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A century of astronomy has revealed the place of the Earth in the universe


A century ago, the Milky Way galaxy was the entire known universe. We had no idea what made the stars shine and it was only known that one star, our own sun, had planetary housing. Of those planets, humans had explored only one: Earth.

"The stellar universe, as we know it … is a flattened organization of clock-shaped stars and nebulae," astronomer Harlow Shapley wrote in the Science News Bulletin, the first version of Science News, in August 1921 (SN : 8/8/1921, p. 3). That glittering pocket watch was the Milky Way, and by the time Shapley wrote this, astronomers were just beginning to conceive that anything could be beyond it.

Today, spacecraft have flown all over the planets of the solar system, taking close-ups of their wild alien faces. It turns out that the solar system contains a cornucopia of small rocky, icy bodies that defied the very definition of a planet. Thousands of planets have been seen around other stars, some of which may have the right conditions for life to thrive. And the Milky Way, we now know, is just one of billions of galaxies.

The last 100 years have brought a series of revolutions in astronomy, each kicking the Earth a little further from the center of things. Along the way, people were not exactly receptive to these blows in the centrality of our home planet. In 1920, the question of whether there could be other "island universes" – galaxies – was the subject of the Great Debate between two astronomers. In the 1970s, when it was shown that Mars had a pink sky, not a blue one, journalists booed. His reaction "reflects our desire for Mars to be the same as Earth," astronomer Carl Sagan later said. And in the 1990s, astronomers almost missed the extrasolar planets that had been hidden in their data because they had adapted their search techniques to find planets more similar to those in our own solar system.

But turning the focus of the Earth opened our minds to new possibilities, new universes, new places where life could exist. The next century of astronomy could bring better views of our cosmic origins and new strategies for finding worlds that other creatures call home.

Misconceptions from past decades suggest that scientists should be careful when predicting what we will find in the future.

“You learn a lot of humility in this business,” says Candice Hansen, a planetary scientist at the Tucson-based Planetary Science Institute. "You always learn a lot more when you're wrong than when you're right."

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More than the Milky Way

In the early twentieth century, conventional wisdom held that the Milky Way was alone. It contained stars, sometimes arranged in clusters, and diffuse spots of light known as nebulae. That was all.

Some nebulae had spiral structures, "appearing in the telescope as vast July 4th pinwheels," as described by Science News Letter, the predecessor of Science News, in 1924. In the 18th century, the German philosopher Immanuel Kant had described nebulae as " Higher universes “or, so to speak, Milky Way.” But in the early 1900s, most astronomers thought drawing that parallel was ridiculous.

"No competent thinker," wrote astronomy historian Agnes Clerke in 1890, "can maintain any nebula to be a stellar coordinate range system with the Milky Way."

By the 1920s, however, that view was already being challenged. As early as 1914, astronomer Heber Curtis of California's Lick Observatory argued that spiral nebulae are not part of the Milky Way, but "star galaxies or starry universes separated inconceivably distant that an entire galaxy becomes an unsolved haze of light."

At the same time, Shapley, of the Mount Wilson Observatory in California, began to show that the Milky Way was inconceivably vast.

Shapley built on the work of Henrietta Leavitt, one of a group of "computer" women at Harvard University who examined photographic plates capturing the night sky. While studying photographs of Magellanic clouds, which we now know are two small galaxies orbiting the Milky Way, Leavitt noticed that certain stars varied in brightness over time, some of them in a peculiar way. "It is noteworthy," he wrote in 1908, that "the brightest variables have the longest periods." In other words, the brightest stars twinkled more slowly.

black and white image of Henrietta Leavitt sitting at a tableIn the early 1900s, astronomer Henrietta Leavitt discovered a feature of certain stars, called Cepheid variables, that helped other astronomers measure cosmic distances. Those stars finally helped prove that the Milky Way is just one of many galaxies.Photo by Margaret Harwood, courtesy of AIP Emilio Segrè Visual Archives, Physics Today Collection, Shapley Collection

This meant that these variable stars, called Cepheids, could be used to estimate cosmic distances. It’s hard to tell how far a cosmic object is: bright-looking stars could be intrinsically faint but close, while faint-looking stars could be intrinsically bright but distant. But all Cepheids in the same cloud should be about the same distance from Earth. That meant that "their periods are apparently associated with their emission of light," Leavitt wrote in 1912. To discover the true brightness of any Cepheid, all an astronomer had to do was measure its scintillating speed. It was a small step to find out the distance.

Shapley used this fact only a few years later, measuring distances to the Cepheids within global groups of stars to discover the position of the sun in the Milky Way. To his surprise, the sun was not in the center of the galaxy but on one side. The Milky Way's star disk was also about 10 times wider than previous astronomers had assumed: about 300,000 light-years in diameter, according to his calculations. (It's been a while; modern astronomers think it's between 120,000 and 200,000 light-years away.)

He and Curtis made their opposing views public at a meeting of the National Academy of Sciences in Washington, D.C., in April 1920, in an event known as the Great Debate. Each had 40 minutes to present their views on whether there is only one or more universes, which we now think of as galaxies.

Shapley, who was about 30 years old and was considered a rising star on the field, was the first. A former journalist who apparently felt uncomfortable talking to crowds, read his argument from a typed script. He barely touched on the question of other universes, focusing on his new measurements of the size of the Milky Way. The implication was that the Milky Way was too big for other galaxies to make sense.

Curtis was an older and respected authority on spiral nebulae, in addition to a gifted speaker. He defended the then standard view that the Milky Way was much smaller than Shapley assumed. But even a large Milky Way should not deny the possibility of other equally large galaxies, he argued. He argued that the spectra of light from the spiral nebulae were similar enough to those in the Milky Way that they could be similar objects.

The two astronomers were partly right and partly wrong.

Galaxies come into view

The big debate was resolved by a young astronomer named Edwin Hubble who worked on Mount Wilson. Hubble also used Leavitt's Cepheid technique to measure cosmic distances, this time finding variable stars in the spiral nebulae themselves.

Hubble began observing the Andromeda Nebula, one of the brightest nebulae in the sky, in the fall of 1923. It used Mount Wilson's 60-inch telescope and its 100-inch telescope, then the largest in the world. Over the next year or so, he studied 35 Cepheids in Andromeda and a different nebula called the Triangle. Their periods were long enough for nebulae to have to be a million light-years away for the stars to look so faint. (We now know that they are more than 2.5 million light-years for Andromeda and 2.7 million for Triangulum.)

black and white image of Edwin Hubble sitting at a tableAstronomer Edwin Hubble, shown here holding a drawing of a galaxy, has shown that there are other galaxies outside the Milky Way.Hale Observatories, courtesy of AIP Emilio Segrè Visual Archives

“Measuring the distance to Andromeda was a big problem because it was the first evidence that there are galaxies beyond ours,” says astronomer Emily Levesque of the University of Washington in Seattle. "It changed what we thought was the shape of our universe."

Some suggestions that the Milky Way was not alone had arisen before that, but Hubble's discovery struck. Even though the Milky Way was as big as Shapley claimed, Andromeda was outside its borders. When Shapley received the Hubble newspaper, he said, "Here's the letter that destroyed my universe."

Science News Letter reported Hubble's finding under the headline "Sky Pinwheels Are Stellar Universes 6,000,000,000,000,000,000 Miles Away" in December 1924 (SN: 12/6/24, p. 2).

“It seems likely that many of the smaller spiral nebulae are even more remote and seem smaller on this account,” says the story quoted by Hubble. "The portion of the universe within the scope of our research consists of a large number of stellar galaxies comparable to ours, scattered over almost empty space and separated from each other by distances of inconceivable magnitude." Here finally was the modern view of the universe.

By the end of the decade, Hubble not only demonstrated that spiral nebulae were "island universes," he also began classifying different types of galaxies and thinking about how they evolved over time. Moreover, he showed that galaxies flew at each other at speeds proportional to their distance. In other words, the universe was expanding.

By the turn of the century, astronomers knew that the universe was dotted with billions of galaxies of all shapes and sizes. In April 1990, NASA launched the first optical space telescope into Earth orbit, giving the world a new perspective on space.

“Instead of these blurry blur of the best observatories on the top of our planet’s mountains,” says planetary scientist Jim Bell of Arizona State University in Tempe, “suddenly the entire realm of the solar system, galaxy, extragalactic … opened up by getting above the atmosphere ".

NASA named the telescope after the scientist who opened the minds of astronomers to the existence of such a universe: the Hubble Space Telescope.

The images he captured during 30 years of operations – clusters of stars, galaxies and nebulae – are so iconic that they are printed on everything from socks and coffee cups to high-fashion catwalk designs. The telescope itself was recently immortalized in the form of Lego.

“It’s what literally everyone has heard about,” Levesque says. Most people today think that Hubble was "the type that built the telescope."

It highlights an image from the beginning of the mandate of the space telescope. In December 1995, the director of the telescope, Robert Williams, decided to train the observatory in a small, dark patch of sky near the handle of the Big Dipper for 10 consecutive days. The resulting portrait of this unbroken piece of sky revealed thousands of unknown galaxies sending their light from farther away than astronomers had ever seen (SN: 20/01/96, p. 36). The universe as Edwin Hubble had imagined it, full of island universes, was captured with a hard look.

As for Henrietta Leavitt, she lost the recognition she deserved for helping beat the Milky Way from her central hanger. A Swedish mathematician wrote to him in 1925 saying that his work "impressed me so deeply that I feel seriously inclined to nominate him for the Nobel Prize in Physics for 1926". He received a response from Shapley, then director of the Harvard College Observatory: Leavitt had died four years earlier.

Steps to Mars

The first liquid-fuel rockets, precursors of which later took robots and people into space, were launched in the 1920s. A century later, robots flew, orbited, or landed on all known planetary bodies in 1920 and some that were not. People walked on the moon and lived in space more than a year at a time. And they talk serious conversations about sending people to Mars.

NASA used to explore other worlds in a clear order, first observing with telescopes and then performing increasingly complex missions: flybys, orbiters, landers, rovers, then people and samples. “We’ve taken all the progression to the moon, in the (last) century,” Bell says. "At some point in this new century, we will add Mars to that list. Everything else in the solar system, we check large chunks of that matrix."

After the Soviet Union launched the first artificial satellite, Sputnik 1, in 1957, space launches were rapid and furious. Many were demonstrations of political and military power. But many of them also had scientific merit. The Soviet spacecraft Luna 3 photographed the side of the moon in 1959, shortly after the founding of NASA. In the spacecraft it flew behind Venus and Mars in the 1960s, sending back the first data about its alien atmospheres and surfaces.

In that same decade, humans landed on the moon and brought back rocks, opening a wide, detailed window into the history of the solar system. The lunar samples from the Apollo missions gave scientists a way to discover how many ancient planetary surfaces there are around the solar system, taught us that the entire inner solar system was bombarded with impacts on its youth, and gave us a history of origin for the moon (SN: 19/07/19 and 20/07/19, page 18).

“Until we started the space program, we really had no idea what geology was elsewhere,” says Hansen of the Institute of Planetary Sciences. “At the turn of the century, they were still debating whether the craters on the moon were impact craters or volcanic boilers. Even right there in our own garden, we didn't know what was going on. "

And the extraterrestrial geology was amazing. Meaningless, planetary scientists had based many of their expectations on other worlds on Earth. The June 1976 Science News cover, the month before NASA's Viking 1 landing, became the first long-lasting spacecraft to land gently on Mars, showing Mars with a Cheez Whiz-colored desert under a clear blue sky. In the sleepless hurry to release the first color images sent by Viking 1, scientists also processed the image to produce a blue sky there.

cover of Science News magazine with an illustration of Viking 1 on MarsBefore NASA's Viking 1 spacecraft landed on Mars in July 1976, Science News and others were contemplating the Red Planet with a blue sky. The sky in Mars is really a yellowish, dusty pink.

But the day after the landing, James Pollack, of the imaging team, told reporters that the Martian sky was pink, probably thanks to the scattered light from the dust particles suspended in the air.

“When we discovered that the sky of Mars was a kind of yellow-pink instead of blue that had been erroneously reported for the first time, the announcement was received by a chorus of boos from the gathered journalists,” Sagan later wrote in the introduction to his popular book Cosmos. "They wanted Mars to be, even in this aspect, like Earth."

Still, Viking landings 1 and 2 brought Mars down to Earth, so to speak. "Mars has become a place," said Viking project scientist Gerald Soffen in an interview for a historic NASA project published in 1984. "It went from a word, an abstract thought to a real place."

Somehow, the views of Mars from the Viking landers were disappointing. The central goal of the mission was to explicitly seek microbial life. It was “a long shot,” journalist Janet L. Hopson wrote in Science News in June 1976 (SN: 5/5/76, p. 374). But "even if no signs of life appear, (biologists) gain their first real perspective on terrestrial biochemistry, the origins of life, and evolution."

The results of the Viking mission's life detection experiments were inconclusive, an almost worse finding than a true negative.

Subsequently, NASA withdrew from searching for life directly. The next 45 years of missions to Mars looked for signs of past water, potentially habitable environments, and organic molecules, rather than living organisms. All of these features appeared in data from Spirit, Opportunity and Curiosity rovers in the 2000s and 2010s.

Now, NASA’s Perseverance explorer, which landed in February 2021, is looking for signs of ancient microbial life. The rover will cache rock samples that a future mission will return to Earth. And the ExoMars rover of joint Russian and European space agencies – named Rosalind Franklin, by the chemist whose work was instrumental in discovering the structure of DNA – aims to look for molecular signatures of life on Mars and just below the surface after its launch in 2022 .

Sagan predicted in 1973 that if he had been born 50 years into the future, the search for life on Mars would already be over. Today, 48 years later, we are still searching.

black and white image of the surface of MarsThe first image taken on the surface of Mars, in July 1976, shows the pavement of NASA's Viking 1 lander and the rocks of a basin called Chryse Planitia.NASA

image of the Perseverance rover on MarsNearly 45 years later, the small Ingenuity helicopter landed with the Perseverance rover and became the first robot to fly in the thin Martian atmosphere. Its blades span 1.2 meters.JPL-Caltech / NASA, Univ. From the state of Arizona.

Exotic moons

The year after the Vikings landed on Mars, another pair of spacecraft launched to bypass almost the entire rest of the solar system from the list of must-have scientists. Astronomers realized that in 1977 the planets would line up in such a way that a spacecraft launched that year could reach Jupiter, Saturn, Uranus, and Neptune one by one, stealing some angular momentum from each world as it advanced. The mission was christened Voyager (SN: 27/08/77, p. 132).

“There’s never been anything like it and there never will be,” says Bell, of the state of Arizona. “It was comparable to the travels of Magellan or Darwin or Lewis and Clark. Only an absolutely profound discovery mission that completely changed the landscape of planetary science in this century. "

Voyager’s views on the outer solar system forced scientists to think outside the “Earth box,” says Hansen, who worked on the mission. “The Voyager imaging team, bless their hearts, would make predictions and then they would be wrong,” she says. "And we would learn something."

Hansen remembers talking to a member of the imaging team as the spacecraft approached Jupiter and its dozens of moons. “He said,‘ Candy, we’ll see craters in (moons) Io and Europe, because we know by density that those are rocky worlds. But not in Ganymede and Callisto, because they are ice, "he recalls. On the contrary, the images showed Ganymede and Callisto covered in craters." It was a moment aha: ice will act as a rock at those temperatures. "Meanwhile, Europe surrounded of oceans and the molten Io had almost no craters.

Jupiter's moons presented "a whole family of exotic worlds previously unimaginable, each radically different not only from their companions but also from the rest of the planet's observer experience," journalist Jonathan Eberhart wrote in Science News in April 1980 (SN: 19/04/80, p. 251).

Before 1979, the Earth was the only one known to scientists in the rocky and geologically active world. But Voyager also changed that view. A member of Voyager's optical navigation team, Linda Morabito, detected a strange mushroom-shaped feature extending to the edge of the Io as she tried to trace the ship's position on March 9, 1979. She consulted with the scientific team and soon learned of who were looking at a gigantic volcanic plume. Io was erupting in real time.

Three planetary scientists had predicted Io's fire before the feathers were discovered. The three suggested that the moon was heated by a gravitational pull and slack between Jupiter and one or two of its other moons, Europa and Ganymede.

But most of the planetary scientific community was stunned. "We take gravity for granted here. Keep your feet on the ground," Hansen says. "But gravity shapes and shapes so many things in so many unexpected ways."

Voyager and later missions to outer planets, such as Galileo on Jupiter in the 1990s and Cassini on Saturn in the 2000s, transformed our view of the solar system in a profound way. They revealed several amazing parts of the solar system where life could exist today.

Voyager hinted that Europe could have an ocean of liquid water under an icy shell. Galileo strengthened that idea and suggested that the ocean could be salty and have contact with the moon’s rocky core, which could provide chemical nutrients for microbial life. NASA is developing a mission to fly beyond Europe. “It won’t surprise me if life is somehow discovered in Europe in my life or in this century,” Bell says.

images of Io, Europa and EnceladusSpaceships have revealed that some moons let out their interior. Jupiter's moon, Io (left), generates magma feathers up to 390 kilometers in the air. The moon of Jupiter Europe (center) and the moon of Saturn Enceladus (right) host underground seas and can bring water into space.From left: JPL-caltech / NASA, Univ. of Arizona; JPL-Caltech / NASA, SETI Institute; JPL-caltech / NASA, Space Science Institute

images of Io, Europa and EnceladusSpaceships have revealed that some moons let out their interior. The moon of Jupiter Io (above) generates plumes of magma up to 390 kilometers in the air. Jupiter's moon Europa (center) and Saturn's moon Enceladus (bottom) host underground seas and can bring water into space.From above: JPL-caltech / NASA, Univ. of Arizona; JPL-Caltech / NASA, SETI Institute; JPL-caltech / NASA, Space Science Institute

Shortly after the arrival of the Cassini spacecraft on Saturn in 2004, scientists realized that the small moon Enceladus expels dramatic plumes of water vapor, dust, and ice crystals into space from a hidden underground sea. That moon also seems like a good place for life.

If the last century of exploration of the solar system was about to tackle alien geology, Hansen says, this next century is going to be about oceanography: getting control over the strange seas of our own solar system.

“I think that’s going to shape a lot of research in the future,” Hansen says. Now that it is clear that these moons have oceans, researchers will ask if they are habitable and eventually if they are inhabited.

Exoplanets have been detected

The first planet sighted outside our solar system (an exoplanet) was so different from anything in our solar system that astronomers were looking for nothing like it.

“Knowing that there really are planets around other stars now seems so trivial to say,” says exoplanet observer Debra Fischer of Yale University. "But we had arguments in 1995 about whether other stars have planets."

Then, when astronomer Michel Mayor of the Geneva Observatory directed his spectrograph into the sky in April 1994, he was silent about his hopes of finding true exoplanets. He was more likely to find brown dwarfs, failed stars that never grew massive enough to burn hydrogen.

His instrument employed a new clever way of hunting other worlds, called the radial velocity technique. Previous exoplanet hunters directly sought the motion of a star in response to the gravity of an orbiting planet, observing whether the star was moving back and forth in the sky. That technique led to several planetary claims, even dating back to 1855, but none of them had endured. Those movements are small; The influence of Jupiter moves the sun only 12 meters per second.

Instead, Mayor and others studied a change in the wavelength of starlight as a star moved back and forth. When a star approaches us, the light changes to shorter or bluer wavelengths; as it moves away, the light becomes redder. By calculating the speed of the back and forth motion of a star, astronomers were able to discover the minimum mass and year length of anything that was pulled from that star.

The shifts the mayor was looking for were still tiny. The search was considered futile and similar to the search for little green men. Therefore, astronomers who explicitly claimed to be looking for planets had difficulty programming observations in telescopes. Brown dwarfs, on the other hand, were considered a legitimate science and would be easier to detect.

Thus, the world was stunned when, in October 1995, the mayor and his student Didier Queloz reported strong evidence not of a brown dwarf, but of a real planet orbiting the sun-like star 51 Pegasi, about 50 light-years from the sun. our solar system.

The new planet was weird. It appeared to be about half the mass of Jupiter, too rugged to be a brown dwarf. But it orbited the star once every 4.23 Earth days, putting it incredibly close to its star. There is nothing like this in our solar system and astronomers had no idea how it could exist.

“The news went through the astronomical community like lightning,” journalist Ron Cowen wrote in Science News, in the first of three stories about the new planet he would write within a month (SN: 21/10/95, p. 260).

51 Peg b, as it turned out, ushered in a new era. “It means there are planets around other sun-like stars, we can find them and they can be the most exciting,” says Lisa Messeri, a Yale anthropologist who studied how astronomers create worlds from pixels and spectra. "The first ones are exciting because they promise there will be second, third and fourth."

Search has been activated. A San Francisco group quickly found two other planets hidden in data that researchers had not yet finished analyzing. The next two planets, 70 Vir b and 47 UMa b, were also more massive and closer to their stars than expected.

The existence of these three worlds, which were named Jupiter hot because their nearby orbits should make them sizzle, transformed the paradigm of what a planet could be. It is clear that our solar system was not the model of the universe.

However, for a few years after 51 Peg b was announced, astronomers debated whether the planet was really there. Perhaps the star’s apparent back and forth was just its outer atmosphere breathing in and out. Those debates diminished as more planets were discovered, but a new technique was needed to convince everyone.

Astronomers have predicted at least since the 1850s that some planets would pass in front of their stars from an Earth perspective. When crossing the face of its star, a planet could reveal its presence by blocking the star's light a little.

But if other solar systems are like ours, transits would be incredibly difficult to detect. Our planets are too small and too far from the sun to cast a large shadow. Hot Jupiters, on the other hand, should block much more light from a star than any planet in our solar system. With the discovery of 51 Peg b, transits seemed not only possible to detect, but almost easy.

The first extrasolar planet in transit was revealed in 1999, when then-Harvard graduate David Charbonneau headed to Colorado to do his dissertation work with astronomer Tim Brown. Brown had built a small telescope on a friend's farm north of Boulder, installing the computers in a reusable chicken coop, to search for planets in transit. However, by the time Charbonneau arrived, the farm had already been sold and the telescope had been moved to a laboratory.

To practice the technique, Charbonneau aimed Brown's telescope at a star, called HD 209458, which already had a suspicious planet. The star's light decreased by about 1 percent and then shone again. That was a clear sign of a planet about 32 percent wider than Jupiter.

Ese descubrimento acabou con todas as dúbidas sobre a existencia de exoplanetas, di Fischer, que traballara co grupo de caza de exoplanetas en San Francisco. "Sucedeu así", di Fischer cun golpe de dedo. O tamaño e a masa combinados do planeta descartaron sen ambigüidade as ananas marróns ou outras explicacións exóticas. "Anda coma un Xúpiter, fala coma un Xúpiter, é un Xúpiter".

Houbo outra vantaxe no método de tránsito: pode amosar a composición da atmosfera dun planeta. Os planetas detectados pola técnica do balanceo eran "pouco máis que fantasmas", escribiu Cowen en Science News en 2007. Eran demasiado pequenos para ser vistos e moi próximos á estrela para ser fotografados directamente.

"Todo o mundo asumira que, se querías (detectar) a atmosfera dun planeta extrasolar, terías que imaxinalo", dixo Charbonneau a Science News. Pero a luz das estrelas que se filtra polo ceo dun planeta en tránsito podería revelar que gases rodean o mundo alieníxena sen necesidade dunha instantánea.

Caza de planetas habitables

Os tránsitos pronto superaron os oscilacións como a estratexia máis fructífera para atopar planeta. Iso foi principalmente grazas ao lanzamento do telescopio espacial Kepler da NASA en marzo de 2009.

A misión de Kepler consistía explícitamente en atopar outras terras. Durante case catro anos, o telescopio mirou a 170.000 estrelas nunha soa porción de ceo para atrapar cantos planetas en tránsito puido. En particular, os seus operadores esperaban planetas do tamaño da Terra en órbitas terrestres arredor de estrelas parecidas ao sol, lugares onde a vida podería existir.

Os anos seguintes foron un boom para os buscadores de planetas. Ao final dos seus case 10 anos de duración, Kepler confirmara case 2.700 planetas e miles de planetas potenciais máis. Findings went beyond the hot Jupiters to worlds the size of Earth and planets in the “habitable zone,” where temperatures could be right for liquid water.

Discoveries came so quickly that a single new world stopped being a news story. Kepler’s data shifted from revealing new worlds one by one to taking an exoplanet census. It showed that hot Jupiters are not actually the most common type of planet; they were just the easiest ones to spot. The most common type makes no appearance in our solar system: worlds between the size of Earth and Neptune, which may be rocky super-Earths or gaseous mini-Neptunes.

And Kepler revealed that there are more planets in the galaxy than stars. Every one of the billions and billions of stars in the Milky Way should have at least one world in its orbit.

But the telescope never really achieved the goal of finding another Earth. Kepler required three transits to confirm a world’s existence. That means the telescope had to stare for at least three years to find a planet orbiting at Earth’s exact distance.

By 2013, after four years of observing, half of Kepler’s stabilizing reaction wheels had failed. The telescope couldn’t maintain its unblinking view of the same part of the sky. Mission scientists cleverly reprogrammed the telescope to look at other stars for shorter spans of time. But most of the planets found there orbited closer to their stars than Earth does, meaning they couldn’t be Earth twins.

Finally, Kepler ran out of fuel in 2018, with no true Earth analog in sight.

Messeri recalls an exoplanet conference at MIT in 2011 where a lot of the conversation was about finding a twin of Earth.

“It was a peak of excitement — maybe we’re going to find this planet in the next three years, or five years. It felt close,” she says. “What’s interesting is, in the 10 years since then, it still feels that close.”

But astronomers had already realized they might not need a true Earth analog to find a planet where life could exist. Rocky worlds orbiting smaller, dimmer stars than the sun are easier to find, and might be just as friendly to life.

Charbonneau again was ahead of the curve, having started a program called MEarth in 2008 to hunt for habitable planets around puny M dwarf stars using eight small telescopes in Arizona (plus another eight in Chile that were added in 2014). Within six months, Charbonneau and colleagues had found a super-Earth dubbed GJ 1214b that is probably a water world — maybe a bit too wet for life.

The European Southern Observatory started the TRAPPIST, for TRAnsiting Planets and PlanetesImals Small Telescope, survey from La Silla, Chile, in 2010. Another telescope, at Oukaïmeden Observatory in Morocco, came online to search for planets orbiting Northern Hemisphere stars in 2016. Among that survey’s discoveries is the TRAPPIST-1 system of seven Earth-sized planets orbiting a single M dwarf star, three of which might be in the habitable zone (SN: 3/18/17, p. 6).

illustration of the TRAPPIST-1 planet systemThe star TRAPPIST-1 hosts seven planets (shown in an artist’s illustration) that all probably have a rocky composition. At least three of the planets could have temperatures that are good for life.JPL-Caltech/NASA

NASA’s successor to Kepler, TESS, or Transiting Exoplanet Survey Satellite, has been scanning the entire sky since April 2018 for small planets orbiting bright nearby stars, including M dwarfs. It spotted more than 2,200 potential planets in its first full-sky scan, scientists announced in March 2021.

These days, astronomers are joining up with scientists across disciplines, from planetary scientists who study hypothetical exoplanet geology to microbiologists and chemists who think about what kinds of aliens could live on those planets and how to detect those life-forms. That’s a big shift from even 10 years ago, Messeri says. In the early 2010s, no one was talking about life.

“You weren’t allowed to say that,” she says. “Astronomers would whisper it to me during fieldwork, but this was not a search for aliens.”

Exoplanet astronomy is on firmer ground now. Its leading figures have won MacArthur “genius” grants. Pioneer planet finders Mayor and Queloz won the 2019 Nobel Prize in physics. The work is no longer hidden away in conferences that are actually about stars. “It doesn’t have to legitimize itself anymore,” Messeri says. “It’s a real science.”

The promise that transiting planets can reveal the contents of their alien atmospheres may soon be fulfilled. NASA’s James Webb Space Telescope may launch this year, after many years of delays. One of its first tasks will be to probe the atmospheres of transiting planets, including those of TRAPPIST-1.

If anything is alive on those absolutely alien, unearthly worlds, maybe the next century will bring it to light.



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