The discovery of giant exoplanets around the planets Kepler-1625b and Kepler-1708b has raised questions. Just as we can assume that stars in the Milky Way have planets orbiting them, moons around these exoplanets should not be uncommon. This increases the difficulty of detecting them. To date, of the more than 5,300 known exoplanets, only two have been found to have moons.
Researchers discovered traces of such satellites for the first time in observations of the planets Kepler-1625b and Kepler-1708b by the Kepler and Hubble Space Telescopes. Now, a new study casts doubt on those previous claims. Scientists from the Max Planck Institute for Solar System Research and the Sonnenberg Observatory in Germany reported in the journal Nature Astronomy that the "pure planetary" explanation for the observations is more convincing.
During the analysis, the researchers used their newly developed computer algorithm Pandora, which facilitates and accelerates the search for external gas clouds. They also looked at what kinds of exomoons could in principle be discovered in modern space-based astronomical observations, and their answers were shocking.
Exomats: a rare phenomenon in observation
In our solar system, the fact that a planet is orbited by one or more satellites is the rule rather than the exception: with the exception of Mercury and Venus, every other planet has such companions; in the case of the gas giant Saturn, researchers have discovered 140 natural satellites so far. Therefore, scientists believe that planets in distant star systems may also have moons. However, so far, there are only two cases with evidence of the existence of such exomoons: Kepler-1625b and Kepler-1708b. This low yield is not surprising. After all, distant moons are much smaller than their parent stars and therefore harder to spot. Moreover, searching for evidence of moons in observations of thousands of exoplanets is extremely time-consuming.
To make searches easier and faster, the authors of the new study used a search algorithm called Pandora. They published their method last year, making the algorithm's open source code available to all researchers. When applied to observations from Kepler-1625b and Kepler-1708b, the results are surprising.
"We wanted to confirm the discovery of exomoons around Kepler-1625b and Kepler-1708b, but unfortunately our analysis showed the opposite," said MPS scientist Dr. René Heller, first author of the new study.
Five years ago, the Jupiter-like planet Kepler-1625b made headlines. Researchers at Columbia University in New York report that there is conclusive evidence that in its orbit is a massive moon with dimensions that would dwarf all satellites in the solar system. The scientists analyzed data from NASA's Kepler space telescope, which observed more than 100,000 stars and more than 2,000 exoplanets during its first mission from 2009 to 2013.
However, in the years following its 2018 discovery, the exoplanet candidate forced astronomers to play a cosmic version of hide-and-seek. First, after the Kepler data were cleaned of system noise, it disappeared. However, further observations with the Hubble Space Telescope revealed clues. Last year, this unusual exoplanet candidate got a companion: Another giant moon, much larger than Earth, orbiting the Jupiter-sized planet Kepler-1708b, according to researchers in New York.
The complex task of detecting exomoons
Dr Rene Heller explained: "Exomats are so far away that we cannot see them directly, even with the most powerful modern telescopes."
Telescopes record fluctuations in the brightness of distant stars in a time series called a light curve. The researchers then looked for signs of satellites in these light curves. If an exoplanet passes in front of a star, it will dim the star by a small amount as seen from Earth. This phenomenon is called a transit, and it repeats regularly with the cycle of a planet orbiting a star. The exoplanets accompanying this planet would have a similar dimming effect. However, its trace on the light curve will be much weaker.
Furthermore, this additional dimming in the light curve will follow a rather complex pattern due to the movement of the moon and planets around their common center of gravity. Additionally, there are other effects to consider, such as planetary-lunar eclipses, natural brightness changes in stars, and other sources of noise generated during telescope measurements.
Still, to detect the moons, the New York researchers and their German colleagues first calculated millions of "artificial" light curves, including the sizes, mutual distances and orbital directions of all possible planets and moons. An algorithm then compares these simulated light curves to observed light curves, looking for the best match. Researchers from the Universities of Göttingen and Sonneberg used their open source algorithm Pandora. The "Pandora" algorithm is optimized for searching for exoplanets and solves this task orders of magnitude faster than previous algorithms.
Still no trace of the satellite
In the case of Kepler-1708b, the German astronomers have now found that the absence of a moon can explain observations just as accurately as the presence of one. Michael Hippke from Sonneberg Observatory is one of the co-authors of the new study. "These data do not indicate the presence of an exomoon around Kepler-1708b."
There is a lot of evidence that Kepler-1625b does not have a giant moon either. Previously, Kepler and Hubble have observed the planet transiting in front of its star. German researchers now believe that the instantaneous brightness changes of a star across its disk, the so-called stellar edge dimming effect, have a crucial influence on the proposed exomoon signal.
This edge-darkening effect looks different when observing Kepler-1625b's parent star through the Kepler or Hubble telescopes. This is because Kepler and Hubble are very sensitive to the different wavelengths of light they receive. Researchers at the Universities of Göttingen and Sonneberg now believe that their modeling of this effect explains the data more conclusively than giant exomoons.
Their extensive new analysis also shows that exoplanet search algorithms often produce unreliable results. Time and time again they "discovered" a moon when in fact only a planet was transiting its host star. For a light curve like Kepler-1625b, the "miss-hit" rate may be around 11%. "The earlier claims by our colleagues from New York were the result of searches for moons around dozens of exoplanets," Heller said. "Based on our estimates, the unreliable results are not surprising at all, almost expected."
strange satellite
The researchers also used their algorithm to predict the types of actual exoplanets that could be clearly detected by the light curves of space missions such as Kepler. According to their analysis, current technology can only detect particularly large moons orbiting planets in wide orbits. Compared with the familiar solar system moons, these moons are oddities: at least twice the size of Ganymede, the largest moon in the solar system, and therefore almost as large as Earth.
"Future observations, such as the first exomoons discovered by the PLATO mission, are certainly going to be very unusual and therefore exciting to explore," Heller said.
Compiled source: ScitechDaily