Scientists have confirmed the existence of two Earth-sized, rocky planets orbiting a star called Kepler-20, 1,000 light-years away in the constellation Lyra.
The planets are the smallest ever confirmed orbiting a sun-like star, and their discovery, reported Tuesday, is an important milestone for NASA's Kepler mission, which faces the technically daunting task of finding small, Earth-like worlds in faraway solar systems that may — or may not — have been able to sustain life in the past.
But the discovery of the planets by the space-based telescope is also a reminder of how little astronomers understand even about the origins of our own solar system, scientists said.
Kepler-20e and Kepler-20f, as the two rocky planets are called, are interspersed with three larger, gaseous planets, believed to be similar to our own Neptune, in an alternating pattern that has astronomers somewhat perplexed. All five planets orbit their sun at a closer distance than Mercury orbits ours.
Astronomers hadn't thought that such a tightly packed planetary system with alternating rocky and gaseous planets could be possible, said Caltech astronomer Mike Brown.
"We love Earth-sized planets, but we knew we would eventually find them," Brown said. But, he added, "Here you have these crazy things that no one would have guessed. It means the canonical ideas we have just don't work."
Scientists don't expect to find rocky planets mixed in with gaseous or icy ones because that isn't the way things are in our solar system, said Harvard astronomer David Charbonneau, a coauthor of two articles reporting the findings, one of which was published online Tuesday in the journal Nature.
The traditional explanation for why the two types of planets don't mingle goes something like this, he said: When the sun formed, there was a leftover pancake of material surrounding it. The planets formed when solid particles from that pancake stuck together and grew like snowballs.
Close to the sun, that solid material was rock, and because there wasn't much of it, the so-called terrestrial planets — Mercury, Venus, Earth and Mars — were relatively small.
At a distance farther from the sun known as the snow line, water also coalesced, leading to the formation of the gas giants Jupiter and Saturn and the ice giants Uranus and Neptune.
The rocky, small planets remained in orbit near the sun. The gassy or icy large planets stayed in place farther away.
"That's the story we tell," Charbonneau said. "And it's increasingly being challenged by these discoveries."
In the last 10 years or so, discoveries of planets elsewhere in the universe — so-called exoplanets — have convinced astronomers that planets actually don't stay put, but instead migrate within their solar systems.
Today, scientists generally believe that the giant planets in our solar system migrated in the past, contributing to the process that's known as the "late heavy bombardment," when asteroids pummeled the Earth and the moon about 4 billion years ago.
"Planets move around a lot, and violently. It's really like a frat party," said Hal Levison, an institute scientist at the Southwest Research Institute in Boulder, Colo., who helped develop the theory.
Charbonneau and his colleagues speculate that the Neptune-like planets orbiting Kepler-20 must have formed in the outer reaches of their solar system and migrated inward toward their observed orbits.
Caltech's Brown, who was not involved in the research, suspected the same.
"To me, this points to a massive jumbling that took place," he said. "Everything started in a normal way" — with rocky planets formed near the star and gassy ones farther away — "and then something happened."
Brown said he wasn't sure what that "something" could be.
But Levison, who also was not involved with the research, said he doubted there would have been room for planetary migrations in the tight-packed Kepler-20 system.
Instead, he suggested that the larger planets were not gas giants at all, but were rocky, just with thick hydrogen atmospheres.
So far, Kepler has identified more than 2,000 planet candidates and confirmed 33 planets. One of these, Kepler-22b, was the first to be located in the so-called habitable zone of its parent star, according to an announcement Dec. 5.
The mission is having a significant effect on the understanding of planetary systems, scientists said.
The fact that it has discovered several solar systems with planets in very tight orbit gives Levison particular pause, he said.
"What makes these systems different than ours?" he said. Or, to flip the question around, what makes our solar system different from those? Why don't we have planets that are closer to the sun?
"It's really weird," Levison said. "I don't understand that."