Further doubts have been raised about the exomoon candidates

In 2017, the astronomy world was buzzing advertisement This exoplanet, Kepler-1625b, likely has its own moon, an exomoon. It was the first hint anyone had seen of an extrasolar moon, and it followed five years later Another filter About the planet Kepler-1708b.

More than five thousand exoplanets have been discovered so far, and we don't know for sure whether any of them have moons orbiting them, which is what made these announcements so exciting. Exomoons provide habitable zones in which we can search for extraterrestrial life, and studying the moons can serve as a valuable window into the composition of the host planet.

But there has been a lot of debate about these extrasolar candidates, as multiple groups have combed through data obtained from the Kepler and Hubble space telescopes.

the Latest paper On this topic, published by astronomers in Germany, they came to the conclusion that extrasolar candidates around Kepler-1625b and Kepler-1708b are unlikely. previous job It also cast doubt on the exomoon candidate around Kepler-1625b.

This is not a clear-cut case, though. David Keeping, leader of the group that made the original discoveries and an assistant professor of astronomy at Columbia University, disagrees with the new analysis. He and his group are in the process of preparing a manuscript that responds to the latest publications.

Needle in a haystack

The most common method for discovering exoplanets is the transit method. This technique measures a star's brightness, looking for a small drop in brightness that corresponds to a planet passing in front of the star.

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Stellar photometry can be extended to search for exomoons, an approach pioneered by Kipping. In addition to the main dip caused by the planet, if the Moon is orbiting the planet, you should be able to see an additional, smaller dip caused by the Moon also blocking some of the star's light.

An example of what transit detection of an exomoon might look like.

Because moons are smaller, they generate a smaller signal, making them more difficult to detect. But what makes this particular case more difficult is that the host stars Kepler-1625 and Kepler-1708 are not that bright. This makes the light dip much fainter, and in fact these systems would have to have large moons to be within the threshold of what the Kepler space telescope can detect.

Models, models, models

Until scientists get more data from James Webb, or future missions like ESA's Plato At launch, it all comes down to what they can do with the existing numbers.

“The relevant aspects here are how the data itself is processed, what physics you factor in when modeling that data, and then what potential false positive signals might be present that could reproduce the type of signal you are looking for,” Eamonn CairnsA senior lecturer in astronomy at the University of Manchester, who was not involved in the study, said: Ars. “I think this whole debate is fundamentally about these questions,” he added.

One key phenomenon that needs careful modeling is known as the stellar limb darkening effect. Stars, including our Sun, appear dimmer at their edges compared to the center due to the effects of the stellar atmosphere. Since this affects the apparent brightness of the star, it is important to understand this in the context of searching for exomoons by measuring the star's brightness.

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“We have models for this, but we don't know exactly how a particular star behaves in terms of the stellar limb darkening effect,” he said. Renee HellerThe study's lead author and an astrophysicist at the Max Planck Institute for Solar System Research, in an interview with the journal Ars. It is possible to infer how particular stars behave, but this is not always trivial. By including improved models of star tip dimming, the researchers found they could explain signals previously attributed to an exomoon.

Data processing is also crucial, especially the type of processing known as scraping. This takes into account long-term variation in brightness data caused by stochastic stellar variation and instrument variability, among others. New research shows that the statistical outcome, whether it's a moon or no moon, depends largely on how this trend plays out.

Furthermore, the authors say that data obtained from the Hubble Telescope, which is primarily the source of the Moon claim around Kepler-1625b, cannot be properly oriented and therefore should not be relied upon in exomoon searches.

Two sides

Until more data is obtained, this will likely remain an ongoing scientific debate without any final conclusion.

Kerins points out that Kipping and his team were very thoughtful in their advertising. “They're very, very careful not to claim this as a cast-iron discovery. They've done exhaustive testing of the data that was given to them, and I really think the difference here has to do with the physics that you put in, how you process the data, and ultimately the fact that the Kepler data set is really on the edge Find exomoons.

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But Heller remains unconvinced. “My impression is that in the Kepler data, we and other teams have also done what is currently possible, and nothing compelling really stands out.”

The number of moons far outnumbers the planets in our solar systemTwo hundred and ninety-eight So far, so it's reasonable to assume that we'll encounter extrasolar moons as we continue to explore the sky. “I think it would be pretty extraordinary if we kept going over the next few years and couldn't find an extrasolar moon,” Kerins said. “I think it's only a matter of time.”

Physical Astronomy, 2023. DOI: 10.1038/s41550-023-02148-s

Ivan Ball is a freelance writer based in the UK, with a PhD in cancer research. He's on Twitter @ivan_paul_.

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