When Voyager 1 snapped a picture of Earth from 3.7 billion miles away, our planet appeared in Carl Sagan's words as a dust mote in a sunbeam - a pale blue dot.
When that picture was taken in 1990, no one knew if our dust mote was alone in a vast, lifeless universe or one of many living worlds.
Now, nearly 20 years later, astronomers are gearing up to seek other pale blue dots around other stars. If they find them, the first thing everyone will want to know is if there's any hope of life.
To figure out what we would look for, a team of scientists turned another spacecraft back on Earth.
The craft had already visited and studied comet Tempel 1 as part of a mission called Deep Impact, and NASA was seeking a new job it could do, said Nick Cowan of the University of Washington.
His group had been involved in searching for planets orbiting other stars, he said, and so they proposed studying our own planet from 30 million miles away.
That might sound far but it's close enough to see Australia with a telescope. To see what Earth might look like from much farther, they took the image and reduced it to a single pixel of light.
It looked blue, but blue light might mean a world of continents and oceans or a toxic ball of methane like Neptune. What would distinguish them that would show up in one pixel?
What set Earth apart, Cowan said, was the subtle color changes it made over its 24-hour rotation.
The continents give off more reddish light, and oceans more blue, and over a 24-hour period, he could see Earth became slightly redder and slightly bluer as oceans and continents rotated in and out of view.
No telescope yet built can take a picture of a planet as small as ours in another solar system, but astronomers are busy designing new telescopes that might. Still, if we want to recognize other earths, it will help to have a good picture of our own.
- Faye Flam