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A debate has sparked among astronomers about the existence of exomoons, the natural satellites revolving around exoplanets that orbit a star outside the solar system.

In 2018, David Kipping, an assistant professor of astronomy at Columbia University, and his team thought they had detected the first exomoon around the exoplanet Kepler-1625b and named it Kepler-1625b I. In 2022, another team, including Kipping, appeared to have discovered a second such non-stellar extrasolar body around Kepler-1708b and aptly dubbed it Kepler-1708b I. 

However, Rene Heller, a scientist at Max Planck Institute for Solar System Research, and Michael Hippke of Sonneberg Observatory wrote in a paper published in Nature Astronomy, "The probability of a moon orbiting Kepler-1708b is clearly lower than previously reported. The data do not suggest the existence of an exomoon around Kepler-1708b." They said the same thing about Kepler 1625b. 

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David Kipping, Alex Teachey and a group of researchers from previous studies have responded to Heller and Hippke's claim. They wrote, "Recently, Heller & Hippke argued that the exomoon candidates Kepler-1625 b I and Kepler-1708 b I were allegedly 'refuted.'" According to them, Hellen and Hippke neglected data, eliminating the exomoon-supporting signal in the Hubble light curves for Kepler-1625b I. 

The two exoplanets at issue, Kepler-1625b and Kepler-1708 b are around 8,000 and 5,500 light years away from Earth, which is an extreme distance. Researchers used a technique similar to the transit method to detect these two exoplanets, initially captured by the Kepler Space Telescope.

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Exomoons are hard to detect, and the only evidence is light curves. However, these curves around exomoons are fainter than those around exoplanets. 

Different researchers employ different techniques, methods, models, and algorithms for the structural analysis of these faint exomoon light curves. Kipping and Teachey claim that Heller and Hippke made errors in their analysis and excluded critical information in their study.

"We demonstrate that their Hubble light curve exhibits ~20 per cent higher noise and discards 11% of the useful data, which compromises its ability to recover the subtle signal of Kepler-1625 b I," write Kipping and Teachey.

However, they added, "We begin by clearly stating, both exomoon candidates may not be real. Our original and continued claim is modest: these objects are candidates for which the data exhibits substantial but not entirely conclusive evidence in favour of exomoons."

Both studies show how challenging the detection of exomoons can be until there are major advancements in telescope technology. Kipping believes this debate between the two research teams is an opportunity to compare methods and conclude on the best approach to discover more about these exomoons. 

(With inputs from agencies)