The Kepler team announced today the discovery of Kepler-452b, an Earth-size planet in the “goldilocks” zone around a Sun-like star. But is it Earthlike?
Kepler-452b sure seems a lot like Earth. This exoplanet has a radius 1.6 times that of our home world and orbits its Sun-like (read: G-type) star every 385 days. That puts it well within the stars’ habitable zone, that fuzzy theoretical region where a planet’s hypothetical surface could support water.
This is the first planet to meet all three criteria (Earth-size, habitable-zone orbit, and Sun-like star), making it the closest cousin to Earth found so far.
But is Kepler-452b really Earthlike? That remains to be seen . . . in fact, it may never be seen. To even begin guessing about a planet’s composition, you have to start with its density, and for that you need its mass. There’s no way to measure mass directly from Kepler data, and the planet is too small and faint for ground-based follow-up. (The planet candidate was confirmed using statistical methods rather than observations.)
What the Kepler team does have is the planet’s physical size. Jon Jenkins (NASA Ames Research Center) and colleagues and others compared that size to other small exoplanets whose size and mass are known, and estimated the planet’s mass to be between 3 and 7 Earth masses.
“We probably won't be able to make a direct mass measurement of this planet anytime soon,” says Jenkins. But what the team can do is refine the planet’s size still further. The planet’s size measurement depends on the star’s size, and measuring that depends on knowing the star’s precise distance, something which the Gaia space telescope will return in a year or two. Even then, Jenkins adds, “we'll have a better idea [of the mass], but it will still be a statistical estimate and not a direct measurement.”
The size measurement and mass estimate put the planet smack in the middle of the dividing line between super-Earths, planets slightly bigger than Earth with rocky surfaces, and sub-Neptunes, planets slightly bigger than Earth with a significant gaseous envelope. As Jenkins put it in a press conference today, “This planet has a somewhat better than even chance of being rocky.”
But that’s not all the team knows about this system. The astronomers are also studying the stellar host: a G-type solar sibling 1.5 billion years older than the Sun, according to stellar evolution models.
“Stars are like people,” Jenkins said at the press conference. “When they’re young, they’re small and dim. When they grow older, they fill out, slow down, and they get brighter.” The star is now 10% larger and 20% brighter than our Sun (though its visible-surface temperature remains the same) — serving as a glimpse into the future of our solar system.
Though the team can’t study the planet’s atmosphere, they consulted planetary geologists and atmospheric scientists to draw the artist’s concept shown above. If it’s rocky with five times Earth’s mass, it would have twice Earth’s surface gravity. It might have significant cloud cover and active volcanoes. Those models also show that the higher influx of radiative energy could soon (in the astronomical sense) put the planet on the brink of a runaway greenhouse effect like the one that boiled away Venus’s oceans a billion years ago.
The Kepler Pipeline
The announcement of Earth’s closest cousin yet comes as part of Kepler’s 7th data release, which netted 521 new planet candidates, including 12 new Earth-size (less than twice Earth’s diameter) planets in their stars’ habitable zone. The tally of confirmed planet has risen to 1,030, up from 1,013 in January.
Kepler has changed its mission, so no new observations are rolling in. It’s the software improvements that enabled researchers to bag new candidates, and an 8th catalog with further improvements is planned for next year.
With a fully automated pipeline, the Kepler team can now mimic the decisions that researchers were making manually. That means that all of the planet candidate detections can be re-evaluated using the full dataset, a feat not possible before. According to Jeff Coughlin (SETI Institute), some 600 new planet candidates were discovered, but another 100 were demoted from candidate status (mostly due to background eclipsing binary stars that mimicked a planet's signal), bringing the total new candidates to 521.
“We still think we have a very high, more than 95%, reliability rate of our planet candidates,” Coughlin adds.
Exoplanet-hunting isn't just for the professionals. Take part in the hunt — read how you can join in Sky & Telescope's March 2014 issue.