NASA has announced the discovery of an Earth-sized planet just the right distance from its star to potentially support life.

“This is the first validated Earth-sized planet in the habitable zone of another star,” said Elisa Quintana, a research scientist at the SETI Institute and the NASA Ames Research Center.

“It is no longer in the realm of science fiction,” Quintana said during a NASA news conference broadcast Thursday.

Nobody has seen this planet, nor will they in the foreseeable future. Its presence was detected when it passed in front of a dim, distant star.

Astronomers, working on large ground-based telescopes to confirm what was found by NASA’s planet-hunting Kepler Space Telescope, say it’s the proper size and distance from its sun to be deemed in the habitable zone.

“We don’t know if the planet is habitable,” said Kepler scientist Steve Howell. “We don’t know if it has an atmosphere, and that probably will not be known,” he said. “That kind of raises the question ‘Oh, what’s the point?’ ” said Howell.

The point, he said, is that the planet exists and others — many of them much closer than this one — await discovery.

To be clear, this planet probably looks nothing like the artist’s conception of it that accompanies the article announcing it in the journal Science.

The illustration shows a Minnesota-like terrain with a lake surrounded by trees that are oddly colored by the weak visible light emanating from a red sun.

It is based on inferences that the scientists investigating Kepler 186f make in their paper on the discovery.

Astronomers expect it is a rocky planet about the size of Earth that orbits its star in the habitable or “Goldilocks” zone — not so hot that water on it would be vaporized and not so cold that it would be frozen solid.

The search for exoplanets orbiting stars other than the sun had, until this announcement, produced no candidates close to Earth’s size that were in the habitable zone. Kepler has discovered giant planets in the habitable zone and Earth-sized planets too close to their suns to sustain water and life, but none with the right stuff for potential habitability.

Unfortunately, it is too far away to ever visit, or even image.

Kepler 186f is the fifth planet found orbiting this particular M dwarf star, about 500 light-years from Earth. A light-year is about 6 trillion miles.

The other four planets, all about Earth’s size, orbit closer to the star, forming “almost a miniature solar system,” said Mark Everett, of the Tucson-based National Optical Astronomy Observatory (NOAO), who helped Kepler scientists verify the discovery.

Everett and Howell, who formerly worked at NOAO, are two of the paper’s 23 co-authors. They were on a team that used the Gemini North and Keck telescopes on Mauna Kea in Hawaii to rule out the presence of a second star in the vicinity of the planet’s host star.

“We found this to be a single star, which simplifies a lot of the assumptions we can make about the properties of the star and the planet,” Everett said.

The planet orbits its sun every 129.9 Earth days. Kepler, which pointed its photometer, or light meter, at the same field of 160,000 stars for more than four years, recorded eight transits of the planet.

The transit method of exoplanet detection uses the slight dimming of a star’s light to determine the size of a planet passing in front of it.

This planet is a lot closer to its sun than the Earth is to ours.

If it were in our solar system, its position would be somewhere near the orbit of Mercury, the hottest planet.

This planet’s sun, however, is smaller and cooler — a little more than half the size and mass of our sun. Energy hitting this planet would be about the same as that hitting mostly frozen Mars from our sun, said Howell.

Howell, said, however, that this planet is twice the size of Mars and could have an atmosphere that would warm its surface and support liquid water.

Astronomers don’t know the planet’s mass and, hence, can’t say for certain that it is a rocky planet. They infer that it is by comparing it to other solar systems and other solar system models. Other planets this size would be Earthlike — rocky and possibly supporting liquid water.

Tom Barclay, a research scientist with the Bay Area Environmental Research Institute, and lead author with Quintana of the paper in Science, said the planet is “more like an Earth cousin, than an Earth twin” because of the different qualities of their parent suns.

This planet is so faint and so far away, said Howell, that follow-up observations to characterize its atmosphere would be fruitless.

Better opportunities may be closer by, NASA scientists said Thursday.

Closer M dwarf stars will be targeted by NASA’s Transiting Exoplanet Survey Satellite (TESS), said Douglas Hudgins, of NASA’s exoplanet exploration program. It will launch in 2017.

The James Webb Space Telescope (JWST), whose near-infrared camera was built at the University of Arizona, will be “ideally suited” to study the atmosphere, composition, weather and habitability of candidates found by TESS, Hudgins said. JWST is set to launch in 2018.

Marcia Rieke, of the UA’s Steward Observatory, said the James Webb telescope wasn’t built with exoplanet investigation in mind, but it’s “a flexible observatory” that can certainly find the telltale signs of habitability — “water, CO2, methane and a whole bunch of other things.”

Rieke headed the team that built NIRCam, the primary near-infrared imager for the telescope. “JWST is going to be a really wonderful machine for doing this kind of science even though it was designed to look for the first light in the universe.”

Rieke said her group recently held a three-day meeting to discuss how NIRCam can best be used in exoplanet discovery and characterization. Exoplanet discovery wasn’t a big topic when NASA first planned JWST, the successor to the Hubble Space Telescope, she said. “It’s quite interesting that something we had never planned on is now going to be such a big deal,” she said.

The search for candidates could come sooner than TESS’s 2017 launch. Howell said the Kepler team is hoping to get the go-ahead from NASA to use its damaged telescope to search for planets around nearby M dwarfs.

Engineers have devised a workaround that involves pointing its solar collectors at the sun and using that force and its remaining reaction wheels to orient it to a star field that includes stars that are “brighter and closer by,” said Howell.

Contact reporter Tom Beal at or 573-4158.