In 2017, astronomers using NASA’s Kepler telescope found
hints of evidence of what they thought could be the first
exomoon ever detected– a moon orbiting a planet outside our
solar system. A lot of people were very excited about the
potential discovery, and the research team requested time on
NASA’s Hubble Space Telescope to see if there was truly a moon.
Now, the data from Hubble is in and processed. Have scientists
found our first exomoon? Looks like they still need more
observations to be a hundred percent sure. The moons in our
solar system are very diverse – some with liquid water, some
that are potentially habitable – so being able to observe moons
in different solar systems would give us a whole new treasure
trove of interesting worlds to study. The team of astronomers
from Columbia University sifted through data from 284 Kepler
measurements of transiting exoplanets. A transit happens
when a planet passes directly between us and its star and
blocks out a small portion of the star’s light, which produces
a dip in the amount of light coming from that star over time,
called a light curve. If there’s a moon on either side of the
planet, we may be able to see a second, smaller dip in the light
curve. Making transit measurements and processing the
data into a light curve can be very tricky though, so
scientists need to observe multiple transits in order to
get convincing evidence of an exoplanet or an exomoon. Out of
the 284 planets the team looked at, they found one planet with
an extra wobble in its light curve that might be a moon. This
planet, Kepler-1625b, orbits its star once every nine and a half
months, so it takes years to observe multiple transits. The
team only had Kepler data from three transits of this
exoplanet, so they sent a proposal to the Hubble Space
Telescope to capture more data of this possible exomoon. Hubble
can measure light curves much more precisely than Kepler, so
Hubble can more definitively confirm or disprove the
existence of this exomoon once it gets multiple transits. So
far, the team of astronomers has collected Hubble observations of
one transit of exoplanet Kepler-1625b. Hubble measured
the light coming from the star over a period of 40 hours. About
7 hours into the observation, the planet started its transit,
which was about an hour earlier than predicted – a tantalizing
piece of evidence that the planet has a large moon, because
a moon would cause the planet to wobble in its orbit. The planet
transited across the face of its star for 19 hours, as expected.
Then, about three and a half hours later, a second smaller
dip in the light curve started. That right there is intriguing
evidence of an exomoon. The Hubble observation unfortunately
concluded before the possible moon finished its transit, so
Hubble hasn’t yet observed a full transit. This light curve
appears like it shows an exomoon, but
we can’t say it’s a definitive detection
until Hubble observes at least one more transit. If this is
actually a moon, what do we know about it? The planet
Kepler-1625b is likely several times the mass of Jupiter. Its
possible moon appears to be about the mass and radius of
Neptune. The ratio between the mass of this planet and its moon
is about the same as between the Earth and our Moon, but scaled
way up. Neptune-sized moons are obviously not something that
exist in our own solar system, and we’re not sure exactly how
such a moon would form. When scientists first started
discovering exoplanets, a lot of them were several times the mass
of Jupiter because big planets are easier to detect, so a
similar thing may happen when discovering the first exomoons.
Finding planets and moons so different from the ones in our
own solar system gives us a lot more knowledge of the many
diverse ways planetary systems can exist. In the near future,
scientists hope to use the Hubble Space Telescope to try to
confirm the discovery of this exomoon, and they’ll use Hubble
for follow-up observations of other planetary systems
discovered by observatories like Kepler and the recently launched
Transiting Exoplanet Survey Satellite, TESS. And then the
upcoming James Webb Space Telescope will be able to take
high-precision measurements in infrared wavelengths, which
combined with Hubble will allow for detailed characterizations
of a whole bunch of really interesting exoplanets and
exomoons. So if you like planets – and honestly, who doesn’t –
stay tuned to learn more about all sorts of fascinating new
worlds.
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www.nasa.gov/hubble @NASAHubble