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The Kepler Mission

"The ways by which (we) arrive at knowledge of the celestial things are hardly less wonderful than the nature of these things themselves."
--Johannes Kepler

How many habitable planets are there in our galaxy? Knowing this number would be a major step on the way to answering the big question: are we alone in the Universe?

On March 7, 2009, NASA launched a space telescope—Kepler—to search for exoplanets and help provide the answer. Kepler is humankind’s first attempt to place life in its cosmic context and seeks to answer the question: are habitable places common in the cosmos? Once we have answered this question, we can attempt to answer the logical follow-up: is there alien life out there?

The Kepler Mission is named for the astronomer Johannes Kepler (1571-1630), who first described the laws of planetary motion with great precision. Kepler derived his laws by observing the planets of our solar system. Now, the survey that carries his name is looking for Earth-like planets in other solar systems.

The Kepler Mission was specifically designed to survey a small part of the Milky Way galaxy and look for Earth-size planets in or near the habitable zone of Sun-like stars. The mission’s ultimate goal is to determine how many of the billions of stars in our galaxy might have such planets.

The mission is looking for planets similar to Earth because the search for life in the universe naturally focuses on life that is similar to what we see on the Earth. For that reason, it seems logical to look for such a phenomenon on planets similar in size to our own and in a relationship to its star paralleling that of the Earth and the sun. While the Kepler mission was aimed at finding planets around Sun-like stars, other types of stars were also observed. Notably, several thousand red dwarf stars were included: these stars are smaller than the Sun and are the most common type of star in our galaxy.

The Kepler Search Field

The Kepler's field of view — and hence, the search space cone — is slightly above the plane of the galaxy’s disk. The scientists who designed the mission selected this field as the best tradeoff between two competing factors. On the one hand, it is important that there be a large number of stars in Kepler’s field of view: the more stars that can be surveyed, the greater the chances of finding habitable planets. On the other hand, the scientists tried to avoid having many background light sources in their field of view, the light from which can reduce the quality of the data.

The Kepler mission is a photometric mission, meaning that it uses the measurement of the brightness of stars over time (photometry) to discover planets that eclipse their host stars periodically. This is the "transit" method that we had discussed previously.

By May 2013, the Kepler Mission had completed its nominal survey, producing four full years of almost uninterrupted photometric measurements of more than 150,000 stars. As of mid-2014, the mission had confirmed 978 planets in 400 star systems and had identified more than 4234 planet candidates.

The figure below shows the observed distribution of planets by size, grouped into categories that mimic the planet types in our own solar system, except for the Super-Earths, which are completely absent “at home.” Kepler observes only transits, therefore it provides only planet sizes. From the periodicity of repeating transits, Kepler also provides the orbital information for each planet, i.e. the planet’s distance from its host star.

Kepler's Discoveries

Planet hunters are still analyzing the data from four years of Kepler observations. However, the preliminary results confirm the past measurements done by limited ground-based surveys, suggesting that planets are generally very common around the stars in our galaxy.

Among its major discoveries, the mission found Kepler-186f orbiting a red dwarf star 500 light-years from Earth. This represented a major milestone because searchers had found other planets in the habitable zone, but Kepler-186f was the first with a size close toEarth’s, being only about 10 percent larger. Moreover, even though Kepler was designed to seek planets orbiting sun-like stars, red dwarfs make up 70 percent of the Milky Way’s stellar population, so we can conclude that many stars in the galaxy might also have Earth-sized planets that are hospitable to life.

In the words of Elisa Quintana, research scientist at the SETI Institute at NASA's Ames Research Center in Moffett Field, California, and lead author of a paper published in Science about the discovery:

By extrapolation, the Kepler survey allows us to estimate that planets with sizes similar to Earth, and located in the habitable zone, are to be found around roughly one in 100 stars. If this figure is confirmed by the final analysis, it would mean that circling the 300 billion stars in our galaxy there must be at least 3 billion habitable planets.

As of mid-2014, in addition to Kepler-186f, the survey made a number of other significant discoveries. Kepler 22-b, discovered in December of 2011, was the first planet to be found in the habitable zone. It is different from 186f in that it is 2.4 times the radius of the Earth. It is 600 light-years away and has a 290-day orbit around its sun.

Then there is Kepler-421b, the exoplanet with the longest known year at 704 days, and Kepler scientists have also made the most precise measurement ever of an exoplanet, determining that Kepler-93b is a super-Earth about 1.5 times the size of our home planet.

Kepler's Changing Mission

Unfortunately, two of the four reaction wheels on Kepler failed (the wheels keep the telescope pointed in one direction), hobbling the spacecraft and preventing it from continuing to fulfill its primary mission. However, the telescope had already functioned beyond its planned time frame, and when scientists huddled to find a new direction for it that could be accomplished with two of the pointing devices instead of four, they developed the “K2” (Kepler 2) mission.

As NASA put it:

"Kepler's loss of a second spacecraft reaction wheel in May 2013 effectively ended data collection in the original Kepler field after 4 years of continuous monitoring. However, all other Kepler assets remain intact and can be used for the K2 mission…On two reaction wheels, K2 is limited to pointing near the ecliptic plane, sequentially observing fields as it orbits the sun."

The K2 Mission offers new possibilities for research. One area that is particularly exciting to astronomers is the opportunity to point Kepler’s superb telescope toward clusters of stars. Studying these clusters, which are physically associated groups of stars, will help astronomers learn about stellar life cycles and the physical properties of stars.

The Kepler Mission represents a giant step forward in the search for life in the universe, at a relatively low cost, and by looking at only a small portion of our galaxy. The astronomer for whom it was named might well be proud of its accomplishments.

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