NASA’s Kepler planet-finding space telescope has discovered three “super Earths” orbiting in the habitable zone of two distant stars, including the one nearest the Earth in size yet discovered.

The planet designated Kepler-62f measures 40% larger than Earth, and is believed to have a rocky composition.

It and its larger, habitable-zone companion, Kepler-62e, orbit a star dimmer and older than the Sun called a K2 dwarf that is 1,200 light years from Earth in the constellation Lyra.

A third exoplanet — Kepler-69c — has been confirmed in the habitable zone of a Sun-like star in the constellation Cygnus some 2,700 light years from Earth.

It measures 70% larger than Earth, and has a 242-day orbit that approximates that of Venus.

“We know of only one star that hosts a planet with life — the Sun,” says Thomas Barclay, the Kepler scientist at the Bay Ares Environmental Research Institute in Sonoma, Calif., who was lead author on a Kepler-69 system paper published Thursday in the Astrophysical Journal. “Finding a planet in the habitable zone around a star like our Sun is a significant milestone toward finding truly Earth-like planets.”

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But Barclay stresses that “we wouldn’t describe these as Earth-like planets ... We simply don’t know if other Earth-like planets are out there yet. We haven’t found any.”

To date Kepler has detected about 2,740 candidate exoplanets by measuring the extremely faint dip in starlight reaching its instruments when a planet passes in front of a star. Of those, 122 plus the seven total planets counted around Kepler-62 and -69 have been confirmed by other telescopes as true exoplanets.

William Borucki of NASA’s Ames Research Center, the Kepler principal investigator, terms the spacecraft a “pathfinder” that will point the way to more detailed discoveries. Earlier this month NASA selected the Transiting Exoplanet Survey Satellite (TESS) mission as an astrophysics Explorer-class space mission.

Capped at $200 million, TESS will scan bright stars within about 30 light years of Earth to find habitable-zone planets that can be better analyzed with spectroscopy. Ultimately, the planned James Webb Space Telescope may be upgraded in space to analyze the atmospheres of some of those planets.

Big telescope

“That’s the edge of what we might be able to do,” Borucki said during a press conference announcing the latest Kepler discoveries. “You need a big telescope. You need a telescope that blocks out the light of a star, because it’s over a billion times brighter than a planet. That’s extremely difficult to do, to the point where you block it off so thoroughly that you can actually see the planet. Once you can do that, then you can actually do spectra and get the composition of the atmosphere.”

To date astronomers can only make educated guesses about the compositions of the exoplanets they are finding, based on similarities to other exoplanets.

While Kepler-62f probably has a rocky structure, there is a chance Kepler-62e may be a “water world,” Borucki said.

“If we look at our own ocean, it is just full of life,” he said. “Speculating as to what those oceans would be like is a puzzle, but one of the things that’s important is to get elements into that water. Some [other Kepler work involves how to] get elements from the rocky into the core to build life.”