Kepler space telescope

The Kepler space telescope is an inactive space telescope launched by NASA in 2009 to find Earth-sized planets orbiting other stars. It was named after astronomer Johannes Kepler. The spacecraft was sent into a special path around the Sun, moving away from Earth. After almost ten years of work, the telescope ran out of fuel, and NASA announced it would stop working on October 30, 2018.

Kepler was designed to look at a part of our Milky Way galaxy to find planets about the size of Earth. It watched the brightness of many stars and looked for tiny drops in brightness that happen when a planet passes in front of its star. This helped scientists find many new planets around distant stars.

Kepler's work changed what we know about worlds beyond our solar system. It gave us important clues about how common Earth-like planets might be around sun-like stars.

History

The Kepler space telescope was part of NASA's Discovery Program of science missions. It was built by NASA's Jet Propulsion Laboratory, with help from Ball Aerospace.

Kepler was launched in 2009 after being delayed several times. The Ames Research Center operated the telescope and studied the data it sent back. Kepler was planned to work for three and a half years, but it needed more time because of noise in the data.

In 2013, two tools that helped point the telescope stopped working. This made it hard to keep collecting data. NASA found a new way for Kepler to continue its work, called the K2 mission. This mission looked for planets around smaller, dimmer stars.

Kepler stopped working in 2018 after running out of fuel. It observed over 530,000 stars and discovered more than 2,600 planets outside our solar system. Its work helped scientists learn more about stars and how planets form. Today, a new NASA mission called TESS continues this search.

Spacecraft design

The Kepler space telescope weighed 1,039 kilograms. It had a special camera with a 0.95-meter lens and a 1.4-meter mirror. When it launched, this was the biggest mirror on any telescope outside Earth orbit. The telescope could see a big part of the sky, about the size of a fist held out in front of you.

The camera was made of many small parts called CCDs. Together, they could take over 94 million pictures. These parts were cooled to work better. The mirror was thin and light but strong for space. It was coated to reflect as much light as possible.

Kepler was built to look for tiny changes in starlight. These changes might show when a planet passes in front of a star. It had to be very careful because the change in light from a planet like Earth is very small.

Kepler orbits the Sun, not Earth. This helps it avoid problems like Earth's shadow and gravity. It slowly moves farther away from Earth.

The telescope was controlled from Colorado, with help from scientists at the University of Colorado. It turned to face the Sun for power and kept its radiator away from the Sun to stay cool.

Sadly, two of the telescope's reaction wheels stopped working in 2013. These wheels help it point accurately. This ended its original mission, but scientists found ways to keep using it for other observations.

Field of view

Kepler looks at one fixed area of the sky. It does not move. You can use a calculator on the mission website to see if an object is in this view and where it would appear in Kepler's photos. Information about possible planets is shared with a program called the Kepler Follow-up Program for more observations.

Kepler's field of view covers 115 square degrees, which is about 0.25 percent of the entire sky. This is roughly the size of two scoops of the Big Dipper. To cover the whole sky, you would need around 400 telescopes like Kepler. The area Kepler watches includes parts of the constellations Cygnus, Lyra, and Draco.

Objectives and methods

The Kepler space telescope studied how planets are arranged around stars. It looked at many stars to find out how common Earth-sized planets are. These planets are in the right place to support life and are called "Goldilocks planets." Kepler also learned about the shapes of planets' paths around stars and how many planets orbit stars that have more than one star.

Kepler focused on finding smaller planets, like Earth, instead of big ones like Jupiter. It watched for tiny drops in starlight that happen when a planet passes in front of its star. By measuring how often and how much the light dims, Kepler could figure out the size and distance of the planet from its star. This method works best for planets that are closer to their stars. Kepler needed to see a planet pass by its star at least three times to be sure it was a planet. Scientists expected to find bigger planets first, with smaller ones like Earth taking longer to confirm.

Planet finding process

Finding planet candidates

After Kepler collected and sent its data, scientists made light curves from the brightness readings. They adjusted these curves to account for the spacecraft's movement. Then, they used software to look for signals that might be caused by planets passing in front of their stars. These possible signals are called threshold crossing events.

Scientists checked these events carefully in two rounds, removing false signals and noise. The remaining events are called Kepler Objects of Interest or KOIs and are stored for more checks. KOIs that pass further checks become Kepler planet candidates. The list of KOIs can change, as new checks might move some from candidates to false positives or back again.

Confirming planet candidates

To confirm that a planet candidate is real, scientists need to rule out other explanations. They often use advanced ground telescopes to check the star’s background and make sure nothing else is causing the signal. Another method is astrometry, which helps determine if the signal comes from a star instead of a planet.

Through other detection methods

Different methods help confirm planets. Doppler spectroscopy uses ground telescopes to detect the wobble in a star caused by a planet’s gravity, especially useful for larger planets. In systems with multiple planets, scientists can confirm them by measuring how the timing of transits changes due to the planets’ gravity. This helps confirm smaller, distant planets.

Circumbinary planets have unique transit patterns caused by two host stars, making them easier to confirm. Scientists also look at changes in a star’s brightness caused by reflected light from planets, which can help find more planets over time.

Through validation

When other methods aren’t possible, scientists confirm planets by showing that the chance of a candidate being a real planet is much higher than it being a false signal. One early method involved observing the transit with other telescopes, like confirming Kepler-22b with the Spitzer Space Telescope.

In 2014, a new method called “validation by multiplicity” was introduced. Since most planets in a system orbit in the same plane, if a star has several planet candidates, it’s very likely a real system. This method is efficient for confirming many candidates quickly. Another tool called PASTIS can confirm planets even with only one observed transit, though it works best for larger planets around bright stars.

K2 Extension

In April 2012, scientists wanted to keep using the Kepler telescope until 2016. In November 2012, NASA finished Kepler’s main work and started a longer mission until 2018 when the telescope ran out of fuel.

Reaction wheel issues

In July 2012, one of Kepler’s four reaction wheels stopped working. On May 11, 2013, a second wheel also stopped, making it hard for Kepler to point correctly. Without three working wheels, Kepler could not look for planets as before. Tests showed the two broken wheels had too much friction. NASA asked scientists for ideas on how to use Kepler’s remaining abilities for other projects. They thought about looking at asteroids, studying star explosions, and finding large planets in new ways. They also thought about changing Kepler’s software to work around the broken wheels.

Second Light (K2)

In November 2013, NASA suggested a new plan called K2, or “Second Light.” This plan used Kepler’s remaining abilities to study star explosions, new stars, asteroids, comets, and more planets. K2 would look at a much larger area of the sky along Earth’s path around the Sun. Objects found by K2 would be part of a catalog called EPIC.

In early 2014, Kepler tested the K2 plan and it worked. From March to May 2014, it collected data from a new area of the sky. In May 2014, NASA approved the K2 mission. On December 18, 2014, K2 found its first new planet, a super-Earth named HIP 116454 b.

In April 2016, Kepler had a problem and went into emergency mode, but it was fixed by April 22. In June 2016, NASA extended the K2 mission for three more years. In August 2018, NASA started the 19th observation campaign after fixing pointing issues. On October 30, 2018, NASA said Kepler had run out of fuel and its mission had ended.

Mission results

The Kepler space telescope worked from 2009 to 2013. In January 2010, it shared its first big discoveries. Over time, it found more and more planet candidates. By November 2018, Kepler had found 5,011 possible planets and confirmed 2,662 real planets. By August 2022, 2,056 more candidates were waiting to be confirmed, bringing the total confirmed planets to 2,711.

2009

In August 2009, NASA shared early results from Kepler. It confirmed a known planet called HAT-P-7b and showed that Kepler was good at finding Earth-sized planets.

Because Kepler looks for tiny changes in starlight, it couldn’t study stars that naturally flickered. In the first few months, Kepler scientists found about 7,500 such stars and removed them from the list. On November 4, 2009, Kepler made these stars’ data public. The first new planet candidate was later confirmed ten years later and named Kepler-1658b.

The first six weeks of data showed five new planets very close to their stars. These included one of the least dense planets found, two small stars called white dwarfs, and Kepler-16b, a planet orbiting two stars.

2010

In June 2010, Kepler released data on most of its 156,000 target stars. Among them, 706 had possible planets, ranging from Earth-size to larger than Jupiter. Details were shared for 306 of these. The data included five groups of planets orbiting the same star, with six extra planet candidates. Only 33.5 days of data existed for most candidates. NASA said data for another 400 candidates would be shared later. This data was published in February 2011.

The findings suggested most candidate planets were smaller than half the size of Jupiter. Small planets with orbits shorter than 30 days seemed more common than large ones. This challenged older ideas. Estimates suggested about 100 million habitable planets might exist in our galaxy. However, Kepler had not actually found any Earth-like planets yet.

In 2010, Kepler found two systems with objects smaller and hotter than their stars. These were likely small stars called white dwarfs.

2011

In February 2011, Kepler announced results from data collected between May and September 2009. They found 1,235 planet candidates around 997 stars. Of these, 68 were Earth-size, 288 were larger than Earth but smaller than Neptune, 662 were Neptune-size, 165 were Jupiter-size, and 19 were bigger than Jupiter. Most were smaller than Neptune.

That same announcement included 54 candidates that might be in the “habitable zone” — where liquid water could exist — including five about twice Earth’s size. Before this, only two such planets were known. These new candidates orbited smaller, cooler stars than the Sun. Among all candidates, 68 were smaller than Earth or all previously found planets.

By December 2011, Kepler had found 2,326 planet candidates: 207 Earth-size, 680 larger than Earth but smaller than Neptune, 1,181 Neptune-size, 203 Jupiter-size, and 55 larger than Jupiter. The number of Earth-size and super-Earth-size candidates grew by 200% and 140% compared to February. Also, 48 candidates were in habitable zones, though this number was lower than before because of stricter criteria.

In December 2011, Kepler found its first Earth-size planets, Kepler-20e and Kepler-20f, orbiting a Sun-like star, Kepler-20.

Based on Kepler’s findings, estimates suggested “at least 30,000” habitable planets within 1,000 light-years. NASA’s Jet Propulsion Laboratory calculated that about 1.4 to 2.7 percent of Sun-like stars might have Earth-size planets in their habitable zones, totaling “two billion” in our galaxy alone.

2012

In January 2012, astronomers reported that each Milky Way star might have at least 1.6 planets on average, meaning over 160 billion planets could exist in our galaxy. Kepler also observed powerful flares from stars, some 10,000 times stronger than a known big flare. The method used to confirm planet Kepler-9d became popular. Kepler also found a planet in a system with four stars, the first of its kind.

By 2012, Kepler had found 2,321 candidates: 207 Earth-size, 680 super-Earth-size, 1,181 Neptune-size, 203 Jupiter-size, and 55 larger than Jupiter. Also, 48 candidates were in habitable zones. Kepler estimated that 5.4% of stars have Earth-size candidates and 17% have multiple planets.

2013

A study published in January 2013 suggested the Milky Way has at least as many planets as stars, totaling 100–400 billion. In January 2013, Kepler announced 461 more candidates. One, Kepler-69c, was an Earth-size planet in the habitable zone of a Sun-like star.

In April 2013, NASA announced three new Earth-size planets — Kepler-62e, Kepler-62f, and Kepler-69c — in the habitable zones of their stars. However, later analysis showed Kepler-69c was more like Venus and probably not habitable.

In May 2013, Kepler stopped working because of a problem with one of its reaction wheels. A second wheel had already failed, and Kepler needed three to work properly. Although it couldn’t collect new data, scientists continued to study the data already gathered.

2014

In February 2014, over 530 more planet candidates were announced, including some nearly Earth-sized in habitable zones. This grew to about 400 more in June 2014.

In February 2014, NASA confirmed 715 new planets using a new method. About 95% were smaller than Neptune, and four were less than 2.5 times Earth’s size and in habitable zones.

In March 2014, a study found small planets with orbits shorter than one day usually had another planet orbiting every 1 to 50 days. These small planets were almost always smaller than twice Earth’s size unless they were a certain type of large planet.

In April 2014, Kepler found Kepler-186f, the first nearly Earth-sized planet in a habitable zone, orbiting a red dwarf star.

2015

In January 2015, the number of confirmed Kepler planets passed 1,000. Two, Kepler-438b and Kepler-442b, were likely rocky and in habitable zones. Also, five small rocky planets were found around an 11.2-billion-year-old star, Kepler-444, making it one of the oldest star systems known.

In July 2015, NASA announced Kepler-452b, a planet near Earth-size orbiting a Sun-like star in its habitable zone.

2016

By May 2016, Kepler had confirmed 1,284 new planets. About 550 were likely rocky. Nine of these were in their stars’ habitable zones.

Data releases

Originally, Kepler planned to share data one year after collection. But this changed, and data was sometimes shared up to three years later. This caused criticism, so the team released the third quarter of data one year and nine months after it was collected. Data through September 2010 was made public in January 2012.

Follow-ups by others

Sometimes, the Kepler team shared lists of possible planets. Using this data, other astronomers confirmed the existence of candidates like Kepler-40b in 2010 and Kepler-39b in 2011.

Citizen scientist participation

Since December 2010, Kepler data was used for the Planet Hunters project, where volunteers searched for planets that computer programs might miss. By June 2011, volunteers had found 69 possible new planets.

In January 2012, a TV program asked volunteers to analyze Kepler data. This led to the discovery of a new planet around a star, named Threapleton Holmes B. By early 2012, 100,000 volunteers were searching over one million Kepler images. They discovered two new planets, PH1b and PH2b, in 2012 and 2013.

In April 2017, a new project called Exoplanet Explorers began. Volunteers helped find 90 planet candidates, including a system with four planets around a Sun-like star, named K2-138.

Confirmed exoplanets

Main article: List of exoplanets discovered by the Kepler space telescope

Main article: List of planets observed during Kepler's K2 mission

See also: List of exoplanetary host stars and List of exoplanets

Exoplanets confirmed using Kepler data include Kepler-39b, Kepler-40b, Kepler-41b, Kepler-43b, Kepler-44b, Kepler-45b, planets around Kepler-223, and Kepler-42. The “KOI” label means the star is a Kepler Object of Interest.

Kepler Input Catalog

Main article: Kepler Input Catalog

The Kepler Input Catalog is a database that people can search. It has about 13.2 million stars in it. This catalog was used for the Kepler Spectral Classification Program and the Kepler mission. But only about one-third of these stars could be seen by the Kepler spacecraft.

Solar System observations

The Kepler space telescope had a special code to share its discoveries about small objects in our Solar System with scientists. In 2013, a plan called NEOKepler was suggested to use Kepler to look for objects close to Earth, especially big rocks that could be dangerous. Because of its special path around the Sun and its wide view, Kepler could see places other telescopes could not. Scientists thought a year-long search could help find these dangerous rocks and also find targets for NASA’s Asteroid Redirect Mission. Kepler’s first find in our Solar System was (506121) 2016 BP₈₁, a big object far beyond Neptune’s orbit.

Retirement

On October 30, 2018, NASA announced that the Kepler space telescope had run out of fuel. After nine years of work, it discovered over 2,600 exoplanets before it was retired. The telescope will stay in its safe orbit away from Earth. The spacecraft was turned off with a "goodnight" command sent from the mission's control center at the Laboratory for Atmospheric and Space Physics on November 15, 2018.