Does a Neptune-Size Exomoon Really Exist Around This Alien Planet?

An artist’s illustration of the exoplanet Kepler-1625b with its hypothesized moon, which is thought to be about the size of Neptune.
An artist’s illustration of the exoplanet Kepler-1625b with its hypothesized moon, which is thought to be about the size of Neptune. (Image credit: Dan Durda)

Six months after its unveiling, uncertainty still swirls around the first serious exomoon candidate. 

Last October, Columbia University astronomers Alex Teachey and David Kipping announced that they had spotted evidence of a Neptune-size satellite orbiting Kepler-1625b, a Jupiter-like world that lies about 8,000 light-years from Earth. 

The duo emphasized that the find, which was made using data gathered by NASA's Kepler and Hubble space telescopes, was tentative. Indeed, Teachey and Kipping described the newly detected object, known as Kepler-1625b I, as a candidate rather than the first confirmed exomoon.

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That caution was warranted, according to two new studies by independent research teams. One of these papers asserts that the detection was likely an artifact of data processing, and the other stresses that the available information and analyses do not support confirmation at this time.

Thanks to the new results, "we do have a pretty definitive answer — that it doesn't exist," said Laura Kreidberg of the Harvard-Smithsonian Center for Astrophysics and the Harvard Society of Fellows, the lead author of one of the new papers.

Teachey and Kipping don't share this view, however. In the coming days, Teachey said, the pair plans to submit a paper that, among other things, analyzes the work done by Kreidberg and her colleagues.

"Suffice it to say, we think the picture is a bit more complicated than is being reported," Teachey told via email. "That is, newer studies are not right by virtue of being new, and I think reporting along the lines of 'the moon isn't there after all' would be a bridge too far given the evidence in hand."

Signals in the light curve?

The recently deceased Kepler space telescope found alien worlds via the "transit method" — by noticing the tiny brightness dips caused when planets cross their host stars' faces from the spacecraft's perspective. (And Kepler did so very proficiently: the telescope is responsible for about 70% of the nearly 4,000 exoplanet discoveries to date.) These dips are evident in "light curves," which map out stellar brightness over time.

Teachey and Kipping noticed some odd deviations in the light curves generated by the 19-hour-long transit of Kepler-1625b, as seen by Kepler. So, the researchers further studied the system using Hubble's Wide Field Camera 3 instrument.

Hubble observed a transit in October 2017 — one that included two substantial anomalies, Teachey and Kipping said. The transit appeared to begin 1.25 hours too early, and the host star's brightness didn't return to normal for a spell after the planet had moved off of the stellar disk. The best explanation for these two features, the researchers said last fall, was the presence of a large moon orbiting Kepler-1625b. 

The two new studies took another look at the key Hubble data. For example, Kreidberg and her colleagues used a different data-processing "pipeline" — one that has proved its mettle repeatedly over the years, she said.

"I have been working on data analysis for this particular instrument on Hubble for about seven years now, so my pipeline has been put through the wringer," Kreidberg told 

No Neptune-size exomoon flowed through this pipeline.

"Even the hint of a signal that they saw doesn't hold up in the new analysis," Kreidberg said. She and her team have just submitted their paper to The Astrophysical Journal Letters. You can read it for free at the online preprint site

The other new study, led by René Heller of the Max Planck Institute for Solar System Research in Germany, did find a brightness dip consistent with the existence of a moon.  

However, "careful consideration of its statistical evidence leads us to believe that this is not a secure exomoon detection," Heller and his colleagues wrote in their paper, which was published this month in the journal Astronomy & Astrophysics

"We find that the exomoon hypothesis heavily relies on a chain of delicate assumptions, all of which need to be further investigated," the researchers wrote. "For the time being, we take the position that the first exomoon has yet to be detected, as the likelihood of an exomoon around Kepler-1625b cannot be assessed with the methods used and data currently available."

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Bumps in the road

Like Teachey and Kipping, the teams led by Kreidberg and Heller detected "transit-timing variations" — signs that something other than the host star may be tugging on Kepler-1625b gravitationally. 

This tug could come from a moon or an undiscovered sibling planet, Kreidberg said. But it's also possible that the signal is an artifact, the result of some type of data-analysis issue, she added.

Indeed, the Hubble data are very tough to interpret, because the transit observations pushed the limits of the Wide Field Camera 3 instrument, Kreidberg said. And she stressed that her work doesn't impugn the findings of Teachey and Kipping. 

The duo "did a meticulous job on a challenging dataset," she said. "This type of thing is natural and how science progresses. Any time you're on the cutting edge of making a measurement, there are bound to be bumps in the road."

Teachey agreed that the Hubble transit data are difficult to deal with. And he said that his and Kipping's forthcoming paper arrives at one of the same conclusions Kreidberg et al. did — that the way the Hubble data were processed led to the two teams' divergent results. Neither study finds fault with the other's analysis, Teachey added; multiple data pipelines can be used, as the three different papers make clear. 

He also emphasized that the Heller-led team spotted the exomoon signal.

"Therefore, at this point the moon signal has been both validated and called into question by other teams," Teachey said. "To me, that suggests that the existence of this moon is still very much an open question, and it warrants further study. I think 2:1 in favor of the moon signal does not invalidate this as a potential exomoon detection. Controversial, but not invalidated. More work is needed."

Both Kreidberg and Teachey stressed that there's no animosity between their teams. Indeed, Kreidberg discussed her study with Teachey and Kipping extensively before submitting it.

"We're pleased to see the continued interest in this system, and at the end of the day we're all trying to get to the truth, which is the most important thing," Teachey said.

Stay tuned!

Mike Wall's book about the search for alien life, "Out There" (Grand Central Publishing, 2018; illustrated by Karl Tate), is out now. Follow him on Twitter @michaeldwall. Follow us on Twitter @Spacedotcom or Facebook

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Mike Wall
Senior Space Writer

Michael Wall is a Senior Space Writer with and joined the team in 2010. He primarily covers exoplanets, spaceflight and military space, but has been known to dabble in the space art beat. His book about the search for alien life, "Out There," was published on Nov. 13, 2018. Before becoming a science writer, Michael worked as a herpetologist and wildlife biologist. He has a Ph.D. in evolutionary biology from the University of Sydney, Australia, a bachelor's degree from the University of Arizona, and a graduate certificate in science writing from the University of California, Santa Cruz. To find out what his latest project is, you can follow Michael on Twitter.