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Taking advantage of the total lunar eclipse
of January 2019, astronomers, using NASA’s

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Hubble Space Telescope, have detected ozone
in Earth’s atmosphere.

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In this observation, Hubble did not look at
Earth directly. Instead, astronomers used

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the Moon as a mirror to reflect sunlight that
passed through Earth’s atmosphere.

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This method serves as a proxy for how they
will observe planets around other stars in

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search for worlds similar to our own.

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Though numerous ground-based observations
of this kind have been done previously, this

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is the first time ultraviolet light passing
through Earth’s atmosphere was observed

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from space. The measurements from this experiment detected the

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strong spectral fingerprint of ozone.

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On Earth, photosynthesis over billions of
years is responsible for our planet’s high

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oxygen levels and thick ozone layer. That’s
one reason why scientists think ozone or oxygen

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could be a sign of life on another planet.

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But finding ozone on distant worlds isn’t
an easy task. Ultraviolet observations like

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this can best be conducted from space telescopes,
above the limiting effects of looking through

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Earth’s skies, because ozone blocks most
ultraviolet light from beyond the atmosphere.

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One of NASA’s major goals is to identify
habitable and inhabited planets. But how would

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we know whether a distant planet has ozone
or not?

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The atmospheres of some extrasolar planets
can be probed if that distant world passes

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across the face of its parent star, called
a transit. During a transit, starlight filters

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through the backlit exoplanet’s atmosphere.

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Chemicals in the atmosphere leave their telltale signature

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by filtering out certain colors of starlight.

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Astronomers have used Hubble to observe the
atmospheres of several gas giant planets that

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transit their stars. But terrestrial planets
are much smaller objects and their atmosphere

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thinner, like the skin on an apple.

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Therefore,
teasing out these signatures is much more difficult.

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To prepare for future studies with larger
telescopes, astronomers used Hubble to conduct

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experiments on a much closer and only known
inhabited terrestrial planet: Earth! Our planet’s

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perfect alignment with the Sun and Moon during
a total lunar eclipse mimics the geometry

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of a transiting terrestrial planet with its
star.

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But the observations were also challenging
because the Moon is very bright, and its surface

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is not a perfect reflector because it is mottled
with bright and dark areas.

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The Moon is also so close to Earth that it
appears to move very quickly in the sky, making

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it harder for Hubble to stay pointed at the
same location.

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However, in spite of the challenges, the experiment
was an incredible success, and Hubble is supporting

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the ongoing quest to find planets that are
similar to our own, and perhaps, one day,

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find signs of life on other worlds.

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[ BEAUTIFUL MUSIC ]