After nine years in deep space collecting data that indicates our sky is filled with billions of hidden planets – more planets even than stars – NASA’s Kepler space telescope has run out of fuel needed for further science operations. NASA has decided to retire the spacecraft within its current, safe orbit, away from Earth. Kepler leaves a legacy of more than 2600 planet discoveries from outside our solar system, many of which could be promising places for life.
"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.”
Kepler has opened our eyes to the diversity of planets that exist in our galaxy. The most recent analysis of Kepler’s discoveries concludes that 20 to 50 percent of the stars visible in the night sky are likely to have small, possibly rocky, planets similar in size to Earth, and located within the habitable zone of their parent stars. That means they’re located at distances from their parent stars where liquid water – a vital ingredient to life as we know it – might pool on the planet surface.
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. 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.
"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 from NASA’s Ames Research Center in California’s Silicon Valley. "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."
Launched on March 6, 2009, the Kepler space telescope combined cutting-edge techniques in measuring stellar brightness with the largest digital camera outfitted for outer space observations at that time. Originally positioned to stare continuously at 150,000 stars in one star-studded patch of the sky in the constellation Cygnus, Kepler took the first survey of planets in our galaxy and became the agency's first mission to detect Earth-size planets in the habitable zones of their stars.
"The Kepler mission was based on a very innovative design. It was an extremely clever approach to doing this kind of science," said Leslie Livesay, director for astronomy and physics at NASA’s Jet Propulsion Laboratory, who served as Kepler project manager during mission development. "There were definitely challenges, but Kepler had an extremely talented team of scientists and engineers who overcame them.”
Four years into the mission, after the primary mission objectives had been met, mechanical failures temporarily halted observations. The mission team was able to devise a fix, switching the spacecraft’s field of view roughly every three months. This enabled an extended mission for the spacecraft, dubbed K2, which lasted as long as the first mission and bumped Kepler's count of surveyed stars up to more than 500,000.
The observation of so many stars has allowed scientists to better understand stellar behaviors and properties, which is critical information in studying the planets that orbit them. New research into stars with Kepler data also is furthering other areas of astronomy, such as the history of our Milky Way galaxy and the beginning stages of exploding stars called supernovae that are used to study how fast the universe is expanding. The data from the extended mission were also made available to the public and science community immediately, allowing discoveries to be made at an incredible pace and setting a high bar for other missions.
Kepler Used Cutting-edge Techniques to Find Earth-sized Exo-Planets
Once the Kepler Space Telescope became fully functional, the centuries-old quest for other worlds like our Earth was rejuvenated by the intense excitement and popular interest surrounding the discovery of hundreds of planets orbiting other stars.
When Kepler was launced, there was clear evidence for a substantial numbers of three types of exoplanets: gas giants, hot-super-Earths in short period orbits, and ice giants. The challenge for Kepler was to find terrestrial planets (i.e., those one half to twice the size of the Earth), especially those in the habitable zone of their stars where liquid water and possibly life might exist.
The Kepler mission used cutting-edge techniques to search for Earth-size and smaller planets around other stars in our neighborhood of the galaxy. In the process, scientists also detected many larger planets.
Kepler was designed to be a special-purpose spacecraft that precisely measured the light variations from distant stars, looking for planetary transits. When a planet passes in front of its parent star, as seen from our solar system, it blocks the light from that star. This is known as a 'transit.' Searching for transits of distant 'Earths' is like looking for the drop in brightness when a moth flies across a searchlight. Measuring repeated transits, all with a regular period, duration and change in brightness, provides a rigorous method for discovering and confirming planets and their orbits – planets the size of Earth and smaller in habitable zones around other stars. Scientists define habitable zones as volumes in space where planets could be that may well have liquid water on their surfaces.
Kepler surveyed four classes of stars – F stars (bigger and brighter than Earth's sun), G stars (similar to our sun in brightness and size) and K and M stars (smaller and less bright than our sun). During its mission, Kepler continuously monitored the brightness of hundreds of thousands of stars in the Milky Way galaxy. The Kepler spacecraft housed a single main instrument called a photometer, that is, a light meter, which could simultaneously measure the brightness variations of stars with a precision of about 20 parts per million.
During its planet hunting mission, Kepler measured at least three transits of a planet to consider it a valid planetary candidate. Then, each star with a candidate planet was observed by ground-based telescopes to eliminate any that may have nearby stars that produce a signal that imitates a planetary transit. Consequently, the announcement of planetary discoveries could only be made after a minimum of several months. For Earth-size planets in orbit around stars similar to our sun, the team waited a minimum of three years to get three transits plus the ground-based observing time.
Original Kepler Mission Scientific Objectives
The scientific objective of the Kepler Mission was to explore the structure and diversity of planetary systems. This was achieved by surveying a large sample of stars to:
- Determine the percentage of terrestrial and larger planets there are in or near the habitable zone of a wide variety of stars
- Determine the distribution of sizes and shapes of the orbits of these planets
- Estimate how many planets there are in multiple-star systems
- Determine the variety of orbit sizes and planet reflectivities, sizes, masses and densities of short-period giant planets
- Identify additional members of each discovered planetary system using other techniques, and
- Determine the properties of those stars that harbor planetary systems.
A Unique Approach -- The Transit Method of Detecting Extrasolar Planets
When a planet crosses in front of its star as viewed by an observer, the event is call a transit. Transits by terrestrial planets produce a small change in a star's brightness of about 1/10,000 (100 parts per million, or PPM), lasting for 2 to 16 hours. This change must be absolutely periodic if it is caused by a planet. In addition, all transits produced by the same planet must be of the same change in brightness and last the same amount of time, thus providing a highly repeatable signal and robust detection method.
Once detected, the planet's orbital size can be calculated from the period (how long it takes the planet to orbit once around the star) and the mass of the star using Kepler's Third Law of planetary motion. The size of the planet is found from the depth of the transit (how much the brightness of the star drops) and the size of the star. From the orbital size and the temperature of the star, the planet's characteristic temperature can be calculated. From this the question of whether or not the planet is habitable (not necessarily inhabited) can be answered.
For a planet to transit, as seen from our solar system, the orbit must be lined up edgewise to us. The probability for an orbit to be properly aligned is equal to the diameter of the star divided by the diameter of the orbit. This is 0.5% for a planet in an Earth-like orbit about a solar-like star. (For the giant planets discovered in four-day orbits, the alignment probability is more like 10%.) In order to detect many planets one can not just look at a few stars for transits or even a few hundred. One must look at thousands of stars, even if Earth-like planets are common. If they are rare, then one needs to look at many thousands to find even a few. Kepler looks at 100,000 stars so that if Earths are rare, a null or near null result would still be significant. If Earth-size planets are common then Kepler should detect hundreds of them.
Considering that we want to find planets in the habitable zone, the time between transits is about one year. To reliably detect a sequence one needs four transits. Hence, the mission duration needs to be at least three and one half years.
The Kepler instrument is a specially designed 0.95-meter diameter telescope called a photometer or light meter. It has a very large field of view for an astronomical telescope —105 square degrees— or about the area of both your hands held at arm's length, in order to observe the necessary large number of stars. It stares at the same star field for the entire mission and continuously and simultaneously monitors the brightnesses of more than 100,000 stars for the life of the mission.
As the quest for a twin Earth was heating up in 2013, astronomers began to find Earth-sized planets orbiting distant stars. An analysis of Kepler data showed that about 17 percent of stars have an Earth-sized planet in an orbit closer than Mercury. Since the Milky Way has about 100 billion stars, there are at least 17 billion Earth-sized worlds out there.
Kepler detected planetary candidates using the transit method, watching for a planet to cross its star and create a mini-eclipse that dimed the star slightly. The first 16 months of the survey identified about 2400 candidates. Astronomers then asked, how many of those signals were real, and how many planets did Kepler miss?
By simulating the Kepler survey, Francois Fressin, of the Harvard-Smithsonian Center for Astrophysics (CfA), and his colleagues were able to correct both the impurity and the incompleteness of this list of candidates to recover the true occurrence of planets orbiting other stars, down to the size of Earth.
"There is a list of astrophysical configurations that can mimic planet signals, but altogether, they can only account for one-tenth of the huge number of Kepler candidates. All the other signals are bona-fide planets," says Fressin.
Altogether, the researchers found that 50 percent of stars have a planet of Earth-size or larger in a close orbit. By adding larger planets, which have been detected in wider orbits up to the orbital distance of the Earth, this number reaches 70 percent.
Extrapolating from Kepler's currently ongoing observations and results from other detection techniques, it looks like practically all Sun-like stars have planets.
The team then grouped planets into five different sizes. They found that 17 percent of stars have a planet 0.8 - 1.25 times the size of Earth in an orbit of 85 days or less. About one-fourth of stars have a super-Earth (1.25 - 2 times the size of Earth) in an orbit of 150 days or less. (Larger planets can be detected at greater distances more easily.) The same fraction of stars has a mini-Neptune (2 - 4 times Earth) in orbits up to 250 days long.
Larger planets are much less common. Only about 3 percent of stars have a large Neptune (4 - 6 times Earth), and only 5 percent of stars have a gas giant (6 - 22 times Earth) in an orbit of 400 days or less.
The researchers also asked whether certain sizes of planets are more or less common around certain types of stars. They found that for every planet size except gas giants, the type of star doesn't matter. Neptunes are found just as frequently around red dwarfs as they are around sun-like stars. The same is true for smaller worlds. This contradicts previous findings.
"Earths and super-Earths aren't picky. We're finding them in all kinds of neighborhoods," says co-author Guillermo Torres of the CfA.
Planets closer to their stars are easier to find because they transit more frequently. As more data are gathered, planets in larger orbits will come to light. In particular, Kepler's extended mission should allow it to spot Earth-sized planets at greater distances, including Earth-like orbits in the habitable zone.
University of Califonia - Berkeley graduate student Erik Petigura, former UC - Berkeley post-doctoral fellow Andrew Howard (now on the faculty of the Institute for Astronomy at the University of Hawaii), and UC - Berkeley professor of astronomy Geoff Marcy reported that the fraction of stars having planets the size of Earth or slightly bigger orbiting within Earth-like orbits may amount to 50 percent.
"Our key result is that the frequency of planets increases as you go to smaller sizes, but it doesn't increase all the way to Earth-size planets – it stays at a constant level below twice the diameter of Earth," Howard said.
Planets one to two times the size of Earth are not necessarily habitable. Painstaking observations by Petigura's team show that planets two or three times the diameter of Earth are typically like Uranus and Neptune, which have a rocky core surrounded by helium and hydrogen gases and perhaps water. Planets close to the star may even be water worlds – planets with oceans hundreds of miles deep above a rocky core.
Nevertheless, planets between one and two times the diameter of Earth may well be rocky and, if located within the Goldilocks zone – not too hot, not too cold, just right for liquid water – could support life.
"Kepler's one goal is to answer a question that people have been asking since the days of Aristotle: What fraction of stars like the sun have an Earth-like planet?" Howard said. "We're not there yet, but Kepler has found enough planets that we can make statistical estimates."
The estimates are based on a better understanding of the percentage of big Earth-size planets that Kepler misses because of uncertainties in detection, which the team estimates to be about one in four, or 25 percent.
To find planets, the Kepler telescope captured repeated images of 150,000 stars in a region of the sky in the constellation Cygnus. The data were then analyzed by computer software – the "Pipeline" - in search of stars that dim briefly as a result of a planet passing in front, called a transit. For planets as large as Jupiter, the star may dim by 1 percent, or one part in 100, which is easily detectable. A planet as small as Earth, however, dims the star by one part in 10,000, which is likely to be lost in the noise, Petigura said.
Petigura spent the past two years writing a software program called TERRA, for Transiting Exo-Earth Robust Reduction Algorithm, which is very similar to Kepler's "Pipeline." The UC Berkeley/Hawaii team then fed TERRA simulated planets to test how efficiently the software detects Earth-size planets.
"It may seem crazy to spend two years re-doing what the Kepler team has already done, but the question we are asking – How many Earth-size planets are we missing? – is so important that we wanted to do it separately for cross-validation," he said.
"Erik is really brilliant," said Marcy, who was Petigura's research mentor when he was still a UC Berkeley undergraduate student. "He has single handedly written data analysis software from scratch, drawing upon the NASA Kepler team's expertise with its Pipeline. The more people working on this, the more independent analyses, the better."
After carefully measuring the fraction of planets missed by TERRA, the team corrected for this and conducted a thorough analysis of 12 of the 13 quarters of Kepler observations freely available on the Internet. They identified 129 Earth-like planets ranging in size from nearly six times the diameter of Earth to the diameter of Mars. Thirty-seven of these planets were not identified in previous Kepler reports.
The analysis confirmed that the frequency of planets increases as the size decreases, which Howard and the Kepler team reported last year. Perhaps one percent of stars have planets the size of Jupiter, while ten percent have planets the size of Neptune. Marcy compared this to rocks on a beach – large boulders are rare, stones are more common, and pebbles extremely abundant.
Unlike the beach, where sand grains and flecks are even more abundant, Petigura's team now estimates that the abundance of planets stops rising at about twice Earth's diameter and remains the same until the size of Earth, the limit of their analysis. This corrects an impression left by Howard's earlier work that the frequency of Earth-size planets actually decreases below twice Earth's diameter.
"A year ago, Kepler had found only a few planets smaller than twice Earth's diameter, far fewer than would be expected if you extrapolate downward from the abundances of larger planets," Petigura said. Accounting for planets that Kepler misses still means that "big Earths" are one third as abundant as would be expected if the rising trend continued below twice the diameter of Earth.
Howard noted, however, that if 17 percent of all stars have Earth-size planets within the orbit of Mercury, "where are they in our solar system? Maybe our solar system is an anomaly compared to the great variety of stars."
Scientists are expected to spend a decade or more in search of new discoveries in the treasure trove of data Kepler provided.
"We know the spacecraft's retirement isn't the end of Kepler's discoveries," said Jessie Dotson, Kepler's project scientist at NASA’s Ames Research Center. "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."
Before retiring the spacecraft, scientists pushed Kepler to its full potential, successfully completing multiple observation campaigns and downloading valuable science data even after initial warnings of low fuel. The latest data, from Campaign 19, will complement the data from NASA’s newest planet hunter, the Transiting Exoplanet Survey Satellite (TESS), launched in April 2018. TESS builds on Kepler's foundation with fresh batches of data in its search of planets orbiting some 200,000 of the brightest and nearest stars to the Earth -- worlds that can later be explored for signs of life by missions such as NASA’s James Webb Space Telescope.
NASA's Ames Research Center in California's Silicon Valley manages the Kepler and K2 missions for NASA's Science Mission Directorate. NASA's Jet Propulsion Laboratory in Pasadena, California, managed Kepler mission development. Ball Aerospace & Technologies Corporation in Boulder, Colorado, operates the flight system with support from the Laboratory for Atmospheric and Space Physics at the University of Colorado in Boulder.