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With over 70 planets identified around distant stars, astronomers are now looking for ways to classify which ones are most like Earth – that is to say, the ones most likely with biological potential. Some initial qualifications are already known: Earth-like planets are likely relatively small, or under the limit of 12 Jupiter-masses. Larger planets would qualify as more of a companion star capable of burning heavy hydrogen and radiating their own nuclear fusion heat. So key entries, like the following, must be made more specific for each new planet candidate found: what are its size, temperature and reflectance (or albedo)? Also, for brighter planets, does the atmosphere have water, carbon dioxide and ozone as signs of potential inhabitability? For astronomers, the issue of planetary brightness is central to most of these classifications.

As recently published in the journal Nature, Princeton scientists have refined the Earth’s light spectra over time (or light-curve). A light-curve graphs how the radiation from an object varies over time. Using this data, researchers aim to make possible the understanding of how Earth-like planets will appear when seen from afar. If Earth were viewed from outside the Solar System, the diversity of its biology—deserts, forests and ocean— would not appear. What would appear to the distant observer of the Earth, unlike less attractive biological candidates like Mars or Venus, would be a flicker. Applying the details of those light fluctuations makes for a way to define habitability based on the light-curve (brightness vs. time).

The science team at Princeton University and the Institute for Advanced Study are participating in the early planning or a NASA mission known as the Terrestrial Planet Finder, a space probe that will scan the skies for planets hospitable to life. What’s unique about their analysis, is that in addition to the unchanging signatures that typically show the Earth-like atmospheric light in chemical bands for water, ozone and carbon dioxide, is also how the spectra changes over time. What they’ve uncovered is that indeed a flickering Earth, and the wide variation in total brightness, has precursors in the biological content of the planet – making it a valuable tool to finding Earth-like planets.

Twinkle, twinkle, tiny, blue Earth

Looking from great distances during a terrestrial day, the most distinctive fluctuations in the Earth’s light-curve is a strong bright/dark cycle. So in our own solar system, to differentiate Earth from less likely biological candidates astronomers can spot the relatively fast changes in reflected light (>10%).

According to Professor Edwin Turner, Director of Princeton University’s 3.5 meter telescope at the Apache Point Observatory, and co-author of the Nature paper: “The large amplitude (tens to even hundreds of percent, depending on band and viewing/illumination angles) and fast time scale (few hours) strongly distinguish the Earth’s light curve from that of other major bodies in the Solar System, which typically show amplitudes of a few to at most 10 percent,” said Turner. “The reason is the mixture of rather dark features (oceans, for example) with very bright ones (cloud systems and continents) of large size on Earth’s surface.”

Seeing the Trees with Computer Models

Prior to the Princeton study, no one could say how much the Earth might flicker, since the chances for distant observation are preciously few. But taking what was known about the reflectance of all kinds of landscapes—from wheat fields to glaciers—the scientist took on the tough problem of reconstructing the particulars of how a day’s rotation on Earth might look from outside the solar system. The results predicted variations in light of up to 150 percent over the course of a day.

These computer models helped the Princeton astronomers identify what is giving rise to these dark-light fluctuations. Different landscapes (sometimes called biomes) have a mixture of different colors and brightness features that make up its distinctive signature. “In our computer model, each point on the planet’s surface is classified into one of several types (water, ice, cloud, bare land, vegetated land, etc),” said Turner. “For each type, we have a function that describes the probability that light striking it from a certain direction will be scattered/reflected into any other direction. It also depends on the wavelength of the light. The computer then simply propagates a huge number of “rays” of light from the model star onto the model planet, figures out the geometry of where they strike and which direction they are reflected into and then adds them up to get a total brightness.”

Once a picture of total brightness and light fluctuations becomes available, then the question arises as to what biological markers might add or subtract from those gross measures. “This is currently an area of active research and discussion,” said Turner. “Spectroscopic indications of atmospheric water vapor and oxygen are now considered to be among the best alternate indicators of biological activity. However, for both these biomarkers and all others, there remains considerable uncertainty about whether they are necessary or sufficient or even useful. This is a very new field!”

What’s Next

Understanding the Earth as it looks from afar is one key to finding new planets with biological potential. But to unravel the temporal and spatial features of other planets requires careful attention. The star for instance is up to millions or even billions of times brighter than any nearby planet and can saturate the field of view. “At present we cannot obtain any relevant data due to the very faint light expected from even the nearest extra-solar terrestrial planets and their very close association (in the sky) with the much brighter primary stars they circle,” said Turner.

Several NASA observational missions in the next decade will enhance the capability to find such candidates. With resolution capabilities 100 times more than the Hubble Space Telescope, the JPL Terrestrial Planet Finder (TPF) mission has reached design stages for formation-flying between multiple imagers across great expanses. By 2015, the European Space Agency will also fly an Infrared Detecting Observatory (called Darwin) which aims to image planets the size of Venus and Earth from 30 light-years away.

“Once TPF or Darwin or other missions/techniques makes it possible to see these objects,” said Turner, “the main challenge will be to obtain high precision and well sampled light curves over a period of at least several rotations (of the planet) and preferably longer.”

Eric Ford (grad student at Princeton) and Sara Seager (postdoc at the Institute for Advanced Study) were collaborators with Ed Turner on this project.