Houston, we have a problem. The fate of everyone's favourite planet-hunting spacecraft, Kepler, is on the line this week, with NASA announcing that it's unexpectedly entered 'emergency mode' (EM).

That means that Kepler is at its "lowest operational" level, and is burning through a huge amount of fuel - and there's a real fear that our best chance at finding potentially habitable outside our Solar System is slipping away. NASA is now scrambling to get the space telescope back up and running from more than 120 million km (75 million miles) away.

"Recovering from EM is the team's priority at this time," wrote mission manager Charlie Sobeck in an update on Friday. "The mission has declared a spacecraft emergency, which provides priority access to ground-based communications at the agency's Deep Space Network."

The last contact the US space agency had with Kepler was on April 4, and there were no indications that anything was wrong with the spacecraft at the time.

NASA still has no idea what could have triggered the entry into EM, but they note that it happened just before they began pointing the spacecraft towards the centre of the Milky Way, to begin a new planetary survey.

Kepler completed is original mission back in 2012, during which it detected almost 5,000 suspected exoplanets, with 1,000 of those now being confirmed by researchers.

After its initial success, Kepler is now on a new mission, known as K2, and has expanded its search to include other interesting astronomical objects, such as young stars and supernovae. In the past year alone, it's discovered something strange and so-far unexplainable orbiting a distant star, the first ever shockwave coming off a supernova, and the most Earth-like exoplanet to date.

With its new focus on the centre of the Milky Way, the spacecraft was going to search for stray planets wandering between stars, any Earth-like planets in the habitable zones of their stars, or strange outer planets at the edge of their solar systems.

"The chance for the K2 mission to use gravity to help us explore exoplanets is one of the most fantastic astronomical experiments of the decade," said the mission's project scientist, Steve Howell.

But the telescope needs to get out of EM to do any of that. And due to Kepler's distance from us, communications are slow. "Even at the speed of light, it takes 13 minutes for a signal to travel to the spacecraft and back," writes Sobeck.

Still, don't lose hope just yet - this isn't the first time Kepler has run into trouble.

There have been several breakdowns over the years, and back in 2013, NASA said the spacecraft was "beyond repair" when the wheels that control where the telescope aims failed. Scientists overcame that by using the Sun's energy to push Kepler's solar panels, which meant they could once again control the direction of the telescope.

So we can at least be confident that the best minds in science are coming up with back-up plans to save the US$600 million space telescope from early retirement.

But it's still an incredibly stressful time for NASA, and we'll be watching closely for updates to see how Kepler fares. Let's cross everything we've got, because we still have so much to learn from our little planet-hunting telescope in the sky. Good luck, little guy!