Astronomers Search For Life

External Links:
Paper from "Nature"
Full paper from ESO


This shows the Sun, Moon and Earth (not to scale) together with the
Earthshine, commonly known as "the old moon in the young moon's arms".
Earthshine is caused by sunlight reflecting off the Earth first to the
Moon and then back again to Earth.

Image Credit: Enric Palle (Instituto de Astrofisica de Canarias, Spain).

An international team of astronomers, including Stefano Bagnulo from Armagh Observatory, has used one of the largest telescopes in the world to pioneer a novel approach to study the surfaces and atmospheres of planets orbiting stars other than the Sun. They have tested the technique on the Earthshine, the light reflected from the Earth by the Moon, and showed that it can reveal the presence of life in our planet. In the not too distant future, this technique could be used to search for life on other worlds. Full details of this work are described in the 1st March 2012 issue of the journal Nature.

"The work exploits a technique called spectro-polarimetry", said Stefano Bagnulo. "Normally, an extra-solar planet is almost completely hidden in the glare of its parent star. This technique uses the fact that the light reflected from the planet is polarised, whereas that emitted by the star is not. The use of a polarising filter allows us to separate out the two sources of light despite the exceptionally close proximity of the planet and the star on the sky."

Put another way, investigating an extra-solar planet with traditional techniques is like trying to study a particle of dust on the surface of a powerful light bulb. But the technique of polarimetry enables astronomers to suppress the bright, unpolarised light coming from the star, allowing them to concentrate on the more interesting signal emerging from the planet.

Michael Sterzik (European Southern Observatory), leader of the team, explained the motivation for their work: "Previous theoretical work had suggested that polarimetry could reveal certain details about an extra-solar planet's atmosphere, especially the various molecular components associated with the presence of life. For example, the simultaneous presence of oxygen and other molecules in certain abundances may only be explicable through the action of living organisms. We therefore wanted to test this technique on the only place in the entire Universe where we are sure that life exists: our home planet, Earth."

In order to obtain an astronomical observation of the Earth, from the Earth, the team used the Moon as a giant reflector. They pointed the Very Large Telescope (VLT) in Chile on that part of the Moon illuminated not by the Sun, but by the reflection of sunlight from the Earth, that is, the Earthshine. This is sometimes called "The old moon in the young moon's arms". It can be easily seen with the naked eye a few days either side of New Moon. Guided by previous theoretical models, these polarimetric observations allowed the team to deduce that part of the Earth was covered by water, part by clouds, and part by vegetation. In other words, they detected an astronomical signature of Life.

"This is the first time that evidence of life on Earth has been detected in polarised Earthshine", says team member Enric Palle, (Instituto de Astrofisica de Canarias and former PhD student of the Armagh Observatory), "This work is an important step to achieving the capability to detect extra-terrestrial life."

FOR FURTHER INFORMATION PLEASE CONTACT: Stefano Bagnulo or John McFarland at the Armagh Observatory, College Hill, Armagh, BT61 9DG. Tel.: 028-3752-2928; E-mail: sbaat or jmfat See also the ESO PR:, and the Universe Awareness (UNAWE) Space Scoop.

Notes for Editors:

Polarimetry: Most astronomical observations measure the brightness (or intensity) of the light coming from stars and other objects, often stretching the light into the familiar rainbow of colours that provides us with information on the nature of the emitting bodies, such as their temperatures and chemical makeup. For example, stars that appear predominantly white or blue will tend to be hotter than those that seem to be red, or yellow like our Sun. This new work exploits a different property of light, called polarisation, which tells us not only how bright an object appears, but also the direction of oscillation of the electromagnetic waves. This can sometimes reveal more about the emitting source and the materials through which it has passed on its journey to Earth.

There are many examples of polarisation all around us. Occasionally, we need to orientate a television aerial to receive a signal from a particular transmitter: by so doing, we are aligning the aerial so that it better picks up the horizontally or vertically polarised signal from that transmitter. Light reflected by certain surfaces such as a wet road, a lake, or a polished table, is polarised, and some people may have noticed that polarised sunglasses (polaroids) suppress part of the reflected light. Polarised light can tell us about the reflecting surface. In the case of the reflected light called Earthshine, it can tell us certain properties of the Earth's atmosphere and surface.

Polarimetry of Extra-Solar Planets: Having shown how the reflected light from Earth into space appears through a polarising filter, and in particular that the polarised light reflected by Earth shows the presence of life, astronomers hope that the same technique may be used in future to search for life elsewhere, such as in distant "extra-solar planets". These are planets orbiting stars other than the Sun, and which are too close to their parent star to be analysed with traditional techniques. Used with powerful telescopes, polarising filters allow us to distinguish the light coming from the planet from the light coming from the star.

Bio-Signatures: Astronomers do not expect to see intelligent forms of life with their telescopes, but hope to detect characteristics associated with life, for instance gases such as oxygen, ozone, methane, and carbon dioxide. While these gases may also occur without the presence of life, their simultaneous presence with the abundances far from chemical equilibrium is only compatible with the existence of life. If life were suddenly to disappear, these gases would quickly react and combine with each other, and these characteristic "bio-signatures" would disappear too.

ESO VLT: This research was carried out with the Very Large Telescope, built and operated by the European Southern Observatory (ESO). The UK is a member of ESO through the Science and Technology Facilities Council.

DCAL: Research at Armagh Observatory is supported by the Northern Ireland Department of Culture, Arts and Leisure.

Full Story: Further details can be found in the paper "Biosignatures as Revealed by Spectropolarimetry of Earthshine", by Michael Sterzik (European Southern Observatory), Stefano Bagnulo (Armagh Observatory) and Enric Palle (Instituto de Astrofisica de Canarias, Spain), published in Nature (1 March, 2012).

Last Revised: 2012 February 29th