system is somehow a freak in the Universe. Only by detecting a population of planet-moon systems can be hope to answer whether the Earth-Moon system is unique and so whether solar systems like our own reside in the cosmos.


Principal Investigator: David Kipping
David Kipping wrote his PhD thesis on the subject of exomoon detection theory at University College London and has single-authored numerous papers on the topic. David devised two new methods to detect exomoons in the form of TDV-V and TDV-TIP (velocity and transit-impact-parameter induced transit duration variations, respectively). These tools are critical in assessing a moon’s mass and sense of orbital motion (prograde or retrograde).

David is now a Donald Menzel fellow at the Harvard College Observatory, where the HEK project servers perform round the clock automated searches for exomoons.

Co-Investigator: Gáspár Bakos
Assoc. Prof. Bakos of Princeton University founded the HATNet project (Hungarian Automated Telescope NETwork), which is one the most successful transiting planet hunting surveys to date. Gáspár’s expertise range from instrumentation, to programming, from observations to theory and we are fortunate to have these talents for HEK.

Co-Investigator: Lars Buchhave
Dr. Lars Buchhave, based at the Neils Bohr Institute in Copenhagen, obtains stellar spectra for the HEK project as well as his own projects. These spectra are then used to refine the stellar parameters and look for the stellar wobble caused by the presence of a planet. By combining this data with the Kepler photometry, HEK can not only confirm candidates, but also dynamically measure the masses and radii of the entire system.

Co-Investigator: Joel Hartman
Joel is an Associate Research Scholar at Princeton and has been instrumental in the success of the HAT project, founded by Prof. Bakos. Joel has recently begun investigating novel non-parametric methods to look for exomoons and provides invaluable support in the interpretation of light curve signals, as well characterizing the host stars.

Co-Investigator: Chelsea (Xu) Huang
Chelsea is a graduate student at Princeton University interested in the analysis Kepler light curve data. Chelsea has developed an independent processing pipeline of the Kepler data which HEK uses in certain cases to vet moon candidate signals. This independent check, including pixel-level diagnostics, allows us to verify potential signals are astrophysical rather than instrumental in nature.

Co-Investigator: David Nesvorný
Based at the Southwest Research Institute in Colorado, Dr. David Nesvorny is a dynamist who has written pivotal papers in fields ranging from Kuiper belt objects to moons, from exoplanets to asteroids. David’s expertise is crucial to the HEK project. Frequently, the dynamical perturbations which can signal the presence of a moon could also be confused with a perturbing planet. David will interrogate the hypothesis of a perturbing planet in such cases, to help us understand the true nature of the planetary system.

Co-Investigator: Allan Schmitt
Allan Schmitt joined the HEK project via This dedicated group of non-professional astronomers inspect Kepler light curves for signs of other planets not found by the automated planet-hunting tools of Kepler. In a similar vein, Allan leads the visual inspection effort on these data for exomoon signals. Exomoon signals are much trickier to spot than new planets but a trained and patient human eye is a powerful tool.

Co-Investigator: Guillermo Torres
Located at the Harvard-Smithsonian Center for Astrophysics, Guillermo Torres is a world renown expert in the analysis and validation of planetary transits and the determination of accurate stellar parameters who joined the HEK project in fall 2014. These expertise aid the HEK project greatly, providing a second check as to the nature of candidate signals and also providing physical parameters for the star which affects the planet and moon terms too.

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About CfA

The Center for Astrophysics combines the resources and research facilities of the Harvard College Observatory and the Smithsonian Astrophysical Observatory under a single director to pursue studies of those basic physical processes that determine the nature and evolution of the universe. The Smithsonian Astrophysical Observatory (SAO) is a bureau of the Smithsonian Institution, founded in 1890. The Harvard College Observatory (HCO), founded in 1839, is a research institution of the Faculty of Arts and Sciences, Harvard University, and provides facilities and substantial other support for teaching activities of the Department of Astronomy. The long relationship between the two organizations, which began when the SAO moved its headquarters to Cambridge in 1955, was formalized by the establishment of a joint center in 1973. The CfA’s history of accomplishments in astronomy and astrophysics is reflected in a wide range of awards and prizes received by individual CfA scientists.

Today, some 300 Smithsonian and Harvard scientists cooperate in broad programs of astrophysical research supported by Federal appropriations and University funds as well as contracts and grants from government agencies. These scientific investigations, touching on almost all major topics in astronomy, are organized into the following divisions, scientific departments and service groups.