The first suspected moon found beyond our solar system may not be quite as it seems. A team of astronomers say the data behind the initial discovery is inconclusive, and suggest an unseen planet is a more plausible explanation.
In 2017 astronomers David Kipping and Alex Teachey from Columbia University in New York said they had found evidence for an exomoon orbiting the exoplanet Kepler-1625b, located about 8,000 light-years from Earth. In 2018, the pair firmed this up, using data from the Kepler and Hubble space telescopes to suggest a moon the size of Neptune could be orbiting this gas giant planet, itself about the size of Jupiter.
But Ren茅 Heller from the Max Planck Institute for Solar System Research in Germany and his colleagues disagree. Astronomers often look for exoplanets by watching for a dip in a star鈥檚 light as the planet passes, or transits, in front. Exomoons should show up as an even smaller dip on top of that. Heller says that the dip attributed to Kepler-1625b鈥檚 moon could instead be a statistical anomaly.
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A second key piece of evidence, namely that the planet crossed its star 78 minutes earlier than expected, hinting at a moon giving it a gravitational boost, could be attributed to an unseen hot, Jupiter-like world听orbiting more closely to the star, says Heller.
鈥淭he exomoon might be there, but it鈥檚 not something you can conclude from the data,鈥� says Heller. 鈥淚 think we are really seeing artefacts here. I鈥檓 maybe 70-30 against an exomoon.鈥�
If such an inner planet does exist, it would orbit at an angle and not transit the star from our point of view, which is why we hadn鈥檛 spotted it so far. It would also be very close to the star: about three percent of the distance between Earth and the sun. Heller鈥檚 team plan to look for gravitationally-induced wobbles in the star鈥檚 position that may suggest the presence of such a planet. If it鈥檚 not there, that would work in the exomoon鈥檚 favour.
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鈥淭his new study agrees with our own conclusion that the current data should not be interpreted as a secure exomoon detection at this time,鈥� says Kipping. He thinks that an unseen planet can鈥檛 explain the moon-like dip, but hopes that follow-up observations will clear things up.
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