Several influences can create a lunar signal in a light curve, even without the presence of a real moon. Credit: MPS/hormesdesign.de
Only two of the more than 5,300 known exoplanets have so far provided evidence of moons orbiting them. During observations of the planets Kepler-1625b and Kepler-1708b carried out by the Kepler and Hubble space telescopes, researchers discovered traces of these moons for the first time.
A new study now raises doubts about these earlier claims. As scientists from the Max Planck Institute for Solar System Research (MPS) and the Sonnenberg Observatory, both in Germany, report today in the journal Natural astronomy“planetary only” interpretations of observations are more conclusive.
For their analysis, the researchers used their new computer algorithm Pandora, which makes searching for exomoons easier and faster. They also studied what types of exomoons can in principle be found in modern space astronomical observations. Their response is quite shocking.
In our solar system, the fact that a planet is orbited by one or more moons is rather the rule than the exception: apart from Mercury and Venus, all the other planets have such companions; In the case of the gas giant Saturn, researchers have discovered 140 natural satellites to date.
Scientists therefore consider it likely that planets located in distant star systems also harbor moons. So far, however, there has only been evidence of such exomoons in two cases: Kepler-1625b and Kepler-1708b. This low yield is not surprising. After all, distant satellites are naturally much smaller than their home worlds and therefore much harder to find. It takes an enormous amount of time to scour observational data from thousands of exoplanets for evidence of moons.
To make the search easier and faster, the authors of the new study rely on a search algorithm that they developed themselves and optimized for the search for exomoons. They published their method last year and the algorithm is available to all researchers as open source code. Applied to observational data from Kepler-1625b and Kepler-1708b, the results were astonishing.
“We would have liked to confirm the discovery of exomoons around Kepler-1625b and Kepler-1708b,” says the new study’s first author, MPS scientist Dr. René Heller. “But unfortunately, our analyzes show the opposite,” he adds.
Hide and seek of an exomoon
The Jupiter-like planet Kepler-1625b made headlines five years ago. Researchers at Columbia University in New York have reported strong evidence for a giant moon in its orbit that would dwarf all moons in the solar system. Scientists analyzed data from NASA’s Kepler space telescope, which observed more than 100,000 stars during its first mission from 2009 to 2013 and discovered more than 2,000 exoplanets.
However, in the years since the 2018 discovery, the exomoon candidate has forced astronomers to play a cosmic version of hide and seek. First, it disappeared after the Kepler data was cleaned of systematic noise. However, clues were found during subsequent observations with the Hubble Space Telescope.
And then last year, this extraordinary candidate for the exomoon had company: according to New York researchers, another giant moon much larger than Earth orbits around the planet Kepler-1708b, the size of Jupiter.
The right match
“Exomoons are so far away that we cannot see them directly, even with the most powerful modern telescopes,” explains Dr René Heller. Instead, telescopes record fluctuations in the brightness of distant stars, the time series of which is called a light curve. Researchers then look for signs of moons in these light curves. If an exoplanet passes in front of its star, as seen from Earth, it obscures the star by a tiny fraction.
This event is called transit and recurs regularly with the orbital period of the planet around the star. An exomoon accompanying the planet would have a similar dampening effect. However, its trace in the light curve would not only be significantly weaker.
Due to the movement of the Moon and planet around their mutual center of gravity, this additional attenuation of the light curve would follow a rather complicated pattern. And there are other effects to consider, such as planet-Moon eclipses, natural variations in the star’s brightness, and other sources of noise generated during telescopic measurements.
To detect the moons, however, the New York researchers and their German colleagues first calculate millions of “artificial” light curves for every conceivable size, mutual distance and orbital orientation of the possible planets and moons. An algorithm then compares these simulated light curves with the observed light curve and searches for the best match. The Göttingen and Sonneberg researchers used their open source Pandora algorithm, which is optimized for the search for exomoons and capable of solving this task several orders of magnitude faster than previous algorithms.
No trace of moons
In the case of the planet Kepler-1708b, the German duo has now discovered that scenarios without a moon can explain observational data as accurately as those with a moon. “The probability of a moon orbiting Kepler-1708b is clearly lower than previously reported,” says Michael Hippke of the Sonneberg Observatory and co-author of the new study. “The data do not suggest the existence of an exomoon around Kepler-1708b,” continues Hippke.
Many reasons suggest that Kepler-1625b also lacks a giant companion. Transits of this planet in front of its star have already been observed with the Kepler and Hubble telescopes.
German researchers now say that the instantaneous variation in the star’s brightness across its disk, an effect known as stellar limb darkening, has a crucial impact on the proposed exomoon signal. The edge of the solar disk, for example, appears darker than the center. However, depending on whether you look at Kepler-1625b’s home star through the Kepler or Hubble telescope, this limb darkening effect is different.
Indeed, Kepler and Hubble are sensitive to different wavelengths of the light they receive. The Göttingen and Sonneberg researchers now say their modeling of this effect explains the data more conclusively than a giant exomoon.
Their new in-depth analyzes also show that exomoon search algorithms often produce false positive results. Time and again, they “discover” a moon when in reality there is only a planet transiting its host star. In the case of a mild curve like that of Kepler-1625b, the “false hit” rate should be around 11 percent.
“The previous exomoon claim by our colleagues in New York was the result of a search for moons around dozens of exoplanets,” says Heller. “According to our estimates, a false positive result is not at all surprising, but almost predictable,” he adds.
Strange satellites
The researchers also used their algorithm to predict the types of actual exomoons that might be clearly detectable in lightcurve space missions like Kepler. According to their analysis, only particularly large moons orbiting their planet in a wide orbit are detectable with current technology.
Compared to the familiar moons of our solar system, they would all be bizarre: at least twice as large as Ganymede, the largest moon in the solar system and therefore almost as large as Earth. “The first exomoons that will be discovered in future observations, such as those from the PLATO mission, will certainly be very unusual and therefore exciting to explore,” says Heller.
More information:
René Heller et al, Large improbable exomoons around Kepler-1625 b and Kepler-1708 b, Natural astronomy (2023). DOI: 10.1038/s41550-023-02148-w
Provided by the Max Planck Society
Quote: Large exomoons unlikely around Kepler-1625 b and Kepler-1708 b, astronomers say (December 7, 2023) retrieved December 7, 2023 from
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