Kepler-16 b was the first planet confirmed to transit, or pass in front of, a pair of stars. Announced in September 2011, it showed that planets can form and survive around binary stars, and it drew instant comparisons with Tatooine, the fictional desert world with two suns in Star Wars. NASA's Kepler space telescope saw it cross in front of both stars, on top of the regular dimming as the two stars eclipse each other. The study, led by Laurance Doyle of the SETI Institute, appeared in the journal Science.
The two stars orbit each other every 41 days. One has about 69 percent of the Sun's mass and the other about 20 percent, and both are smaller and cooler than the Sun. The planet circles both of them every 229 days, at a distance similar to Venus's from the Sun. It has about a third of Jupiter's mass and three-quarters of its width, which makes it similar to Saturn. The system is about 245 light-years away. The orbits of all three bodies lie within half a degree of a single flat plane, which the discovery team took as support for the idea that the planet formed in a disk of gas and dust circling both stars.
How it was weighed is part of its story. Its mass was first worked out from Kepler's light curves, including small shifts in the timing of the stars' eclipses caused by the planet's pull. Another common method, radial velocity, tracks a star's tiny back-and-forth wobble as a planet tugs on it, but it had not been used successfully to find a planet around a binary. In 2022, the BEBOP survey, led by Amaury Triaud, used the SOPHIE spectrograph at the Observatoire de Haute-Provence to detect Kepler-16 b's pull directly. It measured a mass of about 0.31 Jupiter masses, in agreement with the eclipse-timing value. That made Kepler-16 b the first circumbinary planet, meaning one that orbits two stars, detected by radial velocity, and it opened a new route to finding more.
It is no place for life as we know it. Kepler-16 b is a gas planet with no solid surface, and the Smithsonian's announcement estimated its temperature at around minus 73 to minus 101 degrees Celsius.
TerraForge's model, working from its measured mass and size, classifies Kepler-16 b as a cold gas giant at about 197 K with a low habitability score, which agrees well with what astronomers have found. Here, the measurements leave little room for any other reading.