Scientists have long speculated that planets in distant star systems may have moons similar to those in our solar system. However, evidence of exomoons has been scarce, with only two cases, Kepler-1625b, and Kepler-1708b, providing potential traces of such moons. A recent study by researchers from the Max Planck Institute for Solar System Research and the Sonnenberg Observatory in Germany challenges these previous claims, suggesting that the observations can be more conclusively interpreted as planets without moons.

To investigate the presence of exomoons, the researchers developed a computer algorithm called Pandora, which streamlines and expedites the search for exomoons. They also explored the types of exomoons that could potentially be detected using modern space-based astronomical observations. Surprisingly, their analysis revealed that exomoons are likely to be much smaller and harder to find than their host planets if they exist.

In our solar system, it is common for planets to have one or more moons, except for Mercury and Venus. For instance, Saturn has 140 natural satellites. Given this, scientists believe that moons may also exist around planets in distant star systems. However, searching for exomoons is challenging and time-consuming, requiring combing through vast amounts of observational data from thousands of exoplanets.

To simplify and expedite the search, the researchers developed an open-source search algorithm called Pandora, which is available to all researchers. The results were unexpected when applied to the observational data of Kepler-1625b and Kepler-1708b. Instead of confirming the existence of exomoons, the analysis indicated that scenarios without moons could explain the data just as accurately.

The researchers explain that detecting exomoons is indirect, as they cannot be observed directly. Instead, telescopes record fluctuations in the brightness of distant stars, known as light curves. Researchers search for signs of moons by analyzing the dimming effect caused by an exoplanet passing in front of its star. If an exomoon is present, it will cause an additional, more complex dimming pattern in the light curve.

However, the researchers found that exomoon search algorithms often produce false-positive results, mistakenly identifying a moon when only a planet is transiting its host star. In the case of Kepler-1625b, the rate of false positives is estimated to be around 11%. The researchers argue that the previous claim of an exomoon around Kepler-1625b was likely a false positive due to the search for moons around multiple exoplanets.

Furthermore, the researchers used their algorithm to predict the types of exomoons that could be detectable using current technology. They found that only exceptionally large moons in wide orbits around their planets would be detectable. These moons would be at least twice the size of Ganymede, the largest moon in our solar system, and nearly as large as Earth. The researchers anticipate that the first exomoons discovered in future observations will be highly unusual and intriguing to explore.

In conclusion, while the existence of exomoons remains uncertain, the recent study raises doubts about the previous claims of exomoons orbiting Kepler-1625b and Kepler-1708b. The researchers suggest that other factors, such as unconstrained stellar limb darkening and stellar activity in the observational data, may explain the evidence for these exomoons. Further research and technological advancements will be necessary to confirm the existence of exomoons and explore their characteristics definitively.

The research was published in Nature Astronomy.

References

• Heller, R., & Hippke, M. (2023). Large exomoons unlikely around Kepler-1625 b and Kepler-1708 b. Nature Astronomy, 1–14. https://doi.org/10.1038/s41550-023-02148-w
• Max Planck Society. (2023, December 7). Large exomoons unlikely around Kepler-1625 b and Kepler-1708 b, astronomers say. Phys.Org; Max Planck Society. https://phys.org/news/2023-12-large-exomoons-kepler-astronomers.html