Life in the Universe, Reflected by the Moon
This view shows the thin crescent Moon setting
over ESO’s Paranal Observatory in Chile. Credit: ESO/B.
Earthshine – a
poetic, fanciful word for the soft, faint glow on the Moon when
the light from the Sun is reflected from the Earth’s surface,
onto the dark part of the Moon. And as unlikely as it might
seem, astronomers have used Earthshine to verify there’s life in
the Universe: Us. While we already know about life on our own
world, this technique validates that faint light from distant
worlds could also be used to find potential alien life.
“We used a trick
called earthshine observation to look at the Earth as if it were
an exoplanet,” said Michael Sterzik from the European Southern
Observatory. “The Sun shines on the Earth and this light is
reflected back to the surface of the Moon. The lunar surface
acts as a giant mirror and reflects the Earth’s light back to us
— and this is what we have observed with the VLT (Very Large
Sterzik and his team said the fingerprints of life, or
biosignatures, are hard to find with conventional methods, but
they have now pioneered a new approach that is more sensitive.
The astronomers used Earth as a benchmark for the future search
for life on planets beyond our Solar System. They can analyze
the faint planetshine light to look for indicators, such as
certain combinations of gases in the atmosphere – as they found
looking at earthshine – to find telltale signs of organic life.
earthshine, they found strong bio-signatures such as molecular
oxygen and methane, as well as the presence of a ‘red edge’
caused by surface vegetation.
By observing earthshine astronomers can study the
properties of light reflected from Earth as if it were an
exoplanet and search for signs of life. The reflected light is
also strongly polarised and studying the polarisation as well as
the intensity at different colours allows for much more
sensitive tests for the presence of life. Credit: ESO/L. Calçada
Instead of just
looking at the planet’s reflected light, astronomers can also
use spectropolarimetry, which looks at the polarization of the
light. Using this approach, the biosignatures in the reflected
light from Earth show up very strongly.
“The light from a
distant exoplanet is overwhelmed by the glare of the host star,
so it’s very difficult to analyze — a bit like trying to study a
grain of dust beside a powerful light bulb,” said co-author
Stefano Bagnulo from Armagh Observatory in Northern Ireland.
“But the light reflected by a planet is polarised, while the
light from the host star is not. So polarimetric techniques help
us to pick out the faint reflected light of an exoplanet from
the dazzling starlight.”
By looking at
earthshine, the team was able to deduce that the Earth’s
atmosphere is partly cloudy, that part of its surface is covered
by oceans and — crucially — that there is vegetation present.
They could even detect changes in the cloud cover and amount of
vegetation at different times as different parts of the Earth
reflected light towards the Moon.
observations allow us to determine the fractional contribution
of clouds and ocean surface, and are sensitive to
Spectropolarimetry unveils strong biosignatures, visible areas
of vegetation as small as 10%,” the team wrote in their
outside the Solar System depends on two things: whether this
life exists in the first place, and having the technical
capability to detect it,” said co-author Enric Palle from
Instituto de Astrofisica de Canarias, Tenerife, Spain. “This
work is an important step towards reaching that capability.”
“Spectropolarimetry may ultimately tell us if simple plant life
— based on photosynthetic processes — has emerged elsewhere in
the Universe,” said Sterzik. “But we are certainly not looking
for little green men or evidence of intelligent life.”
said that future telescopes such as the E-ELT (the European
Extremely Large Telescope), could provide more detail about the
type of life beyond planets that may exists on another world.
Read the team’s paper, Download
which was published in Nature.
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