On December 13, 2019, the AAA hosted a presentation in the Kaufmann Theater of the AMNH by Alex Teachey, entitled “Exomoons: Kepler’s Hidden Gems.” Teachey is a doctoral candidate in astronomy at Columbia University, researching in particular the use of machine learning in the detection of moons around exoplanets, or exomoons. Teachey shared his knowledge and experience with a packed house at the AMNH.
Teachey began by asking, “Why should we care about space or astronomy?” As he puts it, we are in search of “who we are? where do we come from?” and “where are we going?” These questions transcend all cultures, and various religions have been preoccupied with trying to answer them over thousands of years. Only in the past few hundred years has astronomy begun to provide some concrete answers to them.
As we study other planetary systems, that informs our knowledge of our own. We have discovered a remarkable variety of planetary systems around other stars, and as moons are created at the same time as planet formation, exomoons represent a topic ripe for study. Teachey explained that there are three basic pathways for moon creation: impacts (as was the case for our moon), formation within the circumplanetary disk during a planet’s own formation (for example, Jupiter’s Galilean moons), and capture (Neptune’s largest moon, Triton, is considered to be a captured Kuiper belt object).
This leads to an astonishing diversity of moons, just within our solar system, including some that are large enough to have been called planets if they did not orbit a gas giant rather than a star. Even the larger Kuiper belt objects have moons; moons are everywhere. At least one solar system moon, Europa, even contains more water than the Earth itself. And we have recently discovered that some moons have internal heating due to the gravitational attraction of their parent planet. The presence of water and an internal energy source in some moons greatly increases the number of worlds potentially capable of supporting life, beyond just the rocky planets in a star’s habitable zone.
As Teachey explained, not all tools used in the search for exoplanets will work in the search for exomoons, at least in the short term. Due to technology limitations and extreme distances, although we can use direct imaging to detect a handful of exoplanets around nearby stars, we cannot yet directly image exomoons around such planets.
The radial velocity method measures the slight wobbling of a star created by a planet’s gravitational tug as it moves around the star (or more accurately, as both move around a common center of gravity). This has been a highly successful means to find exoplanets; but again, we do not currently possess the technical ability to measure the tiny wobble of an exoplanet which an exomoon around it would create.
Fortunately, the transit method, made famous by the Kepler and TESS satellites, can in theory detect the presence of exomoons as long as the geometry of the star-planet-moon combination is such that the orbits of both the planet and moon are in line with our view of the star. In such a scenario, we would expect the orbiting exomoon to produce a smaller “dip” in the light curve of the parent star, either preceding or following the larger dip caused by the planet itself. Furthermore, as the exomoon tugs on its planet we might see slight variations in the timing and duration of the planet’s transits across the star’s disk. Analyzing these three effects of transiting exomoons, Teachey identified the first exomoon candidate, Kepler1625b. In October, 2017, he obtained a forty-hour observation of the exomoon candidate using the Hubble Space Telescope. The resulting light curve data show transit timing variations and a second, smaller dip, both of which are consistent with the existence of an exomoon.
Given the magnitude of the discovery, Teachey considered alternative explanations for the light curve data, including instrumental artifacts, starspots masquerading as transits, even the existence of another planet. In Teachey’s view, none of these are likely explanations for the Hubble data, and the most likely interpretation remains the discovery of an exomoon. Ongoing radial velocity measurements of the star will help resolve this mystery, and so far the results appear consistent with the moon hypothesis. Further observations of additional transits will also serve to confirm or not the first discovery of an exomoon.
Teachey’s current research includes developing machine learning to automatically analyze over 1.2 million transit events included in the Kepler data. The machine learning can sift through the voluminous data to identify specific star systems that are worth studying. And Teachey has identified further exomoon candidates.
Exomoons be forewarned: your days hiding from us are now numbered, thanks to Alex Teachey’s research.