Have astronomers just found the first-ever exomoon, a lunar companion of a planet orbiting another star? Definitely maybe.
Using data from NASA’s Kepler and Hubble space telescopes, Columbia University astronomers Alex Teachey and David Kipping report the potential signal of a Neptune-size moon around a planet three times heavier than Jupiter, all orbiting a nearly 10-billion-year-old sun-like star called Kepler 1625 b about 8,000 light-years from Earth. Such a large moon defies easy explanation based on prevailing theories. The findings appear in a study published October 3 in Science Advances, and follow from the duo’s earlier work reported last year that first offered more tentative evidence of the moon.
Lee Billings
I have been following the authors of this study for some time on Twitter – and YouTube – but unfortunately no significant confirmation of this hypothetical double planet has happened in the past two years. The setup is extraordinary by the standards of our own solar system, with a satellite larger than most of our planets, but massive objects are mush easier to detect, so it is natural for the first discovered exomoons to be huge.
I look forward to news about interesting system, but because the James Webb Space Telescope is being constantly delayed it may take years for a definite resolution. Personally I suspect that this configuration will be disproven by further observation, that some other explanation will arise for the peculiar light curve collected so far. The system could contain a pair of co-orbiting planets – although it might be even more unlikely for such a setup to form naturally than the proposed planet-moon pair – or a banal pair of planets in a close orbital resonance that causes the orbital parameters to shift periodically.
All in all, it is difficult to assign a precise probability to the reality of Kepler-1625b-I. Formally, the preference for the moon model over the planet-only model is very high, with a Bayes factor exceeding 400,000. On the other hand, this is a complicated and involved analysis where a minor effect unaccounted for, or an anomalous artifact, could potentially change our interpretation. In short, it is the unknown unknowns that we cannot quantify. These reservations exist because this would be a first-of-its-kind detection—the first exomoon. Historically, the first exoplanet claims faced great skepticism because there was simply no precedence for them. If many more exomoons are detected in the coming years with similar properties to Kepler-1625b-I, it would hardly be a controversial claim to add one more. Ultimately, Kepler-1625b-I cannot be considered confirmed until it has survived the long scrutiny of many years, observations and community skepticism, and perhaps the detection of similar such objects. Despite this, it is an exciting reminder of how little we really know about distant planetary systems and the great spirit of discovery that exoplanetary science embodies.
Alex Teachey & David M. Kipping
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