Mining data from the Kepler space telescope, a group of researchers found a candidate for an exomoon, that is, a natural satellite orbiting a planet outside the Solar System. It’s only the second such detection to date, and astronomers have come pretty close to confirming its existence (99% chance it’s real), but still caution is the watchword.
The work was led by David Kipping of Columbia University in New York and focused on a sample of 70 gas giant planets discovered by Kepler that are in long orbits around their star, lasting more than a year.
A judicious process followed, with statistical analysis of observed planetary transits (marked by the temporary dimming of parent stars as the planet passed in front of them) for signs of a companion exomoon.
The basic method was to contrast the light curve observed by Kepler with that expected if there were a planet and a moon in the same transit. This procedure alone reduced the 70 to 11 possible detections. New statistical tests were applied to the remaining ones, which left only 3, then submitted to specific analyzes for each of them. And that left only one that scientists could not rule out at all: the Jovian planet Kepler-1708 b.
It would have a diameter slightly smaller than that of our Jupiter (90%), and the best fit between models and the observed light curves suggests that it has an exomoon with a diameter 2.6 times that of Earth. It would be a much larger natural satellite than any we’ve ever seen in our Solar System, which looks pretty bizarre and challenges our models of moon formation.
On the other hand, it appears as good company when alongside the only other known candidate exomoon, found in 2018 (by the team of the same David Kipping). It would orbit the gas giant planet Kepler-1625 b, slightly larger than Jupiter, and would be the size of Neptune, that is, about 4 times that of Earth.
Although the false positive rate for the analysis performed by the researchers is a modest 1%, they recall that 70 targets were analyzed in total, which makes the risk of error of 1% not negligible. Like the Kepler-1625 b exomoon, the Kepler-1708 b exomoon should continue to be classified as a candidate until more conclusive data can be obtained.
Still, the fact that there are two similar cases indicates that we may indeed be facing exomoons. Although formation models have difficulty explaining them, it is worth remembering that the same happened with the discovery of the first exoplanets, Jupiter-sized worlds in ultra-short orbits. In the end, they were real. The same can happen with these large exomoons, which bring with them another intriguing possibility: that some of them, orbiting planets in the habitable zone of their star, may be similar to Earth.
This column is published on Mondays, in Folha Corrida.
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