Kepler-186f: Earth's Mysterious 'First Cousin'

By: Melanie Radzicki McManus  | 
The artist's concept depicts Kepler-186f, the first validated Earth-size planet to orbit a distant star in the habitable zone. Here Kepler-186f is orbiting its host star. NASA Ames/SETI Institute/JPL-Caltech

Scientists today still haven't been able to discover whether life as we know it exists on other planets orbiting us. But they're doggedly on the hunt. And thanks to NASA's Kepler mission (which ran from 2009 to 2018), they have more knowledge than ever before about this possibility.

During that time period, NASA's Kepler space telescope and an image sensor array observed more than 500,000 stars. They did this using the transit method — if a star's light dimmed repeatedly at regular intervals, this indicated a planet was orbiting the distant star.


The mission uncovered more than 2,600 exoplanets, which are planets outside our solar system orbiting a star. Hundreds more were discovered after the mission ended, using Kepler data. Planets discovered using Kepler data or through the Kepler space telescope were referred to as Kepler planets. Among those thousands of newly uncovered planets was one called Kepler-186f.

What's Special About Kepler-186f?

The discovery of Kepler-186f in April 2014 was significant because scientists believe planets able to support life will likely be Earth-sized, have a rocky or solid surface, contain liquid water and have a habitable atmosphere — essentially, they'll be planets very much like Earth.

They believe this because, to date, Earth is the only known example in the entire universe of a habitable planet, says Dr. Steve Howell, a senior NASA research scientist who worked on the Kepler mission.


Kepler-186f was the first Earth-size planet found orbiting within its star's habitable zone or Goldilocks zone, the area where liquid water (as opposed to water vapor) could exist on a planet's surface. An orbiting planet can't be too hot nor too cold in order for liquid water to be present.

Many planets had been identified orbiting within their star's habitable zone before Kepler-186f was discovered. But they were all at least 40 percent larger than Earth. In contrast, Kepler-186f had a radius just 1.11 times that of Earth. This small, Earth-like size is important, as it's the smaller planets that tend to be rocky, with terrain that could potentially support trees, plants and land for living. Earth-size planets also tend to have lighter, more breathable atmospheres. In contrast, large planets such as Jupiter and Saturn often have atmospheres filled with inhospitable gasses like hydrogen and helium.

Scientists have yet to determine Kepler-186f's mass and composition, so there's still a lot to learn. But they do know that it's part of a five-planet star system about 500 light-years from Earth in the constellation Cygnus. They also know the planet orbits an M dwarf star (aka a red dwarf) every 130 days. This dwarf star has about half of the mass of the sun.

More than 70 percent of the stars in the Milky Way galaxy are M dwarfs, a star classification indicating stars that are small, cool and dim. Any planet orbiting an M dwarf must be in a relatively tight orbit around the host star for it to receive enough warmth from the star to support life. That can be problematic, as M dwarfs are prone to large flares, or gas ejections, says Howell.

"Those flares, if they're strong enough, can go to a planet world that's very close to them and they could, for example, destroy that planet's atmosphere or life, if life existed on that planet," he says.


How Close to Earth Is Kepler-186f, Really?

Researchers have determined Kepler-186f receives just one-third the energy from its M dwarf that Earth receives from the sun, situating it near the outer edge of the star's habitable zone. This remote locale would help protect it from any large flares, although Howell says this particular M dwarf doesn't appear to have flares, at least from observations to date. This distance may also mean that the planet is not tidally locked, so it might have seasons just as Earth does.

On the negative side, being on the edge of the habitable zone could mean any surface water on Kepler-186f could freeze. So Kepler could be considered more of an "Earth cousin" than an "Earth twin."


Scientists also know Kepler-186f is a darker planet than Earth. At noon, its brightness is believed to be similar to our level of light about an hour before sunset. Yet because they don't know whether Kepler-186f has an atmosphere or its composition, researchers can't determine what sunrises and sunsets would look like here.

Kepler's Companion Planets

And what about those previously mentioned companion planets to Kepler-186f? Named Kepler-186b, Kepler-186c, Kepler-186d and Kepler-186e, these four planets race around the same M dwarf star in a blazing four, seven, 13 and 22 days, respectively. This means they're closer to the M dwarf than Kepler-186f, and their surfaces would be too hot for life to exist as we know it.

Today, the James Webb Space Telescope is keeping an eye on Kepler-186f, among its other celestial duties, probing for additional insights. It's specifically designed to study exoplanet atmospheres and determine their compositions, so hopefully we'll soon know more about this important planet in due course.