of the planet Kepler-452b, the first near-Earth-size world
to be found in the habitable zone of star that is similar to our sun.
But its discovery raises the bar yet again in the search for Earth 2.0, which is a big part of Kepler's mission.
Jenkins said that Kepler-452b has a better than even chance of being a rocky planet (though there's some question about that - read 'Most 1.6 Earth-Radius Planets are Not Rocky'). Its size implies that it's about five times as massive as Earth. He said the planet might be cloudier than Earth and volcanically active, based on geological modeling.
Visiting Earthlings would weigh twice as much as they did on Earth - until they walked around for a few weeks and "lost some serious pounds," he joked.
An artist's impression shows the surface of Kepler 452b.
In the scenario depicted here,
the planet is just entering a runaway greenhouse phase of its climate history.
Kepler 452b could be giving us a preview of what Earth will undergo
more than a billion years from now as the sun ages and grows brighter.
Credit: Danielle Futselaar / SETI Institute
The planet is about 5 percent farther from its parent star than Earth is from our sun, with a year that lasts 385 days. Its sun is 10 percent bigger and 20 percent brighter than our sun, with the same classification as a G2 dwarf.
But Kepler-452b's star is older than our 4.6 billion-year-old home star - which suggests the cosmic conditions for life could be long-lasting.
Models for planetary development suggest that Kepler-452b would experience an increasing warming trend and perhaps a runaway greenhouse effect as it aged, he said.
Kepler-452b's advantages trump the mission's earlier planetary discoveries.
One involved a rocky planet, just a little bigger than Earth, that was found in its parent star's habitable zone - that is, the kind of orbit where liquid water could exist. But that star, known as Kepler-186, is a shrunken red dwarf rather than a close analog to the sun.
Kepler research scientist Jeff Coughlin said it's not clear how hospitable a planet circling a red dwarf might be.
A rocky planet in the right orbit around a sunlike star is a surer bet.
Scientists said Kepler-452b is on the target list for the SETI Institute's search for radio signals from extraterrestrial civilizations, using the Allen Telescope Array in California - but no alien detection has been reported.
This size and scale of the Kepler-452 system compared alongside our own solar system,
plus another planetary system with a habitable-zone planet known as Kepler-186f.
The Kepler-186 system has a faint red dwarf star.
John Grunsfeld, NASA's associate administrator for science, characterized the newly announced planet as the "closest twin" to Earth discovered so far.
However, he said further analysis of the Kepler data may turn up even closer relatives.
Launched in 2009, Kepler detects alien worlds by looking for the faint dimming of a star as a planet crosses its disk. The SUV-sized telescope has spotted more than 4,600 planet candidates.
So far, about 1,000 of those have been confirmed as planets using other methods, ranging from detecting their parent stars' Doppler shifts to carefully measuring the time intervals between the passages of planets.
For Kepler-452b, scientists used ground-based observations and computer models to estimate the mass and confirm the detection to a level of 99.76 percent, Jenkins said.
The findings were due to be published online Thursday (Discovery and Validation of Kepler-452b - A 1.6-R Super Earth Exoplanet in the Habitable Zone of a G2 Star) by the Astrophysical Journal, Jon Jenkins said.
In addition to Kepler-452b, another 521 planet candidates have been added to the mission's checklist - including 12 candidates that appear to be one to two times as wide as Earth and orbit in their parent stars' habitable zones.
Nine of the stars are similar to our own sun in size and temperature, NASA said in a news release.
There's sure to be more to come.
In 2013, Kepler was crippled by failures of its fine-pointing navigation system, but it returned to its planet-hunting mission last year, thanks to some clever tweaking that makes use of the solar wind as an extra stabilizer.