In our own Solar System, it's a common occurrence for planets to be accompanied by one or more moons. With the exception of Mercury and Venus, all other planets have such companions. Saturn, a gas giant, boasts an astonishing 140 natural satellites. Given this, scientists have long speculated that distant star systems may also harbor moons. Yet, the discovery of exomoons has proven to be exceptionally challenging. These distant moons are significantly smaller than their host planets, making them elusive in astronomical observations. Additionally, sifting through the vast amount of observational data for thousands of exoplanets to find evidence of moons is a laborious process.

To overcome these challenges and expedite the search for exomoons, the researchers developed a specialized computer algorithm called Pandora. This algorithm, which was published as open-source code, streamlines and accelerates the hunt for exomoons. When applied to the data from Kepler-1625b and Kepler-1708b, the results were unexpected.

Dr. Rene Heller, the first author of the study, expressed their initial intentions, stating, "We would have liked to confirm the discovery of exomoons around Kepler-1625b and Kepler-1708b." However, their analysis led to a different conclusion.

One of the exoplanets under scrutiny, Kepler-1625b, had garnered attention five years ago when researchers at Columbia University in New York reported strong evidence of a massive moon in its orbit, surpassing all moons in our Solar System in size. The discovery was made using data from NASA's Kepler space telescope, which observed over 100,000 stars during its mission from 2009 to 2013 and identified more than 2000 exoplanets. Subsequently, the exomoon candidate seemed to vanish after systematic noise was removed from the Kepler data. However, it reappeared in further observations with the Hubble Space Telescope. Last year, researchers from New York claimed the existence of another giant moon, even larger than Earth, orbiting the planet Kepler-1708b.

The challenge in detecting exomoons lies in their remote location, making direct observation impossible even with the most advanced telescopes. Instead, astronomers rely on the fluctuations in the brightness of distant stars, known as light curves, to identify potential exomoons. If an exoplanet passes in front of its host star as viewed from Earth, it causes a slight dimming of the star's light. This event, called a transit, occurs periodically in sync with the exoplanet's orbit. A moon accompanying the planet would have a similar dimming effect, but it would be considerably weaker. Detecting these subtle signals is complicated by various factors, including planet-moon eclipses, natural variations in the star's brightness, and other sources of noise during measurements.

To identify exomoons, researchers generate millions of simulated light curves, each representing different sizes, distances, and orbital orientations of potential planets and moons. An algorithm then compares these simulations to the observed light curve, seeking the best match. The German scientists from Gottingen and Sonneberg used their specialized algorithm Pandora, significantly accelerating the exomoon search compared to previous methods.

However, the study cast doubt on the existence of moons around both Kepler-1625b and Kepler-1708b. For the latter, the researchers found that scenarios without a moon could explain the observational data just as accurately. According to Michael Hippke from the Sonneberg Observatory, co-author of the study, "The data do not suggest the existence of an exomoon around Kepler-1708b."

Regarding Kepler-1625b, the German researchers argued that the variation in brightness of the star caused by the limb darkening effect played a crucial role in the proposed exomoon signal. Depending on whether observations were made through the Kepler or Hubble telescopes, this effect appeared differently due to their sensitivity to different wavelengths of light. The scientists believe that their modeling of this effect provides a more conclusive explanation for the data than the presence of a giant exomoon.

Furthermore, the study highlighted the tendency of exomoon search algorithms to produce false-positive results. In cases like that of Kepler-1625b, the rate of these "false hits" was estimated to be around 11 percent. Dr. Rene Heller emphasized, "A false-positive finding is not at all surprising, but almost to be expected."

The researchers also used their algorithm to predict which exomoons could be detectable with current technology. Their analysis indicated that only exceptionally large moons, orbiting at a significant distance from their planets, could be identified using existing methods. These exomoons, compared to the familiar moons of our Solar System, would be extraordinary, being at least twice the size of Ganymede, the largest moon in our Solar System and nearly as large as Earth.

Dr. Rene Heller concluded by stating, "The first exomoons that will be discovered in future observations, such as from the PLATO mission, will certainly be very unusual and therefore exciting to explore."

Research Report:Large exomoons unlikely around Kepler-1625 b and Kepler-1708 b

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