Astronomers have discovered the first Earth-like planets orbiting a star similar to our own Sun.
Kepler-20f could have a thick water-vapour atmosphere but its surface is believed to be too hot for life.
A second planet in the same system, Kepler-20e, is only slightly smaller than Earth and even hotter.
Neither lie within the "habitable zone" where temperatures are just warm enough to allow liquid surface water, increasing the prospects for life.
But scientists are excited by the technical achievement of detecting Earth sized "exoplanets" since it increases the chances of one day finding a potential life-bearing world.
Dr Francois Fressin, one of the astronomers from the Harvard-Smithsonian Centre for Astrophysics in Cambridge, US, said: "It is the first time humanity has been able to discover an object similar to the Earth around a star, so maybe we will be able to find others.
"This could be an important milestone. I think 10 years or maybe even 100 years from now people will look back and ask when was the first Earth-sized planet found. It is very exciting."
A number of extrasolar planets have already been identified with radiuses of 1.5 to twice that of the Earth.
But Dr Fressin pointed out that even these have far more volume than the Earth and it would be wrong to consider them truly "Earth-like".
Earlier this month the reported discovery of Kepler-22b, a planet 2.4 times the size of the Earth situated in the middle of its habitable zone, led to wild speculation about oceans, continents and living creatures.
"You could fit 13 Earths inside Kepler-22b," said Dr Fressin. "The most likely thing is that it's simply a mini-Neptune, not suitable for life.
"Just because a planet lies within the habitable zone that doesn't mean it is habitable."
Like Kepler-22b, the two new planets were found by the American space agency Nasa's Kepler space telescope.
Astronomers detected them by observing the tiny dimming of light that occurred when they passed in front of their parent star.
Both planets are believed to be rocky, with a composition of iron and silicate, and very hot.
Kepler-20f is a baking 426C and Kepler-20e a scorching 726c.