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In 2017, astronomers using
NASA’s Kepler telescope found

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hints of evidence of what they
thought could be the first

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exomoon ever detected– a moon
orbiting a planet outside our

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solar system. A lot of people
were very excited about the

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potential discovery, and the
research team requested time on

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NASA’s Hubble Space Telescope to
see if there was truly a moon.

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Now, the data from Hubble is in
and processed. Have scientists

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found our first exomoon? Looks
like they still need more

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observations to be a hundred
percent sure. The moons in our

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solar system are very diverse –
some with liquid water, some

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that are potentially habitable –
so being able to observe moons

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in different solar systems would
give us a whole new treasure

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trove of interesting worlds to
study. The team of astronomers

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from Columbia University sifted
through data from 284 Kepler

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measurements of transiting
exoplanets. A transit happens

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when a planet passes directly
between us and its star and

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blocks out a small portion of
the star’s light, which produces

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a dip in the amount of light
coming from that star over time,

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called a light curve. If there’s
a moon on either side of the

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planet, we may be able to see a
second, smaller dip in the light

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curve. Making transit
measurements and processing the

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data into a light curve can be
very tricky though, so

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scientists need to observe
multiple transits in order to

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get convincing evidence of an
exoplanet or an exomoon. Out of

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the 284 planets the team looked
at, they found one planet with

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an extra wobble in its light
curve that might be a moon. This

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planet, Kepler-1625b, orbits its
star once every nine and a half

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months, so it takes years to
observe multiple transits. The

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team only had Kepler data from
three transits of this

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exoplanet, so they sent a
proposal to the Hubble Space

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Telescope to capture more data
of this possible exomoon. Hubble

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can measure light curves much
more precisely than Kepler, so

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Hubble can more definitively
confirm or disprove the

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existence of this exomoon once
it gets multiple transits. So

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far, the team of astronomers has
collected Hubble observations of

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one transit of exoplanet
Kepler-1625b. Hubble measured

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the light coming from the star
over a period of 40 hours. About

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7 hours into the observation,
the planet started its transit,

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which was about an hour earlier
than predicted – a tantalizing

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piece of evidence that the
planet has a large moon, because

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a moon would cause the planet to
wobble in its orbit. The planet

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transited across the face of its
star for 19 hours, as expected.

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Then, about three and a half
hours later, a second smaller

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dip in the light curve started.
That right there is intriguing

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evidence of an exomoon. The
Hubble observation unfortunately

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concluded before the possible
moon finished its transit, so

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Hubble hasn’t yet observed a
full transit. This light curve

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appears like it shows an
exomoon, but

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we can’t say
it’s a definitive detection

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until Hubble observes at least
one more transit. If this is

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actually a moon, what do we know
about it? The planet

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Kepler-1625b is likely several
times the mass of Jupiter. Its

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possible moon appears to be
about the mass and radius of

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Neptune. The ratio between the
mass of this planet and its moon

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is about the same as between the
Earth and our Moon, but scaled

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way up. Neptune-sized moons are
obviously not something that

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exist in our own solar system,
and we’re not sure exactly how

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such a moon would form. When
scientists first started

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discovering exoplanets, a lot of
them were several times the mass

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of Jupiter because big planets
are easier to detect, so a

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similar thing may happen when
discovering the first exomoons.

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Finding planets and moons so
different from the ones in our

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own solar system gives us a lot
more knowledge of the many

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diverse ways planetary systems
can exist. In the near future,

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scientists hope to use the
Hubble Space Telescope to try to

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confirm the discovery of this
exomoon, and they’ll use Hubble

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for follow-up observations of
other planetary systems

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discovered by observatories like
Kepler and the recently launched

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Transiting Exoplanet Survey
Satellite, TESS. And then the

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upcoming James Webb Space
Telescope will be able to take

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high-precision measurements in
infrared wavelengths, which

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combined with Hubble will allow
for detailed characterizations

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of a whole bunch of really
interesting exoplanets and

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exomoons. So if you like planets
– and honestly, who doesn’t –

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stay tuned to learn more about
all sorts of fascinating new

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worlds. 

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[music fades out]

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www.nasa.gov/hubble
@NASAHubble