The potential exomoon orbits an exoplanet dubbed Kepler-1625b, which was spotted using NASA's Kepler space observatory in a recent study that identified 284 “transiting planets.” This is the term used for planets that pass between Earth and their home star, resulting in a dimming of the star's light from our point of view.
This transit method can tell astronomers quite a bit about the planets orbiting a given star, but it's much trickier to spot moons orbiting those planets. To get a closer look, the research team requested 40 hours of time with the Hubble Space Telescope, which is about four times as powerful as Kepler.
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By studying the transit data, the researchers were able to discern two signals that suggested the presence of an exomoon. First, they observed a slight reduction in the star's brightness, as the potential exomoon passed in front. About three hours after the planet dimmed the light from the star, Hubble recorded a second smaller decrease in the star's brightness. This is indicative of a moon "trailing the planet like a dog following its owner on a leash," Kipping said in a statement about the new data.
In addition to the transit dimming, the researchers also measured certain gravitational effects that the potential moon would have on its home planet. It seems that the anticipated transit of Kepler-1625b occurred nearly 80 minutes earlier than expected. This suggests that something is tugging on the planet, possibly a moon. The gravitational effect can be measured as transit timing variations, or TTVs, which provide a second method of proving the existing of an exomoon.
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Taken together, these two clues make for a promising lead, but professional astronomers are notoriously precise and careful. Before the exomoon is confirmed, the team plans to comb through data from previous Kepler-1625b transits. In other words, they're going to look at Hubble''s archival “tapes” and see if they can spot the moon from its previous trips throwing shade at the Earth.
Then they'll be closer to determining whether they’ve really found what they’re looking for.