“Earthshine” Discovered: The Search for Earth’s Twin Just Got Easier


Scientists have recently discovered that life leaves an imprint upon a spectrum — that water vapor, oxygen and ozone can be traced.

"Detecting Earth in reflected light is like searching for a firefly from a searchlight that is 2,400 miles distant," according to a panel of astronomers describing the challenges facing the search for other planets in the universe.

With a dramatic new advances, however, astronomers have confirmed an effective way to search the atmospheres of planets for signs of life, vastly improving our chances of finding alien life outside our solar system. Think of it as the method to discover the Earth's "fingerprint" — information about the chemical composition of the Earth’s atmosphere from sunlight that has passed through it.

The spectrum of earthshine also turns up a tentative detection of the so-called ‘red edge’ signature of chlorophyll from the plantlife on our planet that absorb visible light as part of the process of photosynthesis. Beyond about 0.7 microns, the wavelength frequency we can see, plants become highly reflective creating a sharp rise in the red part of the spectrum.  Our sun's color, temperature, and distance from Earth have coaxed photosynthetic plants to absorb most wavelengths of light except for infrared and green, which these plants instead strongly reflect.

Giovanna Tinetti of University College London has estimated that at least 20 percent of a planet’s surface must be covered by plants and free from clouds for the vegetation’s imprint to show up in the global spectrum. Our earthshine spectrum rises toward the blue because the molecules in the atmosphere scatters blue light more efficiently than red light.

A team from the Instituto de Astrofisica de Canarias used the William Herschel Telescope on La Palma  and the Nordic Optical Telescope to take the first transmission spectrum of the Earth.

When a planet passes in front of its parent star, part of the starlight passes through the planet’s atmosphere and contains information about the constituents of the atmosphere. Even though astronomers can’t use exactly the same method to look at the Earth’s atmosphere, the team was able for the first time ever to gain a spectrum of our planet by observing light reflected from the Moon towards the Earth during a lunar eclipse.

The spectrum not only contained signs of life but these signs were unmistakably strong. It also contained unexpected molecular bands and the signature of the earth ionosphere.

“Now we know what the transmission spectrum of an inhabited planet looks like, we have a much better idea of how to find and recognize Earth-like planets outside our solar system where life may be thriving," said Enric Palle, of the Instituto de Astrofisica de Canarias. "The information in this spectrum shows us that this is a very effective way to gather information about the biological processes that may be taking place on a planet.”

“Many discoveries of Earth-size planets are expected in the next decades and some will orbit in the habitable zone of their parent stars. Obtaining their atmospheric properties will be highly challenging; the greatest reward will happen when one of those planets shows a spectrum like that of our Earth,” added Pilar Montañes-Rodriguez, of the Instituto de Astrofisica de Canarias.

Most stars in the nearby universe aren't like the sun. About 80 percent of the Milky Way's stars are dim red dwarfs. As a result, astrobiologists have suggested that photosynthetic plants on worlds orbiting lone red dwarfs could take on hues of red, blue, yellow, purple, or even grayish-black to best absorb the starlight.

"Planets have to be about five times closer than Earth is to the sun to support photosynthesis," he said. "Red dwarf stars are more active, and their solar flares will hit the surface of the planet more often. That's a problem for life."

As a result, O'Malley-James thinks land-based alien plants may evolve natural sunscreens to protect themselves. If the plants are in water, they may temporarily sink when they sense bursts of radiation to protect their vulnerable photosynthetic molecules.

A third of all star systems, meanwhile, contain two or more stars, and it's not known how plants might evolve under mixed light sources.

"Rather than plants using light from, say, both a sunlike star and a red dwarf, we think you'd see them evolve a diversity of colors and use the two types of starlight preferentially," said simulation leader Jack O'Malley-James, an astrobiologist at the University of St. Andrews in the U.K.

"On a world orbiting two red dwarf stars, you'd essentially get one color, and you'd probably see them as black."

Casey Kazan via www.scitech.ac.uk , centauridreams.org and nationalgeographic.com

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