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Our Search for Extrasolar Planets

Our Search for Extrasolar Planets

OUR SEARCH FOR EXTRASOLAR

CONTENTS 2 The next door What have we learned about ’s - exoplanet? 6 The hunt for Earth’s bigger cousins Super- and sub- may host conditions like our home — or not. 12 Why we haven’t found another Earth. Yet. Finding Earth-like planets is still a challenge for astronomers.

NASA/AMES/JPL-CALTECH A supplement to magazine Next-door planets THE EXOPLANET NEXT DOOR There’s no closer to us than Proxima Centauri — and now we know it has an Earth-mass planet in its habitable zone. by John Wenz

THE HUNT FOR HAS, the Kepler catalog, says. “Twenty ago, in some ways, been about the hunt for we were finding the first exoplanets, and it an Earth-like planet — something warm was totally exciting,” he says. Then there where could exist. Headlines tout was the , which each discovery as “the Earth-like found thousands of planets, including planet yet.” Many of those planets are far some in the habitable zone, and some distant. within a few dozen light-years of us. A new discovery published August 24 in And now there’s a planet named hits closer to home, though, with an of 1.3 Earth An artist’s impression of the surface of Earth-mass planet in the habitable zone of right next door, zipping around its star in Proxima Centauri b, drawn as if it has an its star. What’s more, that star is Proxima 11.2 days. Its distance of around 4.3 million . Whether or not an atmosphere Centauri, only 4.24 light-years away. miles (7 million kilometers) from its star exists is not yet known. ESO/M. KORNMESSER This is a landmark discovery: finding a may seem tiny, at less than one-fifth the planet orbiting the closest star to the . distance between and the Sun, but And so far, this exoplanet shows some sim- Proxima Centauri is the runt of the litter in star). In order for instruments to directly European Southern Observatory’s HARPS favor of the solid, concrete evidence. other things that could generate a spurious ilarity to Earth, as it’s roughly the mass of the system. At a diameter image them, planets must be very young instrument, a spectroscope that measures The researchers also were tight-lipped Doppler signal at 11 days, but after looking our planet and in just the right place where, of around 125,000 miles (200,000km), it’s and still glowing, or else very large — not tiny wobbles in a star’s light. about their findings, awaiting the end of at all that, they concluded the variability in if it has an atmosphere, liquid water could only 1.43 times the diameter of . possible with the 5-billlion--old “Instead of applying for a few the peer review process. But as their the data is best explained by the presence exist on the surface. So how was there a planet hiding Proxima b. over a semester and repeating the same intended press conference approached, an of a planet.” This is as in our backyard as it gets. around the closest star to us, just waiting to thing over the years, what we did here is anonymous ESO astronomer leaked the Part of the caginess may arise from a “I think it actually marks a transition,” be discovered? The simple answer: Finding How to find a planet (that try to convince ESO it was worth doing story to the German press, sending Pale 2012 detection of a planet around another Jeffrey Coughlin, a SETI Institute scientist a planet is really hard. Kepler found thou- doesn’t want to be found) intensive campaigns to monitor the star 60 Red Dot into damage control, with the star in the system, Alpha Centauri B. That not involved in the study who assembles sands of planets by staring at 145,000 That’s why the Pale Red Dot project, tasked days in a row, only 20 minutes per ,” team keeping a tighter lid on its findings as planet, aptly named , in a minute region of the sky at the tail end with finding a planet around our nearest principal investigator Guillem Anglada- rumors swirled in the astronomy commu- was too hot to sustain , but instantly of Cygnus, waiting for the 1 percent chance neighbor, had to turn to indirect — but reli- Escudé said in a press conference. nity. Nature was forced to address the became the closest planet to us by default. a planet would pass directly in front of a able — methods of detection. The research- The end result is a planet as close to us rumors in its materials to the press before Or it would have, if it actually existed. star and cause a dip in its light, in a ers chose , a process that looks as any could be, outside of a similar discov- the official announcement. The detection was riddled with prob- method known as transiting. for shifts in a star’s light due to the tug of ery in the same system. Going back to the 2013 detection: the lems, drawn out from spurious data, and The problem with Proxima Centauri b a planet, sometimes called the Doppler The project team, however, has been evidence was initially weak. Subsequent ignored a low signal-to-noise ratio in is that it doesn’t — at least not from method. induces subtle movements cautious every step of the way. They observations bolstered the planet’s case, but search of a sensational new planet, the kind our vantage point. In order to witness a that cause the light of a star to move toward ignored weak evidence of the planet’s exis- it wasn’t until the most recent observation science fiction has long dreamed of. transit, the orbital plane of a planet must the blue end of the light spectrum, which tence stretching back to 2013 in favor of campaign that proof for a planet and not Instead of becoming an Earth-shattering be at or near our line of vision, but not all means it’s moving toward us, or the red end stronger observations over the subsequent some other phenomenon solidified. Given revelation, serious doubts were cast on the solar systems have the same orientation. A of the spectrum, which means it’s moving three years, factoring in other studies of the this history, the team has been methodical detection, which also used radial velocity. star might have all of its planets aligned at away. Based on those changes, researchers star stretching back 16 years. Though fairly every step of the way. “[Pale Red Dot] actually just said the a 90-degree angle from us, with the planets can calculate a mass estimate, and the fre- convinced it’s a planet (above 99 percent “The authors do a great job in their most likely explanation is the presence of a orbiting in such a way that they never pass quency gives an idea of the . certain, in fact) based on their data, the analysis. They follow all protocol and all planet,” Seager says. “If you remember in front of their star, enabling our tele- The planet itself was found over a series researchers still refer to the planet as a can- standard techniques,” , an MIT Alpha Centauri Bb … I just think there’s a scopes to detect them. Some planets have of nights from January 19 to March 31, didate. A possible second planet in the sys- exoplanet researcher and astrobiologist, concern in the community that every This artist’s view shows how Proxima Centauri been found by direct imaging (that is, a 2016, during which Proxima was moni- tem with a 60- or longer orbit only gets says. “And they do say that they looked at retraction looks bad, even though that one may look from just behind Centauri b. ESO/M. KORNMESSER planet appearing in a photo along with its tored closely for subtle variations on the a passing mention in the paper, ignored in all the different types of stellar activity and wasn’t officially retracted.”

2 ASTRONOMY • OUR SEARCH FOR EXTRASOLAR PLANETS © 2016 Kalmbach Publishing Co. This material may not be reproduced in any WWW.ASTRONOMY.COM 3 form without permission from the publisher. www.Astronomy.com This could be especially compounded In transit by the planet’s to its parent The Pale Red Dot team found a planet. It The Proxima Centauri system star. Because of the small separation in the seems to be the right mass and the right system — the distance between Centauri b distance from its star to put something and its star is just 5 percent the distance of somewhat similar to Earth in our cosmic Habitable zone between Earth and the Sun — the same backyard. But reality is way more compli- side of the planet faces Proxima Centauri at cated than that, as seen by the aforemen- Proxima Centauri b all times, much as the same side of the tioned histories of and . : 1.27 Earth masses Astronomers need to observe the planet : 11.19 days faces Earth at all times. However, if Centauri b still has an in greater detail in order to further charac- Sun Proxima Centauri atmosphere, it could reach up terize it. The problem is that Centauri b Mass: 0.12 solar masses to 86° F (30° C) on its sunlit side, and was detected indirectly, and there’s very Distance: 4.23 light-years –22° F (–30° C) on its darker side, bringing little to draw out of the data besides its it into quite Earth-like ranges. orbit and size. The key to preserving an atmosphere is “The planets are so small, the signals are a . Researchers have gone so weak, it takes a huge amount of resources 10 million miles back and forth about if tidally locked plan- to make a detection at all,” Seager says. “If 15 million ets could generate magnetic fields, which you want to do better, it almost needs its Mercury kilometers are a consequence of molten materials in own dedicated telescope just to hammer the planets’ core stirring with its . away at it and do better and better.” Since planets are in lockstep The easiest way to study a planet’s with their star, some believe the cores atmosphere — and easy is a very, very rela- This diagram compares the size of the Proxima Centauri system with our own . The star would be inert. tive term here — is to watch the planet pass is smaller than our Sun, while the planet within just a fraction of Mercury’s orbit. However, In this image of the Alpha Centauri system, the yellow circle shows the location of distant, tiny Proxima because the star is smaller and cooler than the Sun, Proxima Centauri b falls in the habitable zone, Mercedes Lopez-Morales, an astrono- in front of its star and to watch the spectra Centauri. In the system, Alpha Centauri A and Alpha Centauri B, slightly larger and slightly smaller than the Sun, respectively, orbit each other while Proxima orbits both at a distance. SKATEBIKER/WIKIMEDIA COMMONS even that close in. ESO/M. KORNMESSER/G. COLEMAN mer at the Harvard-Smithsonian Center of any gases that distort the star’s light. The for , has modeled the possi- problem with this method is that thus far, bilities of magnetic fields around red no transits have been detected around Thus, the team was rigorous — and permanent smog that ensured the planet dwarf planets, and a picture is gradually Proxima Centauri, though it hasn’t been hurdles. This could show the planet’s own mass star (the Sirius system consists of a transparent — every step of the way. After remained a dry hellscape free of all but the emerging: The planets likely form in the ruled out as a possibility. glow, without the need for starlight. blue giant and an ultra-compact white all, they couldn’t repeat the mistakes of Bb. slightest traces of water vapor. For Mars, outer parts of their solar systems and But to Newton, “it’s basically the best Because of atmospheric distortion, virtu- dwarf, the remnant of a Sun-like star). The end result? A solid detection of an this meant a thin dioxide atmo- migrate in. This means they start out life target for future efforts to look for biosig- ally all astronomy has to be per- Beyond those two, only Proxima Earth-size planet in a place called the sphere with what little water remained rotating, and possibly generating a mag- in the of other plan- formed by space-based telescopes or Centauri’s bigger brothers Alpha Centauri “Goldilocks zone,” because it’s neither too trapped in frozen lakebed buried netic field if they have the right materials ets.” You just need to know how and where high-altitude flying observatories like A and B are larger stars. hot nor too cold for liquid water to exist under oxidized -rich soil, or in - in their core. to look. NASA’s Stratospheric Observatory for Nearly every star is suspected to have a — even if the researchers do use the word sonal floes of brine mixed with trace “On Earth, we’re only here because we Currently, no instrument in space or on Infrared Astronomy, a telescope mounted planet. Some of those could be habitable. If “candidate” to describe a detection with amounts of water. have a magnetic field that shields us from the ground is sensitive enough to pick up to a Boeing 747SP. it ends up that Proxima Centauri b is bar- Kepler-catalog-like certainty. In either case, these planets didn’t last any ,” she says. “Any solar storm reflected light from older and smaller plan- Future space observatories like Webb or ren, then perhaps we’ll have better luck “Because there have been previous long as habitable worlds, at least for any could wipe us out otherwise.” ets. But the James Webb Space Telescope Hubble-like telescopes built for infrared looking at the next star over, Barnard’s claims of other planets (in the system), we life-form beyond a microbe. Once these planets migrate in, their star might be, as will other mega-telescope with apertures of around 3 meters could Star, where planetary detection has had to verify as much as possible that One of the big culprits is the lack of a strips off the early atmosphere of lighter projects currently under construction. also aid in the hunt. remained elusive. It could be that, like [something else] was not causing this can- present-day magnetic field on either planet, elements. But heavier elements — like oxy- Catching these glimmers of light, how- “We can look at the light from the star Proxima Centauri, we haven’t been looking didate signal,” Anglada-Escude said. which, like the shields in Star Trek, gen — could be left behind. The magnetic ever faint, could indicate whether there’s that gets reflected on the atmosphere of for the right kind of planet with the right deflects the worst the Sun and the field shields the planet from the worst an atmosphere, and even what it’s made of. the planet, or we can look at the light in kind of dedication. But is it habitable? can throw at Earth. excesses of its star, which then settles into a “You find the spectrum of the planet, the infrared coming directly from the Or maybe the real solution is at Potential habitability and a lush world of Even if Proxima Centauri b is in the state of relative dormancy it can stay in for and from that you can detect in planet,” Lopez-Morales says. Wolf 359, 7.7 light-years away. Or liquid bodies of water and a thick atmo- habitable zone, it could have had an early trillions of years. the atmosphere of the planet,” Newton says. Lalande 21185, 8.2 light-years away. Or sphere are two very different things. In our atmosphere ripped away by the first billion and other mechanisms could Seager mentions the possibility of using Looking to the future any number of other targets less than a own solar system, three planets are tech- years of violent stellar activity common replenish the atmosphere. With the star less stellar suppression techniques in the Now we know — or know with only a dozen light-years away. nically in the habitable zone. Venus is on with red dwarfs. This means that, even if active, that atmosphere could stick around. future, a process in which blocking the sliver of a percent of doubt — that there’s “If you just make a list of the closest the inner edge, while Mars is in the outer. the planet is in the right place for liquid Lopez-Morales also says that the magnetic light of the star from the vantage point of a a planet slightly more massive than Earth stars to the Sun, there’s a handful there (Hint: The third is Earth.) water, a lack of atmosphere could have field could stay active for billions of years, telescope allows the instrument to gather the next star over. It’s a banner accom- that would make good targets for these Mars and Venus likely had bodies of evaporated that water long ago, leaving a even after a planet becomes tidally locked. more light from the planet or planets plishment, one that has the scientific com- kinds of observations,” Blake says. water at some point in their history. But cold, barren planet of –40° F (–40° C). In other words, the hope for life can stay around that star. munity salivating at the possibilities. We may not need to go clear out to solar winds and other stellar events ripped “The planets are a lot closer to the star alive, even after the brutal first eon of the One of the other possibilities is viewing “To find one around the nearest, best- at 13.8 light-years away to find away layers of lighter elements and evapo- [than ours is to the Sun], so they’re closer to planet’s life. in infrared. Cullen Blake, a University of studied star … maybe we’re just really the next closest potentially habitable rated lakes, , and streams. As the these big energetic events — you’re just “There’s no reason why a planet like this Pennsylvania researcher who studies low- lucky, or maybe there really are just bil- planet. All we need to do is stretch our water boiled away, the escaped potentially getting bombarded with more of could not keep a magnetic field long mass stars and their planets, says in visible lions of M-dwarf planets out there waiting instruments to the limits and take a dedi- into space while the descended and that for potentially habitable planets around enough for life to develop itself,” Lopez- light, “you definitely have a pretty severe for us to find them,” Newton says. cated look for the next pale red dot. bonded with carbon atoms. M-dwarfs,” Elisabeth Newton, a postdoc- Morales says. limit to the distance to which you can see Low-mass stars are the most plentiful in For volcanic Venus, this meant a series toral fellow at MIT who studies red dwarf That means it could hold on to liquid these measurements.” the . Of the 10 closest star systems to John Wenz is the associate editor of of heavy elements and molecules created a systems (also called M-dwarfs), says. water. But does it? Infrared eliminates some of those Earth, only one does not contain a low- Astronomy magazine.

4 ASTRONOMY • OUR SEARCH FOR EXTRASOLAR PLANETS WWW.ASTRONOMY.COM 5 The hunt for Earth’s

Larger than Earth but smaller than , these in-between worlds harbor some surprisingly terrestrial environments. Text and illustrations by Michael Carroll

omewhere between the gas giants Kepler planet-hunting spacecraft used this and the terrestrial Earth-like worlds technique to find thousands of exoworlds. that populate our galaxy lies a A second method, called radial velocity, twilight zone, a region where plan- measures a star’s movement as an orbiting ets defy easy classification. It is a body pulls on it. The planet’s gravity causes Sdimension between gaseous and rocky, a its sun to wobble. When the planet tugs the territory where planet size straddles Earth star away from us, the light becomes red- and Neptune. der; on the opposite side of the orbit, the Several of these recently discovered star gets yanked toward us, and its light hybrid planets offer the most exciting pos- becomes bluer. Astronomers can detect this sibilities for Earth-like conditions on other so-called Doppler shift in a star’s light. worlds. And wherever such environments And the bigger the shift, the more massive exist, the chance that life might gain a foot- the planet must be. hold can’t be ruled out. By combining these two techniques, scientists gain insights into the nature of In search of Earth 2.0 exoplanets. If a planet has twice the mass Finding exoplanets isn’t easy. It’s exceed- of Earth but the same volume, for example, ingly difficult to directly image a planet at it must be very dense and thus rocky. But if interstellar distances because it gets lost in a planet with Earth’s mass has 10 times our the glow of its host star. But astronomers planet’s volume, it must be a low-density, are adept at teasing out planets by closely fluffy world like a small gas or giant. examining the light from distant . Astronomers have charted a wide range Scientists aren’t sure what When a world passes directly in front of of planets orbiting in their host star’s habit- to make of Kepler-452b. The its star from our perspective (a transit), the able zone — the region where liquid water planet’s properties suggest it lies on the border between star dims, and the amount of dimming could exist on a world’s surface — from being a rocky super-Earth depends on the planet’s physical size. The small terrestrials akin to Mercury to rocky and a gaseous sub-Neptune. or gaseous worlds the size of Neptune. If terrestrial in nature, it Frequent contributor Michael Carroll is a Of the worlds with sizes comparable to likely has a thick atmosphere and lots of active volcanoes. science writer and astronomical artist. His latest our home planet, our galaxy may hold book is Earths of Distant Suns (Springer, 2017). upward of 10 billion. Among the exoplanets

© 2016 Kalmbach Publishing Co. This material may not be reproduced in any form without permission from the publisher. www.Astronomy.com WWW.ASTRONOMY.COM 7 Neptune. (Uranus contains 14.5 - Jupiter back from the brink of death. Similar es; Neptune holds 17.1.) These worlds likely migrations may be common in other sys- come with a wide variety of personalities. tems, where sub-Neptunes could form at a Research scientist Mark Marley models large distance and drift starward later. An exoplanet atmospheres at NASA’s Ames Earth-like world that develops close to its Research Center in Moffett Field, sun would have a much higher density California. He believes that sub-Neptunes because it lacks the water content of a planet may turn out to be the most varied of any originating in a system’s cooler outer region. size worlds. “You get bigger than a Elisa Quintana of NASA’s Ames or so, and [planets] all tend to be about the Research Center has been working with a same size because they are dominated by team trying to figure out when a planet their hydrogen- atmospheres. When transitions from being Earth-like to being you get down closer to 1 Earth mass, they’re a gaseous sub-Neptune. “Before we knew of probably all rocky worlds with a little bit of any exoplanets, we had a basic mass-radius atmosphere. But [in this region between relationship based on our solar system. Neptune and Earth], there’s probably a huge Now, we’ve had to throw that away,” she range of what these planets could be like. says. “Theoretical models tell us that the Every one is going to be unique,” he says. transition from rocky super-Earth to gas- Their natures depend on a host of factors, eous sub-Neptune is about 1.5 or 1.6 Earth including their mass, the amount of water radii. Once a planet reaches 2 Earth radii, they possess, and the size of their core. it will be more like a sub-Neptune.” Like Neptune, most sub-Neptunes are Researchers hope to pin down the transi- could be one of the more Earth-like worlds in our galaxy. Its tightly wound orbit around gaseous. Unlike Neptune, however, many tion point as they study more super-Earths. a red dwarf sun places the exoplanet within the star’s habitable zone. Models indicate that under the of these worlds orbit near their primary right conditions, a large would spread across this super-Earth’s star-facing hemisphere. star. This provides astronomers with a How much like home? mystery: How did sub-Neptunes end up Although the discovery of planets with close to their star when they had to form in terrestrial dimensions is exciting, it takes discovered so far, however, the Neptune- component. A third type has a denser core the outer regions of their ? more than size to make an Earth. Even and sub-Neptune-sized worlds are the most than a sub-Neptune but still possesses a Such worlds can be born only beyond the among worlds close to Earth’s size and common. Many of these relatively small sub-Neptune’s extended atmosphere. The so-called snow line, where cool tempera- mass, the “Earth-like” pickings appear to giants qualify as super-Earths. extent of that atmosphere depends on the tures enable them to collect large quantities be slim. Most orbit outside the host star’s planet’s distance from its star — the farther of and gases. habitable zone. Super-Earths and away it orbits, the cooler it will be, and the Planets, it seems, are slippery things, Typical of these is the nasty Earth-sized sub-Neptunes more atmosphere it will retain. Finally, capable of forming in one place and shuf- planet circling the star Gliese 1132. Astron­ Broadly speaking, the term super-Earth larger, high-density super-Earths, some- fling off to another. Our solar system’s omers calculate that Gliese 1132b spans applies to planets that are larger than Earth times called mega-Earths, probably include arrangement of gas and ice giants outside of 1.2 times the radius of Earth and has a mass but still have a rocky surface and a thin major components of and/or . smaller terrestrial worlds apparently is not about 1.6 times as large as our planet, put- atmosphere. The term sub-Neptune refers to the norm across the galaxy. Astronomers ting it on the border between being rocky a small gaseous giant. But uncertainties in Not quite like Neptune developed the Grand Tack model to explain or sub-Neptunian. As Earth-like planets the data mean that the boundary between The ubiquitous sub-Neptunes join the exo- the solar system’s early evolution. The theory go, so far, so good. But scientists estimate these two classes is more blurry than clear- planet menagerie with masses ranging up to proposes that Jupiter and Saturn marched that its surface broils at the temperature of cut. slightly less than our system’s and toward the Sun, but Saturn was able to pull an oven, around 460° F (225° C). Super-Earths seem to be the most com- Just how Earth-like is a super-Earth? Two super-Earths orbit Kepler-62. Both worlds likely have deep oceans of water, though Kepler-62f mon type of exoplanet. Roughly three out Features that contribute to our own world’s (bottom) orbits farther from its star than Kepler-62e (top) and thus may be covered with ice. of every 10 worlds now known fall into this uniqueness offer a good yardstick. First, category. These worlds have no analog in Earth orbits in the Sun’s habitable zone. our solar system. Scientists classify super- Although some super-Earths orbit within pressure rearranges the atoms within it, Another factor that would contribute to Earths strictly by mass without considering the habitable zone of their own star, studies making the rock denser. Without this alter- a super-Earth’s earthiness is a magnetic their composition, nature, or distance from show this may not be enough to beget ation, plates would stall out and cease slid- field. Earth’s rotating molten core gener- their host star. Most of those discovered so Earth-like environments. ing past each other. The change in density ates a field that protects us from energetic far orbit close to their suns — simply is another critical attribute of our home depends on the plate’s makeup. Planets with charged particles. To be Earth-like, a super- because those that orbit close to their suns world because it recirculates the mineralogically different crusts may not be Earth needs to have such a field. are the easiest to detect. The masses of that wash into the and recycles ele- able to maintain a conveyor belt of plates. these worlds range from a low of about 1.5 ments of the atmosphere that have been Second, a super-Earth’s may be too A survey of super-Earths to 2 Earth masses up to a high of 10 Earths. chemically locked into rocks. thick to carry on tectonics. Simulations of Out of the thousands of exoplanets known, Astronomers sort super-Earths into four But recent models contend that super- the pressures within giant Earths reveal that astronomers have found only a few super- categories. Low-density planets contain Earths may not enjoy the benefits of plate thick crusts likely surround most of these Earths with the right characteristics to be large amounts of hydrogen and helium and tectonics. First, it takes the right worlds, putting up a physical barrier to plate potentially Earth-like. One of the closest are referred to as dwarf or sub-Neptunes. smorgasbord to create the planetary jigsaw tectonics. Still, some researchers suggest matches appears to be Kepler-452b. Medium-density super-Earths probably Kepler-22b likely is a rocky planet with a radius about 2.4 times that of Earth. It orbits its host star pattern of shifting plates. On Earth, as one that the increased heat within a super-Earth The first roughly Earth-sized planet are ocean worlds where water is a major near the inner edge of the habitable zone, so it may resemble Venus more closely than Earth. plate slides under another, increasing might be enough to drive plate tectonics. found in the habitable zone of a star similar

8 ASTRONOMY • OUR SEARCH FOR EXTRASOLAR PLANETS WWW.ASTRONOMY.COM 9 EARTH SUB-NEPTUNE

Crust Crust Dissecting new worlds Solid Mantle inner SUPER-EARTH Center core Liquid of mass outer core Star Beneath their Rocky core Planet surface veneers Mantle (water) Comparing the internal structures Hydrogen-helium , a super-Earth, a sub-Neptune, and Neptune Earth reveals that these Cloud decks worlds look as Doppler shifts to the blue Doppler shifts to the red

different on the Mantle (water, Brightness inside as they , and Time do on the NEPTUNE ices) outside. Astronomers know the most about exoplanets observed with both the transit and radial velocity methods. When a planet passes in front of (transits) Rock-ice core its parent star (left), it causes a slight dip in the star’s light. The wobbles induced by a planet’s gravity alter the host star’s radial velocity and show up as shifts in the stellar spectrum (right). In the rare cases when scientists can view a planet with both methods, they get valuable information on the world’s size, mass, and density. ASTRONOMY: ROEN KELLY to the Sun, Kepler-452b is roughly 1.5 times the star that it suffers from a runaway planet orbits close enough to its sun that it perhaps as much as 7 Earth masses. This denser than Earth’s, perhaps similar to larger than Earth. Although it lies slightly like that found on Venus. should be tidally locked, always keeping the purported planet’s size caused astronomers — but cooler than — that of Venus. farther from its star than Earth does from The other two planets — same face toward the star. Depending on its to add a new class to exoplanets: the mega- Some 22 light-years from Earth lies the the Sun, its star (Kepler-452) shines slightly and Gliese 581g — may be more Earth-like, atmospheric composition and surface, it Earth. The world apparently orbits its star triple- . Two of the brighter than ours, so the planet gets just a but astronomers aren’t even sure they exist. might be a barren, Venus-like world, or one with a period of 67 days, placing it near the members are K-type orange dwarfs some- bit more energy than Earth does. Any ter- Both worlds have been detected by multiple with an abundance of water. outer edge of the habitable zone. what cooler than the Sun, while the third is restrial vegetation transported there likely teams, but other researchers have failed to If it has an similar an even cooler red dwarf. The two K-type would thrive in similar lighting conditions confirm them. If real, they would be on the to Earth’s, the globe might be blanketed in Kepler’s reign of glory stars orbit each other; the red dwarf, Gliese and temperatures. shortlist for most Earth-like planets. a thick ice crust. But if the air contains At a distance of 620 light-years, the Sun- 667C, circles them both at a distant 230 AU. That is, if Kepler-452b has a solid sur- Gliese 581g appears to orbit just 0.13 enough greenhouse gases like carbon diox- like star Kepler-22 hosts Kepler-22b. The Gliese 667C appears to have at least three face. The planet’s size hovers right on the (AU; 1 AU is the average ide, temperatures could be substantially planet has the distinction of being the first planets in the vicinity of its habitable zone. edge between a rocky super-Earth and a Earth-Sun distance) from the star. But warmer. The tidally locked world could habitable-zone world discovered by the Perhaps the most intriguing of these is gaseous sub-Neptune. Columbia University because the red dwarf is dim, the planet develop a permanent ocean on the hemi- Kepler spacecraft. Gliese 667Cc, which has a mass less than astronomers Jingjing Chen and David receives roughly the same amount of sphere facing the star, where temperatures With a diameter about 2.4 times that four times that of Earth. This Kipping published a study in The energy as Earth does from the Sun. Its mass would be similar to those in Earth’s tropics. of Earth, it has a density similar to rock, may be a rocky terrestrial, though some Astrophysical Journal in late 2016 that gives may be no larger than 2.2 Earths, barely Another possible world in this system, which means that it may be terrestrial. researchers think it may be a sub-Neptune. the planet only a 13 percent chance of being qualifying it for super-Earth status. The Gliese 581d, appears to be much heavier, Kepler-22b also might have a fairly dense The world circles its sun at breakneck terrestrial rather than gaseous. Models sug- atmosphere and, because it orbits in the speed, completing a circuit in just 28 days. gest that if Kepler-452b is rocky, it probably inner region of its star’s habitable zone, the But because Gliese 667C is a red dwarf, the has a thicker atmosphere than Earth’s and climate may resemble Venus more closely world lies far enough out that liquid water likely would be volcanically active. than Earth. But the planet’s rotation and could exist on its surface. Gliese 667Cc col- A red dwarf sets behind clouds on Kepler-438b. The planet lies close enough to its active star Kepler-452b takes 385 days to orbit its cloud cover could moderate conditions lects about 90 percent of the light that Earth to be exposed to massive stellar flares. If it sun, a year quite similar to Earth’s. But all there. Some recent models point to a surface receives from the Sun. And as with any does not have a magnetic field, the world likely may not be well on this world. Its star is temperature hovering around a comfortable large planet in a habitable zone, it may have experiences deadly levels of . 1.5 older than the Sun and 72° F (22° C). with quite Earth-like environments. radiates more light and heat than it used Farther out in the galaxy, at a distance One of the most Earth-like planets yet to. The planet once was in the center of the of about 1,200 light-years, Kepler-62 boasts discovered is a world with a radius 12 per- and every few hundred days. But if habitable zone, but as the aging parent star five confirmed planets. Two of these reside cent larger than our own. Kepler-438b Kepler-438b has a strong magnetic field, its has warmed, its habitable zone has migrated in the habitable zone of the host orange orbits within the habitable zone of a red surface still might be hospitable. outward, stranding the planet on the inner dwarf star. Both are roughly 1.5 times dwarf, making a circuit every 35 days. If Astronomers have discovered a variety edge. Any oceans it once had likely are larger than Earth, putting them at the bor- Kepler-438b is terrestrial in nature, its mass of exoplanets within their host star’s habit- evaporating into a thick atmosphere. der between Earth-like and super-Earth. would be about 1.4 times Earth’s. Surface able zone. The field seems ripe for the dis- Other possible matches may circle Studies indicate that water likely covers temperatures on this world likely would covery of worlds with thriving biomes , a red dwarf star that lies Kepler-62e in a deep global ocean. And range from 32° to 140° F (0° to 60° C). The beyond our own. The search for life-forms 20 light-years from Earth. Up to five plan- although sibling Kepler-62f also may have a planet suffers from the disadvantage of on Earths of distant suns will be a difficult ets may orbit this star, and three of them large component of water, it lies far enough orbiting close enough to its parent star one, but the detection of a new living world may be super-Earths in the star’s habitable Seen from the surface of a hypothetical nearby moon, super-Earth Gliese 667Cc may be a sub-Neptune, out in the habitable zone that the surface to feel the fallout from the stellar flares would forever change our views of biology, zone. orbits near the zone’s with windy cloudscapes rather than rocky vistas. The planet lies so close to its red dwarf host that it might be frozen, at least at the poles. The common to red dwarfs. In fact, observers planetary development, and the frequency inner edge. It may circle close enough to probably is tidally locked, a situation that may wreak havoc with its banded cloud formations. latter world may have an atmosphere have seen Kepler-438 unleashing radiation of life in the universe.

10 ASTRONOMY • OUR SEARCH FOR EXTRASOLAR PLANETS WWW.ASTRONOMY.COM 11 Pale blue dots

The search for alien Earths is heating up. But there’s still no place like home. Why we by Nola Taylor Redd haven’t found another Earth. AS LONG AS HUMANS HAVE STUDIED PLANETS, they have sought signs of life beyond Earth. Venus and Mars both provided early hopes, but one proved too hot and the other too dry. Today, scientists look beyond the solar system, with the hope of finding Earth 2.0. To narrow their search, scientists have identified a region around stars where rocky planets could hold surface liquid water. This area is known as the habitable zone. For nearly 50 years, the phrase was confined to scientific jargon. With no known exoplanets, the public wasn’t exposed to the term. Yet after the explosion of plan- etary candidates identified by NASA’s Kepler spacecraft in 2010, “habitable zone” sprung out of scientific journals and into the public consciousness. The transition has not necessarily been a smooth one. Habitable zone conjures up a region ripe for life to evolve, though this is not the image the scientific commu- nity intended to evoke. Phrases such as “habitable planet,” “Earth twin,” and “Earth 2.0” have added to the confu- sion, blurring the line between what scientists meant to say and what the general public imagines. Defining the habitable zone The field of exoplanet discovery is fair- ly young, but the term habitable zone itself has existed since at least the 1950s. At that time, it characterized the region around a star in which life-supporting planets can exist. But in the 1990s, not long after scientists discovered the

© 2016 Kalmbach Publishing Co. This material may not be reproduced in any form without permission from the publisher. www.Astronomy.com Yet.WWW.ASTRONOMY.COM 13 KEY EXOPLANETS Jupiter-like e Goldilocks perfect? Habitable zone GLIESE 667C Neptune-like b As many as three of this system’s six Every star’s habitable zone is slightly different. h STARS planets might orbit within the star’s ASTRONOMY: ROEN KELLY AND RICK JOHNSON Mini-Neptune M, red dwarf habitable zone, the region where Super-Earth water could survive in liquid form G, Sun-like on a world’s surface. Terrestrial KEPLER-90 f Uranus d Seven planets orbit c b g this star, making it SOLAR SYSTEM the exosystem with 0.1 AU b c Our system still holds the record for the most e the most known Saturn planets orbiting one star, but future technology g worlds. Astronomers likely will throw that reputation out the window. b d think the innermost e c f e planets are rocky Earth and the others are b 0.1 AU gaseous. Jupiter Mars This system’s outer three giant planets are in a rare c f g 1:2:4 resonance. (In the time it takes the outermost d world to complete one orbit around the star, its Astronomers are finding that this type of system (a neighboring planet completes two orbits and the d 0.5 AU Mars interior of the three completes four.) The only other Neptune-massGliese 687world in a Neptune known example of this resonance occurs with three close-in circular orbit around Gliese 876 a small star) is quite common. 1 AU of Jupiter’s moons: , , and . 10 AU Mercury Venus 0.1 AU

0.1 AU

first planet beyond the solar system, a new lies near the outer edge, yet lacks standing Kasting says, is that different authors have Gliesedue 667C to its ability to help the planet retain frozen world in the mid-2020s. The evolu- “We can only see the atmosphere,” paper established the present-day definition. liquid water on its surface. different definitions for what qualifies as its atmosphere. tion of life on these bodies, originating Ramirez says. “If you just have an ice layer “The conservative definition since 1993 Since the 1990s, scientists have tried to the Goldilocks zone. GlieseAnother 667C key ingredient is the presence completely independently from Earth-life, that‘s sealing everything off, the life is not of the habitable zone is the range of dis- establish firm boundaries. But there is still “We need to get systematic about the of plate tectonics, according to Vlada could help us to understand life on planets going to cut through that ice. You‘re prob- tances from a star at which a rocky planet no hard-and-fast method, as the planet’s choices we make,” Kasting says. Stamenkovic, a post-doctoral candidate at beyond the solar system, Ramirez says. ably not going to see anything with our can maintain liquid water on the surface,” characteristics impact whether or not water the Massachusetts Institute of Technology’s And it would probably change our defini- measurements.” says Jim Kasting, professor at Pennsylvania survives at the surface. Searching for signs of life department of Earth, Atmospheric, and tion of the habitable zone to something less Kepler-90 State University and lead author on that For instance, Kasting points to the With today’s instruments, scientists can Planetary Sciences. On Earth, geological Earth-centric. Earth’s long-lost twin pivotal paper. changing definition of a rocky planet. In only detect if a planet lies within the hab- activity helps the planet regulate the atmo- “It’s possible that planets orbiting While potentially habitable worlds may Because the habitable zone depends on 1993, he says, that meant a planet with a itable zone, and if it has a size similar to spheric levels. Too much of outside what we think of as the habitable exist beyond the habitable zone, a planet the sun it surrounds, its location varies radius up to approximately twice Earth’s, Earth. These two factors do not necessar- this gas results in a runaway greenhouse zone can somehow be inhabited,” says classified as Earth-like would lie within with a star’s age and temperature. The hab- and with no more than 10 times its mass. ily mean that a planet could sustain life. effect; too little creates an icy snowball Aleksander Wolszczan of Pennsylvania that “just right” region. itable zone of a cooler, dimmer M star, for However, a rocky planet orbiting inside planet where the water is frozen over. State University. He was on the first team Recently, NASA announced the discov- instance, lies closer in than it would the habitable zone of its star will likely “If you really want to understand to find an exoplanet. ery of Kepler-452b, a rocky planet in the around a Sun-like star. encourage scientists to take a second, more habitability, you really have to understand Of course, finding a Europa analog habitable zone of a Sun-like star that many It has sometimes been called the “When you talk in-depth look. how the interior of the planet functions,” several light-years away would not play a have dubbed Earth’s closest twin. But “Goldilocks zone” because when it comes “The habitable zone is defined in such a Stamenkovic says. “Plate tectonics are strong role in the search for extraterrestrial when an astronomer deems a world an to water, it’s neither too hot nor too cold. about the habitable zone, way that we can try to find the type of life key to allowing you to open the door to life, because signs wouldn’t be visible to Earth twin, they are only referring to a However, just because liquid water could a person very naturally thinks that would leave atmospheric biosigna- the interior.” today’s instruments. few key features. flow across the plains of a planet does not tures,” Ramirez says. mean that it does — a major distinction. you are talking about a zone That means surface life that changes the Self examination “There’s a lot of planets in the habitable in which everything very air it breathes in a detectable way. If all of this sounds familiar, it is. The goal zone that are likely not to be habitable, or Scientists are already studying alien is to find another planet similar to Earth, at least not be able to have large extended is habitable.” atmospheres, but the techniques are still in because at the present, Earth is the only regions of water on the surface,” says Stephen Kane their early phases. NASA’s Hubble Space planet known to have the conditions right Cornell University planetary scientist Telescope — launched only a few years for life to evolve. Ramses Ramirez, a co-founder of the before any exoplanets had been found — But in the search for habitable worlds, Institute for the Pale Blue Dots. was the first to study these atmospheres. some think that exoplanets may not be Ramirez pointed to a planet in our own Today, Kasting says an Earth-like planet Hubble studies an exoplanet’s atmo- our best bet. solar system that meets the criteria — Mars. tends to be classified with no more than 1.5 sphere as it passes in front of its host star. “I strongly feel that we need to learn The Red Planet lies within the defined hab- times Earth’s radius. Larger bodies are now Light from the star illuminates the atmo- more about our own solar system in order itable zone, whose edge stretches to almost known as sub-Neptunes — extremely gas- sphere, allowing scientists to detect the to understand exoplanets,” Ramirez says. 1.7 times the average Earth-Sun distance, eous worlds with rocky cores. components. He pointed to Saturn’s moon , or astronomical unit (AU). At 1.5 AU, Mars The presence or absence of an atmo- What would make a planet habitable? with its lakes of liquid methane that could sphere, as well as its composition, also helps “It at least has the conditions suitable for act as a solvent, taking over the role water Freelance writer Nola Taylor Redd combined determine if a planet keeps liquid water at life to take hold,” Ramirez says. plays on Earth. Another solar system world an astrophysics degree with a creative writing its surface. If the atmosphere differs from For now, scientists are after Earth-sized with the potential for life to evolve is degree to write about her favorite topic — Earth-like levels of carbon dioxide and planets with carbon dioxide and water as Jupiter’s moon Europa, an icy body thought NASA anything astronomical. You can follow her on water, a planet might hold onto liquid water their primary greenhouse gases. A mag- likely to house a subsurface ocean. NASA From nearby, there are many signs that Earth is inhabited. Reid Wiseman snapped this shot

Twitter at @NolaTRedd at greater distances from its star. The result, netic field is also thought to be important NASA (EARTH ON PAGE24) plans to launch a mission to study the from the International looking north past Cuba toward the Southeastern United States.

14 ASTRONOMY • OUR SEARCH FOR EXTRASOLAR PLANETS WWW.ASTRONOMY.COM 15 Kepler-22b

Kepler-69c Kepler-452b Kepler-62f Kepler-186f Earth

When you stack up the most promising recent exoplanet finds, as illustrated here, it becomes clear none is Earth’s true twin. NASA/AMES/JPL-CALTECH

Spotting vegetation covering the would be another strong suggestion for life, according to Abel Mendez, director of the Laboratory. He says that scientists could detect chlorophyll on exoplanets within the next 20 to 30 years, In 2015, the latest batch of exoplanets from NASA’s Kepler spacecraft were released. This rocky world, Kepler-452b, is thought to be much larger than Earth which would suggest the presence of vegeta- and orbiting a much older Sun-like star. NASA/JPL-CALTECH/T. PYLE tion. He cautioned that such signs could also be confusing. And, if they didn’t correspond with the atmospheric clues that life evolved, astronomers find new planets around “We shouldn’t say Earth-like,” Kasting what some have termed exoplanet fatigue. they could raise even more questions. brighter stars. The James Webb Space says. “We should say, ‘a probable rocky New exoplanets excite little interest unless According to Mendez, finding chloro- Telescope will zoom in on the best candi- planet in the habitable zone.’ ” they are dramatic. As a result, scientists phyll would stretch the limits of human dates from these surveys and seek to clas- American astronomer Harlow Shapley and the media may sometimes feel the technology. sify their atmospheres, revealing more originally called the region the “liquid need to stress unusual potential character- “After that point, we have to come to a information about the distant worlds. It water belt,” possibly a more apt name, in istics over what is actually known. stop,” he says. also will directly image the planets, provid- 1938. Even that could be misunderstood as The field of exoplanets is young com- Scientists could continue to determine ing further into understanding pared to astronomy as a whole, and contin- whether distant worlds were potentially them. ESA’s PLAnetary Transits and ues to grow. habitable, but it would be hundreds or even Oscillations of stars mission, or PLATO, “People want to know the final ques- thousands of years before we could gather will predominantly study terrestrial planets tion,” Kane says. direct evidence that life existed on planets in the habitable zones of Sun-like stars. In their impatience, They want to know if worlds beyond beyond the solar system. Each of these missions will help astron- Earth could house life, if planets beyond NASA’s Kepler telescope has already omers to understand more about the people look only toward the solar system could nourish civiliza- helped to identify more than 4,000 new worlds beyond our solar system. With their that distant answer, and miss tions. In their impatience, people look planets and planetary candidates, provid- help, Kane estimates identifying a true only toward that distant answer, and ing plenty of fresh targets for study. But by the late 2020s or 2030s. what could be learned with miss what could be learned with what while one of the primary goals of Kepler we have today. was to find the frequency of Earth-sized Have we been there, what we have today. “It’s better to understand that is the planets, it wasn’t intended to classify them done that? final question, and along the way to answer as Earth-like or Earth twins, according to If scientists can’t identify more than the many others,” Kane says. Gemini IV Jim McDivitt and Ed White took this picture with a Hasselblad camera and a Stephen Kane of San Francisco State size and location of a planet, then why are Each new exoplanet offers the opportu- 70mm lens as they passed over the Florida Straights in June 1965 following White’s historic first University. Kane also serves as chair of phrases such as Earth twin and Earth-like nity to learn more about how planets — American spacewalk. NASA NASA’s Kepler Telescope Habitable Zone so often heard? One reason has to do with a region where liquid water definitely including Earth — form, or how solar working group, which culls the data from the name itself, which could be considered exists, instead of having the potential to. systems work. Kepler to determine which potentially hab- misrepresentative. And many astronomers agree that chang- By focusing primarily on the habitabil- “When scientists say ‘Earth-like planet continents, and clouds. “There’s a lot more itable planets should be studied in depth. “When you talk about the habitable ing the name would be unlikely to solve ity of a planet, Kane says they’re “trying to in the habitable zone,’ it means they’re talk- than knowing the radius to determine if “It was never meant to be a character- zone, a person very naturally thinks you the problem. “The issue of communica- scoop the real discovery that will be made ing about an Earth-sized planet in the hab- something is Earth 2.0,” Ramirez says. “I izing mission,” Kane says. are talking about a zone in which every- tion is to be solved by educating people,” at least 10 years from now.” itable zone where liquid water can be wouldn’t call anything an Earth twin or Other instruments will help to discover thing is habitable,” Kane says. Mendez says. “Many terms are confusing “One day, we will measure the atmo- sustained,” Stamenkovic says. Earth 2.0 until we can verify if the planet is Earth-like planets. NASA’s Transiting But this isn’t the case, thanks to the at the beginning.” sphere of a that models That’s it — the size and orbit. But the at least orbiting a star like the Sun, confirm Exoplanet Survey Satellite and the European numerous ingredients that factor into mak- But Kane feels the issue goes even will show is unambiguously habitable,” phrase “Earth twin“ or “Earth-like” conjures that it has an ocean, an atmosphere, and Space Agency’s CHaracterising ExOPlanet ing a planet habitable. Some of those also deeper than that. The explosion of new Kane says. “I hate that the reaction will be, an identical twin — a world with oceans, that we’ve found bona fide .” Satellite, or CHEOPS, will both help can be misleading. planets in the last two decades has led to ‘didn’t we already do that 10 years ago?’ ”

16 ASTRONOMY • OUR SEARCH FOR EXTRASOLAR PLANETS WWW.ASTRONOMY.COM 17

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