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Name&Surname: Yusuf Koca School Number: 468 Class: 11/B Subject: Astronomy/Science Contents Speed of Light May Not Be Constant, Physicists Say..............................................................1 Faster-Than-Light Travel Is Possible Within Einstein's Physics...............................................3 Aliens on 1,000 nearby stars could see us, new study suggests.................................................5 Our ‗First Contact‘ With Aliens Will Be With A Superior Civilization....................................8 Possible Super-Earth in the Habitable Zone at Alpha Centuari................................................11 Can Alien Smog Lead Us to Extraterrestrial Civilizations?..................................................16

Speed of Light May Not Be Constant, Physicists Say Textbooks say that the speed of light is constant. But some scientists are exploring the possibility that this cosmic speed limit, a consequence of the nature of the vacuum of space. The definition of the speed of light has some broader implications for fields such as cosmology and astronomy, which assume a stable velocity for light over time. For instance, the speed of light comes up when measuring the fine structure constant (alpha), which defines the strength of the electromagnetic force. And a varying light speed would change the strengths of molecular bonds and the density of nuclear matter itself. A non-constant speed of light could mean that estimates of the size of the universe might be off. (Unfortunately, it won't necessarily mean we can travel faster than light, because the effects of physics theories such as relativity are a consequence of light's velocity). [10 Implications of Faster-Than-Light Travel] Two papers, published in the European Physics Journal D in March, attempt to derive the speed of light from the quantum properties of space itself. Both propose somewhat different mechanisms, but the idea is that the speed of light might change as one alters assumptions about how elementary particles interact with radiation. Both treat space as something that isn't empty, but a great big soup of virtual particles that wink in and out of existence in tiny fractions of a second. Cosmic vacuum and light speed The first, by lead author Marcel Urban of the Université du Paris-Sud, looks at the cosmic vacuum, which is often assumed to be empty space. The laws of quantum physics, which govern subatomic particles and all things very small, say that the vacuum of space is actually full of fundamental particles like quarks, called \"virtual\" particles. These matter particles, which are always paired up with their appropriate antiparticle counterpart, pop into existence and almost immediately collide. When matter and antimatter particles touch, they annihilate each other.

Photons of light, as they fly through space, are captured and re-emitted by these virtual particles. Urban and his colleagues propose that the energies of these particles — specifically the amount of charge they carry — affect the speed of light. Since the amount of energy a particle will have at the time a photon hits it will be essentially random, the effect on how fast photons move should vary too. As such, the amount of time the light takes to cross a given distance should vary as the square root of that distance, though the effect would be very tiny — on the order of 0.05 femtoseconds for every square meter of vacuum. A femtosecond is a millionth of a billionth of a second. (The speed of light has been measured over the last century to high precision, on the order of parts per billion, so it is pretty clear that the effect has to be small.) To find this tiny fluctuation, the researchers say, one could measure how light disperses at long distances. Some astronomical phenomena, such as gamma-ray bursts, produce pulses of radiation from far enough away that the fluctuations could be detected. The authors also propose using lasers bounced between mirrors placed about 100 yards apart, with a light beam bouncing between them multiple times, to seek those small changes. Particle species and light speed The second paper proposes a different mechanism but comes to the same conclusion that light speed changes. In that case, Gerd Leuchs and Luis Sánchez-Soto, from the Max Planck Institute for the Physics of Light in Erlangen, Germany, say that the number of species of elementary particle that exist in the universe may be what makes the speed of light what it is. Leuchs and Sanchez-Soto say that there should be, by their calculations, on the order of 100 \"species\" of particle that have charges. The current law governing particle physics, the Standard Model, identifies nine: the electron, muon, tauon, the six kinds of quark, photons and the W-boson. [Wacky Physics: The Coolest Little Particles in Nature] The charges of all these particles are important to their model, because all of them have charges. A quantity called impedance depends on the sum of those charges. The impedance in turn depends on the permittivity of the vacuum, or how much it resists electric fields, as well as its permeability, or how well it supports magnetic fields. Light waves are made up of both an electric and magnetic wave, so changing those quantities (permittivity and permeability) will change the measured speed of light. \"We have calculated the permittivity and permeability of the vacuum as caused by those ephemeral virtual unstable elementary particles,\" Soto-Sanchez wrote in an email to LiveScience. \"It turns out, however, from such a simple model one can discern that those constants contain essentially equal contributions of the different types of electrically charged particle-antiparticle pairs: both, the ones known and those so far unknown to us.\" Both papers say that light interacts with virtual particle-antiparticle pairs. In Leuchs' and Sanchez-Soto's model, the impedance of the vacuum (which would speed up or slow down the speed of light) depends on the density of the particles. The impedance relates to the ratio of electric fields to magnetic fields in light; every light wave is made up of both kinds of field, and its measured value, along with the permittivity of space to magnetic fields, governs the speed of light. Some scientists are a bit skeptical, though. Jay Wacker, a particle physicist at the SLAC National Accelerator Laboratory, said he wasn't confident about the mathematical techniques

used, and that it seemed in both cases the scientists weren't applying the mathematical tools in the way that most would. \"The proper way to do this is with the Feynman diagrams,\" Wacker said. \"It's a very interesting question [the speed of light],\" he added, but the methods used in these papers are probably not sufficient to investigate it. The other issue is that if there really are a lot of other particles beyond what's in the Standard Model, then this theory needs some serious revision. But so far its predictions have been borne out, notably with the discovery of the Higgs boson. This doesn't mean there aren't any more particles to be found — but if they are out there they're above the energies currently achievable with particle accelerators, and therefore pretty heavy, and it's possible that their effects would have shown up elsewhere. Faster-Than-Light Travel Is Possible Within Einstein's Physics, Astrophysicist Shows For decades, we've dreamed of visiting other star systems. There's just one problem – they're so far away, with conventional spaceflight it would take tens of thousands of years to reach even the closest one. Physicists are not the kind of people who give up easily, though. Give them an impossible dream, and they'll give you an incredible, hypothetical way of making it a reality. Maybe. In a new study by physicist Erik Lentz from Göttingen University in Germany, we may have a viable solution to the dilemma, and it's one that could turn out to be more feasible than other would-be warp drives. This is an area that attracts plenty of bright ideas, each offering a different approach to solving the puzzle of faster-than-light travel: achieving a means of sending something across space at superluminal speeds. There are some problems with this notion, however. Within conventional physics, in accordance with Albert Einstein's theories of relativity, there's no real way to reach or exceed the speed of light, which is something we'd need for any journey measured in light-years. That hasn't stopped physicists from trying to break this universal speed limit, though.

While pushing matter past the speed of light will always be a big no-no, spacetime itself has no such rule. In fact, the far reaches of the Universe are already stretching away faster than its light could ever hope to match. To bend a small bubble of space in a similar fashion for transport purposes, we'd need to solve relativity's equations to create a density of energy that's lower than the emptiness of space. While this kind of negative energy happens on a quantum scale, piling up enough in the form of 'negative mass' is still a realm for exotic physics. In addition to facilitating other kinds of abstract possibilities, such as wormholes and time travel, negative energy could help power what's known as the Alcubierre warp drive. This speculative concept would make use of negative energy principles to warp space around a hypothetical spacecraft, enabling it to effectively travel faster than light without challenging traditional physical laws, except for the reasons explained above, we can't hope to provide such a fantastical fuel source to begin with. But what if it were possible to somehow achieve faster-than-light travel that keeps faith with Einstein's relativity without requiring any kinds of exotic physics that physicists have never seen? In the new work, Lentz proposes one such way we might be able to do this, thanks to what he calls a new class of hyper-fast solitons – a kind of wave that maintains its shape and energy while moving at a constant velocity (and in this case, a velocity faster than light). According to Lentz's theoretical calculations, these hyper-fast soliton solutions can exist within general relativity, and are sourced purely from positive energy densities, meaning

there's no need to consider exotic negative-energy-density sources that haven't yet been verified. With sufficient energy, configurations of these solitons could function as 'warp bubbles', capable of superluminal motion, and theoretically enabling an object to pass through space- time while shielded from extreme tidal forces. It's an impressive feat of theoretical gymnastics, although the amount of energy needed means this warp drive is only a hypothetical possibility for now. \"The energy required for this drive traveling at light speed encompassing a spacecraft of 100 meters in radius is on the order of hundreds of times of the mass of the planet Jupiter,\" Lentz says. \"The energy savings would need to be drastic, of approximately 30 orders of magnitude to be in range of modern nuclear fission reactors.\" While Lentz's study claims to be the first known solution of its kind, his paper has arrived at almost exactly the same time as another recent analysis, published only this month, which also proposes an alternative model for a physically possible warp drive that doesn't require negative energy to function. Both teams are now in contact, Lentz says, and the researcher intends to share his data further so other scientists can explore his figures. In addition, Lentz will be explaining his research in a week's time - in a live YouTube presentation on March 19. There are still plenty of puzzles to solve, but the free-flow of these kinds of ideas remains our best hope of ever getting a chance to visit those distant, twinkling stars. \"This work has moved the problem of faster-than-light travel one step away from theoretical research in fundamental physics and closer to engineering,\" Lentz says. \"The next step is to figure out how to bring down the astronomical amount of energy needed to within the range of today's technologies, such as a large modern nuclear fission power plant. Then we can talk about building the first prototypes.\" Aliens on 1,000 nearby stars could see us, new study suggests

There are about 1,000 star systems where aliens, if they existed, could be watching us from afar, new research suggests. Those 1,004 star systems are in a direct line of sight to our planet, and close enough to us that they could not only detect planet Earth, but also chemical traces of Earthly life. Over the course of the last decade, astronomers have found exoplanets orbiting distant stars using a simple formula: Keep an eye on a star and wait for it to suddenly dim. That dimming is a sign of a planet passing between the star and the telescope. Analyzing how the light changes as the star dims can reveal the chemical contents of the planet's atmosphere. But this method works only for planets whose orbits happen to take them between their host stars and Earth. In a new paper, researchers flipped that formula on its head, asking: Which nearby stars are lined up properly for their inhabitants to see Earth transit in front of the sun? Would any life-forms in those star systems be able to detect signs of us, the living things on Earth's surface? The answer is yes, it turns out, for a great number of nearby stars. \"If observers were out there searching, they would be able to see signs of a biosphere in the atmosphere of our Pale Blue Dot,\" Lisa Kaltenegger, a Cornell University astronomer and lead author of the paper, said in a statement. Planets, it turns out, are common in space. Since researchers first confirmed finding one transiting in front of its star in 1992, astronomers have found 4,292 confirmed planets beyond our solar system, orbiting 3,185 stars, thanks largely to the planet hunting Transiting Exoplanet Survey Satellite (TESS). The James Webb Space Telescope (JWST), slated to launch at some point this decade, should have the precision to study many of those planets in more detail — possibly detecting gases like methane or oxygen in their atmospheres, which would be likely signs of life. What if aliens had their own JWST? Within 326 light-years, the researchers found, there are 1,004 with vantage points to spot Earth. Of those, 508 have viewing angles that would give them at least 10 hours of observational data every time Earth passed between that location and the sun — ideal conditions for spotting this little rocky planet and the signs of life in its atmosphere. \"Only a very small fraction of exoplanets will just happen to be randomly aligned with our line of sight so we can see them transit.\" said Lehigh University astrophysicist Joshua Pepper, co-author of the paper, in the statement. \"But all of the thousand stars we identified in our paper in the solar neighborhood could see our Earth transit the sun, calling their attention.\"

About 5% of the 1,004 stars are likely too young for intelligent life to have evolved, the researchers surmise, even if a planet with habitable conditions orbited them. But the remaining 95% belong to star categories that can sustain life for billions of years, which Earth's experience suggests is long enough for intelligent life to evolve, assuming conditions are right. Most of the stars on the list are toward the farther end of the 326 light-year range, because the zone where Earth's transit is visible gets smaller as you get closer to our solar system. But the closest star on the list is only 28 light-years away. And there are several more nearby stars that are on track to enter the zone where they might spot Earth within centuries. Some are bright enough in the sky to see from Earth. Two stars on the list have known exoplanets. And a red dwarf just 12 light-years from Earth with known exoplanets — known as Teegarden's star — does not currently have the right viewing angle to spot Earth but at its current rate of movement will enter the Earth-spotting zone as soon as 2044. The next step, the researchers wrote, is to focus intelligent life-hunting operations on the 1,004 stars identified in their paper. They specifically mentioned SETI's Breakthrough Listen program, designed to detect communications from advanced alien civilizations.

Our ‘First Contact’ With Aliens Will Be With A Superior Civilization, Say NASA Scientists As They Narrow The Hunt Humans‘ ―first contact‖ with aliens is likely to be with a civilization much more technologically advanced than ours, according to a new NASA-funded study into the search for intelligent extraterrestrial life (SETI). According to the paper published in the specialized journal Acta Astronautica, the easiest way to detect extraterrestrial civilizations is by searching for ―technosignatures‖—evidence for the use of technology or industrial activity in other parts of the Universe. Technosignatures, many of which are based on how Earth might look now, or in the past or future, to alien onlookers, include:  Radio signals, such as the Arecibo message we humans sent in the direction of globular star cluster M13 on November 16, 1974.  The presence of industrial pollution in the atmosphere of a planet. For example, the presence of nitrogen dioxide—as studied recently by the same team of researchers—or the wholly artificial chlorofluorocarbons (CFCs), both of which are evidence for there being a technologically advanced civilization on Earth.  Large swarms of satellites around a planet.  Gigantic space engineering around exoplanets, such as heat shields or ―Dyson spheres‖ that harvest solar energy from the local star.  Crash sites on the Moon or Mars of probes that might have been sent here in a distant past.

However, the study—which was funded by NASA Goddard‘s Sellers Exoplanet Environments Collaboration (SEEC) and the NASA Exobiology program—argues that our search for technosignatures would likely only be successful at finding much more advanced technology than humans can currently create. That raises the spectre of ―contact inequality.‖ ―It seems unlikely that civilizations with a relatively low level of technological development would enter into contact with each other, since that would require either very high sensitivities or highly visible engineering,‖ reads the paper. ―Less advanced civilizations lack the sensitivity needed to detect other civilizations unless they have built very large or luminous structures.‖ In short, we don‘t yet have instruments sensitive enough to definitively find ―another Earth‖ by detecting an alien civilization outright. That‘s despite huge advances in our astronomical instrumentation in the past decade that have revolutionized the science of discovery and study of exoplanets, which now number 4,000+. ―For us to detect such signals at interstellar distances with our current sensitivities, such signals would need to be stronger than those produced by current human civilization, particularly the unintentional ones,‖ read the paper. ―Only those species that have constructed or developed technology is much larger or more luminous than any of our own can be detected with our current astronomical infrastructure.‖ So we‘re looking for massive, unmistakable signs of alien civilisations far more advanced that we are. ―The idea of searching for technosignatures draws upon the technology we have on Earth today and possible extensions of our technology into the future,‖ said Jacob Haqq-Misra, a co-author of the article and chairman of the TechnoClimes 2020 organizing committee. ―This does not necessarily mean that any extraterrestrial technology must be like our own, but imagining plausible extensions of our own future is one place to begin thinking of astronomical searches we could actually do to look for possible technosignatures.‖ The study puts forward a plan, and a new way of classifying the technosignatures as a function of their ―cosmic footprint‖—the relative size scale of a given technosignature in units of the same technosignature produced by current Earth technology. It‘s a measure of how easy to see a technosignature might be from a huge distance. The researchers call this an ―ichnoscale.‖ Scale and scope is tricky since a search for crashed spacecraft on the Moon could easily be done, whereas a search for Dyson spheres in our galaxy would have a billion potential targets, according to the paper. Don‘t get the idea that armies of astronomers and NASA scientists are spending their days and nights searching for traces of extraterrestrial intelligence. They're not. In fact, the renewed interest in ―technosignature science‖ is largely down to the fact that it can be done purely by taking advantage of data that is already being collected astronomical purposes.

For example, many space telescopes and survey satellites—such as TESS—observe stars to see if exoplanet are transiting across them. That‘s exactly the same science that needs to be done to search for technosignatures. The next generation of telescopes—such as the James Webb Space Telescope (JWST), but many others—will also, for the first time, allow a search for so-called biomarkers, evidence for life on other planets. While characterizing the atmosphere of an exoplanet it will by default detect the presence of, say, CFCs or nitrogen dioxide. It‘s therefore something of a free hit, according to the researchers—the modern search for aliens can be all about synergy. A free hit it may be, but it could also be a fruitless one. ―We have no idea whether intelligence is something very common in the Universe or, on the contrary, whether it is extremely rare,‖ said Hector Socas-Navarro, an IAC researcher, the Director of the Museum of Science and the Cosmos, of Museums of Tenerife, and the first author of the paper. ―For that reason we cannot know whether these searches have any chance of success. There is no choice but to search and see what we find, because the implications would be tremendous.‖ Wishing you clear skies and wide eyes.

Possible Super-Earth in the Habitable Zone at Alpha Centuari Astronomers using a new technique may have not only found a super-Earth at a neighbouring star, but they may also have directly imaged it. And it could be nice and cozy in the habitable zone around Alpha Centauri. It‘s much easier to see giant planets than Earth-size planets. No matter which detection method is being used, larger planets are simply a larger needle in the cosmic haystack. But overall, astronomers are very interested in planets that are similar to Earth. And finding them is much more difficult. We thought we‘d have to wait for the ultra-powerful telescopes currently being built before we could directly image exoplanets. Facilities like the Giant Magellan Telescope and the European Extremely Large Telescope will bring enormous observing power to bear on the task of exoplanet imaging. But a team of researchers have developed a new technique that might do the job. They say they‘ve imaged a possible sub-Neptune/super-Earth-sized planet orbiting one of our nearest neighbours, Alpha Centauri A. The team presented their observations in an article in Nature Communications titled ―Imaging low-mass planets within the habitable zone of ? Centauri.‖ The lead author is Kevin Wagner, an astronomer and Sagan Fellow at the University of Arizona. ―These results demonstrate the feasibility of imaging rocky habitable-zone exoplanets with current and upcoming telescopes.‖ While astronomers have found low-mass exoplanets before, they‘ve never sensed their light. They‘ve watched as the planets revealed themselves by tugging on their stars. And they‘ve watched as the light from the stars that host these planets dips when the planet passes in front of the star. But they‘ve never directly imaged one. Until now, maybe. This new detection method comes down to the infrared. One of the challenges in imaging Earth-sized exoplanets in infrared is to discern the light coming from an exoplanet when that light is washed out by all of the background infrared radiation from the star. Astronomers can search for exoplanets in wavelengths where the background infrared is diminished, but in those same wavelengths, temperate Earth-like planets are faint. One method is to look in the near-infrared (NIR) part of the spectrum. In NIR, the thermal glow of the planet is not so washed out by the star. But the starlight is still blinding, and millions of times brighter than the planet. So just looking in the NIR is not a total solution.

The solution may be the NEAR (New Earths in the AlphaCen Region) instrument used in this research. NEAR is mounted on the ESO (European Southern Observatory‘s) Very Large Telescope (VLT) in Chile. It works with the VISIR instrument, also on the VLT. The group behind NEAR is the Breakthrough Watch, part of Yuri Milner‘s Breakthrough Initiatives. The NEAR instrument not only observes in the desirable part of the infrared spectrum, but it also employs a coronagraph. The Breakthrough group thought that the NEAR instrument used on an 8-meter ground-based telescope would allow for better observations of the Alpha Centauri system and its planets. So they built the instrument in collaboration with the ESO and installed it on the Very Large Telescope. This new finding came as a result of 100 hours of cumulative observations with NEAR and the VLT. ―These results,‖ the authors write, ―demonstrate the feasibility of imaging rocky habitable-zone exoplanets with current and upcoming telescopes.‖ The 100-hour commissioning run was meant to demonstrate the power of the instrument. The team says that based on about 80% of the best images from that run, the NEAR instrument is

an order of magnitude better than other methods for observing ―…warm sub-Neptune-sized planets throughout much of the habitable zone of ? Centauri A.‖ They also, possibly, found a planet. ―We also discuss a possible exoplanet or exozodiacal disk detection around ? Centauri A,‖ they write. ―However, an instrumental artifact of unknown origin cannot be ruled out.‖ This isn‘t the first time astronomers have found exoplanets in the Alpha Centauri system. There are a couple of confirmed planets in the system, and there are also other candidates. But none of them have been directly imaged like this new potential planet, which has the placeholder name C1, and is the first potential detection around the M-dwarf in the system, Proxima Centauri. Follow-up observations will have to confirm or cancel the discovery. The researchers say there‘s a possibility that the signal could be an instrument artifact. ―We also discuss a possible exoplanet or exozodiacal disk detection around ? Centauri A,‖ they write. ―However, an instrumental artifact of unknown origin cannot be ruled out.‖ It‘s exciting to think that a warm-Neptune class exoplanet could be orbiting a Sun-like star in our nearest neighbouring star system. One of the Breakthrough Initiatives goals is to send lightsail spacecraft to the Alpha Centauri system and give us a closer look.

But that prospect is out of reach for now. And in some ways, this discovery isn‘t so much about the planet, but about the technology developed to detect it. The large majority of discovered exoplanets are gigantic planets similar in mass to Jupiter, Saturn, and Neptune. They‘re the easiest to find. But as humans from Earth, we‘re predominantly interested in planets like our own. Earth-like planets in a star‘s habitable zone get us excited about prospects for life on another planet. But they can also tell us a lot about our own Solar System, and how solar systems in general form and evolve. If C1 does turn out to be a planet, then the Breakthrough group has succeeded in a vital endeavour. They‘re the first to detect an Earth-like planet by direct imaging. Not only that, but they did it with an 8-meter, ground-based telescope and an instrument specifically designed and developed to detect these types of planets in the Alpha Centauri system. The authors are confident that NEAR can perform well, even in comparison to much larger telescopes. The conclusion of the paper contains a description of the overall sensitivity of the instrument. Then they write that ―This would in principle be sufficient to detect an Earth- analog planet around ? Centauri A (~20 µJy) in just a few hours, which is consistent with expectations for the ELTs.‖ The E-ELT will have a 39-meter primary mirror. One of its capabilities and design goals is to image exoplanets, especially smaller, Earth-size ones, directly.

Of course, the E-ELT will be an enormously powerful telescope that will undoubtedly fuel scientific discovery for a long time, not just in exoplanet imaging but in a variety of other ways. And other gigantic ground-based telescopes will change the exoplanet imaging game, too. What took hours for NEAR to see may take only minutes for the E-ELT, the Thirty Meter Telescope, or the Giant Magellan Telescope to see. NEAR can‘t compete with those telescopes and was never meant to. But if these results are confirmed, then NEAR has succeeded where nobody else has, and for a fraction of the price of a new telescope. Either way, what NEAR has accomplished likely represents the future of exoplanet research. Rather than broad-based surveys like Kepler and TESS, scientists will soon be able to focus on individual planets.

Can Alien Smog Lead Us to Extraterrestrial Civilizations? Last March, when Ravi Kopparapu was still working from his desk at the Goddard Space Center in Maryland, he came across a press release from NASA‘s Earth Observatory. Levels of Nitrogen dioxide (NO₂) had plummeted over China since the country of 1.4 billion instituted strict stay-at-home orders more than a month earlier. He texted his colleague Jacob Haqq Misra with the link: ―Technosignature?‖ he wrote. ―Oh interesting!‖ Haqq Misra replied. The observations had piqued Kopparapu‘s interest, and two months later, still thinking about the ways that modern societies pollute their planet‘s air, he read a paper on the effect of pandemic-related public health measures on atmospheric pollution. Researchers found the same effect playing out in other highly industrialized nations, like South Korea and the United States. The level of NO₂ over urban centers decreased by between 20 and 40 percent from January to April 2020, when many governments were following China‘s lead and mandating that citizens stay at home. Nitrogen dioxide is one of the more prevalent pollutants, a result of combustion and fossil fuel use as well as natural biological processes like soil emissions and lightning. But Kopparapu wasn‘t interested in NO₂ because of its effect on Earth. His focus was light-years away, in the atmospheres of the more than 4,000 known exoplanets in our region of the Milky Way galaxy. The shutdown had shown what atmospheric scientists had struggled to accurately measure up until that point: that the majority—roughly 65 percent—of Earth‘s NO₂ is from nonbiological sources, the combined result of our commuting, manufacturing, and gas and metal refining. If this was the case, Kopparapu wanted to know, would it be possible to

detect this gas in the faraway atmospheres of exoplanets? And if it was, could we be looking at a civilization not unlike our own, that had made use of its own fossil fuels to drive a technological revolution? ―We are producing three times more nitrogen dioxide than what biology and lightning together are producing,‖ says Kopparapu of our own planet. ―So if we see an Earth-like planet and the nitrogen dioxide signal, and we make a model for all of the biological and atmospheric sources possible, and still cannot explain the amount we are seeing on the planet, then one possibility is that there could be a technological civilization.‖ Kopparapu is at the forefront of an emerging field in astronomy that is aiming to identify technosignatures, or technological markers we can search for in the cosmos. No longer conceptually limited to radio signals, astronomers are looking for ways we could identify planets or other spacefaring objects by looking for things like atmospheric gases, lasers, and even hypothetical sun-encircling structures called Dyson spheres. Technosignatures could be observed from Earth or by some of our more ambitious probe concepts, like Starshot—a laser-powered lightsail that could theoretically reach Alpha Centauri in two decades. Eager to explore further, Kopparapu discussed the idea with his colleagues, including Haqq Misra, a senior researcher at the Blue Marble Space Institute of Science, who soon became his coauthor. Their paper, published in late February by The Astrophysical Journal, explored this question using a computer model that mimicked a single column of atmosphere on an Earth-like planet and calculated the odds that we could find traces of NO₂ on one of our galactic neighbors. Their model simulates the exposure of atmospheric molecules to sunlight, specifically four different types of sunlight, modeled off of our own sun, an orange dwarf star, and two M- type stars like Proxima Centauri. Each star emits a unique spectrum of light that interacts with the atmospheres of orbiting planets and causes photochemical reactions. (On Earth, these reactions are what give us an ozone.) When radiation, or light, from the sun heats up molecules in the atmosphere, they enter a temporarily excited state in which a number of things can happen: They can break apart, or they can bond together—and on the ground they can become plant food. Different types of radiation, from other types of stars, could mute or stimulate an NO₂ signal.

Determining the photochemical reactions happening in a faraway atmosphere takes an advanced and extremely fine-tuned telescope fit with a spectrograph. Astronomers have to focus this telescope on a (relatively) miniscule and fast-moving planet as it transits in front of its host star. During this brief window, the telescope can capture the light beaming through the planet‘s atmosphere and break it apart with a prism. The bands of the prism tell us the composition of the atmosphere by way of a unique spectral signature that each element displays, almost like a fingerprint. If an alien civilization had polluted its skies with NO₂, the way we have ours, this would clue us in to their existence. Kopparapu and Haqq Misra concluded that among Earth-like planets, orbiting sun-like stars within a small band called the ―habitable zone‖ that supports liquid water, we could find this signal, if it exists, using the next generation of advanced telescopes. Two highly anticipated NASA telescope concepts, LUVOIR and HabEx, designed with significant improvements in sensitivity and spatial resolution, as well as spectrographs that can focus on multiple objects simultaneously, would be capable of carrying out these types of observations. With the instrumentation earmarked for these missions, we could confidently affirm a signal 30 light years away, after roughly 400 hours of observation. This may sound like a long time, but Kopparapu points out that the Hubble Space Telescope used at least twice that amount of time over a period of three years to conduct the Frontier Fields observations, which took the most detailed photos ever attained of the early universe— thousands of galaxies pixelated across the dark expanse of 13-billion-year-old space. ―If we have a really good candidate in the habitable zone of a planet, around a sun-like star, then we can potentially spend more time on this planet,‖ says Kopparapu. ―The number looks big, but within the context of what we have done before, it is not.‖ Even with enough observation time, there could still be a number of complications. Clouds and aerosols in a planet‘s atmosphere absorb light within the same wavelength region as NO₂, making it plausible that they could mimic the signal outright. At the same time, planets around stars that are slightly smaller than our sun, K and M type stars, could produce a stronger NO₂ signal, since these stars produce less ultraviolet light which can break apart this gas in the atmosphere. That might lead to an overestimate of its prevalence—and an indicator of civilization where there may be none. Biological processes like soil nitrification, wildfires, and lightning also produce NO₂, but research on Earth suggests that these sources provide far less of the overall total than anthropogenic sources—namely the burning of fossil fuels. Still, there are only a small handful of atmospheres from within our own solar system that we‘ve been able to study in any detail that would provide a helpful basis for comparison. Renyu Hu, a planetary scientist and expert in exoplanet atmospheres at the NASA Jet Propulsion Laboratory, says the biggest challenge he sees in identifying NO₂ as a technosignature has to do with the chemical lifetime of the gas in our atmosphere. On Earth, most NO₂ is broken apart by the sun or ―rained out‖ when it transforms into nitric acid, or HNO₃, within 5 to 10 days of being produced. But on other planets, this may differ. ―In exoplanet atmospheres, since their atmospheric conditions could be quite different than Earth, perhaps this NO₂ will have a longer lifetime, and therefore accumulate at a higher

abundance,‖ Hu says. If this exoplanet‘s atmosphere doesn‘t have the same sinks that exist on Earth, it could theoretically mimic the sustained, strong NO₂ signal that we‘d look for as a sign of pollution and an industrial society. For a follow-up study, Kopparapu‘s team is planning on using a more advanced 3D model that would more accurately simulate the atmospheric dynamics we‘d expect to find on another planet. Instead of a single column of atmosphere, the model would simulate the atmosphere as a whole, including more physically accurate cloud heights and movements— improving the researchers ability to vet whether such an NO₂ signal could be mimicked by clouds. Before any of this is possible, NASA needs to prioritize and fund at least one of several next generation telescope concepts, like LUVOIR or HabEX. Both were studied for the forthcoming Planetary Science and Astrobiology Decadal Survey, which will provide research and investment recommendations to NASA and Congress and guide the efforts of the broader scientific community when it is released in spring, 2022. But even if both are prioritized by the survey, they are a long way off—we likely won‘t see these missions get underway before the 2030s. A few decades ago, federal funding for SETI (the search for extraterrestrial intelligence) and for possible radio signals was at an all-time high. In 1961, astronomer Frank Drake published his famous Drake Equation, a formula for estimating the odds of detectable, intelligent life in space, using variables like the number of planets with environments suitable for hosting life, and how many of them might give rise to intelligent creatures. But people following that equation have gotten results ranging anywhere from none to millions. (Last year a set of researchers in the United Kingdom calculated, with unusual specificit y, that there are at least 36 communicating intelligent civilizations in the Milky Way alone.) The SETI Institute was founded in 1984 to help search for that life; in 1999, researchers at the Berkeley SETI Research Center launched the SETI@home project, which allowed people to use their personal computers to help parse patterns in radio telescope data. There was promising astronomical news, too. In 1992, Alexander Wolszczan and Dale Frail, using the Arecibo Observatory‘s radio telescope, discovered the first planets outside of our solar system orbiting a pulsar in the Virgo system. Thousands more were discovered once the Kepler Space Telescope became operational in 2009. On the heels of this came a new line of inquiry: Now that the existence of exoplanets was confirmed, what might they be like? Astronomers began theorizing about the composition of alien atmospheres, specifically what they might be composed of if a planet harbored life. Textbooks are now devoted to the atmospheric physics of faraway worlds, and potential biosignatures—chemical signs of life we could observe in exoplanet atmospheres—are reviewed in top astronomy journals. But some in the scientific community have always been skeptical of technosignatures. Even if there are other civilizations out there, how long could we expect them to send out radio signals? And would we be around to receive them? Over the 13.5 billion year life of our galaxy alone, it‘s entirely possible we‘d miss another civilization‘s lifespan, like ships passing in the night. Plus, the search for extraterrestrial radio signals has recently taken some serious blows. The Arecibo telescope was badly damaged last August by a falling cable, and will now be demolished. The public-facing aspect of the SETI@home project was halted in March, 2020, so that researchers can now crunch through two decades of data.

Kopparapu calls the work he‘s doing now ―atmospheric SETI,‖ an alternative to searching for radio signals from another civilization. ―With atmospheric technosignatures, they don‘t have to do anything actively to communicate with us. They can just go on about their lives and can be completely unaware that we exist while we are observing their planet,‖ says Kopparapu. ―In the next 20 or so years, we may launch space telescopes that could look at the atmospheres and potentially image far away habitable planets. If we can do this within 150 years of industrial civilization, and less than 100 years of developing radio communication capability, how many civilizations have already done this to us within our Earth‘s history of billions of years?‖ Last August, Haqq Misra, the paper‘s coauthor, gathered more than 50 participants, including many prominent astrobiologists and astrophysicists, together for Technoclimes, an online conference at which presenters talked about the latest research into technosignatures, and discussed the focus and framework that such research might follow. ―We‘re kind of in the era now where it‘s possible to ask these questions and not get laughed at by too many astronomers,‖ says Haqq Misra. ―The idea that we can have life on other planets has moved from being science fiction to more close to science reality,‖ says Kopparapu. Kopparapu and Haqq Misra are now at work on a paper that will look at whether an exoplanet with Earth‘s current level of atmospheric chlorofluorocarbons (CFCs), the ozone- depleting chemical present in older refrigerants and aerosols, would be detectable using a future space telescope that takes observations in that wavelength. (Current conceptions of LUVOIR and HabEX would not be capable of this.) Yet there‘s a slight complication— CFCs are industrially produced, and could be a clear indicator of some sort of technological capability. But on Earth we‘ve been fighting for decades to purge them from our atmosphere. The same is true for NO₂—and really all planet-warming pollutants. To survive, we on Earth will have to drastically curtail our emissions to avoid making the planet inhospitable to most life. If this is the case on other planets, and they‘re either fighting to clean up their atmosphere or dying off because they failed to do so, that would further shrink the window of time in which we can actually detect these types of signals— for CFCs, that‘s anywhere from 50 to 150 years, practically a fraction of a second on astronomical timescales. Haqq Misra points out that there are some situations in which a planet could have high levels of CFCs without dooming its inhabitants. Increasing their quantity might actually be desirable on a planet with very little atmosphere, especially if the inhabitants wanted to create an environment that could retain liquid water by doing some large-scale planetary engineering. ―They‘re a potent greenhouse gas, so if we wanted to terraform Mars, one possibility is to put CFCs in the atmosphere,‖ he says. ―Or maybe CFCs aren‘t toxic to whatever organism they are. Or maybe they‘re not biological, they‘re AI.‖ Sara Seager, an astrobiologist at MIT who‘s studied biosignatures and exoplanets for decades, says she‘s glad there‘s another tool in the arsenal. Still, she says that photochemistry is a difficult field and there‘s no magic bullet. Last year, she was part of a team of researchers who announced they‘d found a phosphine signal in Venus‘s atmosphere. On Earth, phosphine is produced by bacteria involved in decomposition, and a small amount is manufactured artificially for use in fumigants or biological weapons. So when astronomer Jane Greaves found what appeared to be phosphine‘s signature on our nearest neighbor, she enlisted an entire team of astrobiologists and chemists to confirm and study the signal. They published their findings in Nature Astronomy in September, 2020,

and though there hasn‘t been a single follow-up paper that offered an explanation of how the chemistry in Venus‘s upper atmosphere could produce phosphine non-biologically, scientists are still far from a consensus on the signal and whether it‘s truly phosphine. ―People still don‘t believe—and the people who wrote the paper, we‘re not believing it‘s a sign of life, either—even though we don‘t know any way that we can produce phosphine without life in an environment like Earth or Venus,‖ Seager says. ―We‘re still not ready to say it‘s life.‖ And Venus is in our backyard. On an exoplanet tens of light-years away, a phosphine signal would be even more difficult to find, and require a higher concentration to be detectable in the first place. ―There‘s this kind of reality check now that it‘s going to be hard to find, and even if we get a strong signal, one could probably always come up with a different explanation,‖ Seager says. For what it‘s worth, there has been some momentum behind a new search for life in space— and it‘s coming from Congress. In the 2018 House Appropriations Bill, Congress directed NASA to include technosignatures as part of its research portfolio, which had not been the case for several decades. Later that year, NASA hosted a three-day Technosignatures Workshop in Houston, bringing together leaders in a number of scientific disciplines to assess the current state of the field and determine a path forward. The new Biden administration has thrown its full support behind the moon-bound Artemis program and the Space Force, but it's not yet clear whether that support will extend to the search for life in the cosmos. Still, Kopparapu is optimistic about the growing support for this kind of research, including the influx of private money from organizations like the Breakthrough Initiative, a multimillion-dollar suite of space programs geared toward discovering extraterrestrial life. A solid signal of alien life, or even technological life, is likely far off—and our acceptance of it further still. Regardless, that notion is what drives many researchers in the field, like Kopparapu and Haqq Misra, to continue their research. Asked what it would feel like to identify a potent signal from a faraway planet, Kopparapu is initially at a loss for words. ―It‘s not a question of if, but when,‖ he says. And maybe also: Who will believe it?