Many eyes were trained toward the night skies with the recent, rare, super blue blood Moon. While such spectacular events in the solar system provide joy for laypeople, astronomers seek to find similar features—beginning with finding a moon—outside the solar system in ongoing efforts to detect conditions for life elsewhere.

An exomoon is a moon orbiting a planet outside our solar system. So far, astronomers have discovered and measured the characteristics of 3,728 planets outside the solar system.₁ They have yet to make an undisputed detection of an exomoon.

An exomoon detection is imminent, however. Furthermore, astronomers have made sufficiently extensive observations to place meaningful limits on exomoon formation models. Such limits already are shedding light on moon design features that are critical for making life possible on a moon’s host planet.

In April of 2014 a paper was published in the Astrophysical Journal announcing the detection of a sub-Earth-mass moon orbiting a free-floating (detached from its host star) gas giant planet 3–4 times the mass of Jupiter.² However, as the authors pointed out in their paper,³ their detection also could be explained by a Neptune-mass planet orbiting a very low-mass red dwarf star. Thus, their exomoon identification was not definitive. They simply detected an exomoon candidate.

In the January 2018 issue of the Astronomical Journal, two Columbia University astronomers and a citizen amateur astronomer published a paper in which they announced the discovery of a much stronger exomoon candidate and placed a limit on the existence of analogs of Jupiter’s large moons (known as Galilean moons) orbiting exoplanets.⁴ This paper was the sixth in a series of papers published by the “Hunt for Exomoons with Kepler” (HEK) project. Kepler is the spacecraft dedicated to finding exoplanets by the transit method.

Exomoon Candidate
The exomoon candidate that the three astronomers found apparently is orbiting the gas giant planet Kepler-1625b. Kepler-1625b is between 5.9 and 11.7 times the mass of Jupiter and orbits its host star with a period of 287 days. The moon, if it proves to be real, is about the size of Neptune (17 Earth masses) and would be orbiting its host planet at a distance of about ten times the diameter of the host planet (about one million miles).

It is possible, however, that Kepler-1625b is more massive than 11.7 times Jupiter’s mass. In that case it might be a brown dwarf star instead of a gas giant planet. Then, the three astronomers would have discovered a planet orbiting a brown dwarf star rather than a moon orbiting a planet. The three astronomers have scheduled time on the Hubble Space Telescope to determine which of the two scenarios is correct.

Galilean Moons
The team selected 284 exoplanets for which astronomers had the highest quality measurements of their physical and orbital properties. They then stacked the data from a total of 6,096 transit events that astronomers had observed for these 284 planets. Next, they performed a rigorous statistical analysis to characterize the exomoon population for the sample of the 284 planets.

The statistical analysis yielded a strong upper limit on the exomoon population for planets orbiting their host stars as close or closer than Earth orbits the Sun. Moons, at least moons as large as the Galilean moons, orbiting such planets must be rare, if they exist at all. The team’s finding is consistent with theoretical work showing that when planets migrate inward toward their host stars any closer than Earth is to the Sun, they lose any moons that they might have possessed.⁵ Thus, the trio concluded that their finding indicates that a large majority of the 284 planets in their sample experienced substantial inward migration.

The paper published by the three astronomers gives the first significant findings from exomoon astronomy. Therefore, it is fair to say that the discipline of exomoon astronomy has been launched. It is also fair to say that these initial exomoon astronomy findings provide yet more evidence for the rare solar system doctrine. The findings make a yet stronger case that the solar system is rare, and likely unique, in its property that its planets experienced exactly the right amounts of inward and outward migration to make advanced life possible within the planetary system.⁶

Featured image credit: Frizaven