Astronomers said the discovery showed that Kepler could indeed find planets as small as our own and was an encouraging sign that planet hunters would someday succeed in the goal of finding Earth-like abodes in the heavens.
Since the first Jupiter-size exoplanets, as they are known, were discovered nearly 15 years ago, astronomers have been chipping away at the sky, finding smaller and smaller planets.
“We are finally there,” said David Charbonneau, an astronomer at the Harvard-Smithsonian Center for Astrophysics, who was a member of the team that made the observations, led by his colleague Francois Fressin. The team reported its results in an online news conference Tuesday and in a paper being published in the journal Nature.
Dr. Fressin said, “This demonstrates for the first time that Earth-size planets exist around other stars and that we can detect them.”
The announcement doubled the number of known Earth-size planets in the galaxy to four from two — Earth and Venus.
The next major goal in the planetary hunt, astronomers say, is to find an Earth-size planet in the so-called Goldilocks zone of a star, where conditions are temperate for water and thus life. We are not there yet.
The two new planets, Kepler 20e and Kepler 20f, are far outside the Goldilocks zone — so close to the star, termed Kepler 20, that one of them is roasting at up to 1,400 degrees Fahrenheit — and thus unlivable.
Although the milestone of an Earth-size planet had long been anticipated, astronomers on and off the Kepler team were jubilant. Geoffrey Marcy of the University of California, Berkeley, another Kepler team member, called the new result “a watershed moment in human history.”
Debra Fischer, a planet hunter from Yale, who was not part of the team, said, “This technological feat is incredibly important because it means that the detection of Earth-size planets at larger distances is technically possible.”
Kepler 20e, the closer and hotter planet, is also the smaller — about 6,900 miles across, or slightly smaller than Venus — and it resides about 5 million miles from its star. The more distant planet, Kepler 20f, also broiling at around 800 degrees, is 10 million miles out from its star. It is 8,200 miles in diameter, about the size of Earth. The two planets are presumed to be rocky orbs that formed in the outskirts of their planetary system and then migrated inward.
Their star, which is slightly smaller and cooler than the Sun, is about 950 light years away from us. Kepler had previously found three larger Neptune-like planets around it, so the new observations bring the total to five so far. All the planets are well inside where Mercury would be in our own solar system, presenting a bounteous system of unlivable planets.
“This is Venus and Earth in a five-planet system,” Dr. Fischer said in an e-mail. “There’s no place like home, and the Kepler data are starting to uncover some mighty familiar architectures.”
Kepler detects planets by watching for blinks when they move in front of their stars. Since it was launched in 2009, it has found 2,326 potential planets, 207 that would be Earth-size, if confirmed as the two reported Tuesday have been.
Confirmation of a planet, however, requires additional observations, usually of its star’s wobbles as it gets tugged by the planet going around. The gravitational pull of planets as small as the Earth on their parent star is too small to measure with the current spectrographs. And so the astronomers resorted to a statistical method called Blender, developed by Dr. Fressin and Guillermo Torres of the Harvard-Smithsonian Center, in which millions of computer simulations of background stars try to mimic the Kepler signal. They concluded that Kepler 20e was 3,400 times more likely to be a planet than background noise, while the odds in favor Kepler 20f being real were 1,370 to 1.
Confirmed (or validated, as the Kepler team likes to say), they join the other planets already known to orbit the star. In a surprise for astronomers who thought they knew how planetary systems form, the orbits of the new planets are sandwiched between the orbits of the older, bigger, gassier ones, a configuration that does not occur in our own solar system.
In an e-mail, Dr. Charbonneau noted: “In the solar system, rocky worlds and gas giants don’t mingle. But in the Kepler 20 system they apparently do.”