It was arguably the biggest news in science this month: A graduate student in Australia discovered the continent of Africa.
What makes Sally Langford’s discovery so remarkable — and worthy of reporting in the journal Astrobiology on April 6 — is not what she saw, but how she saw it. Once a month over the course of three years, Langford stood huddled against the evening chill in lonely Australian farmland and watched as the east coast of Africa shone in the midday sun. Using little more than a backyard telescope and a clever idea, she became the first person in history to see the continents and oceans of Earth by watching their reflections in the Moon.
Anyone who has gazed at the bright sliver of a crescent Moon in clear skies has probably also noticed that the dark side is not entirely dark. The faint glow rounding out the Moon is earthshine: light from the bright day side of Earth shining on the night side of the Moon. Scientists have studied earthshine for decades — but Langford, along with her colleagues at Melbourne and Princeton Universities, decided it would make a perfect test case for detecting oceans and landmasses on a faraway alien world.
Although modern technology has made “exoplanets” commonplace, with almost 350 already known and more found each day, astronomers are still eluded by the holy grail of astrobiology — signs of a planet capable of supporting life like our own. If we hope to find an Earth-like world around another star, we must first be sure we would recognize our own.
Unfortunately, to even the most ambitious of NASA’s future telescopes, a distant exoplanet would be visible only as a tiny speck of light. We could never hope to see an alien landscape in detail — but perhaps we could see the speck brighten and dim as it rotated, the light of its sun reflecting off water and land.
So using a small, amateur-quality telescope hooked up to a low-end digital camera, and running off a car battery on the side of the road, Langford braved the elements as well as the suspicions of local farmers. “I had someone call the cops on me one time because he thought I was going to steal his alpacas.”
The success of the experiment made it all worthwhile. Over the course of each evening, Langford watched the earthshine brighten dramatically when sunlight bounced off the Indian Ocean and dim as the African continent rotated into view. The implications for the exoplanet search are profound: If we can see the effect in earthshine, we might also see it in the light of distant world.
And the experiment was surprisingly inexpensive: The whole set-up only cost about $15,000 — next to nothing for a piece of groundbreaking astronomical research. “Earthshine is such a simple idea that we have known about for such a long time,” says Langford, “but it makes looking at Earth as a ‘prototype’ Earth-like planet so easy! An amateur astronomer could definitely take this data.”
Now that the concept has been proven, plans are underway to do similar earthshine observations of the rest of the globe, so that the lessons learned can be applied to chart the landscapes of bona-fide exoplanets — if we can ever actually see them.
Hopeful astronomers are looking toward a NASA mission called the Terrestrial Planet Finder, or TPF — a space-based planet-imaging telescope capable of picking out the faint light of small exoplanets that would otherwise be lost in the glare of their own suns . “Obviously,” says astronomer Eric Ford, of the University of Florida, “we’re anxious for TPF to be launched.”
Unfortunately, the project is currently on hold due to budget problems. “In principle, humans could probably launch such a mission in roughly a decade,” Ford says. “In practice, I’d guess that it may be even further in the future.”
The wait may be long, but the astronomy community will be ready. In the meantime, we can only speculate that somewhere, many light years away, someone might be watching our own little speck gently flickering in the darkness.
