One of Earth’s most venerable planet-hunters, NASA’s Kepler spacecraft, has gone quiet. Today, the space agency announced that after nearly a decade of staring at the stars, Kepler is out of fuel. Now, the spacecraft will stay in its Earth-trailing orbit, looping around the sun and never coming closer than a million miles from home.
“NASA’s original planet hunter, the Kepler space telescope, has run out of fuel,” Paul Hertz, NASA’s Astrophysics Division director, announced today during a press conference. “This is not unexpected, and this marks the end of spacecraft operations for Kepler and the end of the collection of science data.”
To say Kepler revolutionized our understanding of the cosmos is no overstatement: The mission showed us that extrasolar planets, or exoplanets, circle nearly every star in the sky, and that roughly one-fifth of those worlds are similar to Earth in size and orbit. In other words, Kepler told us that planets are a common consequence of star formation and not the result of some rare, unlikely event. (In addition to piles of planets, Kepler may have recorded the first known alien moon.)
“Thanks to Kepler's amazing legacy, we now know that small planets orbiting in the habitable zone of other stars are plentiful,” says Victoria Meadows of the University of Washington, referencing the region around a star where conditions are favorable for life as we know it. “The future of exoplanet science is very bright.”
Now that Earth’s sharpest planet-hunting eye has closed, astronomers will be busy sifting through and understanding piles of data collected by the spacecraft, plotting new missions and designing new instruments. And with the next generation of planet-spying spacecraft, scientists will be focused on characterizing these worlds that hide in our sky.
After all, the most elusive of astronomical quarries is still out there: a planet elsewhere in the galaxy with signs of life.
“We will be mapping out what these exoplanets are made of, from their interiors to the edges of their atmospheres, in an unprecedented level of detail,” says Caltech’s Jessie Christiansen. “Kepler has lifted the veil on the diversity of planets and planetary systems surrounding us—now it’s time to really explore.”
Mission Full of Milestones
Launched in 2009, Kepler stared at the same swath of northern sky for four years, looking for the short dips in starlight caused by alien worlds marching across their stars’ faces. It shook some 2,300 confirmed planets loose from that starfield, revealing that such bodies are common, that planets rarely live alone, and that the cosmos grows an extraordinary variety of strange and unexpected worlds.
In 2013, though, Kepler lost a crucial bit of machinery that helped it stare, unblinking, at that star-studded patch. Instead of turning off, Kepler began a different set of observations, alternately swiveling to stare at more than half a million stars—as well as planets in our own cosmic backyard—as part of its K2 mission.
But for the last couple years, Kepler has been on borrowed time. Mission managers knew its fuel would run out, and with no way to refill the spacecraft’s gas tank, Kepler fell quiet two weeks ago, as teams scrambled to retrieve the last of the spacecraft’s data.
“We collected every bit of possible science data and returned it all to the ground safely,” says Charlie Sobeck of NASA’s Ames Research Center. “In the end, we didn’t have a drop of fuel left over for anything else.”
Mining Kepler’s Data
While Kepler may no longer beam new data to Earth, its treasure troves are still ripe for mining. More than nine years of data from both the primary Kepler field and secondary K2 observing programs are waiting for scientists to dive in and try and make sense of the first large-scale planetary census ever taken.
One of those scientists is Lauren Weiss, at the University of Hawaii. She’s embarking on a project to examine the planets orbiting a hundred Kepler stars to determine their masses and orbits, the architectures of those systems as a whole, and the ways in which planets end up where they are.
“It’s the only way to understand if our solar system as a whole is common or rare: fully characterizing the inner parts of planetary systems, and then connecting those planets to outer planets that we have yet to discover,” she says.
Meeting the Neighbors
And now that we know our solar system is one among billions, scientists says it’s time to actually get to know some of those faraway worlds—not just in a statistical sense, but to figure out what they’re made of and what their histories are.
“Kepler has taught us that Earth-size planets are practically everywhere,” says Courtney Dressing of the University of California, Berkeley. “It's time to search the habitable zones of nearby stars to find our nearest neighbors.”
NASA’s TESS mission is currently surveying 200,000 nearby stars and looking for the brightest, best planets to characterize. In the future, that characterization work could be done with instruments like the upcoming James Webb Space Telescope or with giant ground-based telescopes currently being designed, such as the European Extremely Large Telescope, the Giant Magellan Telescope, or the embattled Thirty Meter Telescope.
Future missions employing special shades that preferentially block starlight will help Earthlings directly image exoplanets and search them for signs of life, Christiansen says. But for now, there are enough intriguing results to keep astronomers busy.
“We are also using the Hubble and Spitzer space telescopes to investigate exoplanet atmospheres right now, and finding new and interesting results almost every time,” she says.
One of the big motivators for peering deeply into an alien atmosphere is to search for signs of life beyond Earth, including biosignatures that might be written into alien clouds, says Yale University’s Debra Fischer.
Right now, we don’t have instruments capable of easily sniffing out these potential clues. But several missions are being designed that could spot them, such as HabEx or LUVOIR, which are intended to pick apart the light coming from far-off planets and read their chemical composition in a process known as spectroscopy.
“Exoplanet spectroscopy or bust!” Fischer declares.