This story was updated at 2:52 p.m. ET.
A tantalizing group of alien planets that may include the smallest, most Earth-sized world yet seen has been discovered around a star like our sun, NASA announced Thursday.
Observations from the Kepler space observatory confirmed two Saturn-sized planets that orbit a star about 2,300 light-years from Earth. They also revealed a candidate for a possible planet roughly the size of Earth within the same system.
Astronomers have not yet confirmed the potential Earth-like planet, but early analysis suggests it has a radius just 1.5 times that of Earth. The Earth's radius is about 3,962 miles (6,378 km). Additional observations of the planetary system will help confirm the planet's existence, researchers said.
"Our hope is that in the coming days or weeks, we'll be able to be more definitive," said William Borucki, Kepler's mission science principal investigator at NASA's Ames Research Center in Moffett Field, Calif.
For the first time, analysis of Kepler observations was also combined with transit timing and radial velocity observations to estimate the masses of the alien planets.
The system's two larger planets – called Kepler-9b and Kepler-9c – were found to have similar diameters, masses and densities as Saturn. The findings represent the first candidate system of multiple planets found by Kepler to be confirmed as such. [5 Intriguing Earth-Sized Planets]
However, the planets are so close to their parent star that their orbits would fit inside the orbit of Mercury in our own solar system, astronomers said. The Earth-sized world, if confirmed, would be so close to its star that it would look nothing like Earth. It would be hot and likely not habitable.
A study led by Matthew Holman, associate director of the Theoretical Astrophysics Division at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., examined the newly confirmed planetary system. The results are detailed in the Aug. 26 online edition of the journal Science.
The Kepler planets are the second group of alien worlds to be announced this week.
On Tuesday, astronomers with the European Southern Observatory announced the discovery of up to seven planets (though two remain to be confirmed) circling a star 127 light-years from Earth. That find also included a potential Earth-like planet, one that is 1.4 times the mass of Earth.
Astronomers do not yet have a mass estimate for the possible Earth-like world seen by the Kepler observatory, Holman said.
For the time being, the potential third planet about Earth's size remains only a candidate, but if confirmed, it would be the "smallest known planet to date," he added.
"We can say, in terms of its physical size, it would be the smallest, but we don't know its mass yet," Holman told SPACE.com.
Kepler's observations suggest the planet has a radius that is about 1.5 times the radius of Earth, and an orbital period of about 1.6 Earth days, which is much shorter than Kepler-9b and 9c.
It takes the larger planet Kepler 9b about 19.2 days to complete one orbit. The other Saturn-sized world makes one orbit every 38.9 days.
Researchers will need to confirm that the candidate body is orbiting the same star, and is not the result of a so-called astrophysical false positive.
Still, the ability to detect such a small candidate demonstrates the sensitivity of Kepler's instruments, and shows that the mission is on track, he said.
"This shows that we're able to find not only long-period systems, but small planets as well," Holman said. "One of the messages of this work is that Kepler is making progress toward its goal of finding systems of planets that are similar to our own solar system.
Too hot to handle
But, in terms of habitability, the Kepler-9 system may not be the best place to search for life forms of any kind.
"It's very unlikely that these planets are habitable," Holman said.
Holman estimated the temperatures of the two large planets by measuring the amount of light they reflect, which gives a rough idea of how much the planets absorb the parent star's light. Given these assumptions, the Saturn-sized planets are probably 740 degrees Kelvin (872 degrees Fahrenheit) and 540 degrees Kelvin (512 degrees Fahrenheit), he said.
"Those temperatures are far beyond the boiling point of water, so these are not habitable planets," Holman said. "This would not be the first place to look for habitability."
The Kepler-9 system
Holman and his colleagues analyzed seven months of Kepler data, and were able to determine the size of the planets by measuring the amount of light they block when passing in front of the parent star. To estimate their masses, however, the researchers had to observe the gravitational interaction between the two exoplanets.
"What you get from Kepler is just the size – you don't get the mass," Holman said. But in the case of the Kepler-9 system, there is more than one planet transiting the star. We can see the deviation from the times at which the planets transit and measure how much the planets are gravitationally interacting, which is due to their mass."
The Kepler space observatory stares at a patch of the Milky Way that contains more than 156,000 stars, in the constellations Cygnus and Lyra. The spacecraft monitors the stars for subtle changes in their brightness, which could indicate the presence of alien planets passing in front of them as seen from Earth.
By measuring the tiny decreases in the brightness of stars when planets cross – or transit – in front of them, astronomers can determine the size of the planet.
In June, Kepler mission scientists announced that more than 700 candidate exoplanets had been found, including five systems that appear to have more than one transiting planet.
Researchers run these candidates through a set of procedures to verify that the signals are indeed from planets, and are not false positives created by binary stars.
"What is really important here is timing signatures," Holman said. "They are characteristic of the mass of the planets, so we can say with confidence that these are planets and not stars."
Studying the orbits
The two Saturn-sized planets in the Kepler-9 system do not have precisely circular orbits, said Holman, but they are not highly elliptical, either. From detailed modeling and by analyzing the timing variations, the researchers were also able to discern the planets' orbital plane.
"We can verify by the timing variations that the planets occupy a very similar orbital plane," Holman said. "We expect that they have similar co-planarity to our solar system – probably co-planar to 10 degrees or so. But, their orbits are pretty different."
The orbits of the exoplanets can also shine a light onto the formation history of the planetary system, said Alycia Weinberger, an astronomer in the Department of Terrestrial Magnetism at the Carnegie Institution of Washington in Washington, D.C.
Given the planets' timing variations and orbital resonance, they "had to change their relative positions with respect to each other and respect to the star over time," Weinberger said in a media teleconference about the discovery.
Understanding such planetary migration will help astronomers understand the initial conditions that led to the final configuration of the Kepler-9 system, and other planetary systems discovered in the future.
Furthermore, it is possible that there are other planets that transit the same parent star in the Kepler-9 system that have yet to be found.
"It's possible that there are other planets there that don't actually transit the star," Holman said. "If a planet has a more inclined orbit, and doesn't cross the face of the star from our line of sight, we would only detect it from the timing variations."
Holman and his colleagues are hoping to expand upon their study by using Kepler and other data to develop a census of the planets in the system.