Space Team Theorizes Rare Exomoon Discovery

Western University astronomers may have spotted six new moons orbiting planets in solar systems far from our own - an otherworldly discovery so rare it must wait on future technologies to confirm. Until then, however, the mere possibility of the find sparks excitement over our biggest questions about the universe.

"Our own solar system contains hundreds of moons. If moons are prolific around other stars, too, it greatly increases the potential places where life might be supported, and where humankind might one day venture," explained Chris Fox, a Physics and Astronomy PhD candidate who made the discoveries.

The findings of the Institute for Earth and Space Exploration team were recently submitted to the scientific journal Monthly Notices of the Royal Astronomical Society.

Exoplanets are planets orbiting stars other than our Sun; the moons of these planets are called exomoons.

While more than 4,000 exoplanets have been discovered since the mid-1990s, none have a confirmed moon orbiting them, although a number of prime candidates have been identified in recent years.

"We know of thousands of exoplanets throughout our Milky Way Galaxy, but we know of only a handful of exomoon candidates," said Physics and Astronomy professor Paul Wiegert, co-author of the study.

Ranging between 200 and 3,000 light years away, the exoplanets reported in the Western study were discovered using data from the Kepler space telescope - a planet-hunting spacecraft decommissioned by NASA in 2018. They were revealed by the transits (or dimmings) of their star's brightness when an exoplanet passes in front of it. Their moons, however, were not so easily spotted.

"These exomoon candidates are so small that they can't be seen from their own transits. Rather, their presence is given away by their gravitational influence on their parent planet," Wiegert said.

If an exoplanet orbits its star undisturbed, the transits it produces occur precisely at fixed intervals.

But for some exoplanets, the timing of the transits is variable, sometimes occurring several minutes early or late. Such transit timing variations - known as TTVs - indicate the gravity of another body. That could mean an exomoon or another planet in the system affecting the transiting planet.

"Because exoplanets are more massive than exomoons, most TTVs observed to date have been linked to the influence of other exoplanets. But now we've uncovered six Kepler exoplanet systems whose TTVs are equally well explained by exomoons as by exoplanets," Fox explained. "That's why we're calling them exomoon 'candidates' at this point as they still need follow-up confirmation."

Unfortunately, the telescopes needed to confirm these or any of the world's exomoon candidates don't exist - yet.

"We can say these six new systems are completely consistent with exomoons: their masses and orbits are such that they would be stable; they would be small enough that their own transits wouldn't be seen; and they reproduce the pattern of TTVs seen throughout the entire Kepler data set," Fox said. "But we don't have the technology to confirm them by imaging them directly. That will have to wait for further advancements."

The six exomoon candidates are in the star systems known as Kepler Object of Interest (KOI) 268.01, Kepler 517b (KOI-303.01), Kepler 1000b (KOI-1888.01), Kepler 409b (KOI-1925.01), Kepler 1326b (KOI-2728.01) and Kepler 1442b (KOI-3220.01).

Research Report: "Exomoon Candidates from Transit Timing Variations: Six Kepler Systems with TTVs Explainable by Photometrically Unseen Exomoons"

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