Doubts about Exomoon candidates: sky and telescopes

Exomoons, the putative moons of exoplanets, have so far eluded detection. But our own solar system is packed with moons, the largest of which are about 40% the diameter of Earth. Several are warmed by tidal heating and are believed to harbor underground oceans beneath their icy crusts. Therefore, it seems reasonable to infer that moons are also common in other star systems. This is a tantalizing prospect, as some of these satellites could be, if not inhabited, at least habitable.

However, astronomers have so far only found evidence pointing to a few exomoons, and a recent study casts doubt on two of those detections.

When exoplanets transit in front of their host stars, they block some of their light. By measuring these dips in starlight, astronomers can build a picture of the objects causing partial eclipses. Most exoplanets have been found using this technique. transit photometry. Moons could also be found this way, but because their falls can occur before, during, or after those of their planets, they require additional statistical investigation.

Six years ago, a team of astronomers announced the discovery of an exomoon candidate orbiting the Jupiter-sized Kepler 1625b, 7,800 light-years away. Based on Kepler data, this discovery received support from a later Hubble observation. Two years ago, another team with some of the same researchers identified another exomoon candidate orbiting a different giant world, Kepler 1708b, 5,600 light years from Earth. On both occasions, the researchers urged caution in interpreting their results.

Read the September 2020 issue's”The search for the first exomoons” for more information on these (and other) initial detections.

Now, however, a study in Nature Astronomy casts doubt on those exomoon claims, offering an alternative interpretation of the data. Astrophysicist René Heller (Max Planck Institute for Solar System Research, Germany) and citizen scientist Michael Hippke (Sonneberg Observatory, Germany) reanalyzed previous observations, with the help of a new and improved code called Pandora that they developed to model exomoon transits.

For both exomoon candidates, they made 128 models that had only one planet transiting the star, as well as another 128 models that included both a planet and its moon. Both models were based on the known parameters of the exoplanets in question.

Comparing these models, they conclude that Kepler would only have been able to detect moons in wide orbits around exoplanets, specifically orbits beyond 30% of the Earth's surface. hill radius, the limiting distance within which a planet's gravity is stronger than that of its star. Kepler was not designed to detect moons that orbit much closer than that, as the large moons of our solar system do.

However, the supposed exomoon of 1625b orbited very close to its planet in all the transits that Kepler captured.

The new study proposes that the original team could have overestimated the supposed exomoon signal due to dimming of the stellar member, the darkening of a star at its edges. This effect gives objects in transit the appearance of having different sizes, shapes and depths, observed as the “black drop” effect during the transit of Venus and therefore makes it more difficult to separate the signal of a planet from any additional signals.

Heller and Hippke also found that the high confidence that previous astronomers had attributed to exomoon signals depended on the method they had used to eliminate interference, such as that of passing clouds or the vibrations of a telescope. This de-trend It's a challenge, since light from distant stars has all kinds of imperfections, caused by star spots and other random variations. In this case, detrending could have accidentally injected moons into the light curve where none actually existed.

“These moons are extremely difficult to detect, as their signature is very subtle,” says Johanna Vos (Trinity College Dublin), who was not involved in the study. She stresses the importance of each exomoon candidate being “thoroughly investigated, preferably by multiple teams using independent techniques.” She also praises studies like this one for developing “advanced tools to detect and validate potential exomoon signatures.”

Brian Jackson (Boise State University) also calls the study “important and impactful.” And he adds: “The analysis seems rigorous and complete.” The next step in the search for exomoons is to collect more data, he says.

That data may need to come from new instrumentation that can detect exomoons transiting at smaller distances from their host stars. Such visual evidence would put an end to speculation about exomoons, but Heller and Hippke do not believe it can be obtained in either the Kepler archive or the upcoming PLATO exoplanet survey, scheduled for launch in 2026.

Jackson admits surprise at the lack of exomoon detections so far, but notes, “If there's one thing astronomers have learned to expect, it's to expect surprise.”

For the moment, in any case, this is where things stand: despite living in a solar system full of moons, our ability to detect moons elsewhere in the universe remains limited.