Even though astronomers suspect there are millions of Earth-like planets, finding them is fiendishly difficult. Planets only reflect light, they don't produce it. Spotting one at interstellar distances next to their brilliant parent star is close to impossible — like spotting a firefly next to a distant searchlight. It is a task that, until the 1990s, defeated astronomers. Since then, 230 planets have been discovered orbiting other stars but even then they have been observed indirectly — by the planet's gravity making their parent sun "wobble". The wobble effect reveals, roughly, the number of planets, as well as their size and distance from the parent star.

The problem is, only big, Jupiter-like planets — enormous blobs of gas lacking a solid surface, and inhospitable — usually have the gravity to create this effect. That's what makes Gliese special. Like Earth, it is a small planet — 1.5 times Earth's diameter, in fact. Planets of that size are not blobs of gas but rocky. It also orbits its parent sun at a distance where liquid water could exist (the estimated temperature is between 0 and 40 degrees). And liquid water could mean life. It is, in other words, the first potentially habitable planet. "There's probably millions or billions of them out there, but this is the first one to be found," said Malcolm Walter, director of the Australian Centre for Astrobiology at Macquarie University.

"We don't know if it's got an ozone shield or anything like that. "I don't think we know enough about it yet. It's still early days. I don't think we can say anything about interstellar life."

The reason Gliese was discovered was that its parent star is very small — a faint, cool red dwarf. Even small planets can make such stars "wobble". Faint red dwarfs are not the ideal type of sun for Earth-like planets. But, as Mr Walter notes, "any star that can provide enough energy to keep water liquid will do the job as far as I can see … that's the big thing, liquid water". Nevertheless, Gliese would be an odd place. The planet gets enough warmth only because it orbits its faint, cool sun very closely. That produces several interesting side-effects. It has a "year" of only 13 days and tides 400 times stronger than Earth's. It also may be tidally locked — one side scorching, always facing its sun; the other, freezing cold, always facing away, with only a temperate "twilight zone" between capable of hosting life.

The future generation of space telescopes, like the European Space Agency's proposed Darwin mission of 2020, will use special "nulling" techniques to dim the light of stars, thereby enabling direct observation of any planets orbiting them. Even though these planets will be nothing more than specks, their light will tell a huge story. Gliese, presumably, will be on the list. Spectroscopic analysis of the planets' light, says Mr Walter, will reveal what gases make up their atmospheres.

Unfortunately, that's all we'll probably ever glean from Gliese — and from any other planet orbiting another star. It's a case of so near, and yet so far. Our galaxy, containing billions of stars, is 100,000 years wide, one of billions of galaxies in a universe that may well be infinite. That makes Gliese — at 20 light years or 190,000 billion kilometres — one of the Earth's neighbours. But, like Alpha Centauri, it is still enormously distant. Neptune, the most remote planet in our solar system not counting Pluto, is only 4.5 billion kilometres away and it took the space probe Voyager 2, travelling at 16 kilometres a second, 12 years to get there.

The fastest probe launched is New Horizons, now en route to Pluto at 21 kilometres a second. It would take 285,000 years to reach Gliese, and the technology required to significantly speed up spacecraft is still way beyond the realm of feasibility. Even a probe that could travel at the speed of light, the fastest possible speed, would take 20 years, the same time as a message sent from a radio telescope. The Search for Extraterrestrial Intelligence program has been going since 1960 without luck, but as the list of candidate stars grows and the technology improves, who knows?