On Oct. 30, NASA announced that one of the most iconic and influential telescopes ever to exist has run out of fuel. The Kepler Telescope was launched in March 2009 with the intent to discover Earth-sized planets orbiting other stars. Kepler did this using the “transient method” which detects the relative dimming of a star caused by a planet crossing between that star and Earth, thereby blocking some of the incident light. There are a lot of factors that affected Kepler’s ability to detect such events, most of which are simple geometric limitations. Due to the method of transient detection, the star system must be facing us edge-on so that the planet crosses between the star and Earth. This severely restricts our ability to find eligible planet candidates due to the wide breadth of possible orientations that can’t be viewed from our solar system. Additionally, since Kepler was only active for about nine-and-a-half years, planets that have slow orbital periods may not have transited the star during the mission even with the correct orbit. Lastly, Kepler only surveyed a small region of space containing about 150,000 stars, which may sound like a lot, but is only about 0.25 percent of the sky.
Despite all these geometric limitations, the Kepler telescope managed to detect over 2,600 confirmed exoplanets — the formal name for a planet from outside this solar system. This accounts for nearly two-thirds of all known exoplanets that are confirmed. While this is the confirmed number, there are additionally thousands more that may have been detected but have not been verified by scientists due to lack of sufficient data or delays in data processing. It is quite incredible what Kepler has done in revolutionizing our understanding of star systems beyond our own. Prior to Kepler, there were only about 300 known exoplanets but now we understand that nearly every star has a collection of planets, although some systems are notably different than our own.
One of the most significant insights that Kepler taught us was the existence and frequency of “hot Jupiters.” These planets are massive gas giants, similar to Jupiter and Saturn, that orbit even closer to their host star than the Earth orbits to the sun. The repeated detection of such planets has made scientists rethink theories on planetary formation and orbital settlement. Prior to this, gas giants were believed to form far away from their stars to be able to capture enough material to grow to such large sizes. Additional theories have approached planetary migration where such a gas giant may have originated far out but due to gravitational interactions, has been pushed into a tighter orbit.
There are still many questions concerning the data that Kepler has measured but it will have to be followed up by future missions. One such mission is NASA’s Transiting Exoplanet Survey Satellite (TESS), which began taking measurements in July. TESS was designed and built with many things learned from Kepler and so while the telescope may lay dormant out in space, its legacy certainly will not.
