U.S.-based astronomers say they have for the first time discovered a planet on which key conditions for life appear to exist.
The planet is positioned at just the right distance from its sun to support water, and is the right size to have gravity and an atmosphere.
The planet goes by the awkward name of "Gliese 581g" and it is only 20 light years away from earth, in the Libra constellation.
The planet is remarkable, experts say, because it is very similar to Earth in its essential characteristics.
Astronomers at the University of California at Santa Cruz say it's orbiting at just the right distance from its sun to be not too hot or too cold for life as we know it. It offers ideal conditions for the creation of liquid water, that great sustainer of myriad life forms.
It is also of a suitable size -- three times the mass of earth -- to have a gravitational pull to retain an atmosphere, and it has a hard surface.
The university's professor of astronomy, Stephan Vogt, told journalists that "chances for life on this planet are 100 percent."
Experts say that even if Gliese 581g is host to simple bacteria, such extraterrestrial life would change all our ideas about whether we earthlings are sailing through space alone.
Vogt says the comparative ease with which this planet was located indicates that statistically there must be many others in space harboring similar conditions suitable for life.
He says that considering the hundreds of billions of stars in the Milky Way, there may be tens of billions of solar systems containing planets with potential for life.
A practical investigation of Gliese 581g will have to wait however. Even though in space terms it is a close neighbor of earth at 20 light years away, it would take two generations for a spacecraft to reach it.
Vogt gave some extra details of the planet, which was discovered and assessed in the course of 11 years' research at an observatory in Hawaii. It orbits its sun -- a so-called "red dwarf" star -- every 37 days, so its orbiting time is approximately one-tenth that of earth. A red dwarf is a small, relatively cool star.
Also, Gliese 581g does not rotate around its own axis, which means that one side is bathed in perpetual sunlight, the other in darkness. This makes its major surfaces unsuitable for life as we know it, but on the fringes of the two, in the dawn and dusk areas, the climate would be what Vogt called "shirt-sleeve weather."