Kepler-186f: Earth's Enigmatic 'Twin' Across the Stars

Scientists are still in pursuit of discovering whether life exists elsewhere in the cosmos, orbiting other stars. Yet, their relentless search continues. With the help of NASA's Kepler mission (active from 2009 to 2018), our understanding of this possibility has reached unprecedented levels.

During the Kepler mission's operation, the space telescope and its sensor array surveyed over 500,000 stars. Using the transit method, they detected planets by observing periodic dips in a star's brightness, indicating the presence of an orbiting planet.

The Kepler mission uncovered more than 2,600 exoplanets—planets located outside our solar system and orbiting distant stars. Hundreds more were identified after the mission concluded, using the valuable data collected by Kepler. These exoplanets, discovered through Kepler’s observations, became known as Kepler planets. Among these groundbreaking discoveries was Kepler-186f.

What Makes Kepler-186f So Unique?

Kepler-186f's discovery in April 2014 was momentous because scientists believe that planets capable of supporting life will most likely be Earth-sized, with rocky or solid surfaces, liquid water, and a breathable atmosphere—making them similar to Earth in many ways.

This assumption arises from the fact that, so far, Earth is the only planet known to sustain life, according to Dr. Steve Howell, a senior NASA research scientist involved in the Kepler mission.

Kepler-186f made history as the first Earth-sized planet discovered within its star's habitable zone, also known as the Goldilocks zone, where conditions are just right for liquid water to exist on a planet's surface. In this zone, planets are neither too hot nor too cold for water to remain in liquid form.

Before the discovery of Kepler-186f, many planets were identified within their star's habitable zone, but they were all at least 40% larger than Earth. In contrast, Kepler-186f has a radius only 1.11 times that of Earth. This small, Earth-like size is crucial, as smaller planets tend to have rocky surfaces that could potentially support plant life and land. Additionally, Earth-sized planets usually possess lighter, breathable atmospheres, unlike the massive planets like Jupiter and Saturn, which have inhospitable atmospheres dominated by gases like hydrogen and helium.

Although scientists have not yet determined Kepler-186f's mass and composition, they have learned a great deal about the planet. It resides in a five-planet star system approximately 500 light-years from Earth, within the Cygnus constellation. Kepler-186f orbits an M dwarf star (also known as a red dwarf) every 130 days. This dwarf star has about half the mass of the Sun.

Over 70% of the stars in our Milky Way galaxy are M dwarfs—small, cool, and dim stars. Planets orbiting these stars must stay in close proximity to their host to receive enough heat to potentially support life. However, this proximity brings its own challenges, as M dwarfs are known for emitting powerful flares or gas bursts, according to Dr. Steve Howell.

"If these flares are intense enough, they can reach planets in close orbits and potentially strip away the planet's atmosphere or even destroy any life that might exist there," explains Howell.

How Far Is Kepler-186f from Earth?

Researchers have found that Kepler-186f receives only one-third of the energy from its M dwarf star that Earth receives from the Sun, placing it near the outer boundary of the star's habitable zone. This location could shield the planet from any major flares, though Dr. Howell notes that this M dwarf has not shown flares in current observations. This distance might also mean Kepler-186f is not tidally locked, potentially allowing it to have seasons like Earth.

On the downside, being at the outer edge of the habitable zone may cause any surface water on Kepler-186f to freeze. As a result, Kepler-186f might be more accurately described as an "Earth cousin" rather than an "Earth twin."

Scientists have also discovered that Kepler-186f is darker than Earth. At noon, its brightness is thought to be comparable to the light we experience an hour before sunset. However, since the presence of an atmosphere and the planet's exact composition remain unknown, researchers cannot predict what sunrises and sunsets would look like on this distant world.

Kepler-186f's Companion Planets

What about Kepler-186f's companion planets? These include Kepler-186b, Kepler-186c, Kepler-186d, and Kepler-186e, which orbit the same M dwarf star in just four, seven, 13, and 22 days, respectively. This places them closer to the M dwarf than Kepler-186f, making their surfaces too hot for life as we know it to exist.

Currently, the James Webb Space Telescope is closely monitoring Kepler-186f, along with its other celestial tasks, searching for further insights. It’s particularly equipped to study exoplanet atmospheres and analyze their compositions, so we may soon have more detailed information about this intriguing planet.