The researchers, a pair of astronomers at Columbia University, stressed that they only found evidence for the moon’s existence, not the moon itself. To help confirm their potential discovery, they needed that exacting hallmark of science: someone else to replicate their work.
Eventually, someone else tried—with mixed results.
Two separate teams have since delved into the same data. One could only replicate half the evidence. The other found the same signals the Columbia astronomers did, but won’t confirm there’s a moon there. For now, the existence of the exomoon remains uncertain.
“Frankly, I can’t tell you who’s right,” says Alex Teachey, the Columbia graduate student who led the initial study.
The story of the maybe-moon begins about two years ago, with Kepler, a NASA space telescope responsible for uncovering most of the known exoplanets. Before it ran out of fuel and shut down last year, the telescope absorbed the light from thousands of stars in the Milky Way. When something—such as a planet—passes in front of a star, it blocks a tiny fraction of the star’s light. Kepler could spot this faint, brief dimming.
Read: A stunning discovery in the search for moons beyond the solar system
Teachey and his colleague, David Kipping, were sifting through Kepler’s catalog for exoplanets that could have moons. One planet, Kepler-1625b, located about 8,000 light-years away from Earth, seemed more intriguing than the rest—there was something unusual about the light coming from its sun. Teachey and Kipping turned to an even more powerful instrument, the Hubble Space Telescope.
The Hubble observations recorded a dimming as the planet trekked across, as expected. But it began its journey earlier than expected, and the dimming was followed by a second, fainter dip in the light. To Teachey and Kipping, this signal meant that a moon trailed behind, its gravity tugging gently on the planet and shifting its course ever so slightly. An alien astronomer watching the Earth and moon pass in front of the sun would see these same types of blips, too.
The astronomers said the moon, if it existed, was likely the size of Neptune and made of gas. “It looks very convincing on this one detection, but it’s so strange compared to what moons are like in our own solar system that it’s kind of hard to believe it,” Kipping told me last year.
Laura Kreidberg, an astronomer at Harvard and Smithsonian’s Center for Astrophysics, wanted to see it for herself. Kreidberg isn’t an exomoon hunter; she studies the atmospheres of exoplanets, and has extensive experience analyzing Hubble observations. “I have been analyzing data like this for many years, and so I was really curious to see if I put it through my pipeline, if I would get the same answer,” Kreidberg tells me.
Kreidberg emulated the other researchers’ methods. “I did my best to reproduce their analysis as exactly as I could,” she says. Her team confirmed that the exoplanet arrived earlier than expected. But “I could not reproduce that little dip in the brightness that they attributed to the moon,” she says.