The reigning champ of most Earth-sized planets yet is a world called Gliese 581 e, which circles a star that has four planets total. It is called the smallest because of its mass, which is just 1.9 times the mass of Earth, making it the lightest known alien planet to date. The other planets range in mass from 5 to 16 times the mass of Earth.
Credit: ESO/L. Calcada
A new way of comparing the color and intensity changes of light reflected off of Earth's surface to the flickers from exoplanets may help reveal the presence of oceans, continents and ? possibly ? life on alien worlds.
Researchers came up with a color scheme for how our uniquely life-filled, ocean-soaked planet would appear to observers tens of light-years away.
By comparing the changes in observed hues of an alien planet ?as it rotates to this distinct Earthly color palette, "we can infer the surface composition of the [exo]planet," said Yuka Fujii, a doctoral student at the University of Tokyo and lead author of a paper published in the May 4 issue of the Astrophysical Journal.
The method might let astronomers to soon hunt for soil, snow, seas or even plant life on exoplanets that for decades will otherwise remain too faint to directly visualize.
Little light to work with
For today's most powerful telescopes, even a nearby exoplanet is nothing more than a point source of light teased out from the intense glare of its host star. Should a small, Earth-like planet be discovered around another sun, astronomers will not be able to simply look at it and pick out azure seas or verdant rain forests.
The problem: the dearth of light particles, or photons, reflected by exoplanets that then reach across trillions of miles to our telescopes.
The next generation of proposed second Earth-hunting instruments, such as the Terrestrial Planet Finder or the New Worlds Observer, will similarly struggle to resolve much more than just an exoplanetary flicker. "It is hard to obtain enough photons from distant planets for detection" of surface features such as vegetation, said Fujii.
Yet these rare rays of light carry with them telltale signatures, and astronomers have detected various gases in the atmospheres of giant exoplanets using spectroscopy. The new study suggests that enough light may be reflecting off the ground as well to be observationally valuable.
"We cannot directly identify green, blue and red spots on the surface," Fujii said. "But we can observe the total color average over the visible surface of the planet" and compare it to Earth's known values to take a stab at the world's geology or even biology.
Peering into a planetary mirror
Researchers used satellite data to model a baseline for "Earthshine," or the wavelengths of sunlight reflected by major surface types on Earth, including oceans (which covers 71 percent of the planet), vegetation (14 percent), soil, such as deserts (9 percent) and snow and ice (3 percent).
These types reflect different amounts of light at certain wavelengths; snow, for instance, reflects more than 80 percent of light around 0.8 microns, while oceans bounce back photons in the low single digits percentage-wise.
As Earth (or an Earth-like exoplanet) rotates, greater and lesser expanses of these surface types come into view, changing the overall complexion of light reflected out into space in a measurable way.
In this way, astronomers can compare exoplanetary sheens to Earth's to come up with rough estimates for a patchwork of alien seas and savannahs, should they exist.
The varying levels of light representing different land forms are somewhat like the spices and ingredients in a soup, so "it's taking the total light, or the total taste of the soup, and decomposing it into things we know about like soil in the way that a chef knows salt or basil," said Ed Turner, a professor of astrophysics at Princeton University and a co-author of the paper.
A work in progress
For their model, the researchers assumed a twin of Earth at a distance of about 30 light-years ? close by cosmic standards ? and a space telescope with a two-meter mirror observing for an hour a day over two weeks.
"We assumed a fairly nearby planet, but still had to stack the data for two weeks in order to accumulate a sufficient number of photons," said Fujii.
The model relies on many oversimplifications. Clouds, for example, with their light-scattering and ground-feature-obscuring effects, are not yet part of the equation. Alan Boss, an astrophysicist at the Carnegie Institution in Washington, also pointed out that paper ignores "the problem of separating the planet's light from the star's light," perhaps the biggest hurdle in exoplanetary investigation.
Spying blue skies and the red edge?
But even in its preliminary form, the color-detection method shows much promise and Turner said he was "surprised at how well it worked."
The technique could detect both an atmosphere and an ocean simultaneously based on Rayleigh scattering, the light-scattering effect that makes our sky and oceans appear blue and that would similarly tint the light from Earth-like exoplanets.
Color comparisons to Earth could also reveal the so-called red edge.
Plants on Earth have evolved to reflect sunlight in the infrared range that is not energetic enough for photosynthesis but would otherwise heat the plant up; when viewed in infrared, plain green plants are a brilliant bright red, said Turner. This signature would jump out in the stray bits of precious light collected from an exoplanet, he added.
Many astronomers think the red edge is the best shot at detecting alien life (assuming these plants are colored like earthly ones).
"Looking for evidence of the red edge is an excellent means for buttressing the case that an exo-Earth might not only be habitable, but inhabited," said Boss.
Webster Cash, an astronomy professor at the University of Colorado, said the color variation technique is "absolutely going to be useful" in the years ahead.
He pointed out that it could be paired down the road with other recognized ways to sniff out so-called biomarkers of extraterrestrial life, such as the detection of oxygen and other key gases in Earth-like exoplanetary atmospheres via spectroscopy.
For now, the Kepler spacecraft, in its ongoing hunt for habitable planets around 100,000 stars, will not be able to see the red edge or detect surface features because the instrument "has no color information ? all photons are the same for Kepler," Boss said.
Though much refinement is needed for the color-based method, Turner said it could be adapted for use on future terrestrial planet finding satellites that NASA and ESA are exploring.?
"I think we're really on the threshold of being able to study earthlike planets around other stars," Turner said.
And as with horseshoes and hand grenades, when it comes to scanning an exoplanet's light for signs of life, closeness counts.
"If we happen to find an Earth-like planet much closer than 30 light-years, say eight, this would be a lot easier," Turner said.
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