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How do we learn about
a planet's atmosphere?

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How do we know what's in the
atmosphere of an exoplanet?

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The majority of
known exoplanets have

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been discovered because
they partially block

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the light of their host star.

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This is called a transit.

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During a transit some
of the star's light

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travels through the planet's
atmosphere and gets absorbed.

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The light that survives
carries information

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about the planet across
light years of space

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where it reaches our telescopes.

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However, the planet is very
small relative to the star.

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So it is still very
difficult to detect,

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which is why we
need a big telescope

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to be sure to capture
this tiny bit of light.

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So how do we use a telescope
to read transit light?

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Stars emit light at
many wavelengths.

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Like a prism makes a rainbow,
we can separate light

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into its separate wavelengths.

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This is called a spectrum.

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Visible light
appears to our eyes

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as the colors of the rainbow.

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But beyond visible light there
are many wavelengths we cannot

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see.

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Now back to the
transiting planet.

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As light as traveling through
the planet's atmosphere,

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some wavelengths get absorbed.

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Which wavelengths get absorbed
depends on which molecules

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are in the planet's atmosphere.

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For example, carbon
monoxide molecules

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will capture
different wavelengths

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than water vapor molecules.

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So when we look at that
planet in front of the star,

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some of the wavelengths
of the starlight

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will be missing depending
on which molecules

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are in the atmosphere
of the planet.

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Learning about the
atmospheres of other worlds

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is how we identify those that
could potentially support life,

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bringing us another step closer
to answering one of humanity's

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oldest questions, are we alone?

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