Astronomers are still waiting to find an Earth twin amid a plethora of newly discovered worlds, many of them stranger than anything dreamed of in science fiction.

Many are weirder worlds than scientists ever expected to find

A total of 2,700 planetary candidates have been detected by the Kepler space telescope since its launch four years ago, a meeting of scientists was told in London.

Up to 90 per cent of these are likely to be confirmed as true planets orbiting stars beyond the Sun. Since the early 1990s, astronomers have verified the existence of more than 800 ‘exoplanets’.

Many are weirder worlds than scientists ever expected to find, according to Bill Borucki, principal investigator for the Kepler mission.

They include gas giants three times the size of Jupiter – something previously thought to be physically impossible – a super-dense planet that could be covered in oceans of molten iron, and a ‘styrofoam’ world light enough to float in water.

Another Kepler discovery turns out to be only slightly bigger than the Earth’s moon – the smallest ‘exoplanet’ detected so far.

An odd-couple of planetary neighbours have also been found, one rocky and the other gaseous.

Up to half these worlds inhabit multi-planet solar systems, and 40 to 50 of them dwell within the habitable or “Goldilocks” zone – the orbital region where temperatures are right to allow surface liquid water and, potentially, life.

Yet Kepler’s primary goal – to find a possibly life-supporting second Earth – remains elusive.

“There are about 40 to 50 planetary candidates in habitable zones but we have not yet found an Earth-sized planet in the habitable zone,” said Borucki, from Nasa’s Ames Research Centre. “We’ve certainly found a lot of planets in habitable zones, but they’re not Earth-sized.”

Planets the size of Earth and smaller have been detected in hot regions close to their stars, and worlds larger than Earth further out where temperatures are more conducive to life. Some Jupiter-like habitable zone planets might even be circled by life-sustaining moons, said Borucki.

The smaller a planet is, the harder it is to detect at greater distances from its star, he explained.

“They have to have a big signal, and the signal has got to be much larger than the surrounding noise,” he said.

“Each year our data analysis gets moreclever at finding these smaller planets.”

The evidence suggests that, on average, every star has one or more planets. That means there must be at least 100 billion planets in our galaxy, the Milky Way.

But just because there are a lot of planets, that does not necessarily mean life is common among the stars, said Borucki.

“We can’t tell that,” he said. “First of all we haven’t seen any Earths in habitable zones, where it counts. We speculate that there will be some, but that’s only a guess.”

Kepler is watching more than 100,000 stars in the Milky Way looking for evidence of planets crossing or ‘transiting’ the face of their parent star. When this happens a tiny amount of starlight is blocked out, betraying the planet’s presence. The data allow scientists to calculate both the size of the planet and its orbital distance.

A European Space Agency (ESA) mission still in the planning stage could see the first attempt to find evidence of life in ‘exoplanet’ atmospheres.

British scientists are playing a leading role in EChO (Exoplanet Characterisation Observatory), a new space telescope that, if given the green light, will be launched in 2022-24.

EChO’s job will not be to find new planets but to scan the atmospheres of those already detected. It will be able to detect signs of oxygen and carbon dioxide – both possible signatures of life – in the atmospheres of “super-Earths” orbiting cool stars, such as red dwarfs.

“Hopefully EChO will open the route to even more challenging missions later on,” said Giovanna Tinetti, from University College London, who has led the mission proposal.

ESA will make a final decision about whether to adopt EChO next year.

Borucki and Tinetti were attending a discussion meeting on ‘exoplanets’ hosted by the Royal Society in London.