NEW YORK — 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 Aug. 26.

Observations from the Kepler space telescope 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. 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 Mountain View, Calif.

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.

However, the planets are so close to their parent star that their orbits would fit inside the orbit of Mercury in our 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.

The Kepler planets were the second group of alien worlds to be announced in the week of Aug. 23.

Astronomers with the European Southern Observatory announced Aug. 24 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.


Another Earth?

Astronomers do not yet have a mass estimate for the possible Earth-like world seen by the Kepler observatory, according to Matthew Holman of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

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 said.

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.”

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 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.”