The Death of the Kepler Satellite

The Kepler satellite was shut down last week, having operated for 5 years longer than its mission brief. During its lifetime it detected more than 2,500 exoplanets with another 3000 candidates awaiting confirmation, and it will have a profound legacy.

After 9 years in operation, the Kepler space satellite has finally run out of the hydrazine fuel used to control its orientation. NASA has decided to retire the satellite while it is still in a safe orbit far from Earth. Launched in March 2009, Kepler was originally intended to look at one patch of the sky and detect planets orbiting around distant stars. This patch in the constellation of Cygnus contained 150,000 stars.  Four years in, a fix for a mechanical issue led to the development of the K2 mission where the satellite had to be periodically reoriented, expanding its field of view to a further 500,000 stars.

How to Find a Planet

The satellite’s only scientific instrument was a photometer designed to detect drops in brightness that occur when a planet passes in front of a star, blocking some of its light. This way of detecting a planet is known as the transit method. This data alone is not enough to confirm the existence of an exoplanet, so a candidate is then either imaged directly with more advanced telescopes, or, confirmed by detecting a wobble in the movement of parent star due to a shifted centre of mass between the two objects.

Notable examples of exoplanets discovered include ones with surprisingly low densities; planets lying in habitable zones and other multi-planet systems. Aside from exoplanets, Kepler observed white dwarfs, supernovae and, in tandem with the Hubble satellite, a candidate for the first exomoon.

Kepler’s Legacy

Kepler was designed as part of NASA’s Discovery program: the aim, to collect excellent data more frequently with smaller missions, while keeping total costs low. The original lifetime of Kepler was three and half years, but it has lasted 9 years and the data delivered is outstanding. The 2,681 confirmed exoplanets and a further 3000 candidates, including 30 potential Earth-like worlds, will allow astronomers to develop and adjust theories about planet formation. Already we have learnt that the most common size is between that of Earth and Neptune; that many planets are closer to their parents star than expected and that there are more planets than stars.

Kepler also gave ground-breaking access to data to the general public, a scenario nicknamed as Citizen Science.  Since 2010, Kepler was involved with Planet Hunters and Stargazing Live, encouraging people to spot transit events computers might miss. 90 exoplanets were discovered this way and their discoverers were co-authors on research papers.