(CNN) NASA's Kepler Space Telescope has run out of fuel, and its mission has come to an end 94 million miles from Earth, the agency announced Tuesday. The deep space mission's end is not unexpected, as low fuel levels had been noted in July.
The nine-year planet-hunting mission discovered 2,899 exoplanet candidates and 2,681 confirmed exoplanets in our galaxy, revealing that our solar system isn't the only home for planets.
Kepler allowed astronomers to discover that 20% to 50% of the stars we can see in the night sky are likely to have small, rocky, Earth-size planets within their habitable zones -- which means that liquid water could pool on the surface, and life as we know it could exist on these planets.
The final commands have been sent, and the spacecraft will remain a safe distance from Earth to avoid colliding with our planet.
"As NASA's first planet-hunting mission, Kepler has wildly exceeded all our expectations and paved the way for our exploration and search for life in the solar system and beyond," said Thomas Zurbuchen, associate administrator of NASA's Science Mission Directorate in Washington. "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. Its discoveries have shed a new light on our place in the universe and illuminated the tantalizing mysteries and possibilities among the stars."
The Kepler mission was named in honor of 17th century German astronomer Johannes Kepler, who discovered the laws of planetary motion.
Kepler, the telescope, reached more than twice its initial target, accomplishing original mission goals and seizing unexpected opportunities to answer questions about our galaxy and the universe, according to Charlie Sobeck, project system engineer at NASA's Ames Research Center.
Weird and wonderful planets beyond our solar system
An artists's conception shows planet LP 791-18d. The volcanically active planet, which is a similar size to Earth, was discovered by NASA's Spitzer Space Telescope.
This artist's illustration depicts the rocky exoplanet GJ 486 b, which orbits a red dwarf star located 26 light-years away from Earth. Astronomers have detected hints of water vapor in the system, but they can't be sure if it signifies a planetary atmosphere or if it's part of the star.
This illustration shows an Earth-size exoplanet called TOI 700 e, discovered orbiting the small, cool M dwarf star TOI 700, which is located 100 light-years away. Its other Earth-size sibling, TOI 700 d, can be seen in the distance.
TOI 700 d is the first potentially habitable Earth-size planet spotted by NASA's planet-hunting TESS mission.
Artist's impression of the exoplanet WASP-121 b. It belongs to the class of hot Jupiters. Due to its proximity to the central star, the planet's rotation is tidally locked to its orbit around it. As a result, one of WASP-121 b's hemispheres always faces the star, heating it to temperatures of up to 3000 degrees Celsius. The night side is always oriented towards cold space, which is why it is 1500 degrees Celsius cooler there.
This artist's impression shows a close-up view of Proxima d, a planet candidate recently found orbiting the red dwarf star Proxima Centauri, the closest star to the Solar System. The planet is believed to be rocky and to have a mass about a quarter that of Earth. Two other planets known to orbit Proxima Centauri are visible in the image too: Proxima b, a planet with about the same mass as Earth that orbits the star every 11 days and is within the habitable zone, and candidate Proxima c, which is on a longer five-year orbit around the star.
The discovery of a second exomoon candidate hints at the possibility that exomoons may be as common as exoplanets.
This artist's impression shows the football-shaped planet WASP-103b (left) closely orbiting its host star.
This image shows double-star system b Centauri and its giant planet b Centauri b. The star pair is the bright object at top left. The planet is visible as a bright dot in the lower right. The other bright dot (top right) is a background star.
This artist's rendering shows a Jupiter-like planet orbiting a dead white dwarf star 6,500 light-years away from Earth. The planet survived the violent phases of stellar evolution leading to the star's death.
This artist's illustration shows the night-side view of the exoplanet WASP-76b, where iron rains down from the sky.
Astronomers have identified a new class of habitable planets, which they call Hycean planets. These are hot, ocean-covered planets with hydrogen-rich atmospheres.
This artist's illustration shows L 98-59b, one of the planets in a planetary system 35 light-years away from Earth. This planet has half the mass of Venus.
In this artist's illustration, two gaseous exoplanets can be seen orbiting the bright sun-like star HD 152843.
An artist's rendering of TOI-1231 b, a Neptune-like planet about 90 light years away from Earth.
This artist's conception depicts a violent flare erupting on the star Proxima Centauri as seen from the viewpoint of a planet orbiting the star called Proxima Centauri b.
After losing its gaseous envelope, the Earth-size core of an exoplanet formed a second atmosphere. It's a toxic blend of hydrogen, methane, and hydrogen cyanide that is likely fueled by volcanic activity occurring beneath a thin crust, leading to its cracked appearance.
This illustration shows the metaphorical measuring of the density of each of the seven planets in the nearby TRAPPIST-1 system. New measurements have revealed the most precise densities yet for these planets and they're very similar -- which means they also likely have similar compositions.
This artist's illustration shows the view from the furthest planet in the TOI-178 system.
This artist's illustration shows TOI-561b, one of the oldest and most metal-poor planetary systems discovered yet in the Milky Way galaxy. Astronomers found a super-Earth and two other planets orbiting the star.
This massive and distant exoplanet, called HD106906 b, has an elongated and angled orbit that causes it to take 15,000 Earth years to complete one lap around its twin stars.
This is an artist's impression of a free-floating rogue planet being detected in our Milky Way galaxy using a technique called microlensing. Microlensing occurs when an object in space can warp space-time.
This is an artist's impression of exoplanet WASP-189 b orbiting its host star. The star appears to glow blue because it's more than 2,000 degrees hotter than our sun. The planet, which is slightly larger than Jupiter, has a tilted orbit around the star's poles rather than its equator.
For the first time, an exoplanet has been found orbiting a dead star known as a white dwarf. In this artist's illustration, the Jupiter-sized planet WD 1856 b orbits the white dwarf every day and a half.
This illustration shows a carbon-rich planet with diamond and silica as ts main minerals. Water can convert a carbon-rich planet into one that's made of diamonds. In the interior, the main minerals would be diamond and silica (a layer with crystals in the illustration). The core (dark blue) might be made of an iron-carbon alloy.
This image shows a young sun-like star being orbited by two gas giant exoplanets. It was taken by the SPHERE instrument on European Southern Observatory's Very Large Telescope. The star can be seen in the top left corner, and the planets are the two bright dots.
This artist's impression shows a Neptune-sized planet in the Neptunian Desert. It is extremely rare to find an object of this size and density so close to its star.
This is an artist's impression of the multiplanetary system of newly discovered super-Earths orbiting a nearby red dwarf star called Gliese 887.
The newly discovered exoplanet AU Mic b is about the size of Neptune.
This artist's impression shows a view of the surface of the planet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to the Solar System. Proxima b is a little more massive than the Earth.
This is an artist's illustration of an exoplanet's atmosphere with a white dwarf star visible on the horizon. The starlight of a white dwarf filtered through the atmosphere of an exoplanet that's orbiting it could reveal if the planet has biosignatures.
This is an artist's illustration of the Kepler-88 planetary system, where one giant exoplanet and two smaller planets orbit the Kepler-88 star. The system is more than 1,200 light-years away.
Every bit of scientific data collected by Kepler was transmitted to scientists on Earth, and exciting discoveries based on the last bits of data are yet to come, NASA said.
"When we started conceiving this mission 35 years ago, we didn't know of a single planet outside our solar system," said the Kepler mission's founding principal investigator, William Borucki, now retired. "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."
After launching in 2009, Kepler stared at the same spot in the sky for four years. The data revealed that planets are plentiful and diverse, planetary systems themselves are diverse, and small, rocky planets similar to Earth within the habitable zone of their stars are common, said Jessie Dotson, Kepler project scientist at NASA's Ames Research Center.
Four years into the mission, the main goals had been met, but mechanical failures put a sudden end to future observations. The engineers essentially rebooted the mission, devising a way to allow Kepler to survey new parts of the sky every few months. The new mission, studying near and bright stars, was dubbed K2.
K2 lasted as long as the first mission, allowing Kepler to survey more than 500,000 stars total.
Kepler made incredible discoveries, revealing that there are more planets than stars in our galaxy. Kepler watched the very beginning of exploding stars, or supernovae, to gain unprecedented insight about stars and witnessed the death of a solar system.
Astronomers were dazzled by the planets it found, including Kepler-22b, probably a water world between the size of Earth and Neptune. It found inferno-like gas giants, rocky planets, planets orbiting binary stars, Earth-size planets, planets in the habitable zone capable of supporting liquid water on the surface, planets twice the size of Earth, the strangely flickering Tabby's Star, new details about the TRAPPIST-1 planets and, in December, an eight-planet system.
And those discoveries have helped shape future missions. TESS, the Transiting Exoplanet Survey Satellite, launched in April and is the newest planet hunter for NASA. It began science operations in late July, as Kepler was waning, and is looking for planets orbiting 200,000 of the brightest nearby stars to Earth.
Kepler hands off the baton to TESS now, NASA said. The first data from TESS is already being sent to Earth and analyzed.
Later, these planets -- and those found by Kepler -- can be studied by the James Webb Space Telescope, which will be able to look closer at the atmospheres and possibly determine their habitability.
Kepler was able to detect light from stars, but NASA is also studying plans for space observatories that are capable of detecting light from planets. This light would allow astronomers to take the spectrum of a planet and look for signs of habitability -- and life.
"That's the path Kepler has put us on," said Paul Hertz, astrophysics division director at NASA Headquarters in Washington.
Dotson noted, "We know the spacecraft's retirement isn't the end of Kepler's discoveries. I'm excited about the diverse discoveries that are yet to come from our data and how future missions will build upon Kepler's results."