Image caption Scientists are looking for more rocky worlds that could possibly retain liquid water on their surface

The number of observed exoplanets - worlds circling distant stars - has passed 1,000.

Of these, 12 could be habitable - orbiting at a distance where it is neither "too hot" nor "too cold" for water to be liquid on the surface.

The planets are given away by tiny dips in light as they pass in front of their stars or through gravitational "tugs" on the star from an orbiting world.

These new worlds are listed in the Extrasolar Planets Encyclopaedia.

The tally now stands at 1,010 new exoplanets, bolstered by 11 new finds from the UK's Wide Angle Search for Planets (Wasp).

Abel Mendez of the Planetary Habitability Laboratory at the University of Puerto Rico, said that although the number has rapidly increased in recent years, due to a lack of funding this figure is much lower than it could be.

"We have more techniques and proven technology to detect more exoplanets, but the limit has been telescopes, especially space telescopes.

"If we had more funding there would be more telescopes and that count would be much larger by now."

The Kepler space telescope, which spotted many of these worlds in recent years, broke down earlier this year. Scientists still have to trawl through more than 3,500 other candidates from this mission so the number could rapidly increase.

In January 2013, astronomers used Kepler's data to estimate that there could be at least 17 billion Earth-sized exoplanets in the Milky Way galaxy. They said that one in six stars could host an Earth-sized planet in close orbit.

Exoplanets An exoplanet exists outside our Solar System

Many of those found so far are large planets believed to resemble Jupiter or Neptune

The first exoplanet was discovered in 1992, orbiting a pulsar

A few years later, the planet 51 Pegasi B was found orbiting a star similar to the Sun

Hundreds of extrasolar planets have been confirmed since, and thousands more "candidates" await confirmation

The number of confirmed planets frequently increases because as scientists analyse the data they are able publish their results online immediately. But as the finds are not yet peer reviewed, the total figure remains subject to change.

"Each night we get a list of astronomy papers where there might be an exoplanet announcement. When we get that we have to review it," explained Prof Mendez.

This exoplanet catalogue is organised by Jean Schneider, an astronomer at the Paris Observatory. For the past 18 years he has catalogued new exoplanets on the Extrasolar Planets Encyclopaedia.

Fuzzy planets

Others in the field, like Prof Mendez, are then able to review and comment on the findings, which makes it "more dynamic" he explained.

"That's why the other catalogues just lag behind. The review is reliable as it's exactly the same as what the journals do." Prof Mendez told BBC News.

Nasa will only accept those announced in an academic journal, so it updates its list far less frequently. Their tally currently stands at 919 confirmed worlds.

Jean Schneider said it was also important to note that there was "no consensus for the definition of a planet" and that past experience had shown that a few objects declared as planets were actually artefacts or low-mass stars.

"Some objects, like some Kepler planets, are declared 'confirmed planets' but have not been published in [referenced] articles. It does not mean that they will not be published later on, but it introduces another fuzziness in the tally," he added.

Even if there was a generally adopted definition, Mr Schneider said that for some objects "there is a large uncertainty on parameters, so that the planetary nature of the object is uncertain".

For Prof Mendez reaching 1,000 marks an important milestone in the quest to understand the history of the evolution of the cosmos.

A planet next door

"I don't just want to know where the exoplanets are, I want to understand the stars, because they are the hosts for the planets. I want to understand the whole galaxy and the distribution of the stars because everything is connected," he explained.

For him, the most exciting discoveries are Earth-like planets which could be habitable.

"We want to know how unique our planet is, that's a big question and we are now closer than ever," he added.

For Mr Schneider the most interesting is the candidate exoplanet around Alpha Centauri, as it is circling a star only four light-years away.

This planet likely has the same mass as Earth but is outside the "habitable zone" as it circles its star far closer than Mercury orbits our Sun.