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Kepler Spies “Earth’s Cousin”
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Today scientists working with the planet-hunting Kepler telescope announced that the mission has achieved one of its major goals: finding an Earth-sized planet in the habitable zone of a star.

Kepler-186f is the first discovery of its kind – an Earth-sized exoplanet in its star’s habitable zone. The planet was discovered by observing transits that occurred every 130 days. Based on how the star’s light dimmed during the transits, the scientists estimated that Kepler-186f is a planet that is about 10% wider than Earth. The new planet shares the system with four other Earth-sized planets, although these orbit much closer in.

What distinguishes Kepler-186f  from the other planets in the system is that it orbits the host star within the habitable zone. The four other planets orbit less than 0.1 AU from the star, and all have orbital periods under 25 days. Kepler-186f, on the other hand, lies about 0.4 AU out, or about Mercury’s distance from the Sun. Although this would appear to be scorchingly close, the central star Kepler-186 is a red dwarf that is only about the size and mass of the Sun. At Kepler-186f’s distance from the host star, it receives only about a third of the light that Earth does from the Sun.

A comparison of the Solar System to that of the Kepler-186 system. The green area marks the habitable zone for a star of a given temperature.
Credit: NASA Ames, SETI Institute, JPL-Caltech

For that reason, the researchers hesitated to call it Earth-like. During the announcement’s conference call, NASA Ames researcher Tom Barclay said, “Being in the habitable zone does not mean we know this planet is habitable. The temperature on the planet is strongly dependent on what kind of atmosphere the planet has. Kepler-186f can be thought of as an Earth-cousin rather than an Earth-twin.” To support Earth-like temperatures at its distance from the star, Kepler-186f would need an atmosphere much thicker than that of Earth. The properties of its atmosphere, or even whether it has one at all, remain unknown. Should it prove to have an Earth-like atmosphere, the planet would more likely resemble the cold deserts of Mars or the frozen snowball Hoth from Star Wars.

Exoplanet researchers are becoming remarkably adept at using spectrographic techniques to tease out details of exoplanetary atmospheres. When a planet transits across the face of its star, some of the star’s light passes through the planet’s atmosphere. Imprinted in this light are spectrographic features which can tell us the rough chemical makeup of the planet’s atmosphere. Unfortunately, Kepler-186f is too far away for current equipment to make use of this technique, but future developments may make it possible to observe its atmosphere.

The mass and composition of Kepler-186f are also unknowns. One limitation of the transit method is that it can only measure the width and orbital period of a planet. Kepler-186f could be much heavier than Earth if it’s made mostly of iron, or much lighter if it’s mostly water or ice. Elisa Quintana (NASA Ames/SETI Institute), one of the researchers who participated in today’s announcement, explained that models of planetary formation and the masses of other exoplanets in this size range suggest that Kepler-186f is probably a rocky planet.

However, astronomer Victoria Meadows (University of Washington) noted during the press conference that Kepler-186f is too small for astronomers to detect the tug of its gravity on the host star. This is the method that is typically used to measure an exoplanet’s mass. Instead, researchers will need to time the orbital periods of all five planets in the Kepler-186 system, looking for changes caused by the planets tugging on one another. That will take years of careful study.

While Kepler-186f is the first Earth-like and potentially habitable planet to be found, its discovery is not surprising. A previous study, released in September of last year, statistically analyzed the planets already discovered by Kepler to extrapolate how many planets lie in a star’s habitable zone. That study estimated that 22% of all stars have planets roughly the size of Earth within their habitable zones. So why hasn’t one been found before now?

The answer lies in the limitations of planet hunting. The transit method that Kepler uses is probably the best suited for searching, but it isn’t perfect. To confirm a planet, Kepler must observe multiple transits of a planet candidate. For a planet in a star’s habitable zone, transits happen infrequently because of their longer orbital periods. The innermost four planets in the Kepler-186 system were found within the first year and a half of Kepler’s operation, but Kepler-186f escaped confirmation for three years.

Light curve of a transit by Kepler-186f. The transit lasts nearly 6 hours and dims the star by only 500 parts per million. Credit: Quintana et. al. 2014

Light curve of a transit by Kepler-186f. The transit lasts nearly 6 hours and dims the star by only 500 parts per million.
Credit: Quintana et. al. 2014

The type of star an Earth-like planet orbits is also a major factor. Bigger stars produce more light. Red dwarfs are ideal for finding small planets, because the planets produce a bigger dip in the amount of light received from the star. If Kepler-186f were orbiting a more Sun-like star, it might have easily been lost in the glare.

Kepler’s planet hunting days ended in August of last year. The spacecraft had four reaction wheels, three of which were needed to keep the spacecraft pointed in the same direction. Two of these reaction wheels failed, leading NASA to end the mission. NASA is currently considering returning Kepler to service with a new mission named K2. Instead of keeping its cameras focused towards the same field, Kepler will instead point at four fields a year, using sunlight pressure to help keep the spacecraft oriented. That mission is currently being examined by NASA’s senior review panel. Kepler’s successor, TESS, is currently under development for a 2017 launch.

Also check out Kepler-186’s entry in the Open Exoplanet Catalog to see the planets in motion, and a Google Hangout with the discoverers!

Featured image credit: NASA Ames/SETI Institute/JPL-Caltech 

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About The Author
Justin Cowart
Justin Cowart is a geologist interested in Earth and Solar System history. As a geologist, he spends hist time looking at the ground, but in his free time he looks to the skies as an amateur astronomer.

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