My first Magazine Sky & Telescope - 01.2019 | Page 12

NEWS NOTES
Hubble Boosts Case for Exomoon
ASTRONOMERS REPORTED last year
that they might have found the fi rst
known moon outside our solar system
(S&T: Dec. 2017, p. 13). Now, those
same researchers think their claim just
got stronger. New observations from the
Hubble Space Telescope lend credence
to the idea that a moon roughly the size
and mass of Neptune orbits the Jupiter-
size exoplanet Kepler-1625b.
“I think we have to be very, very cau-
tious,” says Stephen Kane (University
of California, Riverside). “But it is very
encouraging.”
Alex Teachey and David Kipping
(Columbia University) got their fi rst
hints of an exomoon after looking
through data on 284 planetary systems
from the Kepler space telescope. The
duo found one that looked promising: a
gas giant that passes in front of its star
every 287 days, paired with a hint of a
second, smaller transit from a potential
moon. But they needed more data to be
sure. So they used the Hubble Space Tele-
scope to look in October 2017, when the
planet was expected to transit again.
They weren’t disappointed. The
hoped-for second transit came a few
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JA N UA RY 2 019 • SK Y & TELESCOPE
hours after the exoplanet’s. What’s
more, the planet started its transit
earlier than expected, another potential
sign of a planet and moon orbiting each
other and changing places with time.
The team published their fi ndings Octo-
ber 3rd in Science Advances.
“[They] did a marvelous job of
examining the data,” says René Heller
(Max Planck Institute for Solar System
Research, Germany). As to the existence
of the exomoon, “I remain skeptical.”
It’s not that exomoons are unlikely.
And it’s not that the signal is too small
to be reliable. Being a Neptune-size
world, the putative moon blocks an
amount of starlight that’s on par with
many exoplanets already discovered.
Heller stresses instead that it is critical
to understand all the potential sources
of noise in the data — the telescope, the
instruments, and the star itself.
The researchers do address many of
these issues. One test was to look for
the transit in multiple wavelengths. If
the star were fl ickering, then red and
blue starlight would dip by different
amounts. But these transits look the
same in all colors.
An artist’s impression of
exoplanet Kepler-1625b
transiting its star with
moon in tow
There is also the matter of the exo-
moon’s unusual size. While the ratio
in masses between the moon and the
planet is about the same as that of our
Moon and Earth — about 1% — the
entire system is roughly 1,000 times as
massive as ours.
“[This] is so extraordinarily different
from what we’re used to,” Kane says.
“It raises a whole lot of questions about
how did this come to be.”
There are three presumed vehicles for
creating a moon: an impact on a planet,
a circumplanetary disk of gas and dust,
or capture of a passing body. Each
method explains different moons in
the solar system, but all three are tough
to reconcile with this situation. “This
moon, if confi rmed, must have formed
through a yet-unknown astrophysical
mechanism,” Heller says.
The next step is to get more data. The
researchers have applied for time on
Hubble in May 2019, when the planet
is expected to transit next. One more
transit that confi rms the moon is in
the right position based on its predicted
orbit could clinch the case.
■ CHRISTOPHER CROCKETT
EXOPLANETS