EARTH is not alone. Or at least it has started to look less like a lonely one-of-a-kind. Scientists working on data from the planet-searching Kepler probe have announced 1,284 newly validated exoplanets and 1,327 other objects that are more likely than not to be planets orbiting distant stars. This is the largest number of such exoplanets that have ever been announced in one go, and among them were nine planets that could be something like our home planet. They are described as orbiting “the Goldilocks” zone (not too hot, not too cold, like the porridge), a distance from their stars where water and life might be possible.

Estimates now suggest that there are around 100 billion planets in the whole of the Milky Way. Over 3000 exoplanets have been confirmed by all detection methods, including Kepler. But we know little about most of these planets, including those Kepler has identified. They are just glimmerings of potential - but they provide a list of candidates which other scientists can follow up and investigate from down here on Earth. Such research includes "planetary characterisation" which Professor Ken Rice of Edinburgh University is involved in - a project that revolves around HARPS-N, a 'planet-searcher spectograph' based in the Canary Islands, whose measurements can be used to find out the size and density of planets.

For people like Rice these exoplanet figures are not surprising. For a while, astronomers and astrophysicists have been expecting that Kepler, which has been observing a patch of sky containing 150,000 stars since it was launched in 2009, was likely to announce a big haul. Planets, Rice says, are common. “They are everywhere - we can just point the telescope up and we’ll find one.”

What are exoplanets?

Their name may make them sound strange and unfamiliar, but exoplanets are merely planets orbiting round a star other than our own. The certain knowledge that the galaxy is bursting with such planets, in all different colours, types and sizes, is relatively new. The first exoplanet was discovered in 1988. This is something says Dr Beth Biller, an exoplanet scientist involved in a project taking images of exoplanets at Edinburgh University, worth marvelling it. “It was only a few decades ago that the very first planet was detected and I think people thought maybe we’ll discover tens or hundreds of these and Kepler has found thousands.”

It has now been estimated that there are around 10 billion exoplanets just in the Milky Way galaxy. Some are gaseous, some dense, some orbit around two stars. Even rocky planets vaguely similar to earth are actually quite common. As Professor Ken Rice of Edinburgh University puts it: “It appears that every star will have some sort of rocky companion. Of course the thing we still don’t know is how many of those are going to be in the right place to potentially have life.”

What are exoplanets like?

Almost as mind-blowing as the notion that there may be another Earth, are the revelations around the vast variety of exoplanets that are entirely unlike our planet. These are other worlds, utterly alien to us, mostly completely inhospitable. There is for instance, HD 189773b, 63 light years from Earth, which glows with the azure blue of light that is reflected from particles of silicate in the upper atmosphere. Gravity causes these particles to form glass shards that whizz around the planet on winds of 4,000 miles per hour. Or there is Gliese 436b, a planet which has a constant temperature of about 800 degrees Fahrenheit, way higher than the boiling point of water, but is still covered in ice due to extremes of gravity. Or, then there's great granddaddy of planets, PSR B1620-26 b, also known as the Genesis planet, around 13 billion years of age, the oldest planet that we have ever discovered.

Edinburgh University’s Dr Beth Biller’s research revolves around taking pictures of planets around other stars – none of which are remotely like Earth. Yet all are fascinating. “The planets I look at are quite far out, further out than Jupiter. They are very much not habitable. But they are just interesting in and of themselves. Some of them have crazy weather.” Part of the draw for her is, she says, “just to study other worlds – and that’s what these are, other worlds”.

How long till we visit Earth 2?

At the centre of the rising field of exoplanet study, is the search for what are called “Earth-like planets” and, ultimately, what some refer to as an “Earth twin”, a planet orbiting around a star like the sun, at the same distance as the earth is from the sun, and roughly the same radius and mass. As Professor Ken Rice of Edinburgh University describes: “We don’t yet have anything like that. We are finding planets that have the right composition as the earth in terms of density but we’re not finding them in the right place.”

However, Natalie Batalha, a Kepler mission scientist, has calculated that the recent Kepler results suggest that there could be 10 billion planets that are roughly Earth-sized, and possibly habitable, in the galaxy.

An Earth twin is, in effect, the Holy Grail of this field. Such a planet might be habitable, or even already inhabited. It could be somewhere we might want to send a space craft. It has the potential to ignite the imagination, kick off a fresh new space race and the development of long distance space travel. Batalha has estimated that the nearest habitable planet could be as little as 11 light-years away. Some believe this is possibly reachable by some kind of craft in our lifetime. After all, recently Stephen Hawking announced a plan to send a tiny “nanocraft” called Starchip to our nearest stellar neighbour, Alpha Centauri, which is 4.4 light-years away. As Hawking put it: "The limit that confronts us now is the great void between us and the stars but now we can transcend it, with light beams and light sails and the lightest spacecraft ever built we can launch a mission to Alpha Centauri within a generation.”

To send humans to visit, of course, is far more of a challenge. Apart from anything else, one of the biggest problems might be how to carry the quantities of fuel to take them there.

But, in any case, we have yet to find that twin. Some believe that we will do so within the next ten years, though this, perhaps, relies on the coming to fruition of a European Space Agency mission - PLATO (Planetary Transits And Oscillations of Stars) - due to launch into space in eight years time. Unlike Kepler, which looked at faint stars, PLATO has been designed to look at the brightest stars, and with the specific aim of finding Earth-like planets. As Dr David Brown of the University of Warwick, an exoplanet scientist working on the project puts it: “PLATO has been designed specifically to find rocky earth-like planets that could have liquid water, around stars like the sun. That is what excites me. That is one of the things I would love to see finally happen. It is only a matter of time. We’re so close at the moment that I want to say sooner than ten years.”

However, such an Earth-like planet might not necessarily be host to life, and even if it is, it may not be the advanced intelligent life form we dream of communicating with. As Brown conjectures: “I think there is life out there. I think the odds are stacked for there being life out there, but they’re also stacked in favour of it being plant or bacterial.”

Will they be anything like the planets in sci-fi?

Before we knew anything about these billions of planets out there, or even of their existence, we were imagining them – from Priplanis, temporary home to the Robinsons, in the sixties television series Lost In Space to Stanislaw Lem’s Solaris, covered by a sentient ocean, or Altair IV in the sci-fi classic Forbidden Planet. Science fiction writers and film directors have conjured up countless worlds, some harsh, desolate or terrifying. Such stories ignited the imagination of many of the current generation of exoplanet scientists. Dr David Brown, of the University of Warwick, for instance, recalls that it was being a sci-fi fan that inspired him to study astronomy. “It was things like Star Trek and Star Wars," he said. "The idea of exploring other planets, other civilisations.” These two monolithic and very mainstream space series have delivered to us a stream of other worlds. From hot-desert-like Tatooine, home of Luke and Anakin Skywalker, to the Genesis planet of Star Trek.

Often the planets imagined in science fiction have been pretty inhospitable. Arrakis, for instance, the sand-covered planet of Frank Herbert’s Dune. Particularly uninviting is LV-426, the cold, rocky storm-ravaged wasteland of a planet, featured in the Alien series, a place with an atmosphere that can only be tolerated for short period. Then there is, Iain M Bank’s Nasqueron, a gas giant, yet nevertheless home to the Dwellers, a long life-spanned species.

But space contains paradises too - one being Pandora in the film, Avatar, actually not a planet, but a forested moon, orbiting the gas giant Polyphemus in the Alpha Centauri system. It is a perfect Eden, with all nature interconnected, until humans come along and cause inevitable trouble. Will the truth be stranger than these fictions? Very probably.