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In May 2016, an international
team of astronomers announced

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the discovery of three small
exoplanets orbiting an ultracool

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dwarf star just 40 light-years
away, called TRAPPIST-1. Now,

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the Hubble Space Telescope has
taken the first steps toward

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characterizing the atmospheres
of two of these planets. This is

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the first time anyone has taken
spectroscopic measurements of an

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earth-sized exoplanet. A lot is
unknown about these planets.

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Their atmospheres could range
from a thin, tenuous atmosphere

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like Mars to a mostly gaseous
body composed of hydrogen and

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helium like Neptune. If there
were large, inhospitable

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hydrogen-helium atmospheres on
these planets, the Hubble Space

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Telescope would be able to
detect it through measuring

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changes in the host star’s
spectrum of light as the planets

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pass between us and the star. On
May 4th, Hubble had the rare

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opportunity to observe a transit
of both TRAPPIST-1b and

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TRAPPIST-1c as they crossed the
disk of their star just minutes

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apart. After analyzing the data,
the results are in: these two

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earth-sized planets likely do
not have thick hydrogen-helium

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atmospheres. They are probably
not miniature gas giants, so

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that increases the likelihood
that they could be rocky bodies

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like Earth. This still leaves
several remaining possibilities

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for their atmospheres. Possibly
they have thick atmospheres like

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Venus. Perhaps they have almost
no atmospheres at all, like

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Mars. Or maybe they have
moderate-sized atmospheres with

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oxygen and water vapor like
Earth. The next step in

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distinguishing between these
remaining atmospheric scenarios

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will be to use the Hubble Space
Telescope to follow-up on these

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two planets and detect whether
or not these atmospheres have

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molecules like water vapor or
methane. Then after the James

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Webb Space Telescope is launched
in 2018, Webb will be able to

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observe farther into the
infrared wavelengths to measure

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other important gases like
carbon dioxide or ozone, and

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with its increased resolution
will even be able to provide

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data on atmospheric temperature
and pressure. So we’ll soon have

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a much more thorough
understanding of the atmospheres

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of these planets. The TRAPPIST-1
planetary system is one example

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of how telescopes on the ground
and in space are making huge

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strides in discovering and
characterizing planets with

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similarities to our own and are
on the hunt for evidence of

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alien life.