After nearly 10 years of searching beyond our solar system for alien worlds called exoplanets, the Kepler Space Telescope has been officially resigned to the heavens.
NASA retired Kepler on Oct. 30 after the groundbreaking space observatory reported that it had finally exhausted its fuel supply. Launched in 2009, the mission was designed to have enough fuel to last six years. Instead, NASA engineers managed to squeeze another three and push the space telescope beyond what many thought possible.
"This was not unexpected," Paul Hertz, astrophysics division director at NASA headquarters in Washington, D.C., said during a press conference. "All fuel-limited spacecraft are known to be fuel-limited when we launch them. We can estimate their lifetimes, and we're not surprised when they run out of fuel, but when they run out of fuel, then we get to have a celebration of all the great science they have done."
A galaxy sprinkled with exoplanets
While scanning the Milky Way for exoplanets, Kepler discovered more than 2,600 alien worlds — with most looking absolutely nothing like the eight-planet arrangement of our solar system.
"The most common size of planet Kepler found doesn't exist in our solar system — a world between the size of Earth and Neptune — and we have much to learn about these planets," NASA wrote in a statement. "Kepler also found nature often produces jam-packed planetary systems, in some cases with so many planets orbiting close to their parent stars that our own inner solar system looks sparse by comparison."
But not all was out of the ordinary. In 2017, artificial intelligence combing through Kepler's massive data haul discovered two exoplanets that brought the total known orbiting its host star to eight. The system, called Kepler 90, marked the first time a numerical twin to our own solar system had been observed.
"The Kepler-90 star system is like a mini version of our solar system," Andrew Vanderburg, a NASA Sagan Postdoctoral Fellow and astronomer at the University of Texas at Austin, said in a statement. "You have small planets inside and big planets outside, but everything is scrunched in much closer."
A legacy that lives on
According to the Kepler mission's founding principal investigator, William Borucki, before work on the space telescope began some 35 years ago, astronomers could only theorize as to the existence of planets outside our solar system.
"Now that we know planets are everywhere, Kepler has set us on a new course that's full of promise for future generations to explore our galaxy," he added.
It's a legacy that NASA has already built upon, launching Kepler's successor — TESS, or the Transiting Exoplanet Survey Satellite — last April. The new advanced space observatory is presently scanning an area of the night sky nearly 400 times larger than what was covered by its predecessor, roughly 85 percent of the sky.
"Kepler was all about doing a census: How common are planets in general? What is the size distribution of planets like? Are Earth-sized planets common?" Stephen Rinehart, the project scientist for TESS at NASA, told The Verge. "TESS is really optimized for knocking on doors in the neighborhood and saying, 'Hi, how are you? What is this planet actually like?'"
Over the course of its two-year prime missions (Kepler's initial mission was only scheduled for four), TESS is expected to detect more than 20,000 new exoplanets. In September, mission officials announced that it had already discovered its first, a rocky exoplanet roughly twice the size of Earth and located some 60 light-years away.
Now, as NASA shuts down the last of Kepler's instruments, the agency is celebrating the astounding discoveries made by the observatory and those yet to come from the trail through space it helped blaze.
"Not only did it show us how many planets could be out there, it sparked an entirely new and robust field of research that has taken the science community by storm," Thomas Zurbuchen, associate administrator of NASA's Science Mission Directorate, said. "Its discoveries have shed a new light on our place in the universe, and illuminated the tantalizing mysteries and possibilities among the stars."