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This artist's rendition provided by NASA shows the Kepler space telescope. Kepler is designed to search for Earth-like planets in the Milky Way galaxy. ((AP/NASA) )

Four hundred years ago German astronomer Johannes Kepler published his Astronomia nova, a seminal work that explained the movement of the Earth and other planets around our sun, creating a new awareness of how our solar system worked. Now scientists hope a spacecraft bearing Kepler's name will answer another fundamental question of astronomy: are we unique?

Scheduled to launch no earlier than March 6, the Kepler spacecraft will be the latest telescope to train its gaze on distant stars, looking for signs of planets outside our solar system.

But unlike the Canadian MOST spacecraft or France's COROT telescope — the two satellites currently in orbit capable of detecting planets passing in front of, or transiting, our view of stars — Kepler's mission will be focused on a particular kind of planet: one like Earth.

Since the first planet outside our solar system was definitively detected in 1995, astronomers using space and ground-based telescopes have found some 330 extra-solar planets, or exoplanets.

Most of these have been huge gas giants and often they've been detected by inference: the planets are so large their gravitational influence causes a telltale wobble in the star. Some planets have been spied when they passed in front of our view of the star, causing a subtle dimming of the light reaching us. A select few very large exoplanets have been seen directly.

Finding a twin to our own planet — with a similar size, orbit, and neighbouring star —  has proved far more difficult, says York University astronomer John Caldwell.

Specifically, Kepler will be looking for small rocky planets orbiting sun-like stars at a distance where liquid water could be maintained on the surface. Liquid water is believed to be essential for the formation of life.

"We don't know if there are any planets like us in the universe," said Caldwell, the lone Canadian scientist on the Kepler mission team.

"Hopefully, Kepler will give us a better idea of how typical or how unique we are," he said.

So few Earth-like planets have been discovered because their small size and distance from the star makes them poor candidates for observation through their gravitational influence.

And spotting a transiting planet is a rarity to begin with, as a planet's orbit has to be nearly perfectly aligned with our line of sight. The further away the planet is from its parent star, the less likely it would be that its path would cut across our line of sight. For a planet as far away as Earth is from the sun, the chance it will transit its star is less than one per cent.

The second limiting factor is size: the smaller the planet, the smaller the impact it will have on the light reaching us from the star.

NASA says an Earth-like planet transiting a star like our own would have an effect similar to the dimming one might see if a flea were to crawl across a car's headlight viewed from several kilometres away.

To capture such minute changes, Kepler will cast a wide net, monitoring 100,000 stars along the Orion spiral arm of our Milky Way galaxy continuously and simultaneously.

Kepler can do this because its telescope has a 1.5-metre-diameter primary mirror — the largest telescope ever sent out of Earth's orbit — allowing it to look at an unprecedentedly large field of view, approximately 12 degrees of arc in diameter.

Capturing starlight from the region of the northern sky near the constellations Cygnus and Lyra, Kepler will focus that light on a 95-megapixel array of charge coupled devices — silicon light-sensitive chips like those used in today's television cameras — that will record the light reaching it and be able to discern subtle changes, such as when a planet transits its star.

The telescope is scheduled to spend about three and a half years looking at the region of space, since before it is able to confirm the existence of exoplanets like Earth, it will need to witness at least two orbits around a star. Planets closer to a star, like Mercury in our own solar system, will be detected much more quickly.

For Caldwell, 64, who spent years working on the science team for the Hubble space telescope, Kepler is the one last big project in a long career. The first excitement for scientists will come after the first three months of the mission, he said, when the spacecraft will turn from its view of the region of space and point towards the Earth to transmit its data.

Most of the stars and planets observed will be 600 to 3,000 light years away from Earth, with fewer than one per cent closer than 600 light years. (A light year is roughly 10 trillion kilometres.) So exploration of potential Earth-like planets will remain the stuff of science fiction. But Caldwell said once we know where to look for these planets, future telescopes equipped with devices such as spectrometers might be able to provide us with information on atmosphere composition, for example.

As to how many planets Kepler might find, Caldwell isn't one to speculate.

"Before they were discovered, nobody thought it was possible to find a Jupiter-sized planet so close to its star, and yet we've seen many of these 'hot Jupiters,' " he said.

"I think we'll be surprised."