Sky & Telescope - September 2020 - 37

the team analyzed the results from Hubble, they predicted
could be stellar activity or another planet. But the candidate
that the next time the planet transited its host star, the
withstood every test.
moon would be on the opposite side of its orbit, with a transit
"We really felt like we had turned over every stone that we
preceding that of the planet itself. So they put in a second
could possibly think of," Teachey says. "We wanted to be at
least as hard-nosed about this as anyone else. Because it really Hubble proposal, but the committee denied it.
"That would have been, like, a complete slam dunk," Kipdoes us no good to put out a sensationalist conclusion that's
ping says. "At that point, it would have been a discovery."
not going to hold up to scrutiny."
Unfortunately, it was the team's only opportunity. The
The team decided the best way to bolster the discovery was
more you project these planetary system models into the
to train the Hubble Space Telescope on the star for 40 hours
future, the greater the uncertainty. "So if we are going to look
in October 2017. And sure enough, Hubble, whose sensitivity
for it again, we kind of have to get back to scratch and do a
is four times greater than Kepler's, spotted a secondary dip in
completely blind search," Kipping says. "It's not even guaranlight after the planet crossed the star. Moreover, the planet
teed to transit."
also started its transit 77.8 minutes ahead of schedule - a
Instead, the existence of Kepler-1625b I will remain an
sign that something was tugging gravitationally on the planet.
open question.
Both signals were consistent with an exomoon, but
"It's a very frustrating situation," Kipping says. "I really
Kipping and Teachey remained circumspect. As did other
tore my hair out many a night thinking about that - how
astronomers.
"The data wasn't a slam dunk," says Laura Kreidberg (Cen- if we'd gotten that observation, at least then we would have
known, even if we hadn't seen the moon. I could have been
ter for Astrophysics, Harvard & Smithsonian). "It was kind
able to sleep at night thinking, 'Okay, we can move on.'"
of right on the edge of signiﬁcance. And so that tripped some
wires of like, 'Okay, let's put on our skeptical-scientist hat
and make sure this thing is real.'"
Harboring Secrets
Kreidberg reviewed the data using her own analysis and
Other claims have been even more difﬁcult to verify.
conﬁrmed the planet's early transit, but she didn't detect the
In 2013, David Bennett (then University of Notre Dame)
moon's secondary dip. The results left her unconvinced that
and his colleagues discovered what looked like a planet ﬂoatthe moon exists.
ing freely through the galaxy. Though typically invisible, the
Meanwhile, René Heller (Max Planck Institute for
exoplanet brieﬂy revealed itself when it happened to pass in
Solar System Research, Germany) and his colleagues confront of a background star - a chance alignment that caused
ﬁrmed both the early transit and the secondary dip. But the
the foreground planet to act as a magnifying glass to focus
researchers argue that it could be due to a second unseen
and brighten the distant star's light. A small anomaly in the
planet rather than an exomoon.
spike of light appeared to come from a moon in tow behind
Today, Heller remains uncertain. "I have my doubts,
the planet. But because the discovery was based on a
but I wouldn't be surprised if it turns out that it
one-time event, it's impossible to conﬁrm.
really is a moon," he says.
Then in 2015, Matthew Kenworthy
There is only one way to settle the issue:
(Leiden Observatory, The Netherlands) and
more data. To transform the candidate
Eric Mamajek (then University of Roch"That tripped some
into a discovery, the team needed to
ester) claimed that an exomoon might
wires of like, 'Okay,
make a prediction based on their model,
have carved a gap in the ring system
let's put on our skeptiand then verify that prediction.
surrounding a distant Saturn-like exocal-scientist hat
It should have been easily done. After
planet. But observers have yet to see the

and make sure this
thing is real.'"
-LAURA KREIDBERG

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Sky & Telescope - September 2020

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