On October 3, 2018, using data from the Kepler Space Telescope, NASA announced that astronomers have acquired captivating evidence of a possible moon orbiting an exoplanet. The discovery of an Exomoon would mark the first of its kind, a moon orbiting a planet outside of our solar system.
The candidate is located in the Cygnus constellation, a gas giant planet(b) orbiting the star Kepler-1625, some 8,000 light-years away from Earth.
Though exciting, researchers have insisted that this is merely a hypothesis awaiting confirmation by follow up observations by the Hubble Space Telescope.
Our solar system contains eight planets and about two hundred moons, which begs the question: if we’ve discovered nearly 3000 exoplanets, then why haven’t we confirmed a single moon? As one could gather, they are extremely difficult to find. This potential exomoon was detected using the transit method, the same procedure used for finding exoplanets when they cross in front of their host star, temporarily dimming the star’s light only for just a moment, visible to any telescope or observatory watching. Exomoons are slightly harder to detect than their companion exoplanets naturally due to their smaller size, as a result creating a weaker transit signal. Other difficulties include the shift in the moon’s position with each transit as it orbits its planet.
Astronomers from Colombia University have analyzed the data taken from 284 exoplanets discovered by Kepler with wide orbits lasting about a month or longer around their host stars.This candidate remains the only instance of a transit signature with fascinating anomalies, which might just indicate the existence of a moon.
Following these initial findings, the team continued to intensively examine the planet, using the transit method to acquire better and more precise data on dips in light. After the first 19 hour transit ended, a second, smaller transit, among other observations, was detected by Hubble 3.5 hours later, consistent with the trailing behavior of a gravitationally-bound moon.
A paper published by the journal Science Advances reports the unusually large size of the candidate moon, comparing it to Jupiter, unlike any moon known to exist in our solar system. At the time when the first exoplanets were discovered, naturally the largest ones (mainly gas giants) were found first. The same seems to apply to exomoons, allowing researchers and experts to revisit theories on the creation of moons and planetary development.
The Exoplanet Kepler-1625b lies within the habitable zone of its star, its moderate temperature allowing for the existence of water on terrestrial surfaces. It is several times the mass of Jupiter, and if this candidate moon turns out to be gaseous rather than rocky as some have surmised, it could have further implications to how moons form through a different unknown process.
The future of exomoon hunting appears bright. Wide orbits with infrequent and long transits mark the ideal indicators for planets that may host moons. There currently exist but a handful of such planets within the Kepler database, though equipment to push the boundaries even further are finalizing before deployment just around the corner.
The powerful James Webb Space Telescope will have the ability to detect candidate exomoons with significantly greater detail than with Kepler, even spotting really tiny moons as well. So, you know, no one’s left out.