Viewed from a star in some other corner of the galaxy, Earth would be a speck, a faint
blue dot hidden in the blazing light of our sun.

Would there be any hint of that speck’s amazing diversity of life? According to a paper
in the Aug. 30 issue of Nature, a savvy alien would find at least one important clue: an
interesting flicker in the pale blue light.

While our neighbors Venus and Mars would reflect a fairly even glow, Earth would put
on a little show. Earth’s light would brighten and dim as it spins, because oceans,
deserts, forests and clouds — which are all too small to be seen from such a distance
— reflect varying amounts of sunlight. The variations, it turns out, are so strong and
distinctive that a surprising amount of information could be taken from a simple ebb
and flow of light.

Scientists at Princeton University and the Institute for Advanced Study conducted a
detailed study of Earth’s reflections not for insights into an alien’s view of our home
planet, but as a way for human scientists to learn about distant planets that may be
like our own. They are participating in the early planning for a NASA mission known as
the Terrestrial Planet Finder, a space probe that will scan the skies for planets
hospitable to life.

“If you looked at our solar system from far away, and you looked at the terrestrial
planets — Mercury, Venus, Earth and Mars — one of the quickest ways to see that Earth
is unique is by looking at the light curve,” said Ed Turner, professor of astrophysics
and a co-author of the study. “Earth has by far the most complicated light curve.”

Eric Ford, a graduate student, and Sara Seager, a member of the Institute for
Advanced Study, developed the idea in collaboration with Turner.

The standard thinking in the field had been that most of the information about an
Earth-like planet would come from spectral analysis, a static reading of the relative
component of different colors within the light, rather than a reading of changes over
time. Spectral analysis would reveal the presence of gasses such as water vapor,
carbon dioxide and oxygen, in the planet’s atmosphere.

Looking at the change in light over time does not replace spectral analysis, but it could
greatly increase the amount of information scientists could learn, said Turner. It may
indicate, for example, the presence of weather, oceans, ice or even plant life.

“It’s just one more tool, one more approach to a very tough problem,” said Turner.

Although the idea that a planet’s light would vary seems straightforward, the three
scientists had no idea whether that variation would be large or small or what it would
look like. After all, there are precious few opportunities to look at Earth from afar, noted
Turner. He and colleagues reached their conclusions by studying existing research on
the light-scattering properties of everything from cornfields to ocean waves. They then
invented computer models of Earth that incorporated the data. The results showed
variations in light of up to 150 percent over the course of a day, with characteristic
signatures for different terrestrial features such as deserts, forests and oceans.

Turner said he and colleagues will continue to refine the idea. One possible way to
test their conclusions is to measure changes in how much light from Earth is reflected
off the moon, a phenomenon known as earthshine. But the real test will be if and
when someone finds the first Earth-like planet. That moment could come in the next
decade or so. NASA is exploring several alternatives for a planet-finding mission that
would launch in 2012 or beyond, and also is seeking plans for smaller projects that
could be launched in just a few years.

With characteristic understatement, Turner noted that if Earth-like planets were found
“they would presumably be objects of tremendous interest.”

Contact: Steven Schultz


Princeton University

Related link

  • 30 August 2001: Characterization of extrasolar terrestrial planets from diurnal photometric variability, Nature (abstract. Fee required for access), Nature (abstract. Fee required for access)

    “Terrestrial planets orbiting in the habitable zones of stars – where planetary surface conditions are compatible with the presence of liquid water – are of enormous interest because they might have global environments similar
    to Earth’s and even harbour life. The light scattered by such a planet will vary in intensity and colour as the planet rotates; the resulting light curve will contain information about the planet’s surface and atmospheric properties. Here we report a model that predicts features that should be discernible in the light curve obtained by low-precision photometry.”