A viewer who answered the call has helped spot a world that appears to be circling a star dubbed SPH10066540.

The planet is described as being similar in size to our Neptune and circles its parent every 90 days.

Chris Holmes from Peterborough found it by looking through time-lapsed images of stars on Planethunters.org.

The website hosts data gathered by Nasa's Kepler space telescope, and asks volunteers to sift the information for anything unusual that might have been missed in a computer search.

"I've never had a telescope. I've had a passing interest in where things are in the sky, but never had any more knowledge about it than that," Mr Holmes told BBC News.

"Being involved in a project like this and actually being the one to find something is a very exciting position."

Chris Lintott from Oxford University who helps organise Planethunters.org added: "We're ecstatic. We've been groaning under the strain of all these people who want to help us, which is exactly how it should be."

The public participation project was launched last year, but it got a huge fillip when it was featured in the popular Stargazing series' return to BBC Two on Monday.

Volunteers have tripled to more than 100,000 people, and the number of images inspected has now reached a million.

The new planet candidate's status will need more checking, but it looks strong, said Dr Lintott.

"It would be our fifth detection since we started and our first British one as well," he added.

The Kepler space telescope, launched in 2009, has been searching a part of space thought to have many stars similar to our own Sun.

It looks for the periodic dip in light that results every time a planet passes in front of one of those stars.

These so-called transits have to be observed several times before a planet will be confirmed. For the orange dwarf star SPH10066540, five such events have now been seen in the Kepler data.

Mr Holmes found a pass; the Planethunters team then looked deeper into the Kepler archive and found it had made other transits before and after.

The candidate has a radius about 3.8 times that of Earth, and orbits its parent star at a distance of 55 million km - a separation similar to that between Mercury and our Sun.

This means the planet is probably too hot to support life.

"Kepler is trying to answer the question: 'how many planets are there in our Milky Way Galaxy?'" explained Dr Lintott.

"Now, you can build an algorithm to search through the data but the chances are it will have some systematics - it may be missing some things. Planethunters is the ultimate check. If the computers don't find the planets, the humans will; and it helps us to be sure that we're getting a true picture of the planet population in the Milky Way."

bbc.com