When Spaceships Pass in the Night: Exoplanets and the Search for Life

This piece won 2nd Prize in the Eleventh Annual Joan and Arnold Seidel Griffith Observer Science Writing Contest. It can be read in Volume 87, Issue #10 of the Griffith Observer.

We are surrounded in every human pursuit by the ghosts of those who came before us. Every book we read links to moments past, as does every oil painting and every mark of graffiti along a bridge; every building inhabited before us, built by someone else’s hands; every science arising from ancient Greece, ancient Mesopotamia, ancient stories passed down about constellations before humans learned to write.

For a hundred years now, we’ve broadcast bits of culture over radio waves. We’ve sent our pop songs, our daytime news, and our nightly television spectacles every which way—including up into space. Past broadcasts bubble out from our planet at the speed of light. Carly Rae Jepsen plays prominently to stars ten lightyears away, while Phil Collins’ “In the Air Tonight” now echoes at the star Capella.

Our broadcasts aren’t the only signals marking up the universe. No matter what direction we gaze in the sky, a dim glow rushes back to us over microwaves, a low-intensity form of light. This “cosmic microwave background” is a remnant of the early universe, from around 13.8 billion years ago. But if the universe has been around for billions of years, shouldn’t there be more? Shouldn’t there be alien broadcasts, alien pop songs—ghostly messages from worlds before ours?

Though the question of whether we’re alone has been asked for centuries, we’ve only started to search in the second half of the 1900s, thanks in part to the late Carl Sagan. As a science popularizer, Sagan worked to dispel UFO myths and create interstellar greetings to potential aliens. As an astrophysicist, he explored the possibility of life on Mars, Venus, and the moons of Jupiter and Saturn, as well as life on “exoplanets,” worlds that orbit other stars. We haven’t found evidence of life on these planets yet—but the search has barely begun.

Although Carl Sagan died in 1996, just four years after the first exoplanets were discovered, his vision of finding life in the universe carries on at Cornell University’s Carl Sagan Institute. Because the search for life bridges many different fields, the Institute attracts members from across biology, Earth science, computer science, math, engineering, and even English departments to analyze exoplanets and communicate with the public. Dr. Lisa Kaltenegger is the director of the Carl Sagan Institute, and Thomas Mitchell is an astronomy undergraduate who helps with the Institute’s publicity. Both study exoplanets at Cornell, at different career stages and with different focuses.

Mitchell has worked with Hubble data to confirm an exoplanet in a triple star system, but his current research involves “hot Jupiters.” These planets resemble our Solar System’s Jupiter in composition, but orbit their stars once every ten days, close enough to cook the planets to thousands of degrees Fahrenheit. Mitchell feeds his program points of data—things like mass, radius, density, and orbit—and the program returns a graph of the “light curve.” Light curves represent a planet’s “transit,” a dip in starlight as an exoplanet passes in front of its host star, blocking a bit of the star from view. Mitchell says that “changing all the little knobs on the program” allows him to compare computer-generated light curves to actual observations of planets, estimating what their masses and densities might be.

Dr. Kaltenegger and her team conduct similar research. They, too, create computer models of exoplanets—some hypothetical and some real—to find out if life on another planet could produce signs we might detect on Earth. All it takes is a little starlight.

When exoplanets transit in front of their star, space telescopes collect the tiny amount of light passing through any atmosphere the exoplanet has. When scientists examine this starlight, they’re able to deduce what elements it passed on the way, forming a luminous fingerprint of the planet’s atmosphere. Scientists call this fingerprint the planet’s “spectra.”

“Specifically, my team’s research is modeling the spectra of Earth-like planets around other stars,” Dr. Kaltenegger says. “If there were life—and it could be a very different kind of life on that planet—would it look different from an inhospitable world to our telescopes?”

Dr. Kaltenegger marvels at the perspective young adults and children seem to have regarding life on other worlds. “A lot of people actually assume there is life already,” she says. The first exoplanet around a Sun-like star was confirmed in 1995, and people born after this date “have never lived in a world where we didn’t know for sure there were other planets…. Is it really such a jump to say, if there are other planets, if they’re the right distance, if they’re the right size, some of them must have life?”

After 13.8 billion years, with an estimated 300 million planets able to support life in the Milky Way, it seems impossible for us to be the first.

Or maybe we’re afraid to be alone.

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In 2022, the number of discovered exoplanets surpassed 5,000. Within the tables of data downloadable from the NASA Exoplanet Archive are swathes of host stars—from dwarfs to giants to middle-of-the-road, prime-of-their-life stars, like our own Sun. There are exoplanets in these tables that orbit two stars, maybe even three stars; there are “rogue planets,” likely ejected from the disks of young stars, that shoot through an endless night; there is a planet that takes one million years to orbit its star; there is a planet that takes 49 minutes to orbit its pulsar; there are planets with moons and there are planets without; there is a red-hot planet where gems may fall from clouds of metal and a deep blue dot where glass may rain sideways.

Artist’s conception of exoplanet HD 189733 b, a deep blue dot where glass may rain sideways. Its color comes from clouds of silicate particles, whipped up above winds over 5,000 miles per hour. Image Credit: ESO/M. Kornmesser

Some of these planets—the more typical, less extreme ones—have a chance at hosting life. They’ll need to be not too far, not too close, but just the right distance from their host star: in the “Goldilocks” or “habitable” zone where liquid water could exist on a planet's surface. The star will have to treat its planet kindly, lest it strip away the planet’s atmosphere, or heat the planet so intensely it becomes a “lava world.” Even if the planet avoids these extremes and organisms manage to thrive, the world those organisms call home may look quite different from ours.

Travel to a red dwarf and the sky might go gray. The star’s reds and oranges turn any plants a deep purple, even black. Water molecules tend to absorb red and infrared; even a puddle might look murky, with oceans an immediate abyss. If life on Earth is thought to have emerged from oceans, life near a red dwarf might need to stay at the surface to suck up what little light it can.

Red dwarfs comprise a majority of the Milky Way’s stars, so it makes sense to consider them when looking for life. But red dwarfs emit less energy than the Sun, meaning that any habitable red dwarf planets would need to orbit closely. Being too near a star introduces a new host of problems: the planet may become synchronously locked, meaning that the same side of the planet always faces the star. This could cause an extremely hot side in perpetual daylight, as well as an extremely cold side in perpetual night.

For life to have a chance, a red dwarf planet would need an atmosphere at least ten percent as thick as Earth’s. An atmosphere would help equal out the hot and cold sides, with clouds both trapping and deflecting heat. On the hot side, clouds could act as a reflective sunshield on the dashboard of a car, while on the dark side clouds could be a warm blanket—but also a cloak, obscuring our view of life below the cover. We could avoid the cloud issue entirely if life evolved on the planet’s moon, rather than the synchronously locked planet. But this may be harder to detect.

As we stray even further from Sun-like stars, our chances of finding life seem to shrink. A few exoplanets might exist around blue or white supergiant stars. But supergiants have a time limit, living at most 30 million years. While that may seem like a long time, on Earth it took at least a few hundred million years for bacteria to evolve from the first cells, and 225 million years for humans to evolve from the first mammal. Supergiant planets in the habitable zone might make for nice vacation spots, but not for alien nurseries.

One of the first exoplanets, discovered in 1992, orbits a pulsar. Pulsars, by all accounts, are the opposite of gracious host stars. They are what remains from the cores of stars many times larger than the Sun after supernova explosions blow away the outside. What with the pulsar’s lethal gamma rays and X-rays, these planets seem like even worse nurseries than ones that orbit supergiant stars. But even here, life might find a way. With an atmospheric pressure as great as that at the bottom of Earth’s oceans, life could be protected on a pulsar planet, assuming such a planet could hold on to all that gas.

The issue with these extreme examples—supergiant, pulsar, and red dwarf exoplanets—is that even if microbes evolve, intelligent civilization may require much more than a little energy and a little bit of nutrients. If life evolves only at the bottom of an ocean, or beneath an atmosphere millions of times thicker than Earth’s, will it ever grow to build tools and technology? Could a creature crawling the thermal vents of the ocean floor ever rise high enough to see the stars? Nestled so deeply in their planets, we might not be able to detect these life forms at all.

“We have no idea how often life develops, and we also have lively arguments in the biology community whether or not life always has to go through oxygen, and whether it always has to become multicellular and complex—or if it could happily stay in a single-celled organism stage,” Dr. Kaltenegger says.

She adds that even if life exists in a place we can see, it might not show up in our measurements. “For the first two billion years, [life on Earth] didn’t have any unique signs on our world either, because it mostly produced CO2 and methane, and those you can also get with geology, out of volcanoes.”

Perhaps more likely than being entirely alone is to be surrounded, but still lonely—our neighbors shrouded in the deep, or even just beyond our reach—hidden in plain sight.

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When the pandemic hit in March 2020, I was a senior at a St. Louis high school. My last moments in the school building were spent at the astronomy club. I felt like a mentor to the underclassmen in the club. I didn’t realize I would never see them again.

I traveled over 700 miles away to my freshman year of college at Cornell University, hoping to have a fresh start, but I struggled to meet new people. Sometimes, five days would pass without seeing any friends in person. My only meaningful conversation was with the person serving green beans at the dining hall.

I didn’t know him yet, but fellow astronomy major Ben Jacobson-Bell was also struggling that year, across the country in Berkeley, California. He’d decided not to attend his freshman year in person. But many of his friends had opted to leave home, and it was hard to connect to new friends over Zoom. “I don’t know if home feels like the worst option because that’s what I did, or if I would’ve been equally miserable either way,” he says. “Everybody’s lives had been put on hold, and that means you don't want to do anything. You just want to exist until that’s over.”

Ben Jacobson-Bell, 2022-23 Treasurer and 2023-24 Vice President of the Cornell Astronomical Society, walks toward Cornell University's historic Fuertes Observatory.

When the pandemic’s grip began to ease, both Jacobson-Bell and I joined the Cornell Astronomical Society, which operates Cornell’s Fuertes Observatory. Its hundred-year-old telescope finally gave us a good look at objects once lost to Bay Area and St. Louis lights. This year, Jacobson-Bell became the club’s treasurer and I became the outreach coordinator, opening the observatory every Friday night for the public. Jacobson-Bell says his goal as treasurer is to make “the little old building on the top of the hill more valuable and inviting to the people who visit every week.”

“For a lot of people coming through, [space] feels untouchable. So our job is to connect that to life on Earth and show them how exciting space can be, and how it can be meaningful to their lives,” he says.

Occasionally, we get questions about whether we think aliens exist. Sometimes visitors want a big answer—they want to know if we think big spaceships, big intergalactic superhighways built by incomprehensible beings, could exist somewhere. I tend to tell them there’s probably something out there, but maybe it’s microbes, or maybe we can’t see the planets they’re on. Maybe they just orbit beyond our view.

Dr. Kaltenegger co-authored a paper on the idea. She and astrophysicist Jackie Faherty from the American Museum of Natural History wanted to know what planets in our galaxy might be able to detect us, as if Earth were the exoplanet and humans were aliens. The plane of our Solar System is a flat disk, along which every planet orbits; if we extend that disk out into the night sky, around 1,715 nearby stars could have detected Earth in the past 5,000 years by watching it transit in front of the Sun. This comes with a few caveats: Dr. Kaltenegger and Dr. Faherty only included stars within around 325 lightyears of Earth, and only seven of these stars are known to have exoplanets. There’s also the fact that stars constantly move, and none of them can stay within the disk of Earth-sight forever.

“It’s really like ships passing in the night—because this vantage point is gained and lost, and you only hold it for about 1,000 years on average,” Dr. Kaltenegger says. “There are stars with known planets in the habitable zone that could’ve seen us 3,000 years ago but would’ve lost that vantage point about 900 years ago. And there are other systems that will have to wait for thousands of years before they see us, but we already know they’re there.”

This isn’t to say aliens couldn’t detect us through other means. Humans have many other ways of discovering exoplanets, but not all are as robust as the transit method. We might be missing certain exoplanets simply because we aren’t in the right line of sight, just as those exoplanets might be missing us.

We are ships in the night, but also ships in an ocean of time and space. I tell the observatory visitors that maybe there have been spacefaring societies, but maybe they succumbed to extinction long before we arose; maybe there are societies searching at the same time as us, but they live so far away that any message sent over rippling light would take thousands of years to reach us; in each case their ghostly waves a remnant of the past, haunting us like the ancients do on Earth.

Sometimes, the visitor just wants to know if Independence Day could happen.

Artist’s conception of exoplanet 55 Cancri e, also known as planet Janssen. This “lava world” is synchronously locked to its Sun-like star. Silicate clouds on the planet’s dark side would sparkle with reflections of its molten surface. Image credit: NASA/JPL-Caltech

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In a poem titled “Hubble Photographs: After Sappho,” Adrienne Rich spoke of the distance between us and other galaxies. She wrote

These impersonae, however we call them

won’t invade us as on movie screens

they are so old, so new, we are not to them

we look at them or don’t from within the milky gauze

of our tilted gazing

but they don’t look back and we cannot hurt them

Galaxies are much larger and farther away than any intelligent life we’d find on nearby planets, but the concept is the same. With such vast distances and timescales, is it even possible that malevolent aliens could hurt us? What limits does the universe impose, if any, on how fast aliens could get here?

Science fiction stories like Star Trek and Star Wars feature conveniently faster-than-light travel. But without warp speed or wormholes that bridge distant points in space, we’re limited. “From our knowledge of physics right now, we don’t think we can go faster than the speed of light,” Dr. Kaltenegger says. For this reason, she’s not worried about any alien invasions. “The distances in the Cosmos also give us a buffer against this fear.”

“The safest thing to do would be to not broadcast anything, and not try to find anything. But that would be kind of miserable,” Jacobson-Bell says. “It would be like staying in our homes forever. At least to me, it seems so much more preferable to put yourself out there and learn more about the universe.”

Even if aliens were malevolent, Dr. Kaltenegger points out that humans haven’t done much to mark ourselves as cosmically special, beyond our hundred years of radio waves. “What if there’s many, many other planets with life out there? Why would anyone want to come visit us? We have boots on the Moon, but not anywhere else.” She implies we’re not an intergalactic fascination—“Yet!”

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Light speed may feel like a limit, but 21st-century humans are far from hitting the wall. The current record for fastest spacecraft is held by the Parker Solar Probe, clocking in at over 300,000 miles per hour. But this is still less than one percent the speed of light. If light were to travel at the cruising speed of a commercial jet, our fastest spacecraft would be slower than a scuttling ant.

Cosmic speed bumps slow every spacecraft. Rockets must work tirelessly against the pull of Earth’s gravity, and any spacecraft with its sights beyond our Solar System must work against the Sun, too. A heavy spacecraft requires more fuel to accelerate, but storing more fuel just makes the spacecraft heavier. By the time our Solar System probes pass the outer planets, they’ve slowed down to a garden snail’s pace against the light-speed jet.

The twin Voyager spacecraft, launched in 1977, are two of these star-bound snails. Their speed has suited them just fine: the Voyagers were built to collect data in the outer Solar System. Though their final planetary encounter took place in 1989, the twins still send messages to Earth from 18 and 22 light-hours away—meaning that light waves take just as many hours to reach them from Earth. The Voyagers may seem far away after 45 years, but light surpasses them in less than a day.

What if we want to travel faster? The nearest stars are on the order of lightyears away, not light-hours; at their snail’s pace, the Voyager spacecraft will not reach another star for tens of thousands of years. If we want to reach for the stars, we might need to level out the cosmic speed bumps—and remove the heavy fuel entirely.

Breakthrough Starshot is a proposal to send a fleet of one thousand tiny “light sails” to the stars. Light sails are a lot like boat sails, harnessing particles of light rather than wind and removing the need to carry fuel onboard. Recent light sails, such as JAXA’s IKAROS and The Planetary Society’s LightSail 2, have accelerated by reflecting particles from the Sun. But to reach higher speeds, future light sails might use arrays of lasers on Earth.

After a few minutes in a laser beam, a future sail similar to Cornell University’s Alpha CubeSat prototype might reach one-fifth the speed of light. Compared to the commercial jet, that’s faster than a hundred miles per hour. Perhaps a light sail is a car on the Autobahn—reaching the nearest star system, Alpha Centauri, in just twenty years.

There are certainly hurdles to jump before laser light sails can take flight. The sails will need to be lightweight, but sturdy enough not to disintegrate in a high-energy beam; their computers will need to weigh less than a paperclip and fit on the tip of a finger; their surfaces will need to withstand four lightyears’ worth of space dust. But not every sail needs to last. Each one would have its own computers and its own ability to beam data back to Earth.

Artist’s conception of the surface of Proxima b. Proxima b orbits within the habitable zone of the nearest star to Earth, a red dwarf named Proxima Centauri—which appears as the brightest star in this planet’s sky. Alpha Centauri A and B can be seen as two tiny dots to the right of Proxima Centauri, forming the rest of the triple star system. Image Credit: ESO/M. Kornmesser

The fraction of a thousand sails that do survive will whiz past a triple star system. Alpha Centauri A and B are Sun-like stars; the third star, Proxima Centauri, is a red dwarf with at least three exoplanets. Two of these planets are within the habitable zone, but likely experience the same problems as other “habitable” red dwarf planets: being synchronously locked and risking high radiation. Even if a light sail at Alpha Centauri doesn’t turn up evidence of life, the up-close data may teach us valuable information about exoplanets.

From a sentimental side, the sails could also serve as tiny messenger pigeons, carrying a piece of home into the dark. Affixed to each of the twin Voyager spacecraft was a golden phonograph record with images, sounds, and music from Earth; in case alien life exists out there, shouldn’t we include a greeting on our interstellar light sails, too? One lightweight option might be to send pictures of Earth within holograms, the kind seen on passports, or on trading cards whose pictures change as one tilts one’s head. A spherical hologram on the laser-beam side might help stabilize the sail; on the other side, aliens could shuffle through thousands of images just by shifting their viewing angle.

The chances that any alien civilization will find our minuscule ships in the vastness of space are astronomically low. An object as small as a spacecraft would be much harder to detect than humanity’s constant broadcast buzz. But interstellar messages aren’t always about who finds them. With every message and every spacecraft, we’re entrusting ourselves to the Cosmos.

We send pieces of home in hopes someone will find it, but also because it makes the space a little less vacant.

Promotional artwork for Alpha CubeSat at Cornell University’s Space Systems Design Studio. Alpha CubeSat aims to demonstrate new light sail technologies, paving the way for a future mission to our nearest star system, Alpha Centauri. Floating to the left of planet Earth are the International Space Station, Alpha CubeSat, and its light sail. Above is the Voyager spacecraft; to the left are hypothetical light sails and the triple stars of Alpha Centauri. Permission was granted by artist Stephanie Young to reproduce this image.

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Even if a holographic message could hold thousands of pictures, it’s hard to choose which ones to send. Which people, places, and pieces of art would we choose to represent ourselves? Would an alien civilization find any resemblance between these pictures and their own?

“It’s easy to think ‘wow, there’s so much here’—but that’s really just confined to this tiny speck of dust floating out in space,” says astronomy undergraduate Ben Jacobson-Bell. “If we’ve got all this history, all this stuff right here, then how much history, how many artistic movements and literary movements have there been in the universe?”

Among astronomers I’ve spoken to, most don’t believe the first life we detect on an exoplanet will be technologically advanced. Carl Sagan Institute undergraduate Thomas Mitchell agrees with this consensus. “My guess is that it’s not going to be a ‘War of the Worlds.’ My guess is it’s going to be some kind of microbial life,” he says. “Even finding a single-celled organism out in the universe is a very huge deal…. It can teach us a lot about our place in the universe.”

And if we are alone, “then I think it’s our duty to learn as much about the universe” as we can, Mitchell says. “To share that knowledge with everyone, just in the hope that it doesn’t die out.”

The search for life is hard. Pandemics may hamper our progress, clouds may cover our views, and vast distances inhibit our telescopes, plant us firmly within our own bubble. But just because it’s hard at the moment doesn’t mean it always will be, that limitations can’t be overcome—or that these limitations even need to be negative.

“Even though there’s this huge distance between us, we are connected, because we can already watch,” Dr. Kaltenegger says. Since light from our planet might take thousands of years to reach someone else, “our past is somebody’s future and somebody else’s past is our future.”

As we whirl through space, and as the other stars whirl too, Dr. Kaltenegger says “we will lose some planets, but we’ll also find new ones. I think that’s just a beautiful visualization that everything in the universe moves, and we move with it, and we are embedded in this beautiful dance.”

Mitchell doesn’t mind the limitations either. “Sure, we don’t know what 96% of the universe is made of, but you have to remember, we came from the universe itself. We weren’t handed any of this on a silver platter. We are starstuff, as Carl Sagan would say—we evolved from primordial elements into single-celled organisms, which then got more complex, and we came from the universe itself. We had to learn all this stuff through trial and error,” he says. “We came from all that, and did all that, in what is essentially the blink of an eye on a cosmic time scale—take another blink and see where that gets us.” ✶

Sources and references: https://docs.google.com/document/d/1cHZhnLEeXhDLziGx0NamFodl0YOhbvPUGHzHHZuejFw/edit?usp=sharing