Telescope orbiting the Sun has lost part of its stabilising system, making it too inaccurate to hunt for another Earth

Nasa's hunt for other planets that could harbour life faces a setback: its Kepler space telescope is broken and, since it is in orbit around the Sun, may prove impossible to fix.

The failure of stabilising systems on the spacecraft could mean an end to the $600m mission's search, although Kepler has already outlived official expectations and the space agency is not yet ready to call it quits. The telescope has discovered scores of planets but only two so far that show strong signs of being habitable.

"I wouldn't call Kepler down and out just yet," said Nasa sciences chief John Grunsfeld.

Nasa said Kepler had lost two out of four wheels that control its orientation in space, meaning it can't point at stars with the same precision.

In orbit around the sun, 40m miles (65m kilometres) from Earth, Kepler is too far away to send astronauts on a repair mission like Nasa did to the Hubble telescope. Engineers on the ground are trying to restart one of Kepler's faulty wheels or find a workaround. The telescope could be used for other purposes if it can no longer track down planets.

Kepler was launched in 2009 in search of Earth-like planets. So far it has confirmed 132 planets and spotted more than 2,700 potential ones. Its mission was supposed to be over by now but in 2012 Nasa agreed to keep it running through 2016 at a cost of about $20m a year.

In April Kepler scientists announced the discovery of a distant duo that seems ideal for some sort of life to flourish. The other planets found by Kepler haven't fitted all the criteria that would make them right for life of any kind, from microbes to humanoids.

While ground telescopes can hunt for planets outside our solar system, Kepler is much more advanced and is the first space mission dedicated to that goal.

For the past four years Kepler has focused its telescope on a faraway patch of the Milky Way where there are more than 150,000 suns, recording slight dips in brightness that give away a planet passing in front of a star.

Now "we can't point where we need to point. We can't gather data," deputy project manager Charles Sobeck said.

Scientists said there was a backlog of data they still needed to analyse even if Kepler stopped looking for planets. "I think the most interesting, exciting discoveries are coming in the next two years. The mission is not over," said chief scientist William Borucki at the Nasa Ames research centre in northern California, which manages the mission.

Scientists who have no role in Kepler mourned the news, saying the spacecraft may not be able to determine how many Earth-size planets are in the "Goldilocks zone" where it's not too hot or too cold for water to exist in liquid form on the surface. They praised the data collected by Kepler so far but said several more years of observations were needed to analyse it all.

"This is one of the saddest days in my life. A crippled Kepler may be able to do other things but it cannot do the one thing it was designed to do," said Alan Boss of the Carnegie Institution of Washington.

In 2017 Nasa plans to launch Tess – the Transiting Exoplanet Survey Satellite – designed to search for planets around nearby stars.