TRANSIT
A FIELD GUIDE TO THE KEPLER SPACE TELESCOPE
Ten chapters on the mission that changed the answer to an ancient question. Before Kepler, we did not know whether planets were common or rare. Kepler pointed one camera at one patch of sky, watched 150,000 stars without blinking for four years, and measured shadows a ten-thousandth of a percent deep — enough to prove there are more planets than stars in the galaxy. Scroll to begin.
BEGIN — CHAPTER 01 →When a planet crosses its star, the star gets very slightly dimmer
If a planet's orbit is edge-on to us, the planet passes in front of its star once each orbit and blocks a tiny fraction of its light. For an Earth-sized planet crossing a Sun-like star, that dip is about 0.01% — one part in ten thousand — lasting a few hours.[1]
See the same dip repeat on a regular schedule and you have an orbit; the depth gives the planet's size, and the period gives its distance from the star. The whole method rests on measuring brightness with extreme, unwavering precision.
A spacecraft built to do a single measurement extremely well
Kepler launched on a Delta II on March 7, 2009, into an Earth-trailing orbit around the Sun with a 372-day period.[1] It carried exactly one instrument: a 0.95-metre photometer with a 95-megapixel detector, and no ability to observe individual targets on demand. Its entire design served one statistical experiment.
A fixed field between Cygnus and Lyra, watched for four years straight
Kepler held a single 115-square-degree field of view near the plane of the Milky Way, chosen so its stars stayed in view year-round and never passed behind the Sun.[1] It recorded the brightness of about 150,000 stars every 30 minutes, continuously.
Continuity was everything: a planet in a one-year orbit only transits once a year, so proving it takes years of an unbroken record. A single long gap could hide a world.
A wide-field camera, and four wheels holding it perfectly still
Kepler is a Schmidt telescope — a design that trades a correcting lens for a very wide, clean field. Behind the optics sits the largest camera NASA had flown. Select a component.
The galaxy has more planets than stars
From its field of 150,000 stars Kepler identified thousands of planets and planet candidates, enough to do statistics on the whole galaxy. The conclusions: essentially every star has at least one planet; small, rocky planets are the most common kind; and a sizeable fraction of Sun-like and red-dwarf stars have a roughly Earth-sized planet in the zone where liquid water could exist.[2]
Habitable-zone rocks, packed systems, and a real Tatooine
Kepler-22b (2011) was the first confirmed planet in a star's habitable zone. Kepler-186f (2014) was the first Earth-sized planet found there. Kepler-11 turned out to have six planets orbiting closer to their star than Venus is to the Sun. And Kepler-16b, orbiting two stars at once, made the twin-sunset sky of science fiction a measured fact.[2]
You need three working wheels to hold a telescope still in three dimensions
Kepler's precision pointing depended on four reaction wheels, of which three had to work. One failed in July 2012; a second failed in May 2013.[1] With only two wheels, the telescope could no longer hold its field steady enough to detect a transit, and the original mission was over — though by then it had already gathered enough data to keep scientists busy for a decade.
Balancing the spacecraft against the pressure of photons
Engineers found a fix: point the spacecraft nearly along its orbit so that sunlight pressing evenly on the body acts like a gentle third wheel, holding the unstable axis. Kepler could then stay steady enough to observe — but only along the plane of its orbit, and only for about 80 days before the Sun moved into view.[1] The K2 mission ran a series of these campaigns from 2014 to 2018, adding several hundred more confirmed planets plus supernovae, star clusters and Solar System bodies.
Retired with more than half of all known exoplanets to its name
The K2 balancing act still needed thrusters to unload momentum, and in 2018 the hydrazine ran out. NASA retired Kepler on October 30, 2018, leaving it in a safe orbit around the Sun.[1] Its final tally: more than 2,600 confirmed planets — over half of all exoplanets known at the time — and a settled answer to whether other worlds are common. They are.
The middle of the exoplanet century
Kepler did not find the first exoplanet and will not find the last. It answered the question of how many, and handed a galaxy full of targets to the missions built to study them one at a time.
Sources cited throughout this guide
Bracketed numbers in the text above correspond to the sources below.