NASA's Kepler space telescope has found two new planets orbiting a distant sun-like star, and the researchers who made the find say these two are the size of Earth or smaller. That's a first in the search for extraterrestrial life.
If the discovery holds up under scrutiny by other scientists, it could be a very big deal. Earth-sized planets are considered critical in the search for life elsewhere in the universe, but until now, scientists said their instruments were not sensitive enough to detect them.
"Theoretical considerations imply that these planets are rocky, with a composition of iron and silicate," wrote Francois Fressin of the Harvard-Smithsonian Center for Astrophysics, the leader of the team that made the discovery. "The outer planet could have developed a thick water vapour atmosphere."
The team is publishing its report today online in the journal Nature.
The two newly-found planets, called Kepler-20e and Kepler-20f, are much too far away to be seen directly. They circle a star about 950 light-years away. Scientists measured the miniscule dimming of their host star as they passed in front of it, and then did the math to figure out how large they are likely to be and in what orbits they move.
Kepler-20e and f are probably too hot to be friendly to life -- one of them circles its sun in just six Earth days, and the other does it in 19. But the simple fact that they've been found, say the scientists, is reason to expect that others like them exist.
"We would be remiss if we did not do our best to find more planets just like our own," said Alan Boss of the Carnegie Institution for Science, in an email to ABC News. "That does not mean that we necessarily think that only exact Earth twins could be inhabited, just that we at least had better be able to find Earth twins, and then along the way we will be certain to uncover all sorts of other types of exoplanets that should be habitable, and perhaps even inhabited."
Scientists know, from looking at Earth's solar system, that rocky worlds like ours are a precious commodity. If a world is too small (think of Mercury or Earth's moon), any atmosphere will escape into space before life could possibly form. If a world is too large (think of Jupiter or Neptune) it's likely to be all atmosphere, a giant ball of gas or slush that thickens quickly as you plunge beneath its cloud tops, but probably has no solid surface where living things could thrive.
Just two weeks ago scientists reported a planet orbiting a different star, right in the middle of its so-called habitable zone. That planet was believed to be much larger (10 times as massive as Earth), but its temperature was estimated at an average of 72 degrees Fahrenheit -- perfect for liquid water, considered essential for life as we know it.
Put the two finds together, say scientists, and chances are good that some day soon we will find a planet of just the right size and temperature to have at least a chance of being a lively place.
"In less than 20 years, we have gone from not knowing if any other planets exist in the universe, to being able to look out at the night sky and realize that essentially any star we can see has at least one planet, and a good number of those are likely to be habitable," said Boss. "That is a revelation that has not yet dawned on the general public, and even astronomers are having their minds blown when they think about it."