With better and better observations of exoplanets—planets orbiting other stars—the dream of discovering life on other worlds is much closer to reality than it has ever been before. However, in many ways we're still in the early days of exploration: despite the number of exoplanet discoveries, information about their surfaces and chemical compositions are still scanty.

A test run for inferring the presence of life on other planets would be to confirm the existence of life on Earth, using only astronomical measurements of some sort. Partly due to the lack of appropriate space-based instruments, nobody has made a successful observation of life on Earth using data that's as limited as what we'd get when studying another world. A new study by Michael F. Sterzik, Stefano Bagnulo, and Enric Palle attempts just that with an unusual source of light: Earthshine, or light from Earth reflected back to us by the Moon.

Planets do emit a small amount of infrared light that originates from surface heating and internal sources of warmth. But most of the light we can observe from them originates from their host star, and is reflected off their surfaces. By studying the spectrum of this reflected light, astronomers can determine things about the planet's atmospheric and surface composition. The specific wavelengths (colors) that are absorbed and the total amount of light reflected back—the planet's albedo—provide indications of the atmosphere's chemistry and possibly other details, such as whether the surface is solid or fluid, and if solid, possible terrain types.

Signs of life

It should theoretically be possible to detect hints of life using signals in this reflected light. Biological molecules such as chlorophyll polarize light that falls on them. While biochemicals are hardly alone in that property, life on Earth uses molecules of a single chirality—the twist in orientation of atoms in the chain, analogous to the difference between a human's left and right hands. Therefore, light reflected by vegetation exhibits circular polarization in the direction of the chlorophyll's chiral orientation.

Non-biological molecules do not behave this way, so the presence of a preferential polarization of light in the reflection from a planet might just be a sign of life.

Using the Very Large Telescope (VLT) in in Chile, Sterzik, Bagnulo, and Palle measured both the spectrum and the polarization of Earthshine. This type of measurement, known as spectropolarimetry, has not been performed on light from Earth before, largely because there are no suitable instruments in space for that purpose.

Earthshine presents its own difficulties: the Moon tends to depolarize much of the light that reaches it, removing part of the signal the researchers were looking for. In addition, reliable measurements can only be performed when the Moon is near the first or last quarter (half-Moon phase): Earthshine appears on the "dark" portion of the Moon's disc, but visible-light observations must be done at night to keep the pesky Sun's interference at a minimum, ruling out effective observations during new Moon. During the quarter Moon phase, only half of Earth's surface reflects sunlight toward the Moon, so the amount of available Earthshine is less than ideal.

Searching for signals in the Earthshine

The team performed its observations on two days: April 25 and June 10 of 2011. Once the Earthshine spectropolarimetry data was in hand, the researchers compared it to three theoretical models of different surface types and cloud coverage. Specifically, they considered water (in the form of oceans), landforms with minimal vegetation (such as deserts, tundra, and ice), and land with heavy plant coverage (e.g., forests). They varied the proportions of each surface type by small increments until they got the best match for the spectrum and polarization data.

Their strongest results showed seven and three percent vegetation coverage on April 25 and June 10, respectively. Since Earth's surface is watery, the observations also turned up 18 and 46 percent ocean coverage, along with 72 and 50 percent cloud coverage. (Obviously the large uncertainties are not necessarily a bad thing: any indication liquid water content and cloud coverage on an exoplanet is pretty exciting.)

With so many possible terrain types, the fraction of polarized light due to vegetation was very small. Two of the three models the researchers used failed to indicate any plant matter in the June data, showing how difficult this observation is. Even with the more substantial results from the April data, the amount of polarized light directly attributable to life was less than one percent of the total polarization in the Earthshine.

However, as a proof of principle, this observation is a step in a positive direction. The problems with using Earthshine are large, but they are correctable with further observations—and can be eliminated entirely by using space-based or Moon-based observatories. When we get around to real experiments, we won't actually be trying to detect any light reflected off an exoplanet's moon, which simplifies matters.

Enhanced theoretical models can also help, potentially accounting for more details in terrain types. While there is no guarantee that hypothetical life forms elsewhere in the cosmos will have the same chiral biochemistry we have on our home world, spectropolarimetry may indeed offer the best chance we have of finding anything that's out there.

Nature, 2012. DOI: 10.1038/nature10778 (About DOIs).