New candidate for exomoon discovered
Almost all planets in our solar system – and even some dwarf planets – are orbited by moons. In other star systems, however, astronomers have not yet been able to definitively confirm a single moon. Is it because there are no moons there? Certainly not – our observational technology simply isn’t ready yet. But an article published in Nature Astronomy now introduces at least one new candidate for an exomoon. If confirmed as an exomoon, Kepler-1708 b-i – which is 2.6 times larger than Earth – could provide a missing piece of the puzzle for understanding the formation and evolution of extrasolar planetary systems.
The last candidate was Kepler-1625 b-i, a celestial body with about 19 Earth masses – hardly comparable to our Earth’s moon. Kepler-1708 b-i is much closer to our idea of a “moon”. Cool giant planets orbiting at some distance from their star, like Jupiter or Saturn, are a preferred place for the formation of moons. Such planets, however, are difficult to detect using the transit method – the most widely used method for finding exoplanets, which involves observing the small dip in brightness that a planet-moon system produces as it passes in front of its star.
David Kipping and colleagues examined the exoplanets detected in transit by the Kepler Space Telescope for traces of exomoons. They focused on 70 gas giant planets with cool temperatures (less than 300 Kelvin, equivalent to about 27 °C) orbiting their respective stars at a distance greater than that between the Sun and Earth, with an orbital period of more than a year. After a rigorous review, the authors found only one signal, and that was around an exoplanet the size of Jupiter, designated Kepler-1708 b. This signal is best explained by the existence of an exomoon around Kepler-1708 b, designated Kepler-1708 b-i, although this signal could still be an artifact with a 1% probability.
The researchers therefore note that further evidence is needed to confirm the reality of Kepler-1708 b-i’s signal and its status as a possible exomoon. Understanding the origin of such large moons, however, the authors say, will pose a challenge to planet formation theories. The heaviest moon in the solar system, Ganymede, is just over one-fiftieth the mass of Earth.