Meet 1708b-i: Exomoon of a Jupiter-sized Planet?
Exomoon is a moon orbiting an exoplanet. Just as planets in our solar system have moons orbiting them, exoplanets too can have moons orbiting them. But finding an exoplanet itself is a difficult task, let alone an exomoon.
Exomoons represent a critical link for Astronomers to understand exoplanets and their evolution. Recently, a team of Researchers did an exomoon survey of 70 cool, giant transiting exoplanets found by Kepler.
The host planet of the yet to be confirmed exomoon 1708b-i is a Jupiter-sized exoplanet orbiting a Sun-sized star at a distance of 1.6 A.U.
The search for exomoons was driven by the excess in the number of moons in our own solar system. One of the most promising methods to detect exomoons is to focus on transiting planets: worlds that eclipse their star periodically and form a large portion of detectable exoplanets.
Another factor is that due to dynamic considerations, exomoons are mostly thought to be found around cool exoplanets but the current surveys lead to an under-representation of such candidates due to observational bias in such transit surveys. To date, not enough is known on the existence of these exomoons.
Around the interiors, i.e. in orbits closer to the parent star, the probability of finding exomoons is quite less. But when we start looking towards the exteriors, planets with longer orbital periods have a greater probability of hosting an exomoon, one such example being ~ 1au Jupiter-sized planet, Kepler 1625-b, which was reported to have a timing variation in its transit signature which were similar to a Neptune-sized moon.
Out of the 70 planets which they surveyed, they only found a single candidate which had the potential of being an exomoon, Kepler 1708-b. Thus, Kepler 1708b-i, the yet-to-be-conformed exomoon joins Kepler 1625-b-i as an example of an unexpectedly large exomoon candidate.
The radius of Kepler 1708 b-i is 2.16 times the radius of Earth, thus making it huge by standards of moons in our solar system.
Compared with Kepler 1625 b-i, Kepler 1708 b-i is smaller in size and on a smaller orbit, and its orbit is more consistent with a coplanar geometry. Although the reality of Kepler 1625 b-i remains unclear (it is also a yet-to-be-confirmed exomoon candidate), the existence of this second candidate challenges us to consider the origins of such large moons.
For finding an exomoon one of the primary observable effects is the time transit variation of the host planet, which is due to the gravitational pull of its moon.
Another observational evidence is that the shadow of the exomoon can eclipse either the star or the planet, which leads to changes in the flux on top of the transit signature. These changes can occur in or out of orbit and result in a complex shape.
Another important observation for an exomoon is that the planet should have a circular orbit because elliptical orbits occur due to planet-planet scattering which has the potential to strip exomoons.
Although there are other reasons for the formation of an elliptical orbit, the dynamic region of stability in an elliptical orbit is very less and hence the planets with elliptical orbits are generally rejected.
If the above conditions are satisfied by a given candidate, it further undergoes an eccentricity test before being considered as an exomoon candidate. Further, they undergo a secondary test, after which 3 candidates emerged as probable exomoon candidates out of the original pool of 70.
Using the light curve analysis and studying the data they narrowed it down to a single candidate Kepler 1708 b-i. Still, further study is required to confirm whether Kepler 1708 b-i is actually an exomoon or not, but the current study seems promising as it identified a potential exomoon candidate, and Kepler 1708 b-i along with previously identified Kepler 1625 b-i have opened a new field of exomoons to the scientific community. Further studies in this field are going to reveal answers to some exciting questions may we find more such exomoons.
The paper “An exomoon survey of 70 cool giant exoplanets and the new candidate Kepler-1708 b-i” was published in Nature Astronomy.