Scientists think they found the moon of an alien planet, which would be the first time we’ve ever seen an “exomoon.” The problem is that we’ll never really know if that’s what astronomers saw.
Researchers discovered the possible exomoon using gravitational microlensing — a technique that detects distant objects regardless of the light they emit — to take advantage of chance alignments between stars. When a foreground star passes between Earth and a more distant star, the closer star can act like a magnifying glass on the more distant one. If that star has a planet whizzing about it, the planet will brighten or dim the light of the distant star. But in some cases, the distant object could be a free-floating planet instead of a star, giving scientists the ability to measure the mass of the planet relative to its companion.
In this case, scientists could have spied a small star circled by a planet 18 times Earth’s mass — or they could have spotted, for the first time, a planet bigger than Jupiter teamed up with a moon weighing less than Earth.
But because this encounter was completely random, it’s impossible to know if we finally laid eyes on an exomoon. So, the search continues.
An exomoon, or extrasolar moon, is a natural satellite that orbits an exoplanet or other extrasolar body.
It is inferred from the empirical study of natural satellites in the Solar System that they are likely to be common elements of planetary systems. The majority of detected exoplanets are gas giants. In the Solar System, the gas giants have large collections of natural satellites (see Moons of Jupiter, Moons of Saturn, Moons of Uranus and Moons of Neptune), therefore it is reasonable to assume that exomoons are equally common.
Though exomoons are difficult to detect and confirm using current techniques, observations from missions such as Kepler have observed a number of candidates including some that may be habitats for extraterrestrial life.
In December 2013, a candidate exomoon of a free-floating planet MOA-2011-BLG-262, was announced, but due to degeneracies in the modelling of the microlensing event, the observations can also be explained as a Neptune-mass planet orbiting a low-mass red dwarf, a scenario the authors consider to be more likely. This candidate also featured in the news a few months later in April 2014.