The hunt for extraterrestrial life is about to enter a new dimension with the launch of a space probe designed to seek out Earth-like planets.

The Kepler telescope will spend 3½ years locating planets by measuring the brightness of stars and the slight dimming as objects pass in front of them.

Hundreds are expected to be identified during the mission and an estimated 50 of them will be the right size and in orbits suitable to support life.

Results from the mission are predicted to define mankind’s place in the Universe by establishing whether planets that can sustain life are common or if Earth is an extraordinary exception.

If, as expected, several dozen Earth-like rocky planets can be found over the next 3½ years, the chances are that life is common across the Universe. If, on the other hand, few or even no Earth-like planets show up, the chances of mankind being alone in the Universe will be greatly increased.

Discoveries provided by the space telescope will, astronomers maintain, determine the direction of space exploration for many years to come by showing where astronomers should look to seek out extraterrestrial life.

Jon Morse, of Nasa, said: “It’s the first mission capable of finding Earth-sized planets. Kepler will push back the boundaries of the unknown in our patch of the Milky Way. Its findings may fundamentally alter humanity’s understanding of itself.”

The $591 million (£413 million) project is designed to measure the brightness of 100,000 stars in the Milky Way and, by recording when they dim fractionally, detect the passage of planets across their faces. Only a small proportion of planets have an orbit which, from the perspective of Earth, pass in front of their stars, but enough should do so to ensure plenty are detected.

The mission’s prime aim is to find Earth-sized planets in the “habitable zone” of each solar system, the region where an Earth-like planet would be close enough to the star to be warm but far enough away that water will not boil away. The region, often described as the Goldilocks zone, is “not too hot, not too cold, but just right”.

A key element of the project will be Kepler’s ability to detect planets that orbit their stars in about a year, a good sign of a planet in the habitable zone.

In the brightest solar systems, those with Type A stars, Earth-like planets could be detected tens of thousands of light years distant. Those orbting Type G stars, like the Sun, could be detected at about a thousand light years away.

Once planets have been identified by Kepler, ground-based telescopes will be trained on them to confirm their identify and establish their likely composition.

The mission will put to the test a claim made last week by Alan Boss, of the Carnegie Institution in Washington, that planets are so common that most stars similar to the Sun will harbour a satelite capable of sustaining life. Detecting life is beyond the specifications of the mission but among the planets it identifies are expected to be several where life is capable of surviving. “Kepler won’t find ET but it’s helping to find ET’s home,” William Borucki, of the Nasa Ames Research Centre in California, said.

On board the spacecraft, intended to be launched on March 5, will be the biggest camera ever put into space with an aperture a metre wide and a capacity of 95 million pixels to record what it sees, using the same technology that makes digital photography and home camcorders possible. Whereas ordinary cameras have detectors the size of a thumbnail, the ones used in Kepler are a square foot.

Other research projects have identified more than 300 extraterestrial planets so far but all are substancially bigger than Earth and most are gas giants.

Kepler, named after the 16th and 17th-century German astronomer Johannes Kepler, who studied planetary motion, will be able to find small planets — possibly as small as Mercury but, unless Nasa astronomers are way out in their calculations, definitely those the size of Earth.

Professor Debra Fisher, of San Francisco State University, said that the project was a significant step towards realising a dream shared by astronomers around the world, that of taking the first photograph “of a pale blue dot by a star”.

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