Exploring the Enigma of Cosmic Mysteries: Reimagining Massive Exomoons

The discovery of gigantic exomoons in the vicinity of Kepler-1625b and Kepler-1708b has cast a shadow of uncertainty.

Just as it is widely accepted that stars within our Milky Way are encircled by planets, the idea of moons orbiting these distant exoplanets should seem plausible. However, the quest to identify these moons has proven to be a formidable challenge. Out of over 5300 known exoplanets, only two have been confirmed to possess moons. A recent data analysis underscores the nuanced nature of scientific findings, revealing that beneath every assertion lies a degree of uncertainty, akin to a suspenseful narrative.

Delving into Exomoon Exploration

Previous observations of Kepler-1625b and Kepler-1708b by the Kepler and Hubble space telescopes initially unveiled hints of these elusive exomoons. However, a recent study raises doubts about these earlier claims. Researchers from the Max Planck Institute for Solar System Research and the Sonnenberg Observatory, both situated in Germany, report in the journal Nature Astronomy that interpretations pointing to the existence of exomoons may be less certain than previously thought.

To conduct their investigation, the scientists harnessed their newly devised computer algorithm named Pandora, designed to expedite the search for exomoons. They also contemplated the kinds of exomoons that modern space-based astronomical observations could theoretically reveal, and the results were rather surprising.

Exomoons: A Rarity in Sight

Within our own Solar System, planets are frequently accompanied by one or more moons, with Mercury and Venus being notable exceptions. For instance, the gas giant Saturn boasts an impressive 140 natural satellites. Consequently, scientists extrapolate that planets in distant star systems might similarly possess moons. However, definitive evidence of exomoons exists for just two planets—Kepler-1625b and Kepler-1708b. The scarcity of such findings is unsurprising, given that these distant satellites are significantly smaller than their host planets, rendering them exceptionally challenging to detect. Moreover, sifting through the extensive observational data of thousands of exoplanets in pursuit of moon evidence is a laborious endeavor.

Pandora: The Search Algorithm for Exomoons

To streamline and expedite this search, the researchers rely on their custom-made search algorithm, Pandora. This algorithm, published last year and available for use by all researchers as open-source code, yielded astonishing results when applied to the data from Kepler-1625b and Kepler-1708b.

Dr. René Heller, the lead author of the study, notes, “We would have liked to confirm the discovery of exomoons around Kepler-1625b and Kepler-1708b, but unfortunately, our analyses show otherwise.”

Unraveling the Celestial Game of Hide and Seek

Kepler-1625b, a planet akin to Jupiter, made headlines five years ago when researchers at Columbia University in New York presented compelling evidence of a massive moon in its orbit. However, subsequent investigations introduced a cosmic version of hide-and-seek as the exomoon candidate seemingly vanished from the Kepler data, only to reappear during further observations with the Hubble Space Telescope. Last year, the New York researchers reported the presence of another colossal moon orbiting Kepler-1708b, surpassing Earth in size.

Detecting Exomoons: A Complex Endeavor

Dr. René Heller emphasizes the challenge in directly observing exomoons, given their tremendous distance from Earth, even with the most advanced telescopes. Instead, telescopes record variations in the brightness of distant stars, creating a time series known as a light curve. Researchers scour these light curves for signs of moons. When an exoplanet passes in front of its host star from Earth’s perspective, it causes a slight dimming of the star’s brightness—a phenomenon known as a transit. An exomoon accompanying the planet would also induce a similar dimming, albeit fainter.

Moreover, the interplay between the moon and planet, revolving around their common center of gravity, generates a complex pattern of dimming. Additional factors such as planet-moon eclipses, variations in the star’s brightness, and other sources of noise during telescope measurements must be considered.

To detect exomoons, both the New York researchers and their German counterparts generated millions of simulated light curves, encompassing various planet and moon sizes, orbital distances, and orientations. An algorithm subsequently compared these simulated light curves with the observed data to identify the best match. The researchers from Göttingen and Sonneberg utilized their open-source Pandora algorithm, which proved significantly faster than previous algorithms in solving this intricate task.

No Traces of Moons

Regarding Kepler-1708b, the German researchers concluded that scenarios without a moon can explain the observational data as effectively as those including a moon. Michael Hippke from the Sonneberg Observatory and co-author of the study asserts, “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.”

Kepler-1625b’s situation is no different. Previous observations with the Kepler and Hubble telescopes captured the planet’s transits in front of its star. The German researchers argue that the instantaneous brightness variation of the star across its disk, known as stellar limb darkening, significantly influences the proposed exomoon signal.

The complex nature of limb darkening, varying depending on whether observed through Kepler or the Hubble telescope due to their sensitivity to different wavelengths of light, led the researchers to conclude that their modeling of this effect provides a more convincing explanation for the data than the presence of a massive exomoon.

Their extensive analyses further reveal that exomoon search algorithms often produce false-positive results. In cases like Kepler-1625b’s light curve, the rate of false positives is estimated to be approximately 11 percent. Dr. Heller comments, “The earlier exomoon claim by our colleagues from New York was the result of a search for moons around dozens of exoplanets. According to our estimates, a false-positive finding is not at all surprising, but almost to be expected.”

Unusual Satellites

Using their algorithm, the researchers also projected the types of exomoons that could be definitively detectable in light curves from space missions like PLATO. Their analysis indicates that only exceptionally large moons in wide orbits around their planets would be discernible with current technology. These moons, in comparison to the familiar moons within our Solar System, would be atypical, measuring at least twice the size of Ganymede, the Solar System’s largest moon and nearly as large as Earth.

Dr. Heller anticipates, “The first exomoons discovered in future observations, such as those from the PLATO mission, will undoubtedly be extraordinary and thus, highly intriguing to explore.”

Reference: “Large exomoons unlikely around Kepler-1625 b and Kepler-1708 b” by René Heller and Michael Hippke, 7 December 2023, Nature Astronomy.
DOI: 10.1038/s41550-023-02148-w

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