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NASA’s Hubble Space Telescope
has looked for evidence of

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atmospheres around several
earth-sized planets in the

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TRAPPIST-1 system, including
three that are in the star’s

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habitable zone, where liquid
water could exist on the

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surface. A lot of astronomers
and space enthusiasts were very

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excited by the discovery of the
seven Earth-sized planets of the

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TRAPPIST-1 system. The planets
orbit an ultracool dwarf star

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about 40 light-years away.
Scientists have theories about

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what these planets may be like
and whether they could support

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life, but we won’t know for sure
until we get more comprehensive

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observations of this system,
including data on the planets’

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atmospheres. As a planet in the
TRAPPIST-1 system passes between

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us and the star, it blocks out a
small portion of the star’s

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light. Telescopes like Hubble
can look at changes in specific

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wavelengths of light, which
provide clues to the composition

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and size of the planet's
atmosphere. Hubble observations

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in May, 2016 of TRAPPIST-1 b and
c showed that these planets do

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not seem to have thick, puffy
hydrogen-rich atmospheres. This

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indicates a higher chance that
they are rocky, terrestrial

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planets rather than
mini-gas-giants. Hubble then

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observed planets d, e, f, and g
in December 2016 and January

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2017 in near-infrared
wavelengths, and the results

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were similar. Hubble found no
sign of thick, puffy

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hydrogen-rich atmospheres for
any of the four planets. The

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data suggest that there isn’t
this gas-giant-like atmosphere

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for planets d, e, and f. The
data from this round of

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observations was not as strong
for planet g, so while there’s

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no evidence for a thick
hydrogen-rich atmosphere on g,

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the researchers are not yet
ruling it out. Planets e, f, and

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g orbit at distances where
temperatures would allow for

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liquid water, while d is likely
a little too hot. Hubble has yet

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to take observations of planet
h, which is outside the system’s

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habitable zone. To summarize –
Hubble has not seen evidence of

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thick, hydrogen-rich atmospheres
for planets b, c, d, e, and f,

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and of those five planets, e and
f are in the habitable zone.

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Planet g needs more data, and
Hubble has not yet looked at

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planet h. It’s worth noting
though, that even the planets

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outside the habitable zone still
might be able to have liquid

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water somewhere on its surface
in certain conditions. It’s also

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worth noting that if any of
these planets have high-altitude

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clouds and hazes, that would
block Hubble’s ability to detect

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a thick, hydrogen-rich
atmosphere, but such an

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atmosphere is not likely to
exist on these planets. Many

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possibilities remain for what
types of atmospheres these

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planets have, or whether they
even have atmospheres. The

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TRAPPIST-1 planets could have
compact atmospheres similar to

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Mars, Venus, Earth, or something
entirely different. Researchers

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hope to use Hubble’s ultraviolet
capabilities to look for

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evidence of water vapor or
methane, and NASA’s upcoming

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James Webb Space Telescope will
look in the far-infrared to

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further characterize these
atmospheres. Future telescopes

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also hope to look for hints of
whether the planets are

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habitable and if life could be
present. The TRAPPIST-1 system

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provides the best opportunity we
currently have to study

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Earth-size exoplanets. Over the
next few years, Hubble and other

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telescopes will work together,
each contributing important

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observations. For the first time
ever, we’ll have an in-depth

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understanding of a set of
terrestrial planets outside our

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solar system. 

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www.nasa.gov/hubble
@NASAHubble