Searching for Exoplanets: The Kepler Mission

kepler mission
The Kepler mission looks for planets around other stars by detecting slight changes in the brightness of the light as a planet orbits around the star. NASA/Kepler

The hunt for worlds around other stars is on! It all began in 1995, when two young astronomers Michel Mayor and Didier Queloz announced the confirmed discovery of an exoplanet called 51 Pegasi b. While worlds around other stars had long been suspected, their discovery paved the way for other ground-based and space-based searches for distant planets. Today, we know of thousands of these extra-solar planets, also referred to as "exoplanets".

On March 7, 2009, NASA launched a mission designed specifically to look for planets around other stars. It's called the Kepler Mission, after the scientist Johannes Kepler, who formulated the laws of planetary motion. The spacecraft has discovered thousands of planet candidates, with more than a thousand of its objects now confirmed as actual planets out in the galaxy. The mission continued to scan the sky until late 2018, when it finally ran out of coolant and was commanded to shut down by mission team members.

How Kepler Searches for Exoplanets

There are some major challenges to finding planets around other stars. For one thing, stars are large and bright, while planets are generally small and dim. The reflected light of planets is simply lost in the glare of their stars. A few really large ones that orbit far from their stars have been "seen" by the Earth-orbiting Hubble Space Telescope, for example, but most others are too difficult to detect. That doesn't mean they aren't there, it just means astronomers had to come up with a different method to find them.

The way Kepler does it is to measure the dimming of a star's light as a planet orbits around it. This is called the "transit method", so called because it measures light as the planet "transits" across the face of the star. The incoming light is gathered in by a 1.4-meter-wide mirror, which then focuses it into a photometer. That's a detector sensitive to very tiny variations in light intensity. Such changes may well indicate that the star has a planet. The amount of dimming gives a rough idea of the size of the planet, and the time that it takes to make the transit gives data about the speed of the planet's orbit. From that information, astronomers can figure out how far away the planet is from the star.

Kepler orbits the Sun well away from Earth. For its first four years on orbit, the telescope was pointed at the same spot in the sky, a field that is bounded by the constellations Cygnus, the Swan, Lyra, the Lyre, and Draco, the Dragon. It watched a part of the galaxy that is about the same distance from the center of our galaxy as the Sun lies. Within that tiny region of sky, Kepler found thousands of planet candidates. Astronomers then used both ground- and space-based telescopes to focus in on each candidate for further study. That's how they've confirmed more than a thousand candidates as actual planets.

In 2013, the primary Kepler mission was stopped when the spacecraft began having problems with the reaction wheels that help hold its pointing position. Without fully functioning "gyros", the spacecraft could not keep a fine lock on its primary target field. Eventually, the mission resumed, and commenced on its  "K2" mode, where it is observing different fields along the ecliptic (the apparent path of the Sun as seen from Earth, and also defines the plane of Earth's orbit). Its mission remains roughly the same: to find planets around other stars, to determine how many Earth-size and larger worlds there are around a wide variety of star types, how many multiple-planet systems exist in its field of view, and to provide data to determine the properties of stars that do have planets. It will continue operations until sometime in 2018, when its on-board fuel supply will run out.

Other Findings by Kepler

Not everything that dims a star's light is a planet. Kepler has also detected variable stars (which go through intrinsic variations in their brightness NOT due to planets), as well as stars undergoing unexpected brightening due to supernova explosions or nova events. It has even spotted a supermassive black hole in a distant galaxy. Pretty much anything that causes dimming of starlight is fair game for Kepler's detector.

Kepler and the Search for Life-Bearing Worlds

One of the big stories from the Kepler mission has been the search for Earth-like planets and in particular, habitable worlds. Generally speaking, these are worlds that have some similarity to Earth's size and orbit around their stars. They may well be terrestrial worlds (meaning they are rocky planets). The reason is that planets like Earth, orbiting in what's called the "Goldilocks Zone" (where it's not too hot, not too cold) might be habitable. Given their position in their planetary systems, these kinds of worlds might have liquid water on their surfaces, which appears to be a ​requirement for life. Based on Kepler's findings, astronomers have estimated there could be millions of habitable worlds "out there".

It's also important to know which kinds of stars would host a zone where habitable planets could exist. Astronomers used to think that single stars much like our Sun were the only candidates. The discovery of worlds similar to Earth's size in habitable zones around not-exactly-just-like-the-Sun stars tells them that a wider variety of stars in the galaxy could harbor life-bearing planets. That finding may well turn out to be one of Kepler's more enduring accomplishments, well worth the time, money, and effort undertaken to send it out on its voyage of discovery.