Artist’s impression of the exoplanet Kepler-1625b transiting the star with the candidate exomoon in tow. Image: Dan Durda

Scientists may have detected the first moon orbiting a planet outside our solar system, known as an "exomoon."

Why it matters: Scientists are hunting for exoplanets that could harbor conditions suitable for supporting life. Exomoons could also be hosts of alien life in some form, according to the authors of the new study, published today in the journal Science Advances.

The details: The moon is about the size of Neptune, according to the researchers, and is orbiting a Jupiter-sized gas giant planet known as Kepler-1625b, located about 8,000 light-years away from Earth. However, the study's authors caution that their findings, gleaned using data from two space telescopes, should not be viewed as a slam dunk case.

How they did it: The study's authors, Alex Teachey and David M. Kipping from Columbia University, identified the exomoon using NASA's Kepler space telescope's survey of transiting planets that pass between a star and an observer, which can cause a a temporary dimming of a star's light. They also utilized 40 hours of observations from the Hubble Space Telescope, which allowed them to view multiple transits of exoplanet Kepler-1625b and tested their hypothesis against data from both.

What they found: The initial data suggested that an exomoon may be orbiting Kepler-1625b, which Teachey and Kipping previously hypothesized in a 2017 study. However, the new study includes additional observations from Hubble, which is more powerful than Kepler.

  • This allowed them to study the transits more closely, revealing both a dimming of the star as Kepler-1625b passed in front of it and a timing variation in when the planet begins its transit.
  • These observations further suggested the existence of a large exomoon interacting with the planet.

"[T]he planet appears to transit one and a quarter hours too early, that’s indicative of something gravitationally tugging on the planet," Kipping said of the anomalies observed, and his interpretation of them.

"The second anomaly is an additional decrease in the star’s brightness after the planetary transit has completed. The location, shape and depth of this event appear consistent with a Neptune sized moon transiting in front of the star."
— David Kipping, Columbia University

What they're saying: René Heller, an astrophysicist with the European Space Agency who was not affiliated with the new study, said that the evidence is bolstered compared to the same researchers' first study on an exomoon orbiting this particular planet. However, he still has significant doubts about the findings.

  • "The new Hubble data has much better quality than the Kepler data, and the statistical evidence for an exomoon interpretation has formally increased. ... What I would have liked to have seen in this new study is a prediction of the planet-moon orbital configuration during one of the next transits — something to verify or falsify," Heller explained.

Thomas Zurbuchen, NASA's associate administrator, said the moon's size could require a rethinking of how moons form in the first place. "If confirmed, this finding could completely shake up our understanding of how moons are formed and what they can be made of," he said in a statement.

Yes, but: Teachey also urged caution in being too confident about the new findings. "The first exomoon is obviously an extraordinary claim and it requires extraordinary evidence," he said. "Furthermore, the size we’ve calculated for this moon, about the size of Neptune, has hardly been anticipated and so that too is reason to be careful here."

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