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Space boffins used data from Nasa's WISE spacecraft to determine the nucleus sizes of comets, and estimate the number out there.

Some comets in the distant parts of the solar system, between 186 billion miles (300 billionkms) away from the sun, known as the Oort Cloud, can be up to 60 miles across.

However, they may only pass the sun every 200 years or so, making them harder to study.

Comets are made up of materials, including rock and ice, left over from the formation of the universe, and it was previously thought there were less than there actually are.

If anything of a few miles or more across hit the Earth, it could destroy the entire planet, wiping out all life.

The findings published in the Astronomical Journal, also found more comets than thought are leaving the Oort Cloud and passing the sun.

A Nasa spokesman said: "NASA's WISE spacecraft, scanning the entire sky at infrared wavelengths, has delivered new insights about these distant wanderers.

"Scientists found that there are about seven times more long-period comets measuring at least 0.6 miles (1km) across than had been predicted previously.

"Researchers also observed that in eight months, three to five times as many long-period comets passed by the Sun than had been predicted."

James Bauer, lead author of the study and now a research professor at the University of Maryland, College Park, said: "The number of comets speaks to the amount of material left over from the solar system's formation.

"We now know that there are more relatively large chunks of ancient material coming from the Oort Cloud than we thought."

The Oort Cloud is too distant to be seen by current telescopes, but is thought to be a spherical distribution of small icy bodies at the outermost edge of the solar system.

The density of comets within it is low, so the odds of comets colliding within it are rare.

Long-period comets that WISE observed probably got kicked out of the Oort Cloud millions of years ago.

Amy Mainzer, study co-author based at NASA's Jet Propulsion Laboratory, Pasadena, California, said: "Our study is a rare look at objects perturbed out of the Oort Cloud.

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A meteor captured above the erupting Klyuchevskaya Sopka volcano

"They are the most pristine examples of what the solar system was like when it formed."

Astronomers already had broader estimates of how many long-period and Jupiter family comets are in our solar system, but had no good way of measuring the sizes of long-period comets.

That is because a comet has a "coma," a cloud of gas and dust that appears hazy in images and obscures the cometary nucleus.

But by using the WISE data showing the infrared glow of this coma, scientists were able to "subtract" the coma from the overall comet and estimate the nucleus sizes of these comets.

The spokesman added: "The existence of so many more long-period comets than predicted suggests that more of them have likely impacted planets, delivering icy materials from the outer reaches of the solar system.

"Researchers also found clustering in the orbits of the long-period comets they studied, suggesting there could have been larger bodies that broke apart to form these groups.

"The results will be important for assessing the likelihood of comets impacting our solar system's planets, including Earth."

Ms Mainzer added: "Comets travel much faster than asteroids, and some of them are very big.

Scientists found that there are about seven times more long-period comets measuring at least 0.6 miles (1km) across than had been predicted previously.

NASA spokesman

"Studies like this will help us define what kind of hazard long-period comets may pose."

Last year research published in the Royal Astronomical Society journal of Astronomy and Geophysics warned there were hundreds of comets in the far reaches of the solar system measuring 60 miles plus and even as long as 160 miles, known as centaurs.

Scientists behind the study said much had taken place into locating as many near-earth asteroids, which vary from tens of metres to many miles wide, as possible.

However, little has been done to plan for the threat of the centaurs the research team claimed.

Professor Bill Napier, co-author of the report from the University of Buckingham, said: "In the last three decades we have invested a lot of effort in tracking and analysing the risk of a collision between the Earth and an asteroid.

"Our work suggests we need to look beyond our immediate neighbourhood too, and look out beyond the orbit of Jupiter to find Centaurs.

If a Centaur deviated towards the Earth's atmosphere, it would be expected to break up, showering the planet with huge chunks of rock and ice that could each devastate a continent and send tsunamis across the seas.

A 10-mile comet or asteroid which struck the Yukon peninsula in Mexico 65million years ago is thought to have wiped out most of the dinosaurs, so if a 60-mile comet hit is it is unlikely anything could survive.

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A meteor streaked across the sky of Russias Ural Mountains

In September 2015 former journalist turned pseudo scientist Graham Hancock published book Magicians of the Gods which warned a 20-mile long fragment of a larger comet was still in the solar system and could strike Earth in just 15 years.

His book claimed that it was part of a bigger comet that struck between 11,600 and 12,800 years ago, wiping out civilisations.

Then, this April, a study by the University of Edinburghs School of Engineering published in the University of the Aegean's International Journal of Mediterranean Archaeology, appeared to have vindicated his claims of the comet strike 13,000 years ago that caused a mini ice age.

Much of Mr Hancock's claims stemmed from evidence found during a dig at an ancient site in Turkey known as Gobekli Tepe, near the border with Syria.

It is more than two times older than Stonehenge, but was a much more skilled construction with astronomical carvings, which Mr Hancock said also told of the ancient comet strike.

The University of Edinburgh's study said the carvings at Gobekli Tepe do show a comet strike, in 10,950BC.

Dr Martin Sweatman of Edinburgh University said: "One of the pillars at Gobekli Tepe seems to have served as a memorial to this devastating event probably the worst day in history since the end of the Ice Age."

More here:

The deadly comets that NASA says could one day hit Earth with devastating effects - Express.co.uk