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The potential exomoon revolves around an exoplanet dubbed Kepler-1625b, which was spotted by NASA's Kepler Space Observatory in a recent study of 284 "planets in transit". This term is used to refer to the planets that pass between the Earth and their star attenuating the star's light from our point of view.
This method of transit can say a lot about astronomers about planets orbiting a given star, but it is much harder to spot moons in orbit around these planets. To take a closer look, the research team asked for 40 hours of time using the Hubble Space Telescope, which is about four times more powerful than Kepler.
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By studying the transit data, the researchers were able to discern two signals suggesting the presence of an exomoon. First, they observed a slight reduction in the brightness of the star, the potential for exomoon being passed in front. About three hours after the planet dimmed the starlight, Hubble recorded a second smaller decrease in star brightness. This is indicative of a moon "stalking the planet like a dog following his owner on a leash," Kipping said in a statement on the new data.
In addition to transit attenuation, the researchers also measured some gravitational effects that the potential moon would have on its original planet. It seems that the expected transit of Kepler-1625b took place nearly 80 minutes earlier than expected. This suggests that something is pulling on the planet, maybe a moon. The gravitational effect can be measured in the form of transit time variations, or TTV, which constitute a second method to prove the existence of an exoon.
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Taken together, these two clues are promising, but professional astronomers are notoriously precise and cautious. Prior to confirmation of the exomoon, the team plans to review data from previous Kepler-1625b transits. In other words, they are going to look at Hubble's "tapes" of archives and see if they can see the moon from his previous travels by casting a shadow over the Earth.
Then they will be closer to determining if they have really found what they are looking for.
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