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Illustration of Kepler-1625b with a moon: This is what the duo could look like if they exist

Photo: Illustratration: Dan Durda / AP

Kepler-1625b and Kepler-1708b were thought to be the home planets of the first exomoons ever discovered. Researchers have now re-analysed data and come to the conclusion that they may not be moons at all. The study was published in the journal Nature Astronomy.

"Exomoons are so far away that they cannot be seen and detected directly, even with powerful telescopes," said first author René Heller, according to a press release. Instead, telescopes record the brightness fluctuations of distant stars. In it, the researchers are looking for clues to moons. When an exoplanet passes in front of its star as seen from Earth, it obscures the star by a tiny fraction at regular intervals during this so-called transit. An exomoon would show up similarly.

Exoplanets are celestial bodies that lie outside our solar system. Astronomers count several thousand of them. On the other hand, there is little evidence of exomoons. In 2018, researchers from Columbia University in New York reported in the journal Science Advances about observations at a distance of 8000 light-years, which they believed to be the first known exomoon.

In the area of the constellation of the Swan, for example, they had looked at a star called Kepler-1625 with the Hubble telescope. This is slightly heavier than our Sun and has a radius almost twice as large. The scientists had noticed that the star – viewed from close to Earth – regularly dimmes several times. On the one hand, the exoplanet Kepler-1625b, a gas giant the size of Jupiter, is responsible for this. A moon orbiting the exoplanet may also be to blame for further darkening, it was said at the time. Other clues were provided by researchers about possible moons on Kepler-1708b.

However, experts from the Max Planck Institute for Solar System Research (MPS) in Göttingen and the Sonnenberg Observatory now write that a "moon-free" interpretation of the data is more obvious. For the analysis, the team used an algorithm that simplifies and accelerates the search for exomoons, according to the study's press release. The researchers applied the algorithm to the measurement data of Kepler-1625b and Kepler-1708b. "We would have liked to confirm the discovery of exomoons around Kepler-1625b and Kepler-1708b," Heller said. "Unfortunately, however, our evaluations show otherwise."

In the case of the planet Kepler-1708b, scenarios without a moon could explain the measurement data just as accurately as those with a moon, the study says. "The probability of a moon orbiting Kepler-1708b is clearly lower than previously reported," said co-author Michael Hippke. "The data do not suggest the existence of an exomoon around Kepler-1708b."

In the case of Kepler-1625b, there is also much to suggest that it does not have a moon, according to the team. The reason: In the original evaluation of the combined data from the telescopes, the center-edge darkening was not sufficiently taken into account. This refers to the phenomenon that less radiation reaches Earth from the edge of a star than from its center. For example, the edge of the sun appears darker in photographs.

Depending on whether the host star of Kepler-1625b is observed through the Kepler or Hubble telescopes, the center-edge dimming looks different. This is due to the range of wavelengths of the light that the telescopes process. Modelling this effect is a more coherent theory than a giant exomoon, the researchers argue.