Image 1 - Earth's Reflection From Moon Helps Search For Life Outside Our Solar System
March 1, 2012

Earth’s Reflection From Moon Helps Search For Life Outside Our Solar System

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An international team of astronomers has found evidence of life in the Universe — right here on Earth — after pointing one of the world´s largest telescope at the Moon.  While the observation might seem trivial, it has helped perfect a technique that could lead to future discoveries of life outside our solar system.

The researchers used the European Southern Observatory´s Very Large Telescope (VLT) to look at Earthshine reflected from the moon.

“We used a trick called earthshine observation to look at the Earth as if it were an exoplanet,” said Michael Sterzik (ESO), lead author of the paper about the work published March 1 in the journal Nature.

“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,” he explained.

The astronomers analyzed the faint earthshine light to search for indicators, such as certain combinations of gases in the Earth´s atmosphere, that are the telltale signs of organic life.

The method establishes the Earth as a benchmark for the future search for life on planets elsewhere in the Universe.

Such biosignatures, or fingerprints of life, are hard to find with conventional methods.  But the new approach is more sensitive, allowing researchers to analyze not only how bright the reflected light is in different colors, but also the polarization of the light -- an approach known as spectropolarimetry.

By applying this technique to earthshine observed with the VLT, 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. But the light reflected by a planet is polarized, 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,” said Stefano Bagnulo  of the Armagh Observatory in Northern Ireland, co-author of the study.

The astronomers studied both the color and the degree of polarization of light from the Earth after reflection from the Moon, as if the light was coming from an exoplanet.

They determined that the Earth´s atmosphere is partly cloudy, part of its surface is covered by oceans, and that 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.

“Finding life 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 of the Instituto de Astrofisica de Canarias in 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," Sterzik concluded.

“But we are certainly not looking for little green men or evidence of intelligent life.”

The next generation of telescopes, such as the E-ELT (the European Extremely Large Telescope), may well be able to bring us the extraordinary news that the Earth is not alone as a bearer of life in the vastness of space.

Earthshine can easily be seen with the unaided eye, and is spectacular in binoculars.  It is best observed when the Moon is a thin crescent, about three days before or after new Moon. In addition to the bright crescent, the rest of the lunar disc is visible, dimly illuminated by the bright Earth in the lunar sky.


Image 1: This view shows the thin crescent Moon setting over ESO´s Paranal Observatory in Chile. As well as the bright crescent the rest of the disc of the Moon can be faintly seen. This phenomenon is called earthshine. It is due to sunlight reflecting off the Earth and illuminating the lunar surface. 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. This picture was taken on 27 October 2011 and also records the planets Mercury and Venus. Credit: ESO/B. Tafreshi/TWAN (

Image 2: This very realistic view of the Moon is a rendering based on detailed maps of the reflectivity and height of the lunar surface from NASA´s Lunar Reconnaissance Orbiter. The bright crescent is directly illuminated by the Sun but the rest of the disc is glowing faintly in light reflected from Earth – the earthshine. Credit: ESO/NASA/M. Kornmesser


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