There are more than 200 moons in our Solar System, but their relatively small sizes make similarly sized extrasolar moons (exomoons) very hard to detect with current instruments. The best exomoon candidates so far are two nearly Neptune-sized bodies orbiting the Jupiter-sized exoplanets Kepler-1625b and Kepler-1708b, but their existence has been contested. In new research, astronomers reanalyzed the Hubble and Kepler data used to identify these two exomoon candidates.
An artist’s impression of the gas giant Kepler 1625b with its large moon, Kepler 1625b-i; the pair has a similar mass and radius ratio to the Earth-Moon system but scaled up by a factor of 11. Image credit: Sci.News.
From the discovery of Jupiter’s four principal moons in 1610 by Galileo Galilei, which triggered the Copernican revolution, to the discovery of cryovolcanism on Saturn’s moon Enceladus as evidence of continuing liquid water-based chemistry in the outer Solar System, moons continue to deliver fundamental and fascinating insights into planetary science.
The detection of moons around some of the thousands of extrasolar planets known today has, thus, been eagerly anticipated for over a decade now.
So far, two possible exomoon detections have been put forward, both of which had originally been claimed in data from NASA’s Kepler space mission.
The first candidate corresponds to a Neptune-sized moon in a wide orbit around the Jupiter-sized planet Kepler-1625b, which is in a 287-day orbit around the evolved solar-type star Kepler-1625.
The second exomoon claim has recently been announced by the same team. It is around the Jupiter-sized planet Kepler-1708b, which is in a 737-day orbit around the solar-type main-sequence star Kepler-1708.
“Exomoons are so far away that we cannot see them directly, even with the most powerful modern telescopes,” said Dr. René Heller, an astrophysicist at the Max Planck Institute for Solar System Research.
“Instead, telescopes record the fluctuations in brightness of distant stars, the time series of which is called a light curve.”
“Astronomers then look for signs of moons in these light curves. If an exoplanet passes in front of its star as seen from Earth, it dims the star by a tiny fraction.”
“This event is called a transit, and it re-occurs regularly with the orbital period of the planet around the star.”
“An exomoon accompanying the planet would have a similar dimming effect. Its trace in the light curve, however, would not only be significantly weaker.”
“Due to the movement of the moon and planet around their mutual center of gravity, this additional dimming in the light curve would follow a rather complicated pattern.”
“And there are other effects to be considered, such as planet-moon eclipses, natural brightness variations of the star and other sources of noise generated during telescopic measurements.”
In order to detect the exomoons, Dr. Heller and his colleague, Dr. Michael Hippke from Sonneberg Observatory, calculated many millions of ‘artificial’ light curves for all conceivable sizes, mutual distances and orbital orientations of possible exoplanets and their moons.
They then compared these simulated light curves with the observed light curve and looked for the best match.
They used their open-source algorithm Pandora, which is optimized for the search for exomoons and can solve this task several orders of magnitude faster than previous algorithms.
In the case of Kepler-1708b, the authors found that scenarios without a moon can explain the observational data just as accurately as those with a moon.
“The probability of a moon orbiting Kepler-1708b is clearly lower than previously reported,” Dr. Hippke said.
“The data do not suggest the existence of an exomoon around Kepler-1708b.”
“There is much to suggest that Kepler-1625b is also devoid of a giant companion.”
“Transits of this planet in front of its star have previously been observed with the Kepler and the Hubble telescopes.”
The researchers argue that the instantaneous brightness variation of the star across its disk, an effect known as stellar limb darkening, has a crucial impact on the proposed exomoon signal.
“The limb of the solar disk, for example, appears darker than the center,” they said.
“However, depending on whether you look at the home star of Kepler-1625b through the Kepler or the Hubble telescope, this limb darkening effect looks different.”
“This is because Kepler and Hubble are sensitive to different wavelengths of the light that they receive.”
“Our modeling of this effect explains the data more conclusively than a giant exomoon.”
The team’s paper was published in the journal Nature Astronomy.
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R. Heller & M. Hippke. Large exomoons unlikely around Kepler-1625b and Kepler-1708b. Nat Astron, published online December 7, 2023; doi: 10.1038/s41550-023-02148-w
