The imagined appearance of the surface of Kepler-186f

Danielle Futselaar/Science

NASA has announced the discovery of an Earth-sized planet in the habitable zone of a distant star that astronomers call “a landmark on the road to discovering habitable planets orbiting stars besides our Sun.”

Nobody has seen this planet, nor will they in the future, but astronomers, working on large ground-based telescopes to confirm what was found by NASA’s planet-hunting Kepler Space Telescope, say they’ve found the first planet of the proper size and distance from its sun to be deemed habitable.

“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, said Howell, is that the planet exists where it exists. That means others, a lot of them a whole lot closer than this one, are out there waiting to be found.

To be clear, this planet probably looks nothing like the artist’s conception of it that accompanies the announcement. Not that anyone will ever prove it wrong.

The illustration shows a Minnesota-like terrain with trees around a lake, under a dull, red sun along with the four other planets of this miniature solar system.

It is based on assumptions that the scientists investigating Kepler 186-f are carfeul to not make in their paper on the discovery, being published today in the journal Science.

Astronomers expect, but can't prove, that 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 that sweet spot and Earth-sized planets  outside it, but this discovery is a first.

Kepler 186-f is the fifth planet found orbiting an 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 Tucson-based National Optical Astronomy Observatory, who helped Kepler scientists verify the discovery.

Everett was part of a team including Howell that used the Gemini North Telescope 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."

The planet orbits its sun every 129.9 Earth days. Kepler, which pointed its 0.95-meter photometer (a telescope that functions as a light meter) at the same field of 1500,000 stars for more 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 closest and hottest planet.

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

The Science paper published today says: “The intensity and spectrum of the star’s radiation place Kepler-186f in the stellar habitable zone, implying that if Kepler-186f has an Earth-like atmosphere and water at its surface, then some of this water is likely to be in liquid form.”

The threat to this planet’s theoretical viability is that it orbits at the outer reaches of Kepler 186’s habitable zone, which could lead to freezing that theoretical water. 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.

That’s assuming there is water present in the first place.

Astronomers don’t know that. They also 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 rocky.

This planet is so faint and so far away, said Howell, that follow-up observations to answer those questions would be fruitless.

But the mere fact that it exists lends credence to theories that Earth-sized planets exist in abundance in our galaxy.

Howell and the Kepler team are hoping to get the go-ahead from NASA to use their damaged telescope to search stars closer to us.

Kepler had a good run for four years, discovering nearly 1,000 exoplanets of various sizes.

In May 2013, the second of its stabilizing reaction wheels stopped working, meaning it could no longer continue pointing in the same direction.

Engineers have devised a workaround that involves pointing its solar collectors at our 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.

Find a planet like this one in the habitable zone of a bright star in our own galaxy, said Howell, and the real fun begins.