The first two Earth-like worlds orbiting another star have been detected, although neither are believed to be suitable for life.
But if the planets had water in the past, there's a good chance they could have hung on to it long enough for life to take hold, Linda Elkins-Tanton, with the Carnegie Institution in Washington DC, told reporters on a conference call Tuesday.
The planets, known as Kepler-20e and 20f, are the smallest planets found so far by NASA's Kepler telescope, which was launched three years ago to look for Earth-sized worlds around other stars.
The newly discovered planets have diameters of 6,900 miles and 8,200 miles -- equivalent to 0.87 times Earth -- slightly smaller than Venus -- and 1.03 times Earth.
They join three Neptune-sized gas giant planets circling the parent star Kepler-20, which is slightly smaller and dimmer than the sun. The system is located about 1,000 light years away in the constellation Lyra.
While Kepler-20e and 20f are not thought to be habitable, it is "the next major milestone along the road to finding habitable planets around stars beyond the sun," said Kepler project scientist Nick Gautier, with NASA's Jet Propulsion Laboratory in Pasadena, Calif.
The two new discoveries were found by a team led by Francois Fressin of the Harvard-Smithsonian Center for Astrophysics, using NASA's Kepler orbiting space telescope. The results were published online in the journal Nature.
Finding the relatively small planets was a feat.
Kepler monitors more than 150,000 stars for tiny wobbles in light. The wobbles signal a planet as it passes in front of the star and dims the light reaching the telescope.
The discovery follows the announcement earlier this month of Kepler-22b, a planet around twice the diameter of Earth that is orbiting the right distance from its parent star for liquid water to exist on its surface. Water is believed to be a key ingredient for life.
"Kepler-22b has the right temperature, but it is too big. What we're announcing today are just the right size, but too hot. But you can bet that the hunt is on to find a planet that combines the best of both worlds, a true earth twin," said astronomer David Charbonneau with Harvard University.
Unlike our solar system where the rocky planets Mercury, Venus, Earth and Mars orbit closer in to the sun than the gas giants Jupiter, Saturn, Uranus and Neptune, the five planets circling Kepler-20 are a mix of rocky and gas planets and all orbit closer to their parent star than Mercury is to the sun.
"The architecture of that planetary system is crazy," Charbonneau said. "Rocky planets and gas giants happily mingle together. This is the first time we've seen anything like this."
"I really want to dare my fellow astronomers to try to explain how this system could have formed," he said.