NASA’s renowned planet-hunting Kepler space telescope has reached the end of its lifetime. It has run out of fuel and no longer can point itself at stars to determine if planets are in orbit around them (exoplanets). The announcement comes as no surprise. Kepler worked more than twice as long as expected, revolutionizing scientific understanding of planetary system formation along the way. Scientists will continue to study the data it returned and from newer spacecraft.

NASA Astrophysics Division Director Paul Hertz made the announcement during a teleconference today that included Bill Borucki, Kepler’s principal investigator. Hertz praised Borucki’s determination in getting the mission approved and launched. Noting that NASA is better known for its large space telescopes like Hubble, Hertz pointed out that NASA has a portfolio of missions of varying sizes and complexity. Borucki fought hard to convince his peers and NASA that the mid-sized Kepler needed to be built “and boy are we are glad he did that.”

Kepler stared at stars in a particular patch of the sky over the course of its 9.5 year lifetime. In all, it looked at about 5,000 stars and discovered 2,681 confirmed planets. Kepler data suggests another 2,899 potential planets that ground-based astronomers will continue to study and determine if they are, in fact, planets.

Before Kepler was launched, theories existed about the number and types of planets that might be orbiting other stars, but there was little hard data. Now there is a great deal and it is clear that planets are very common. They take many forms, big and small, close to their host star or far away. How many might be “Earth-like” is still debatable because scientists have not reached consensus on what that really means. Hertz said that NASA does not provide any definitive list of how many of the planets discovered by Kepler might be Earth-like, but a scientific paper published several years ago listed 30. Since then, however, additional analysis using data from other ground- and space-based instruments, including Europe’s Gaia astrometric space observatory, have lowered that to between 2 and 12, he said.

That’s a small number, but Kepler looked at only a few stars cosmically-speaking. Extrapolating the Kepler results, NASA estimates that “20-50 percent of the stars visible in the night sky are likely to have small, rocky planets similar in size to Earth, and located within the habitable zone of their parent stars.”

Scientists will continue to search for planets using the Transiting Exoplanet Survey Satellite (TESS) launched earlier this year, the James Webb Space Telescope currently scheduled for launch in 2021, and future spacecraft. But there is still much to learn from the Kepler data. Scientists will spend “a decade or more in search of new discoveries in the treasure trove of data Kepler provided.”

Thomas Zurbuchen, the head of NASA’s Science Mission Directorate, said Kepler “wildly exceeded all our expectations” and “sparked an entirely new and robust field of research that has taken the science community by storm…”

Borucki explained during the teleconference that they expected to find thousands of planets and they did, but they were not the size anticipated. “We expected to find more Jupiters,” but instead the most common size is between that of Earth and Neptune, of which there are none in our solar system.

The end of Kepler comes at the same time as two other NASA space telescopes, Hubble and Chandra, have experienced problems, but Hertz stressed it is all coincidental. Hubble and Chandra had problems with the gyroscopes that allow them to point in the right direction, but both have recovered. They and Kepler are old spacecraft far beyond their design lifetimes. The same is true of another NASA spacecraft, Dawn, that is nearing the end of its life after 11 years exploring the asteroid Vesta and the dwarf planet Ceres.

Nevertheless, NASA has more than 60 operational science missions right now, a “golden age of NASA science” according to Hertz.

Kepler project systems engineer Charlie Sobeck said Kepler’s transmitters soon will be turned off so it will not cause unintentional interference. The approximately $700 million spacecraft (including operations) will remain in a safe and stable orbit around the Sun forever.