On June 6, NASA announced that the Kepler space observatory was reaching the end of its life. Because the propellants needed to keep the unmanned exoplanet hunter pointed in the right direction are running out, mission control put the spacecraft into a state of semi-hibernation that allowed it to transmit stored data before beginning a final phase of observations. Now that Kepler is approaching its final days, let's look back at its remarkable nine-year career and how it has changed our views about the universe.

For all its success, Kepler isn't exactly a household name and it certainly doesn't loom large in popular culture. But with its record of finding thousands of planets beyond our solar system, it might one day end up in the history books as the robotic equivalent of Christopher Columbus or Captain Cook – and with just as much impact on how we see ourselves and our world. That's a lot to load on what is essentially a giant camera, but Kepler is a one very exceptional Box Brownie.

Since it was launched in 2009, Kepler has become arguably the most important telescope since Galileo trained his eight- or nine-power telescope on the skies in 1609. According to NASA, Kepler and its extended K2 mission has so far found 2,723 exoplanets, with 30 of these confirmed to sit in the habitable zone around their stars where liquid water – and, possibly, life – can exist.

To put this into perspective, before Kepler only a handful of exoplanets were confirmed. In fact, so little was known about planets outside of the solar system that it wasn't even certain if they were commonplace. To answer this, Kepler, also known as Discovery Mission 10, was designed to carry out a survey of one section of the Milky Way to look for exoplanets, in particular ones in the habitable zone the size of the Earth or smaller, and to provide data to show how many of the hundreds of billions of stars in the galaxy have these planets.

Changing the universe

An astronomical survey may seem about as earthshaking as doing a population count of potato bugs, but the impact of the Kepler mission is an example of something simple having profound impact on human history. In the case of Kepler, the impact may be the greatest in over four hundred years.

Galileo's crude spyglass is easily out-performed today by a cheap pair of binoculars, but his observations over the course of a couple of months in 1609 revolutionized our conceptions of the universe. Previous to Galileo, accepted scientific ideas didn't leave any room for worlds other than the Earth and the Moon. Mankind lived in a neatly ordered, confined universe with defined rules and surprisingly small dimensions.

Putting things very simply, the universe was conceived as being a huge crystal sphere surrounded by Heaven. The outer shell of this sphere held the fixed stars and inside this were a series of concentric crystal shells for each of the five known planets, the Sun, and the Moon. Inside of all of these spheres, at the very center, was the Earth. This was essentially the universe's rubbish disposal with everything earthy and profane being drawn to the center and everything rarefied and pure rising to Heaven.

This sounds a bit silly and clumsy when boiled down to a couple of sentences, but centuries of refinement from Aristotle to St. Thomas Aquinas had produced a self-contained, consistent cosmology that explained how the universe worked.

The only problem was that this system didn't have any place for other worlds aside from the Moon and possibly the Sun. It simply wouldn't have made any sense for there to be other worlds because that would require separate, completely autonomous sets of concentric spheres. This sounds very much like our modern hypotheses about multiverses with the added difficulties of how to account for a universe with more than one center to attract things to.

Galileo's telescope didn't destroy this view of the universe, but it did give it an almighty intellectual thump from which it never recovered. Instead of being a perfect, reflecting silver sphere, the Moon was covered in "seas," craters, and mountains, while the Sun had spots on it like tarnish. When he looked at the planets they showed discs, indicating that they were spheres like the Earth. Worse, Venus had phases like the Moon, demonstrating that it really was a sphere rather than a disk.

But what really smashed things up was Jupiter, which had four moons orbiting around it. That was a real problem. If everything revolved around the Earth, then how could four moons revolve around Jupiter? What held them in place? Why didn't they collide with the crystalline sphere that held Jupiter?

In many ways, these simple telescopic observations had a greater impact than Copernicus' assertion that the Earth revolved around the Sun, rather than the other way around. Up until the 19th century, when star measurements finally confirmed it, you could argue that the Copernican system was just a simplified version of the old geocentric one that made astronomical calculations easier, but wasn't necessarily true. With Galileo's telescope, scientists were faced with objective facts that had to be accounted for.

Before Kepler

The Kepler mission is having a similar impact on how we view the universe and our place in it. Before Kepler, the only example of a planetary system was our own, and we weren't entirely sure what that meant. Until recently, the solar system seemed a very orderly place. There were small rocky planets in the interior and large gas giants in the outer regions, with some comets and asteroids thrown in. Discoveries like the Kuiper Belt and the Oort cloud expanded this, but it still held together as a system.

The thinking went that if other planetary systems exist, they will be organized like ours, and the planets will be very similar to the rocky planets and gas giants in our solar system.

This view was supported by the accepted theories of how the solar system formed. The current model is that the Sun and the planets formed out of a cloud of gas and dust that slowly accreted, with the Sun at the center of a rotating disk from which the planets formed. Eventually, the Sun ignited to produce a warm inner system and a cold outer system. The inner zone produced the small, rocky planets, while the area beyond the "frost line" created the conditions that allowed the gas giants to form without their atmospheres boiling away.

Meanwhile, astronomers spent almost two centuries before Kepler looking for exoplanets, but without much in the way of results. This is because the Earth's surface is a dreadful place for planet hunting. Not only did the annoying tendency of the Earth to rotate with the Sun rising and setting make it impossible to watch any particular star continuously, but the distorting effects of the atmosphere were overwhelming.

The result was that time and again, a possible exoplanet would be discovered only to be debunked by something as mundane as errors introduced by cleaning the telescope. Even more galling, some exoplanets actually were discovered, but it wasn't possible to confirm that fact until decades later.

Pulsar planets

But there were successes. In 1992, the first confirmed exoplanet was announced. This was found revolving about the star PSR B1257+12, which is 2,300 light years away in the constellation of Virgo. It was found by the Polish astronomer Aleksander Wolszczan on February 9, 1990 using the Arecibo radio telescope.

Why a radio telescope? Because PSR B1257+12 is no ordinary star, but a pulsar, and the presence of the planet and two others later found was deduced by how its gravitational pull threw off the usually very precise timing of the pulsar as it rotated once every 0.006219 seconds.

But what really perplexed astronomers was that these planets even existed. Pulsars are the remnants of supernovae – stars that explode with more energy than an entire galaxy puts out. How those planets were there is still a mystery. Were they captured by the pulsar after it formed? Did they somehow survive the titanic blast? No one is sure.

The impossible planet

But even a pulsar planet was nothing compared to what came next. In 1995, the discovery of the first exoplanet to orbit a main sequence star (one similar to the Sun) was confirmed by Michel Mayor and Didier Queloz of the University of Geneva. With the official designation of 51 Pegasi B, this was a planet larger than Saturn that orbited a star in the constellation of Pegasus 50 light years from Earth.

Finding another gas giant orbiting another star would have been exciting, but 51 Pegasi B was the biggest puzzle yet. This planet has half the mass of Jupiter, but it is closer to its parent star than Mercury is to our Sun and has a year of only four Earth days. Worse, it's tidally locked, meaning that one side always faces its star, resulting in temperatures of over 2,000° F (1,100° C) on the light side, and the dark side being hundreds of degrees cooler – causing continuous storms between the two hemispheres with wind speeds of thousands of miles per hour.

This is not only spectacular, it shouldn't be there. According to the standard model of planet formation, a gas giant can't possibly be so close to its principal, but there 51 Pegasi B was. This posed the question, was this an oddity, or was the truth something else entirely? To find out, astronomers needed a sample size several orders of magnitude larger.

That was where Kepler came in.

Mission Kepler

Kepler wasn't the first space mission designed to hunt for exoplanets, but it could have been. Originally conceived in the 1990s, the Kepler mission wasn't given the green light until 2001 after a proposal was submitted for the fifth time. This was followed by eight years of delays and cost overruns that allowed the French Space Agency to launch its Convection, Rotation et Transits planétaire (CoRoT) mission in 2006. A much less ambitious mission than Kepler, this ran through to 2013 and found 32 confirmed exoplanets before it shut down due to an unexpected malfunction.

Meanwhile, Kepler was being developed not as an alternative to Earth-based exoplanet hunters, but as a dedicated survey craft whose mission was to feed likely candidates to the ground telescopes, which would make closer observations of and confirm if they are, in fact, planets.

There are a surprisingly large number of ways to find exoplanets – each with its strengths and weaknesses, but when they work in concert, they provide a powerful set of overlapping tools that can separate the cosmic wheat from the interstellar chaff. There isn't enough time to go into all of these in any detail, but what they all suffer from is the need to focus on individual stars for long periods of time.

Transit photometry

The method that the Kepler Mission used is called transit photometry, which is a fancy way of saying that it hunts for exoplanets by looking for eclipses.

Imagine that the Moon reflected no light at all. That it was a completely black object that absorbed all light that hit it. No matter what time of the month it was, we wouldn't be able to see it, so you'd probably think we wouldn't know that it was there. However, we would know very well that the Moon exists because two to four times a year the Moon passes in front of the Sun, blocking some or all of the light coming from it.

In terms of evidence, a total solar eclipse is a pretty spectacular example of "conclusive."

This is what happens on a less dramatic level with transit photometry. If a planet is orbiting another star and the plane of its orbit is on a line with our solar system, then it will sooner or later pass in front of its principal star's disc. When it does so, or transits, it blocks off part of that light like the Moon does the Sun.

It's not a lot. The Earth passing in front of the Sun would block only 0.008 percent of its light, but it is detectable. By plotting the brightness of a star over the time of transition, it's possible to not only detect a planet, but also deduce its orbital characteristics and its radius. Combined with other detection techniques, it's even possible to learn about its mass, density, structure and its atmosphere (if any).

The world's biggest camera

Transit photometry works and is the most successful technique to date, but why build a new space telescope? True, earthbound observatories might have problems, but why not use the Hubble or some other?

The answer lies in the field of vision. At any one time the Hubble can only cover a tiny portion of the sky about the size of the full Moon as seen from Earth. This is no problem in most missions, but there's no way to tell which stars have exoplanets or when one will make a transit. A telescope could be looking at a star for 200 years before it saw such an event – then have to wait another 200 years to confirm it.

Needless to say, the US Congress would take a dim view of financing the Hubble to spend up to 400 years looking at one star, so some alternative is needed. This is even truer when you consider a few facts about potential exoplanets.

The first thing is that the transit method only works if the plane of the exoplanet's orbit is in line with the solar system. For planets with small orbits, that works out to only 10 percent of all stars and odds get worse as the orbits get bigger. Also, the planets have to be within about 3,000 light years for the light dips to be detectable. And this isn't helped by the fact that some stars naturally vary in brightness to the point of causing the transit method to produce as many as 40 percent false positives for single-planet systems that need to be confirmed by independent methods.

What this boils down to is that in order to find exoplanets, a space telescope can't look at one or two stars at a time, but needs to take in tens or hundreds of thousands simultaneously on a round-the-clock basis over a period of many years.

This is where Kepler came in. At its most basic, Kepler was a very simple mission. Where other deep space probes carried a raft of instruments, such as imagers, magnetometers, radiation counters, infrared sensors, micrometeoroid detectors and the like, Kepler had just one instrument – nothing more or less than the biggest camera ever sent out of Earth orbit at the time of its launch.

It was also a very wide-angle camera capable of monitoring the brightness of about 150,000 main sequence stars – 90,000 of which are Sun-like – simultaneously in its fixed field of vision. This was pointed at a very specific part of the sky in the region of the Cygnus and Lyra constellations that lies along the line of the Milky Way galaxy's disk. This region is in the same general neighborhood as the Sun, so the stars there are of a similar age and should have a similar composition. Equally important, the stars there are relatively close by at between 600 and 3,000 light years, so the transit method has a better chance of working because more light is gathered from each one, reducing the noise levels of the signals.

Another reason for pointing at just one area of the sky is that it allowed the engineers to make the Kepler spacecraft both stable and simple. To do this, the spacecraft itself is surprisingly stripped down and basic. Built by Ball Aerospace & Technologies, Kepler had a launch weight of only 1,052.4 kg (2,320 lb), with half the mass taken up by its sole scientific instrument – the photometer.

The Photometer

Kepler's photometer consists of a Schmidt telescope – a special wide-angle telescope that is favored by astronomers for sky surveys. It's the ninth largest Schmidt telescope ever built and at launch was the largest telescope ever sent into deep space. However, it's called a photometer rather than a telescope because it doesn't take pictures in the accepted sense of the word. Instead, it measures the intensity of the light it takes in, producing curves on a graph displaying those measurements over time.

The heart of the photometer is its 1.4-m (4.6-ft) primary mirror made of lightweight, ultra-low-expansion glass with an enhanced silver coating. This is mounted on three focusing mechanisms for very fine adjustments that, once set, require no power to stay in place. The light gathered from this mirror feeds to the 0.95-m (3.1-ft) Schmidt corrector plate, which gives Kepler a field of view of about 105 square degrees. By comparison, the Hubble telescope can only manage 10 square arc-minutes, or about a sixth of a degree.

Halfway between the primary mirror and the corrector is the clever part of Kepler, the focal-plane array. This consists of a set of 42 charge-coupled devices (CCDs) just like those found in smartphones and digital cameras, though these have a combined resolution of 95 megapixels and are cooled to - 85° C (-121° F) by a series of passive heat pipes. These feed into 84 data channels that are converted into a digital format by the onboard computer, then stored for up to 60 days in a solid-state memory while awaiting the next monthly transmission window.


But none of this would mean anything if the photometer isn't kept as near to absolutely still as possible. To achieve this, Kepler was designed with as few moving parts as possible. In fact, the only moving parts are the reaction wheels that keep the telescope pointed in one direction. Liquids can also destabilize a spacecraft if they're allowed to slosh about, so Kepler has only a small ration of hydrazine for the attitude thruster that's held still by a pressurized membrane. This is also why it doesn't have an engine to change its orbit.

In addition, the 10.2-m² (109.8-ft²), 1.1-kW solar array that also acts as a heat shield is fixed in place even before launch, as are the radio antennae for sending and receiving data. This is why Kepler only transmits data periodically. To do so, Kepler must turn itself bodily for a high-rate transmission to Earth.

Even the orbit chosen for Kepler was designed for maximum stability. By having the spacecraft trailing the Earth, it was subjected to minimal gravitational effects and other forces that would normally have applied torque and shifted Kepler's attitude. The only major source of torque that had to be considered was from the solar winds, and by balancing the spacecraft against those winds, they actually act as a stabilizer – much like setting the sails on a boat.

Start of the hunt

Kepler launched on March 6, 2009, at 7:49 pm PST (March 7, 03:49 GMT) atop a United Launch Alliance Delta II rocket from Space Launch Complex 17B at Cape Canaveral Air Force station in Florida and entered service on May 12 after a shakedown cruise. It's currently in a heliocentric, Earth-trailing orbit with a period of 372.57 days, which is slowing changing over time.

According to NASA, the main objectives of the Kepler mission were to seek out extrasolar planets and determine something of their properties; find as many terrestrial and larger planets in or near the habitable zone and determine their frequency; learn the size of the planets, how they are distributed in their systems and their orbits; estimate the frequency of planets in multiple star systems; and return spectral data on the parent stars.

However, Kepler is not, technically, a planet hunter. It's more of a preliminary surveyor. This is because Kepler tends to send back a high percentage of false positives, so each "planet" found was merely a candidate that had to be confirmed by ground observation. It's bit like gold mining. What you find might look like gold, but you still need to send it off to the assay office to determine if it's real gold or iron pyrite, aptly called "fool's gold."

As Kepler sent back data, the candidates were identified and then the information was sent to various ground or space observatories for confirmation. These observations use methods other than transits, so they are also able to learn more about the planets and their stars, including their size, mass, and age.

The confirmation process is complex and meticulous. Put simply, it involves a process of elimination by looking for nearby objects that could have contaminated the light curve or if the star in question is a red giant or a variable star that could produce a planet-like light curve. Using the Kepler data as a starting point, those responsible for confirmation use astrometry to measure how the parent star moves under the influence of the unseen planet, Doppler spectroscopy to seek out shifts in the light spectrum, the timing of successive transits that may shift if the candidate planet is orbiting more than one star, and look for signs of reflected light from the suspected planet.

Mission extension and losing a wheel

The Kepler mission was originally slated to last for 3.5 years, but an unexpectedly high noise level in the data from the area surveyed and the spacecraft itself prompted NASA in April 2012 to extend it through 2016 to meet all the mission objectives. That wasn't an unforeseen contingency and the mission profile was drawn up to allow for this, but it wasn't long before things began to go pear-shaped.

A key component in keeping Kepler pointing rigidly in the right direction is a set of four reaction wheels – little gyroscopes that manage the spacecraft's attitude without using the thrusters. Unfortunately, in July 2012, one of the four reaction wheels failed.

Like most spacecraft, Kepler has a high degree of redundancy built in, so the loss of one of the wheels was more annoying than alarming. The other three wheels could easily do the job just as well. Then on May 11, 2013, a second wheel failed. This put the entire mission in jeopardy, but things quickly went from bad to worse in July when the last two wheels started to malfunction within four days of one another, with both showing signs of increased friction.

Without at least three functioning wheels, the Kepler mission as originally planned was doomed, so on August 2, 2013, NASA started soliciting ideas for new missions for the Kepler spacecraft while a full systems evaluation was conducted. Two days later, the current mission was abandoned and an engineering report was ordered.

K2 - Second Light

In November 2013, this evaluation and brainstorming led to abandoning the Kepler mission in favor of a new one called K2 or "Second Light." This involved a jury-rigging operation of sorts to help Kepler regain some of its stability without using its reaction wheels.

NASA engineers managed this by using the solar panel like a sail. Previously, the panel had acted as a way of nullifying the torque caused by the solar winds. Now it was being used by setting the spacecraft at such an angle that the solar panel balanced against the wind in such a way that the spacecraft always pointed at the same angle relative to the ecliptic. In practical terms, this mean that it always pointed the same way in relation to the path it was traveling on.

The good news was that it meant Kepler could continue making observations. The bad news was that it could point at a limited a series of areas as it circled the Sun. In addition, the photometer could still maintain a precision of about 300 parts per million as compared to its previous 20 parts per million – a decrease, but an acceptable one and later testing using the two remaining wheels brought this up to 44 parts per million.

This salvage operation meant that Kepler could continue to look for hot planets revolving around bright stars as well as small planets around small, bright stars. It could identify potentially habitable planets orbiting bright M-dwarfs close to the solar system as well as supernovae in their early stages of collapse.

By December 2014, K2 found its first exoplanet, the super-Earth called HIP 116454 b and the mission was so successful that in June 2016 it was extended for three years, or past the point when Kepler's thrusters would run out of propellant, forcing an end to operations.

The results

Up to now, Kepler has found 2,723 confirmed exoplanets with many more likely to follow in the years to come as many more candidate planets are studied. Of course, Kepler didn't find all these planets at once. Instead, it was a slow, cumulative process because many candidates required several transits to provide enough data for a positive result.

It wasn't until a number of months into the mission that first planet candidates were found and, as expected, these were large, Jupiter-sized planets orbiting close to their stars. As the Kepler kept watching, more data was collected and more discoveries be made, but it soon became clear that things weren't exactly as had been predicted.

What Kepler showed was that most exoplanets are smaller than Jupiter and most are even smaller than Neptune. In fact, the largest group were planets larger than Earth, but smaller than Neptune. What's significant is that these mini-Neptunes are unknown in the solar system and, even more interesting, these planets sit in a size/mass category that they may be made entirely of water without any kind of rocky core.

In addition, Kepler found that the planet Mercury in our solar system, which is the closest planet to the Sun, would be a distant outliner in other planetary systems. The Kepler data shows that a large number of exoplanets have orbital periods much shorter than Mercury's 88 days. A surprising number are super-Jupiters with periods that measure not in days, but in hours. One even has a "year" of 168 minutes.

Another surprise is a super-Neptune orbiting very close to a star that's only 5 to 10 million years old. This means that either such gas giants can form very quickly, or they form far from their stars and then migrate inwards. But a bigger surprise was the discovery of super-Earths – Earthlike planets that are larger than Earth. Such planets aren't found in the solar system. However, because so little is known about their composition or structure, the question remains whether these are large rocky planets like Earth or small gas giants like Neptune.

The finding of more exoplanets showed that the basic theory of plaet formation is correct – planets form from gas and dust condensing into a protostar with a rotating disc. In fact, it appears that most stars have planets revolving around them with 22 to 40 percent of systems having more than one planet, perhaps even larger because of the fact that some planets are very hard to detect at present.

What wasn't expected was that the structure of the systems would vary so much. Instead of looking like a solar system with rocky inner planets and gaseous outer planets, the structure of exoplanet systems seems to be random. Also, some systems are much more compact than was thought possible with most of their planets within 0.1 AU (9.3 million mi, 15 million km) of their stars and some within 0.02 AU (1.8 million mi, 3 million km). This was thought to be impossible because the systems would be too unstable, but that turns out not to be the case.

There was one bit of good news for Star Wars fans – planets like fictional Tatooine with more than one sun in the sky turn out to be real. In fact, half of the Sun-like stars in the galaxy are part of binary star systems and Kepler found 11 that have planets revolving around both stars in complex orbits. According to the data, the moon of one of these planets might be habitable. Whether there are Ewoks on it remains to be seen.

Some of the exoplanets found were downright odd. One has a density so small that it's less than that of helium, but for some reason doesn't collapse under its own weight. Others were brighter than their parent stars, suggesting that they aren't planets, but white dwarfs. Some worlds are made of diamonds. Others are so hot that has seas of molten lava, clouds of titanium waft through the atmosphere, and it rains rocks.

Towards the end of the Kepler mission, Astronomers could start making informed guesses about our galaxy – that it contains from 100 to 400 billion planets, 100 million of which are Earthlike. 500 million planets are in the habitable zone with 30,000 habitable planet within a thousand light years of Earth with the closest within 12 light years and up to 2.7 percent of Sunlike stars have Earthlike planets inside their habitable zones. That boils down to two billion Earth analogue planets. If the other galaxies are anything like ours, that works out to one sextillion Earth analogues in the Universe.

That's a lot of potentially habitable real estate.

Why bother searching for exoplanets?

But what is the significance of these exoplanets? What difference does it make that a rock or a ball of gas orbits another star hundreds of light years away? Who cares?

The easiest answer is the usual one about exploration being deep in the human DNA –the burning need to seek out new life and new civilizations, to boldly split infinitives that no one has split before. And there's a lot to be said about that. Even if one doesn't adhere to the more romantic tinge of this, exploration does pay dividends.

For example, when Europeans went mad about 600 years ago and started crossing oceans, climbing mountains, traversing deserts, and heading off into the unknown in a way that no one really cared about before, much of it, like looking for the poles, seemed a bit daft at the time, but it did alter our world beyond recognition. And we've seen the same thing in the past century where dabbling in some interesting bit of science or engineering led to things like nuclear power, antibiotics, smartphones, and artificial intelligence.

But with exoplanets, the work of the Kepler and K2 missions has already had a definite impact on our world that one day, as we've said, could be comparable to that caused by Galileo's telescope. Thanks to Kepler, we now have a much better understanding of how many planets the Milky Way may be home to, the structure of planetary systems, and how the solar system compares.

Another effect of Kepler has been its impact on the search for extraterrestrial life. Not only has the spacecraft found planets, or even potentially habitable planets, but at least one phenomenon raised some real hairs on the back of the head because it could be the result of not life, but intelligence.

The star KIC8462852 was showing transits where the star dipped in brightness by up to 20 percent. Worse, the curves were very complex and irregular. Ruling out a planet, one likely explanation is that the star is home to a swarm of giant comet 100 km (62 mi) in diameter that orbited the star in an eclipsing cloud.

Another alternative put forward is that these are giant artificial structures built by some super advanced civilization. Perhaps the inhabitants of KIC8462852 are in the middle of building a Dyson sphere – an artificial shell that encompasses an entire star to capture its entire energy output while providing the builders with living space 550 million times greater than that of the entire Earth's surface. Though this is highly speculative and almost certainly not the case, it does show what fascinating stuff the Kepler data could reveal.

On a more conservative tack, Kepler has found a surprising number of Earthlike planets sitting inside the habitable zones of their stars. This doesn't mean that these planets support life, because the term "habitable" as used by scientists is so generous that in the solar system it not only applies to the Earth, but to Venus, which has a toxic atmosphere and is hot enough to melt lead, and Mars, where if life ever did exist it never rose above the level of very primitive microbes.

To determine if a planet really is habitable in the sense of actually being able to support life, we'll need to learn a lot more about both the nature of the exoplanets, but also about our own Earth and why it has life. We must answer the question of whether life arises very easily if water is present, or if it depends on a complicated mix of factors like orbiting a specific type of star, having a magnetic field, having the right mix of elements, having just enough water and no more, or having a large moon that generates tides.

Since this is a frighteningly complex problem, it might be better to sieve through the Kepler data to find planets where it may be possible to directly detect biosignatures like free oxygen in the atmosphere.

This search for life is important not only because it answers of the question "Are we alone?" It also will one day have a very direct impact on the human race. There will be a time many millions of years in the future when the Earth will be uninhabitable. When that happens, we'll need to find a new home. Since the chances of terraforming other planets in the solar systems turns out to be highly unlikely, to say the least, it may be that these exoplanets first seen by Keper cold one day be Nova Terra.

Then, to go from the sweepingly dramatic to the more relatable, Kepler has inspired a series of travel posters extolling the holiday potential of such exotic locations like twin-sunned Kepler-16b, the hypothetical scarlet jungles on Kepler-186f, and other destinations revealed by Kepler and other telescopes. All you need to book a tour package is a major credit card and for physicists to crack faster than light travel.

The future

Kepler may be approaching the end of its days, when it will be shutdown to prevent its radio from becoming a hazard to other spacecraft or the Deep Space Network, but that's not the end of exoplanet hunting. In April 2018, NASA launched the Transiting Exoplanet Survey Satellite (TESS), which will look for exoplanets across the entire sky. ESA has announced that it will launch its Ariel mission to study exoplanet atmospheres and the Gaia satellite continues to map the Milky Way. Meanwhile, even CubeSats are getting in on the exoplanet hunting act.

Then there are new observatories and instruments on Earth to help in the hunt and to verify the discoveries made by the growing constellation of spacecraft. Exoplanet hunting is shifting into a higher gear, with the prospect that the number of confirmed exoplanets going from thousands to tens or even hundreds of thousands in the near future.

In all, Kepler represents a tremendous sea change. In less than 10 years, we've gone from knowing of only a few exoplanets to catalogues of thousands. Exoplanet hunting has gone from an eccentric astronomical backwater to a major field of study. More than that, Kepler has helped change how we view the universe.

It will take decades to complete the analysis of Kepler's data and decades more to sort out the significance of what it has to say. It can even be argued that the full impact of Kepler may take centuries to play out, just as the work of early scientists still reverberate to this day. But one thing is certain – thanks to Kepler, we've begun to meet the neighbors.

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