Earth’s cousin(s)

Just a month before K2 was approved, in April 2014, scientists announced Kepler had helped find the type of planet everyone was looking for. About 500 light years from Earth sits a star called Kepler-186. It has four inner planets that are less than 1.5 times the size of Earth. The fifth planet, Kepler-186f, is roughly the size of Earth, right in the star’s habitable zone, where liquid water can exist.

It was the first-ever discovery of a potentially Earth-like planet that might support life. The big difference is Kepler-186f’s star—an M-class dwarf smaller and dimmer than the Sun. At high noon, it would only shine as bright as the Sun at sunset. A year on Kepler-186f lasts just 130 days, and whether or not the planet could support life depends on its atmosphere.

But that was just the beginning. A year later, scientists found a similar planet orbiting a star more like our own. And there are now 30 confirmed planets less than twice the size of Earth in their stars' habitable zones.

Even under the modified K2 mission design, the spacecraft still had to use its thrusters to point towards Earth to send home data. In March 2016, NASA said that Kepler probably had just a little more than two years of fuel remaining. Right on schedule, in March 2018, the agency warned the end was near. In early July, the spacecraft was so low on fuel that NASA halted the current observing campaign—campaign number 18 out of an originally planned 10 for K2—to allow the spacecraft to hibernate until its next scheduled contact with the Deep Space Network on 2 August. The hope was that Kepler would have enough fuel left to turn towards Earth one more time and send home its precious data.

It did. The mission moved on to campaign 19, which, according to NASA updates, was somewhat successful. But Kepler's fuel reserves are so low, there will be no twentieth campaign. The mission is over.

Fortunately, Kepler has already passed the planet-finding baton to a new space telescope, TESS, the Transiting Exoplanet Survey Satellite. TESS launched in April, and is already collecting science data. Back on Earth, NASA and the National Science Foundation are preparing to follow up on Kepler and TESS exoplanet discoveries using the WIYN telescope on Kitt Peak in southern Arizona.

The next step is to try and figure out the composition of some of these Earth-sized exoplanets’ atmospheres. That work will be done with the next generation of ground-based telescopes, as well as the James Webb Space Telescope.

If we find a true, Earth-like planet orbiting another world, Kepler will have laid the groundwork for the discovery. We won’t be able to thank it, but we’ll know it’s out there, drifting silently around the Sun—a monument to our desire to know our place in the cosmos.