Kepler-186f (exoplanet)

Kepler-186f is an exoplanet, or planet outside the Sun’s gravitational pull, orbiting the red dwarf (M dwarf) star Kepler-186, and was the first Earth-sized planet discovered in the habitable zone of another star. Its discovery by the Kepler spacecraft (for which it and its red dwarf are named) was announced on April 17, 2014. Although it is located 490 light-years away from Earth and unavailable for further exploration even by telescopic means in the near future, Kepler-186f’s discovery was an important milestone in space exploration for proving the existence of planets of its type. Furthermore, public interest in the planet and exoplanets in general, including the search for potential extraterrestrial life, is instrumental in securing funding for further missions.

Background

The Kepler spacecraft, named for the German astronomer Johannes Kepler, was launched on March 6, 2009, as part of NASA’s Discovery Program of science-based missions. Its goal was to find exoplanets, or planets outside of Earth’s solar system. Kepler itself was designed to be a simple craft, consisting primarily of a sensitive photometer to monitor the brightness of the stars in its field of view. Specifically, the photometer was aimed at a fixed field of view in order to continually monitor approximately 150,000 main sequence stars, or stars with a stable balance of nuclear fusion at the core and gravitational forces (most stars are main sequence stars). That data was transmitted by the craft to Earth for analysis.

The periodic dimming of a given star indicates planets in transit around that star, and modern data analysis and Kepler’s imagery are precise enough to calculate specific characteristics of those planets based on this dimming. By early 2016, Kepler had discovered well over two thousand exoplanets through this method. Though exoplanets had been assumed to exist for hundreds of years, it was not until the late twentieth century that the first confirmed discoveries were made. Even then, for years technological limitations meant that detections were rare and often controversial. Most of the planets first detected were gas giants very close to their stars, making them unlikely candidates for Earth-like conditions, and therefore potential life. The Kepler mission vastly expanded the number of known exoplanets, including those in the habitable zones of their stars.

The habitable zone of a star, or circumstellar habitable zone, is the area around a star in which the amount of radiant energy from the star is just right that a planet orbiting in that range could theoretically, with sufficient atmospheric pressure, support liquid water. Planets that are too close to a star would be too hot, and water would boil off; planets that are too far would be too cold, and any water would remain ice. For these reasons, the habitable zone is sometimes called the Goldilocks zone.

It is important to remember, however, that the fact that a planet occurs in the habitable zone does not guarantee that it is inhabited by extraterrestrial life or is habitable by humans, only that it is possible that it could be. Being in the habitable zone does not guarantee the presence of atmospheric pressure or the presence of liquid water, which are considered prerequisites for life as we would recognize it.

Overview

Kepler-186 is a main sequence red dwarf. Red dwarfs are one of the most common types of stars in our galaxy, but are too dim to be easily seen from Earth. They are much cooler and dimmer than our Sun, and so Kepler-186’s habitable zone is naturally closer to the star than our Sun’s. Kepler-186f is only 32.5 million miles (52.4 million kilometers) from Kepler-186, compared to Earth’s average distance of about 93 million miles (150 million kilometers) from the Sun. Its proximity to its star means that Kepler-186 appears larger in the exoplanet’s sky—about a third larger than the Sun appears in Earth’s sky—but even at that proximity, it receives less light than Earth does. The effects of that light—that is, what the sky would actually look like on Kepler-186f, how bright it would appear from the surface, and what color it would be—are unknown, since those things all depend on atmospheric conditions. The same is true of Kepler-186f’s temperature. Not enough is known of the planet’s composition and atmosphere to determine whether, beyond size, it is relatively Earth-like or not.

It is possible to determine the Kepler-186f’s orbit. A year on the exoplanet takes only 130 Earth days to orbit the red dwarf. The length of the day (the time it takes for the planet to rotate) is undetermined, however, as is the presence or absence of an axial tilt like Earth’s, which would contribute to seasonal differences in temperature. Such details may be key factors in the potential evolution of life.

There are four other known planets in the Kepler-186 system, all of them closer to the star: Kepler-186b, c, d, and e. The four inner planets are assumed to be tidally locked, meaning that one side of the planet constantly faces the star. Kepler-186f is far enough away that this is probably not the case; however, many believe that its proximity to the star means that it rotates very slowly, and that its day could be Earth weeks long. Given the importance of circadian rhythms to Earth life, this would presumably have a drastic effect on the evolution of any life that might exist on the planet.

As exciting as Kepler-186f is, its great distance from Earth seems to rule out any further information in the near future. Neither current telescopes nor even hypothetical telescopes in current development have the capacity to determine the mass or atmosphere of a planet that far away. Investigations by the SETI Institute, which searches for signs of extraterrestrial intelligence, have found no radio emissions from the Kepler-186 system. For the twenty-first century, the main legacy of Kepler-186f is what its existence proves: that other Earth-sized planets in habitable zones exist, and that we have the means to discover them. Given that in May 2016 NASA announced that advances in data analysis had resulted in roughly doubling Kepler’s number of exoplanet discoveries, future advances in optics and data analytics will yield even more productive missions.

Bibliography

Encrenaz, Thérèse. Planets: Ours and Others: From Earth to Exoplanets. Hackensack: World Scientific, 2014. Print.

Johnson, John Asher. How Do You Find An Exoplanet? Princeton: Princeton UP, 2016. Print.

Lammer, Helmut, and Maxim Khodachenko, eds. Characterizing Stellar and Exoplanetary Environments. New York: Springer, 2015. Print.

Linde, Peter. The Hunt for Alien Life: A Wider Perspective. New York: Springer, 2016. Print.

Nordseth, Anna. "Six Exoplanets in Our Universe that Could Support Life Other than Earth." Discover, 31 July 2024, www.discovermagazine.com/the-sciences/6-exoplanets-in-our-universe-that-could-support-life-other-than-earth. Accessed 15 Nov. 2024.

Perryman, Michael. The Exoplanet Handbook. New York: Cambridge UP, 2014. Print.

Pont, Frédéric J. Alien Skies: Planetary Atmospheres from Earth to Exoplanets. New York: Springer, 2014. Print.

Seager, Sara. Exoplanet Atmospheres: Physical Processes. Princeton: Princeton UP, 2010. Print.

Stevenson, David S. Under a Crimson Sun: Prospects for Life in a Red Dwarf System. New York: Springer, 2013. Print.