Chris Fox & Paul Wiegert
Dept. of Physics and Astronomy, Western University, London ON CANADA
The Institute for Earth and Space Exploration (IESX), London ON CANADA

Updated 24 November 2020, originally posted 24 June 2020

In a paper submitted to the scientific journal Monthly Notices of the Royal Astronomical Society, Canadian astronomers report on eight new exomoon candidiates. Planets discovered to be orbiting stars other than our Sun are called 'exoplanets' and the moons of these planets are called 'exomoons'. "We know of thousands of exoplanets throughout our Milky Way Galaxy," says Western University's Dr. Paul Wiegert, a co-author of the study, "but we know of only a handful of exomoon candidates."

The right panel shows an exoplanet transiting in front of its star. On the left is a simulation of the view from a telescope at Earth. Even through the most powerful telescopes, distortions due to our atmosphere and/or diffraction (which affects even space telescopes like the Hubble Space Telescope) may completely blur out the planet. But the periodic dimming of the star remains, signalling the presence of an unseen planet. Thousands of planets have been discovered this way by the Kepler planet-hunting spacecraft.

The exoplanets reported on in this study were themselves were discovered by the Kepler planet-hunting spacecraft, and revealed by their periodic 'transits', the slight dimming of the star's brightness when an exoplanet passes in front of it. The exomoon candidates reported in this new study are so small, that they can't be seen from their own transits, but rather their presence is given away by their gravitational influence on their parent planet.

If an exoplanet orbits its star undisturbed, the transits (dimmings) it produces occur precisely at fixed intervals. But for some exoplanets, the timing of the transits are variable, sometimes occuring several minutes early or late. Such 'transit timing variations' or TTVs, indicate that the gravity of another body —an exomoon or another planet in the system— is affecting the transiting planet.

"Because exoplanets are more massive than exomoons, most TTVs observed to date have been linked to the influence of other exoplanets. But now we've uncovered eight Kepler exoplanet systems whose TTVs are equally well explained by exomoons as by exoplanets." explains the PhD candidate who made the discovery, Chris Fox of Western University's Department of Physics and Astronomy and the Institute for Earth and Space Exploration (IESX). "That's why we're calling them exomoon 'candidates' at this point, they still need follow-up confirmation."

How an exomoon changes the pattern of dimmings (transit centres). The red case shows a planet with a moon. Because the planet and moon both orbit their centre of mass (small white dot) the planet does not start and end crossing the star at the same times as it would if it had no moon, shown by the blue case. The right panel shows brightness measurements of the star with their small inevitable uncertainties, These uncertainties can hide the presence of the dimming caused by the small moon. Click for a larger version or watch it on youtube.

Unfortunately the telescopes needed to confirm these or indeed any of the world's exomoon candidates don't exist — yet. Says Fox "We can tell you these eight new systems are completely consistent with exomoons: their masses and orbits are such that they would be stable; they would be small enough that their own transits wouldn't be seen, and they reproduce the pattern of TTVs seen throughout the entire Kepler data set. But we don't have the technology yet to confirm them by imaging them directly: that will have to wait for further advancements."

The eight exomoon candidates are in the star systems known as Kepler Object of Interest (KOI) 268.01, Kepler 517b (KOI-303.01), Kepler 1000b (KOI-1888.01), Kepler 409b (KOI-1925.01), Kepler 809b (KOI-1302.01), Kepler 857b (KOI-1472.01), Kepler 1326b (KOI-2728.01) and Kepler 1442b (KOI-3220.01). Artist's conceptions of these exomoon systems, to scale with our own Earth-Moon system, are shown lower on this page. The candidate systems are all around planets that are relatively hot because they are closer to their star than Earth is to the Sun, and are not expected to be in their star's habitable zone.

Said Fox, "Our own solar system contains hundreds of moons. If moons are prolific around other stars too, it greatly increases the potential places where life might be supported, and where humankind might one day venture."

Artist's conceptions of the candidate exomoon systems, together with the Earth-Moon system for comparison. The systems are to scale and the camera is located the same distance away in all cases to give an intuitive feel for the system scales. The distance to the central star is indicated in Astronomical Units (AU), the Earth-Sun distance. The reflex motion of the planet around its centre of mass due to the candidate moon is also to scale. Click on the animation for a larger version or watch it on youtube.

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