Astronomers have always known that discovering moons around planets outside the solar system wouldn’t be easy — but the current debate in planetary science circles shows just how difficult it is to discover so-called exomoons.
The story begins in 2018, when astronomers, including David Kipping, an assistant professor of astronomy at Columbia University, thought they had discovered the first extrasolar moon. The topic was found around the exoplanet Kepler-1625b, a Jupiter-like world located about 8,000 light-years from Earth. It was initially observed by the Kepler space telescope.
Upon its alleged discovery, Kepler-1625b was named “Kepler-1625 b I”. Later, this was further confirmed by data from the Hubble Space Telescope. Then, in 2022, another team, also including Kipping, appears to have scouted A.J second The outer moon, this time using the Kepler space telescope alone. This object was supposedly seen orbiting Kepler-1708 b, a gas giant planet located about 5,400 light-years from Earth and with a mass 4.6 times that of Jupiter. The second possible exomoon has a similar name to the first; It was named “Kepler-1708 b I.”
Related: This is not the moon! Scientists are skeptical about the proposed detection of the first exomoons
These two moons were unlike anything found in the solar system. Both showed sizes larger than Earth’s, for example, making them similar in size to a class of exoplanets called “mini-Neptunes.” In this respect, the first exomoons seem to mirror some of the first exoplanet discoveries, which included things scientists didn’t expect to see like Jupiter-sized worlds housing their own stars and planets orbiting rapidly rotating neutron stars called pulsars.
Although the discoverers of Kepler-1625 b I and Kepler-1708 b I—let’s call them the “pro-exomoon team”—remained open-minded about the existence of moons, and to this day continue to encourage healthy skepticism, it seemed certain that humanity had stumbled upon On its first exomoons after decades of determining the positions of only exoplanets.
That was until late 2023, when a separate team of astronomers led by solar system research scientist Rene Heller of the Max Planck Institute — the “no exomoon team,” if you will — cast doubt on the two exomoon discoveries in a paper published in the journal Nature Astronomy. .
Kipping and his colleagues have now responded, producing a paper that appears in the pre-print arXiv paper repository, and defending their exomoon findings.
“I think Kepler-1625b and Kepler-1708b are absolutely viable candidates for extrasolar existence, and I think we’ve shown that convincingly,” Kipping told Space.com. “So what do I think is really going on, and why haven’t they gotten to the moon?”
Paradox in the outer moon
The technique used to discover these two exoplanets is similar to the transit method, which has brought together a large group of more than 5,000 planets in the exoplanet catalog so far.
The transit method relies on detecting a slight dip in light coming from a planet’s parent star, created when that world crosses or “crosses” the face of that star from our view of the universe. The same principle applies to exomoons, although on a much smaller scale. If these moons are in exactly the right position around the planet they are orbiting as that planet transits its star, this should also cause a reduction in light, though less significant.
However, such a small drop in light is the evidence that points to the presence of Kepler-1625 b I and Kepler-1708 b I to the pro-exomoon team. However, because this drop in light output caused by a transit of one of the outer moons is so small, it cannot be seen directly. Instead, it requires powerful computer algorithms to extract them from telescope data.
Kipping says that both 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 could be due to how the two teams worked. Deal with that data through the algorithms used.
Kipping told Space.com that the “no-exomoon” team may have missed Kepler-1708 b I because of the software they chose to analyze the Hubble and Kepler telescope data. Although the program is related to the one he and the ‘exomoon’ team used, it is still a little different.
“The software packages we used are almost twins of each other. Their package is much newer. It has only been around for a couple of years, while the package we use has been around for about ten years or so, maybe more,” Kipping explained. “The real answer is that their algorithm is not able to find the moon. “That doesn’t mean the moon isn’t in their data.”
Kipping also suggests that the “no-exomoon” team used its software, which is usually very reliable, outside of its default mode, but rather in a mode that was sensitive to the number of steps used to process the data. This may explain why exomoons were missed during their calculations.
For Kepler-1625 b I, Heller and his team proposed a “non-exomoon” effect called “stellar limb dimming,” which means the edges of the star are darker than its center, affecting the proposed exomoon signal. Heller’s team argues that the darkening of the stellar limbs would actually explain observations of the parent star better than the darkening caused by the presence of an exomoon.
Kipping argues that this is not a valid case against exomoon candidates because he and his “pro-exomoon” team had already taken into account such darkening of the fringes when they first proposed the existence of Kepler-1625 b I.
“We took into account the blackness of the edges in the original paper, so it’s not like we messed up and forgot about it,” he explained. “I think that puncture point is kind of a red herring in my book; “It does not affect the argument for an exomoon.”
Rene Heller, principal investigator of the exomoon debunking team, looked at the paper presented by Kipping and his team, but was not impressed by the belief in the existence of Kepler-1625 b I and Kepler-1708 b.
“I don’t see anything new in the paper that would change my opinion about Kepler-1625b and Kepler-1708b being planets without large moons. Their new paper is mostly about our work and trying to find weaknesses in our line of thinking. This is what it is,” Heller told Space.com. com: “It is a natural and very welcome process in modern science, although in this case I do not see any significant progress.” “I refute the idea of their refutation to refute their claim. I think the debate is still unresolved, and that’s okay. Let’s move forward!”
The idea of moving forward is something Heller and Kipping agree on, at least for now.
Are Kepler-1625 b I and Kepler-1708 b so strange?
The main reason these exomoons appear in transit mode is that they are massive objects the size of mini-Neptune that can range from 1.6 times to 4 times the diameter of Earth. If they’re there, they’re there huge.
Kipping thinks this is part of what might make them so unusual that they cannot be widely accepted as the first exomoon discoveries.
“I think there’s a lot of skepticism about Kepler-1625 b I and Kepler-1708 b I because they’re weird. They’re both like these little moons the size of Neptune, right? And everyone’s wondering, who ordered this? How does the universe make such weird things,” he said. “So this argument doesn’t help at all. “They are very damaged now.”
Kipping now intends to use the James Webb Space Telescope (JWST) to search for exomoons that are closer to the moons we see inside the solar system.
“My strategy, and perhaps the wrong strategy, is let’s see if we can get some more familiar moons with the James Webb Space Telescope, which are similar to moons in our solar backyard like Io and Europa,” he said. “Hopefully this will build confidence that we saw these exomoons at the time, that they were really interesting, and that we should go back and look at them again.”
Kipping is very clear that the disagreement between the team’s conclusions does not reflect poorly on either group of scientists regardless of the final answer, “exomoon” or “no exomoon.” Instead, it simply shows how challenging it will be to detect moons around exoplanets until significant advances in telescope technology are achieved.
“It’s very difficult work, and I won’t pretend otherwise,” he said. “When we look for exomoons, we are looking for a signal that none of these telescopes were designed to detect.” “We think we can get this kind of science out of them. But it pushes these instruments, especially Kepler and Hubble, to the limits of what they’re capable of doing.”
“So the decisions you make and the choices you make about how you use algorithms, how you handle data, can make the difference between success and failure.”
Kipping believes this disagreement between the two teams provides exomoon-hunting scientists the opportunity to compare methods and converge on the best approach to hunting these small, distant objects.
Hopefully, an improved method for exomoon detection will one day unite these two groups of talented and enthusiastic scientists around an exomoon discovery they can agree on.