Astronomers have always known that detecting moons around planets outside the solar system would be a minor feat, but current debate in planetary science circles shows how difficult it will be to detect these so-called exomoons.

The story begins in 2018, when astronomers including Columbia University associate professor of astronomy David Kipping believed they had discovered an exoplanet for the first time. The object was found around the exoplanet Kepler-1625b, a Jupiter-like world located about 8,000 light-years from Earth. It was detected for the first time using the Kepler space telescope.

Following the supposed discovery, the satellite Kepler-1625b was aptly named “Kepler-1625 b I”. This was later confirmed by data obtained from the Hubble Space Telescope. Then, in 2022, another team, including Kipping, appears to have found out. second exomoon, this time only with the help of the Kepler space telescope. This object is believed to have been detected orbiting Kepler-1708 b, a gas giant with a mass 4.6 times that of Jupiter, located approximately 5,400 light-years from Earth. The potential second echo had a name similar to the first; It was named “Kepler-1708 b I”.

These two moons were unlike anything else in the solar system. For example, both were larger than Earth, making them similar to a class of exoplanets called “mini-Neptunes.” In this context, the first exoplanets appeared to reflect some of the first discoveries of exoplanets, which included things scientists didn’t expect to see, such as Jupiter-sized worlds stuck in their stars and planets orbiting fast-spinning neutron stars. pulsars.

This seemed incredibly certain, although the discoverers of Kepler-1625 b I and Kepler-1708 b I—let’s call them the “pro-exomoon team”—kept an open mind about the existence of moons and expressed healthy skepticism to this day. After decades of searching exclusively for exoplanets, humanity has found its first outer moons.

This situation lasted until the end of 2023; A separate team of astronomers led by Max Planck Institute for Solar System Research scientist René Heller (the “no-echomoon team,” if you will) questioned the discovery of both economons in a paper published in the journal Nature. Astronomy. .

Kipping and his colleagues responded with a research paper defending their findings on the exomoon, now available in a preprint version of the arXiv article repository.

“I think Kepler-1625b and Kepler-1708b are extremely valid exomoon candidates, and I think we’ve made a strong case for that,” Kipping told Space.com. “So what do I think actually happened and why didn’t they reach the moon?”

Exomoon discrepancy

The technique used to detect these two exoplanets is similar to the transit method, which has accounted for more than 5,000 planets in the exoplanet catalog to date.

The transit method relies on detecting a small gap of light from a planet’s parent star, created when that world passes or “passes” across the face of that star from our vantage point in space. The same principle applies to echoons, albeit on a much smaller scale. If these moons are in the right position around the planet they orbit, this should cause a smaller dip in light as that planet passes in front of its star.

But such a small malfunction is a clue for the proexomoon team, suggesting the existence of Kepler-1625 b I and Kepler-1708 b I. However, this drop in light output resulting from the passage of the echo is so small that it cannot be seen directly. Instead, powerful computing software algorithms are required to extract this from telescope data.

Kipping says his “pro-exomoon” team and Heller’s “non-exomoon” team used the same data from the same telescopes, but the disappearance of Kepler-1625 b I and Kepler-1708 b I may have something to do with how the teams processed them. Data using the algorithms used.

Kipping told Space.com that the “non-exomoon” team may have missed Kepler-1708 b I because of the software they chose to analyze data from the Hubble and Kepler telescopes. While the software is related to the software he and the pro-exomoon team use, it is slightly different.

“The software packages we use are almost identical. Their packages are much newer. “This has only been around for a few years, the one we use has been around for about a decade, maybe even longer,” Kipping explained. “The real answer is that their algorithms can’t find the moon. “It’s not like the moon isn’t included in their data.”

Kipping also suggests that the non-exomoon team uses their generally very reliable software outside of the default mode, but rather in a mode that is sensitive to the number of steps used to process the data. This may explain why echoons were missed in their calculations.

Heller and his team proposed an effect called “stellar limb dimming” for the “no other” Kepler-1625 b I; This means that the edges of the star are darker than its center, which affects the proposed exomoon signal. Heller’s team suggests that the dimming on the star’s flank actually explains observations of the host star better than the dimming caused by the presence of the outer moon.

Kipping argues that this is not a valid argument against exomoon candidates because he and his “pro-exomoon” team had already described this type of limbic dimming when they first proposed the existence of Kepler-1625 b I.

“We took limb blackening into account in the original paper, so it’s not like we messed up and forgot about it,” he explained. It does not affect the argument in favor of an exomoon.

René Heller, principal investigator of the Exomoon debunking team, reviewed Kipping and his team’s report but was not convinced that Kepler-1625 b I and Kepler-1708 b I existed.

“I don’t see anything new in the paper that would change my opinion that Kepler-1625b and Kepler-1708b are planets without large moons. Their new paper essentially looks at our work and tries to find weaknesses in our reasoning. “This is a natural and welcome process in modern science, but in this case I don’t see any significant progress,” Heller told Space.com. “I reject the idea that our refutation of their claims disproves their rebuttal. I think the debate remains unresolved, and that’s okay. Let’s continue!”

Heller and Kipping agree to move on, at least for now.

Are Kepler-1625 b I and Kepler-1708 b too “weird”?

The only reason these ecomoons appear during transit is because they are massive objects the size of a mini-Neptune, which can have a diameter between 1.6 and 4 times that of the Earth. If they’re there, they’re huge.

Kipping believes this is part of what makes them so unusual that they cannot be widely accepted as the first exomoon discoveries.

“I think there’s a lot of doubt about Kepler-1625 b I and Kepler-1708 b I because they’re both weird. They both look like mini moons of Neptune, right? And everyone asks: “Who ordered this? “How does the universe create such strange things?” said. “And that argument is of no use. They are very damaged now.”

Kipping now plans to use the James Webb Space Telescope (JWST) to look for outer moons that are closer to the moons we see in the Solar System.

“My strategy, and perhaps the wrong strategy, is to see if we can get satellites with JWST that are similar to those like Io and Europa that are in our solar backyard,” he said. “Hopefully this will strengthen the confidence that we saw these exomoons back then and that they were really interesting and that we should go back and look at them again.”

Kipping makes clear that the disagreement between the team’s findings does not reflect badly on any group of scientists, regardless of whether the final answer is “ekomoon” or “no ekomoon.” Instead, it shows how difficult it will be to detect moons around planets outside the solar system until significant advances are made in telescope technology.

“It’s a very difficult task, and I can’t pretend otherwise. “When we look for echoeons, we’re looking for a signal that none of these telescopes are designed to detect,” he said. “We think we can extract such science from them. However, this pushes these instruments, especially Kepler and Hubble, to the extremes of their capabilities.

“So the decisions you make and the choices you make, how, what algorithms you use, how you process data, can make the difference between success and failure.”

Kipping believes this disagreement between the two teams gives exomoon scientists an opportunity to compare methods and find the best approach to hunt for these small distant objects. Hopefully, an improved method for detecting an exomoon could one day lead to these two groups of talented and passionate scientists coming together to find an exomoon they can agree on.