Intelligent life probably exists on distant planets — even if we can’t make contact, astrophysicist says
Recently released Navy videos of what the U.S. government now classifies as “unidentified aerial phenomena” have set off another round of speculative musings on the possibility of aliens visiting our planet. Like other astrophysicists who have weighed in on these sightings, I’m skeptical of their extraterrestrial origins. I am confident, however, that intelligent life-forms inhabit planets elsewhere in the universe. Math and physics point to this likely conclusion. But I think we’re unlikely to be able to communicate or interact with them — at least in our lifetimes.
Wanting to understand what’s “out there” is a timeless human drive, one that I understand well. Growing up in poorer and rougher neighborhoods of Watts, Houston’s Third Ward and the Ninth Ward of New Orleans, I was always intrigued by the night sky even if I couldn’t see it very easily given big-city lights and smog. And for the sake of my survival, I didn’t want to be caught staring off into space. Celestial navigation wasn’t going to help me find my way home without getting beaten up or shaken down.
From early childhood, I compulsively and continuously counted the objects in my environment — partly to soothe my anxieties and partly to unlock the mysteries inside things by enumerating them. This habit earned me nothing but taunts and bullying in my hood where, as a bookish kid, I was already a soft target. But whenever I looked up at a moonless night sky, I wondered how I might one day count the stars.
By age 10, I’d become fascinated, even obsessed, with Einstein’s theory of relativity and the quantum possibilities for the multiple dimensions of the universe it opened up in my mind. By high school, I was winning statewide science fairs by plotting the effects of special relativity on a first-generation desktop computer.
So perhaps it’s not surprising that I have gone on to spend much of my career working with other astrophysicists to develop telescopes and detectors that peer into the remote reaches of space and measure the structure and evolution of our universe. The international Dark Energy Survey collaboration has been mapping hundreds of millions of galaxies, detecting thousands of supernovae, and finding patterns of cosmic structure that reveal the nature of dark energy that is accelerating the expansion of our universe. Meanwhile, the Legacy Survey of Space and Time will make trillions of observations of 20 billion stars in the Milky Way.
What we’re discovering is that the cosmos is much vaster than we ever imagined. According to our best estimate, the universe is home to a hundred billion trillion stars — most of which have planets revolving around them. This newly revealed trove of orbiting exoplanets greatly improves the odds of our discovering advanced extraterrestrial life.
Scientific evidence from astrobiology suggests that simple life — composed of individual cells, or small multicellular organisms — is ubiquitous in the universe. It has probably occurred multiple times in our own solar system. But the presence of humanlike, technologically advanced life-forms is a much tougher proposition to prove. It’s all a matter of solar energy. The first simple life on Earth probably began underwater and in the absence of oxygen and light — conditions that are not that difficult to achieve. But what enabled the evolution of advanced, complex life on Earth was its adaptation to the energy of the sun’s light for photosynthesis. Photosynthesis created the abundant oxygen on which high life-forms rely.
It helps that Earth’s atmosphere is transparent to visible light. On most planets, atmospheres are thick, absorbing light before it reaches the surface — like on Venus. Or, like Mercury, they have no atmosphere at all. Earth maintains its thin atmosphere because it spins quickly and has a liquid iron core, conditions that lead to our strong and protective magnetic field. This magnetosphere, in the region above the ionosphere, shields all life on Earth, and its atmosphere, from damaging solar winds and the corrosive effects of solar radiation. That combination of planetary conditions is difficult to replicate.
Still, I’m optimistic that there have been Cambrian explosions of life on other planets similar to what occurred on Earth some 541 million years ago, spawning a cornucopia of biodiversity that is preserved in the fossil record. The more expert we become in observing and calculating the outer reaches of the cosmos, and the more we understand about how many galaxies, stars and exoplanets exist, the greater the possibility of there being intelligent life on one of those planets.
For millennia, humans have gazed in wonder at the stars, trying to understand their nature and import. We developed telescopes only a few hundred years ago, and since then the dimensions of our observable universe have expanded exponentially with technological advances and the insights of quantum physics and relativity. Beginning in the early 1960s, scientists have tried to calculate the odds of advanced extraterrestrial life. In 1961, researchers at the NASA-funded search for extraterrestrial intelligence (SETI) developed the “Drake Equation” to estimate how many civilizations in the Milky Way might evolve to develop the technology to emit detectable radio waves.
Those estimates have been updated over the decades, most recently by Sara Seager’s group at MIT, based on observations of exoplanets outside our solar system by successive generations of advanced space-based telescopes — such as the Kepler Space Telescope, launched in 2009, and NASA’s MIT-led Transiting Exoplanet Survey Satellite, launched in 2018. Detecting the presence of life on exoplanets requires large telescopes outfitted with advanced spectroscopy instruments, which is what the James Webb Space Telescope will deliver when it launches in November.
In 1995 the first exoplanet was discovered orbiting Pegasus 51, 50 light-years distant from Earth. Since then, there have been more than 4,000 confirmed discoveries of exoplanets in our galaxy. More important, astronomers agree that almost all stars have planets, which radically improves the odds of our discovering intelligent life in the universe.
At the low end of consensus estimates among astrophysicists, there may be only one or two planets hospitable to the evolution of technologically advanced civilizations in a typical galaxy of hundreds of billions of stars. But with 2 trillion galaxies in the observable universe, that adds up to a lot of possible intelligent, although distant, neighbors.
If only one in a hundred billion stars can support advanced life, that means that our own Milky Way galaxy — home to 400 billion stars — would have four likely candidates. Of course, the likelihood of intelligent life in the universe is much greater if you multiply by the 2 trillion galaxies beyond the Milky Way.
Unfortunately, we’re unlikely to ever make contact with life in other galaxies. Travel by spaceship to our closest intergalactic neighbor, the Canis Major Dwarf, would take almost 750,000,000 years with current technology. Even a radio signal, which moves at close to the speed of light, would take 25,000 years.
The enormity of the cosmos confronts us with an existential dilemma: There’s a high statistical likelihood of intelligent life-forms having evolved elsewhere in the universe, but a very low probability that we’ll be able to communicate or interact with them.
Regardless of the odds, the existence of intelligent life in the universe matters deeply to me, and to most other humans on this planet. Why? I believe it’s because we humans are fundamentally social creatures who thrive on connection and wither in isolation. In the past year, many of us felt the hardship of isolation as deeply as the threat of a potentially fatal infectious disease. Enforced seclusion during the pandemic tested the limits of our tolerance for separation and made us acutely aware of our interdependence with all life on Earth. So, it’s no wonder that the idea of a trackless universe devoid of intelligent life fills us with the dread of cosmic solitary confinement.
For a hundred years, we’ve been emitting radio signals into space. For the past 60 years, we’ve been listening — and so far, in vain — for the beginning of a celestial conversation. The prospect of life on other planets remains a profound one, regardless of our ability to contact and interact with them. As we await evidence of extraterrestrial intelligence, I draw comfort from the knowledge that there are many powerful forces in the universe more abstract than the idea of alien intelligence. Love, friendship and faith, for example, are impossible to measure or calculate, yet they remain central to our fulfillment and sense of purpose.
As I head into my mid-50s, I look forward with an infinity of hope to the moment when humans will finally make contact with extraterrestrial intelligence — in whatever far-flung star system they may live, and in whatever century or millennium moment that momentous meeting may occur. Until that day, I have no doubt that generations of young humans around the globe will continue to stand watch, looking skyward with the same sense of amazement and wonder that intoxicated me as a young boy.
Hakeem Oluseyi, president-elect of the National Society of Black Physicists, has taught and conducted research at MIT, University of California at Berkeley and the University of Cape Town. His memoir, “A Quantum Life: My Unlikely Journey from the Street to the Stars,” co-written with Joshua Horwitz, was published last week.
Do we really want to know if we’re not alone in the universe?
It was near Green Bank, W.Va., in 1960 that a young radio astronomer named Frank Drake conducted the first extensive search for alien civilizations in deep space. He aimed the 85-foot dish of a radio telescope at two nearby, sun-like stars, tuning to a frequency he thought an alien civilization might use for interstellar communication.
But the stars had nothing to say.
So began SETI, the Search for Extraterrestrial Intelligence, a form of astronomical inquiry that has captured the imaginations of people around the planet but has so far failed to detect a single “hello.” Pick your explanation: They’re not there; they’re too far away; they’re insular and aloof; they’re zoned out on computer games; they’re watching us in mild bemusement and wondering when we’ll grow up.
Now some SETI researchers are pushing a more aggressive agenda: Instead of just listening, we would transmit messages, targeting newly discovered planets orbiting distant stars. Through “active SETI,” we’d boldly announce our presence and try to get the conversation started.
Naturally, this is controversial, because of . . . well, the Klingons. The bad aliens.
“ETI’s reaction to a message from Earth cannot presently be known,” states a petition signed by 28 scientists, researchers and thought leaders, among them SpaceX founder Elon Musk. “We know nothing of ETI’s intentions and capabilities, and it is impossible to predict whether ETI will be benign or hostile.”
This objection is moot, however, according to the proponents of active SETI. They argue that even if there are unfriendlies out there, they already know about us. That’s because “I Love Lucy” and other TV and radio broadcasts are radiating from Earth at the speed of light. Aliens with advanced instruments could also detect our navigational radar beacons and would see that we’ve illuminated our cities.
“We have already sent signals into space that will alert the aliens to our presence with the transmissions and street lighting of the last 70 years,” Seth Shostak, an astronomer at the SETI Institute in California and a supporter of the more aggressive approach, has written. “These emissions cannot be recalled.”
That’s true only to a point, say the critics of active SETI. They argue that unintentional planetary leakage, such as “I Love Lucy,” is omnidirectional and faint, and much harder to detect than an intentional, narrowly focused signal transmitted at a known planet.
These critics add that it’s bad form for scientists to attempt such interstellar communication without getting permission from the rest of humanity. Plus there’s the question of what, exactly, a message to the stars ought to say.
Thus one of the greatest scientific mysteries — Are we alone in the universe? — leads to a thorny political and cultural question: Who speaks for Earth?
‘A waste of time’
This discussion about the proper protocols of communicating with aliens is not the most mainstream scientific debate ever concocted. But it got a lot of attention here in San Jose at the annual meeting of the ultra-mainstream American Association for the Advancement of Science.
Astronomer Jill Tarter, a pioneer of SETI who is neutral about the more active approach, organized a symposium on the topic. Before the symposium, two advocates of the idea, Shostak and Douglas Vakoch, appeared at a press briefing alongside science-fiction writer David Brin and planetary scientist David Grinspoon.
“Active SETI is a reflection of SETI growing up as a discipline,” said Vakoch, a clinical psychologist who is the SETI Institute’s director of Interstellar Message Composition. “It may just be the approach that lets us make contact with life beyond Earth.”
But Brin, a signer of the petition protesting the campaign for active SETI, said we don’t know what’s out there and shouldn’t presume that aliens are benign. He said there are roughly 100 scenarios to explain why we haven’t heard from the aliens so far. About a dozen of those scenarios are unpleasant, he said.
Vakoch countered that Brin was being inconsistent, because he collaborated on a message that will be carried into space by NASA’s New Horizons spacecraft after its fly-by of Pluto later this year.
“No one is going to get it!” Brin interjected. (The spacecraft is very slow in the galactic scheme of things and will journey for eons into the void of interstellar space.)
As the scientists debated one another, a white-haired, bespectacled man in the back of the room listened quietly: Frank Drake.
He is 84 years old, the beloved dean of the SETI field. He is the Drake of the famous Drake Equation, the formula he scribbled down in 1961 in advance of a meeting in Green Bank. His equation offers a technique for estimating the abundance of communicative civilizations.
He parked himself on a bench in a corridor and, bracketed by a clutch of reporters, held forth for 30 minutes. He said he thinks it’s too soon to engage in active SETI. We don’t know enough.
“I think it’s a waste of time at the present. It’s like somebody trying to send an e-mail to somebody whose e-mail address they don’t know, and whose name they don’t know.”
Odds of someone out there
When Drake plugs his estimates into the Drake Equation (and who is more entitled to do so?), he comes up with 10,000 alien civilizations that we could detect if we looked in the right places with the right techniques.
“It’s 10,000 that we can detect. There are a lot more,” Drake clarifies. “A lot more young ones that can’t be detected because they don’t have the technology, and there are older ones that have technology that is so good that they don’t waste any energy.”
The Drake Equation has endured despite being rather ungainly at first glance:
N=R*· f p · n e · f l · f i · f c · L
Drake also created the Arecibo Message, a simple binary encoded message broadcast into space by the Arecibo radio telescope in Puerto Rico in 1974. The message encodes several things: the numbers 1 to 10, the basic chemistry of life on Earth, the double helix structure of DNA, Earth’s population, a graphic of the solar system, a human figure, and a graphic of the Arecibo radio telescope and its dish’s dimensions. (Ramin Rahimian for The Washington Post)
It’s not as complicated as it looks. The number (N) of detectable civilizations is the product of seven factors: the rate of star formation (R*), the fraction of stars with planetary systems (f p ), the average number of habitable planets per planetary system (n e ), the fraction that actually have life (f l ), the fraction that have intelligent life (f i ), the fraction with communicative civilizations (f c ) and the average longevity of the communicating phase of such civilizations (L).
Exoplanets — outside our solar system — were first discovered in 1995. NASA’s Kepler Space Telescope and other observatories in space and on the ground have found more than 1,000 planets in the years since. Astronomers say it’s likely that our galaxy has tens of billions of “habitable zone” planets. And of course (channeling Carl Sagan) our galaxy is just one of billions and billions of galaxies.
But after the first three factors in the Drake Equation, we enter the murk. How many of those potentially habitable planets out there actually have life? No one knows, because we don’t yet know how life began on Earth. How likely is it that simple, microbial life will evolve into complex, multicellular organisms and eventually into creatures with large brains? We don’t know, because we have only the one data point of life on Earth.
Do intelligent creatures tend to be communicative and potentially detectable? No idea. And finally, there’s that ominous “L” at the end of the equation: Do technological civilizations tend to survive a long time?
“Those factors are just completely unknown. It’s a great way to organize our ignorance,” Tarter says.
Why, a reporter asked Tarter, should we try to pick up signals from an alien civilization?
“We’re curious how many different ways there are to do this thing called life,” she said. “And we’re curious if it’s possible for us to have a long future.”
That’s because we’d most likely find a very old civilization, not a young one. It’s a matter of statistical probabilities. The universe is 13.8 billion years old. If we pick up a signal, it is unlikely to be from a civilization that has only recently become communicative.
Tarter isn’t discouraged by SETI’s null result to date. She says our ability to detect signals, though much improved since 1960, remains limited.
An artist’s illustration of Kepler-186f, the first validated Earth-size planet to orbit a distant star in the habitable zone, a range of distance from a star where liquid water might pool on the planet’s surface. (Nasa/Jpl-Caltech/T. Pyle/EPA)
“We’ve explored one eight-ounce glass of water out of the ocean,” she says.
We’ve explored one eight-ounce glass of water out of the ocean,” she says.
But you hear something different from Geoff Marcy, an astronomer who has found many of those exoplanets, and who also came to San Jose to discuss results from the Kepler mission. Marcy — who, like David Brin and Elon Musk, signed the petition to protest efforts in active SETI — said it is striking that we have found all these distant planets but no evidence at all of intelligent civilizations.
“The absence of strong radio beacons, television broadcasts, robotic spacecraft, obelisks on the moon — all of those absences add up to give us the suggestion that our galaxy is not teeming with technological life,” Marcy said.
After the active SETI symposium at the AAAS convention in San Jose, the interested parties reconvened for a Valentine’s Day workshop at the SETI Institute up the road in Mountain View. Bottom line: No one’s going to be beaming signals to the aliens anytime soon.
“We need tools to enable true global deliberation and then action,” Tarter said in an e-mail summarizing the workshop. She pointed out that this active SETI issue echoes another debate that got a lot of attention at the AAAS meeting: whether to inject aerosols into the upper atmosphere to reflect sunlight and combat global warming. No one’s going to do that, either, in the near future, but suddenly people are discussing these basic issues of planetary management and global decision-making.
Rogue alien-hunters can always go it alone, of course — and they have. For example, a Russian astronomer, Alexander Zaitsev, has repeatedly beamed messages to nearby stars. Even NASA has gotten into the act, beaming the Beatles song “Across the Universe” toward the star Polaris in 2008 (“I see that this is the beginning of the new age in which we will communicate with billions of planets across the universe,” Yoko Ono said, according to the NASA news release).
Frank Drake has dabbled in active SETI himself. It was just a stunt, a proof-of-concept, on April 16, 1974, at the dedication ceremony of the rebuilt Arecibo Observatory in Puerto Rico. He transmitted an encoded message — one that described the elements that make up DNA, the planets in our solar system, the size of a human being, etc. — toward a star cluster in the constellation Hercules. The star cluster is about 25,000 light-years away.
The odds that anyone will get that message are vanishingly small, but Drake did catch grief from Britain’s Astronomer Royal at the time, Sir Martin Ryle, who thought it was reckless. Drake shrugs and says, “Anyone who’s even 100 years ahead of us [in technology] could detect our run-of-the-mill transmission.”
Drake said he doesn’t worry, as some do, that we would become depressed by contact with a superior civilization. Children aren’t depressed by the company of adults, he says. He compared SETI to doing research on ancient civilizations on Earth, such as the Greeks and the Romans.
“We’re going to do the archaeology of the future,” Drake says. “We’re going to find out what we’re going to become.”
Intelligent Alien Life Is Rare In The Universe, Study Concludes
You might be forgiven for getting excited whenever NASA announces a finding that has to do with the possibility of alien life.
Whether it’s water spurting from the icy moons Europa or Enceladus, or the possibility of microbes living on Earth-like planets like those of TRAPPIST-1, we have a host of exciting possibilities all over the universe for life. But remember, this discussion is mostly limited to microbial or bacterial life, not intelligent life as we know it.
Is E.T. actually out there? There are numerous discussions calculating the probabilities of intelligent life, the most famous being the Drake Equation that estimates the number of active and communicating intelligent civilizations in our own galaxy, the Milky Way. But a new study says it is quite unlikely that intelligent life exists.
The study in large part looks at the history of our own Earth, which had a number of unique evolutionary events (such as the asteroid that killed off the dinosaurs some 66 million years ago, allowing mammals and humans to occupy the dinosaurs’ niche.)
“Intelligent life in the Universe is exceptionally rare, assuming that intelligent life elsewhere requires analogous evolutionary transitions [to Earth],” reads the new study in the scientific journal Astrobiology.
The study points out that on Earth, intelligent life emerged “on a timescale similar to that of Earth’s lifetime.” The Earth is about 4.5 billion years old and it was only about 5 million years ago (or 0.5 billion years ago) that intelligent life arose, the study authors say. And the window of intelligent life’s existence here appears to be narrow, as in less than 1 billion more years from now, the sun will become so luminous that surface temperatures on Earth will increase beyond what could support life on our planet.
The authors also pay attention to transition times between key evolutionary markers of Earthly life, such as the origin of life (or abiogenesis) and markers of increasingly complex life forms such as eukaryotic (a type of cell used by mammals), multicellular and intelligent life. Some of these transitions may be quite unusual, especially the eukaryotic cells that emerged on Earth a billion years after their more simple precursors, called prokaryotic cells. Others may be more common, such as multicellular life that may have originated independently more than 40 times.
But what the authors say we must pay attention to is the chain of events that led from the simplest forms of life to the intelligent forms of life we have today. The sequence of evolution alone appears to be unlikely, leading to an oft-quoted passage from Stephen Jay Gould that the chances of intelligent life are “vanishingly small” if we were to rerun the sequence of Earth’s evolution from the start, over and over again.
But there are limitations in making these estimates, the authors note. “The timings are subject to a sample bias,” they said, meaning the bias that we only know of one planet — our own — with intelligent life. “In particular, we can only observe evolutionary transitions that occurred rapidly enough to fit within Earth’s habitable lifetime,” they added, noting that these transitions may take even more time on other planets.
Another limitation is how unpromising other solar systems may be to any life at all. M-dwarf stars, previously thought to be promising candidates to host life-friendly planets, turn out to be flaring stars that may spew deadly radiation often enough to kill any chances of life on rocky planets.
While this story only captures some of the arguments the authors put forth, their overall conclusions do point to an unlikelihood for intelligent life overall. So while Earth-sized planets appear to make up a noticeable fraction of planets in the universe, it is unclear how many of them are hosted in solar systems that allow water and stable conditions on their surface for billions of years, just like our own planet.
That said, the universe is big. And to paraphrase the movie Contact from 1997, which discussed intelligent alien life, we live in a big universe and it seems that only having a single instance of intelligent life would be an awful waste of space.
Life in the Universe: What are the Odds?
As humanity casts an ever-wider net across the cosmos, capturing evidence of thousands of worlds, an ancient question haunts us: Is anybody out there?
The good news: We know vastly more than any previous generation. Our galaxy is crowded with exoplanets – planets around other stars. A healthy percentage of them are small, rocky worlds, of a similar size and likely similar composition to our home planet.
The ingredients in the recipe for earthly life – water, elements associated with life, available sources of energy – appear to be almost everywhere we’ve looked.
Now the bad news. We have yet to find another “Earth” with life, intelligent or not. Observing signs of possible microbial life in exoplanet atmospheres is currently just out of reach. No convincing evidence of advanced technology – artificial signals by radio or other means, or the telltale sign of, say, massive extraterrestrial engineering projects – has yet crossed our formidable arrays of telescopes in space or on the ground.
And finding non-intelligent life is far more likely; Earth existed for most of its history, 4.25 billion years, without a whisper of technological life, and human civilization is a very late-breaking development.
Is there life beyond Earth? So far, the silence is deafening.
“I hope it’s there,” said Shawn Domagal-Goldman, a research astronomer at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “I want it to be there. I’ll be planning a party if we find it.”
Domagal-Goldman co-leads a team of exoplanet hunters who, in the years and decades ahead, are planning to do just that. Working with scientists across NASA, as well as academic and international partners, his team and others are helping to design and build the next generation of instruments to sift through light from other worlds, and other suns. The goal: unambiguous evidence of another living, breathing world.
While the chances of finding life elsewhere remain unknown, the odds can be said to be improving. A well-known list of the data needed to determine the likely abundance of life-bearing worlds, though highly conjectural, is known as the “Drake equation.”
Put forward in 1961 by astronomer Frank Drake, the list remains mostly blank. It begins with the rate of star formation in the galaxy and the fraction of stars that have planets, leading step-by-step through the portion of planets that support life and – most speculatively – to the existence and durability of detectable, technological civilizations.
When Drake introduced this roadmap to life beyond Earth, all the terms – the signposts along the way – were blank.
Some of the first few items are now known, including the potential presence of habitable worlds, said researcher Ravi Kopparapu from Goddard, also a co-leader of Domagal-Goldman’s team. He studies the habitability and potential for life on exoplanets.
If we develop and launch a powerful enough space telescope, “we could figure out if we have advanced life or biological life,” he said.
Finding a planet that’s ‘just right’
Drake’s list can be a good conversation starter, and a useful way to frame the complex questions around the possibility of other life. But these days, scientists don’t spend a great deal of time discussing it, Domagal-Goldman said.
Instead, they use a narrower yardstick: the habitable zone.
Every star, like every campfire, has a definable zone of radiated warmth. Too close, and your marshmallow – or your planet – might end up as nothing more than a charred cinder. Too far away, and its surface remains cold and unappetizing.
In both cases, “just right” is more likely to be somewhere in between.
For a planet, the habitable zone is the distance from a star that allows liquid water to persist on its surface – as long as that planet has a suitable atmosphere.
In our solar system, Earth sits comfortably inside the Sun’s habitable zone. Broiling planet Venus is within the inner edge, while refrigerated Mars is near the outer boundary.
Determine the distance of an exoplanet from the star itself, as well as the star’s size and energy output, and you can estimate whether the planet falls within the habitable zone.
For larger, hotter stars, the zone is farther away; for smaller, cooler stars, it can be very close indeed. Finding these “just right” planets in the habitable zone is one of the keys to finding signs of life.
“If they fit within these parameters, they could potentially support a temperate environment,” said Natasha Batalha, a research scientist at the NASA Ames Research Center. “Therefore it would be incredibly interesting to study their atmospheres.”
Batalha’s specialty, in fact, is finding ways to read exoplanet atmospheres – and building computer models to better understand them.
“That is the next step, the next frontier,” she said.
The habitable zone concept is not yet definitive. Small, rocky worlds like ours that orbit other stars are too far away to determine whether they have atmospheres, at least using present-day technology.
That’s where teams like the one co-led by Kopparapu and Domagal-Goldman come in. The space telescopes and instruments now on their drawing boards are meant to be powerful enough to peer into these atmospheres and identify the molecules present. That will tell us which gases dominate.
We could find a small, rocky, watery world around a Sun-like star with an atmosphere of nitrogen, oxygen, and carbon dioxide: a little like looking in a mirror.
“To search for life anywhere, we have this ‘follow the water’ approach,” Domagal-Goldman said. “Anywhere you find water on Earth, you find life. Whether it’s life on Mars, ocean worlds, or exoplanets, water is the first signpost we’re looking for.”
For now, the habitable zone remains a kind of first cut in the search for life-bearing worlds.
“The habitable zone is a very useful tool for mission design,” said Rhonda Morgan of NASA’s Jet Propulsion Laboratory in Southern California.
She studies how to use the data gathered so far on exoplanets to refine designs for future space telescopes.
Over the past quarter century, thousands of exoplanets have been confirmed in a Milky Way galaxy that likely holds trillions. Thousands more will come to light in the years ahead. Tools like the habitable zone will help planet hunters sort through these growing ranks to pick the most likely candidates for supporting life.
“We are in a position now where we can propose a potential, future mission that would be capable of directly imaging an Earth-like planet around a nearby, Sun-like star,” she said. “This is the first time in history that the technology has been this close, probably less than 10 years from launch.”
Still, we might need something beyond the habitable-zone concept for more extreme cases. It won’t help much, for instance, with “weird” life – life as we don’t know it. Living things on other worlds might use vastly different chemistry and molecular compounds, or even a solvent other than water.
“This is one of the questions we get from the public often: If there are aliens, how are we going to recognize them if they’re really weird?” Domagal-Goldman said. “How do we find what we would consider to be weird life? And how do we make sure not to be tricked by strange, dead planets that look alive – mirages in the desert?”
Life on planets around other stars also might be hidden in a subsurface ocean encased in ice, invisible even to our most powerful space telescopes. Moons of Jupiter and Saturn are known to harbor such oceans, some revealing through remote sensing at least a few of the characteristics we expect for habitable worlds.
Some “exo-moons” also might be habitable worlds, as in the film, “Avatar.” But even proposed, future instruments are unlikely to have sufficient power to detect atmospheres of moons around giant exoplanets.
Still, the habitable zone is a good start, a way to zero in on signs of life made familiar by our fellow organisms here on planet Earth.
A shortcut to finding lifeforms like ourselves, of course, would be to intercept tech-savvy communications. Searches for signs of intelligent life have been underway for decades.
In recent years, among NASA scientists, such potential signs have acquired an intriguing new name: technosignatures.
Evidence of a communicative, technological species somewhere among the endless fields of stars could come in the traditional form: signals by radio or optical light waves, or from some other slice of the electromagnetic spectrum.
But scientists imagine many other forms. An exoplanet atmosphere might show signs of synthetic gases, such as CFCs, revealing an industrial species like us.
Or maybe we’ll see the glimmer of something like a “Dyson sphere,” popularized by physicist Freeman Dyson: an epic-scale structure built around a star to capture the lion’s share of its energy.
Such possibilities remain speculative. For now, we have no real answer to a disturbing question from another 20th century physicist, Enrico Fermi.
Where is everybody?
The question has fueled more than 70 years of debate, but boils down to a simple observation. Our Milky Way galaxy has plenty of stars, plenty of planets, and plenty of time to develop intelligent lifeforms – some of whom might well have had billions of years to develop interstellar travel.
Yet so far, we’ve seen no sign of such technology, nor heard a peep of conversation. Why is the cosmos so profoundly silent?
“If life had so much time to evolve, why haven’t we found it?” Batalha asks, to summarize the question. “Why isn’t life just crawling everywhere in the galaxy, or the universe? It could be a combination of a lot of things. Space travel is very difficult for us.”
Vast amounts of energy would be needed just to get us to our nearest neighboring star, Proxima Centauri, she said. “It would just be incredibly expensive, and require a lot of resources.”
And once we – or some other civilization – reached such a distant destination, she said, another problem would be perpetuating the travelers’ existence into future generations.
Experts offer many reasons why somebody, or something, might be out there, yet beyond our detection. On the other hand, the ultra-cautious might remind us that, while a lifeless cosmos seems unlikely, we have exactly zero information one way or the other.
Still, scientists like Kopparapu say they like our chances of finding some form of life, and are hard at work on the telescopes and instruments that could make that future, party-starting epiphany a reality.
“It’s not a question of ‘if,’ it’s a question of ‘when’ we find life on other planets,” he said. “I’m sure in my lifetime, in our lifetime, we will know if there is life on other worlds.”
washingtonpost.com, “Intelligent life probably exists on distant planets — even if we can’t make contact, astrophysicist says.” by Hakeem Oluseyi; washingtonpost.com, “Do we really want to know if we’re not alone in the universe?” By Joel Achenbach; forbes.com, “Intelligent Alien Life Is Rare In The Universe, Study Concludes.” By Elizabeth Howell; exoplanets.nasa.gov, “Life in the Universe: What are the Odds?”;