by Sarah Scoles
When we think of potential peering creatures within our own solar system, we usually think first of Mars. People have hypothesized habitation there since the late 1800s at least. And thanks to probes and rovers like Curiosity, Spirit, Opportunity, Viking, and other strongly named robots, scientists know of water ice that sits on the planet’s surface, liquid saltwater that flows down dunes, fertile soil, and ancient oceans. Mars may have a boring landscape and no good trees (or bad ones, for that matter), but it has—or has at least had, in the past—conditions that terrestrial beings could survive. That’s why scientists are still looking for life there, despite having been disappointed twice before.
The first blow came in the 1970s, when two Viking landers ventured to Mars, taking along with them four life-detection experiments. Safely on the surface, four experiments tested for the presence of organic compounds and metabolism—ones that contain carbon (which, as far as we know, is actually a thing necessary for life). Three came up empty. But one, called the Labeled Release Experiment, did provide a positive result: it seemed to have detected some organism taking in energy and expelling waste (just like you!). Together, these experimental contradictions—in which instruments found no carbon, but one perhaps found beings eating and excreting—were deemed “inconclusive.” The inconclusive conclusion that still holds today, although a study from 2010 suggested the experiments may have had no chance to detect carbon, even if it were there: the salts that spike Martian soil may have destroyed any extant organic compounds when scientists heated them up.
Still, nothing in the experiments, especially not a something, screamed, “Life, for sure!”
The second dashed hope happened in 1996, when a team of American scientists scouring for meteorites found one from Mars. It had formed around 4.1 billion years ago, when both Mars and Earth both were still “young” planets. Mars was nicer back then, hadn’t been hardened and stripped of its atmosphere by the solar system’s hard knocks. It still had liquid water sloshing around its surface. But around 17 million years ago, the piece of rock—fondly known as ALH84001—shot from Mars into space. After whizzing around and past who knows what, it eventually fell to Earth 13,000 years ago. It only took those scientists 12,980 more years to find it.
And when astronomer David S. McKay of NASA stared closely at the rock with his scanning electron microscope, he saw something strange: fractalesque patterns. He’d seen the same patterns on this planet: they were fossils, from microbes pressing themselves against the rock. These meteoritic squiggles, he said, were evidence of ancient Martian biology—the Red Planet had been alive. McKay published a paper saying as much in Science. The world lost its mind, and as evidence of that, I present you with this: President Bill Clinton went on television to announce the news to the American people—and if you know how often presidents call press conferences about science, you know how special this moment must have been.
But ALH84001’s fame was not to last. Other scientists showed that they could create such “fossils” chemically and geologically in the lab. And while that doesn’t mean the meteorite doesn’t have fossils, it did mean the meteorite doesn’t necessarily have fossils. It could be life, or it could just be a geochemical process, albeit a Martian one. While McKay hung on to his conclusions, most—finding no other evidence in support of his hypothesis—moved on. But there’s a lasting legacy of sorts from the scientific frenzy: So many scientists wanted to study so little rock that it became necessary to invent or perfect tools for studying small samples. Those tools live on and continue to be improved upon and miniaturized for future studies on other worlds.
Other places in the solar system make even better microbial real estate than Mars does. Saturn’s largest moon, Titan, is about 160 degrees Fahrenheit colder than Earth ever gets. But it has a nitrogen and methane atmosphere, and lakes and oceans made of methane. While that might sound awful to you or me, there are organisms here on Earth that thrive in methane environments, so it would follow that there could be extraterrestrial methanophiles, too.
Europa, one of Jupiter’s satellites, seems to have liquid water oceans beneath its icy exterior, pressing up against it just like the water of Antarctica presses up against its ice floes. Perhaps, at the bottom of all that, Europa has its own version of black smoker beasts, or sheets of algae-like biomasses clinging to the ice’s underside, or animals that feed on the energy from the planet’s gravitational flexing (if you have seen the movie Europa Report, you might also have some more sinister Galilean aliens in mind). Many scientists, including some at the SETI Institute, hope to send a probe there to find out.
Beyond our solar system, sending probes isn’t really practical at the moment. But in the past 25 years, scientists have discovered that many, many, many planets do exist out there—more planets than stars. This is a huge philosophical shift for science, scientists, and the rest of us.
It wasn’t long ago that scientists thought other planets might not exist at all beyond the worlds within our own solar system, let alone be habitable. Around the time that the Cyclops Report, the SETI bible, came about and Tarter joined SETI, astronomers didn’t yet know if hypothetical aliens would have anywhere to live. “I can’t imagine what it’s like to finish graduate school and begin this search, up against a complete abyss of knowledge,” says exoplanet astronomer Debra Fischer, who now heads a planet-hunting project called EXPRES. “There’s nothing to hang on to. What kind of courage it takes to look!”
And first, before the field focused on aliens themselves, it made sense to look for their worlds. “The first thing that we asked ourselves was, ‘Oh, well, are there any other planets out there?’” says Tarter.
At that time, scientists hadn’t abandoned the idea that perhaps planets formed when two stars passed close to each other and one stole a string of material from the other, pulling it out like a thread from a sweater. “If that were, in fact, the correct explanation, planets were going to be really rare,” says Tarter. The competing model—the one that holds sway today—says planets form from the disk of material left after a dense portion of a giant molecular cloud collapses under its own gravity to form a rotating protostar. In this disk, small bits of dust and rock smash into each other and snowball, eventually, into Jupiters and Mercuries and—perhaps sometimes—Earths. This cosmic billiard game lasts until the protostar turns on and its radiation and particle winds sweep away any of the remaining disk. The planets left behind continue to interact, as the final architecture of the planetary system is established.
But scientists, being scientists, knew that the only way to tell how rare or abundant planets are was to set about finding some. But how?
Astronomers’ first attempts used the precise position of stars in the sky. By mapping their tiny movements from our perspective, a method called astrometry, astronomers thought they would perhaps be able see the tiny tugs of planets pulling their stars around using gravity. But the precision needed for these measurements wasn’t really achievable at that time.
Another method, called photometry, watches the light from stars to see if they dim on a regular basis. Just as you can pass your finger over a bulb and block out some of its light, so too do planets when they pass in front of their stars. Astronomer Bill Borucki began working on ways to make this method reality. His quest that would eventually—after much fitting and starting—lead to the Kepler Space Telescope and an embarrassment of exoplanetary riches. But not until decades after he first dug into the idea. A space telescope was needed to avoid the atmosphere’s blurring effects, but doing things in space takes a long time and a lot of money.
Instead, a third method dominated until the Kepler spacecraft actually launched in 2009. It’s called a radial velocity search. In this kind of hunting, which Fischer’s EXPRES uses, telescopes watch for the see-sawing of a star as its planets tug it around, just as they do in astrometric searches. But instead of looking for a change in the star’s actual position, they look for a change in the light waves. When the star moves
slightly toward Earth, its light waves get squished, becoming bluer; as it scoots back away, its light waves get spread out, becoming redder. You can hear this phenomenon in audio form when you listen to an ambulance go by: its siren pierces at a high pitch as it moves toward you, then wah-wahs down in pitch as it passes by.
As happens amazingly often in astronomy, the first planet anyone ever found for sure didn’t come from any of these methods. And it lived around a star, along with two other planets, that surely wouldn’t support life: a pulsar. A pulsar is actually a dead star, an ultradense hulk of neutrons left after a supernova explosion. Astronomers Dale Frail and Alexander Wolszczan were watching pulses of radio waves coming from the pulsar B1257+12. These pulses should have pinged the telescope at highly regular intervals like the tick-tick of an atomic clock. But the scientists noticed that something was pulling the pulsar around, delaying the pulses sometimes and sending them to Earth early at other times. It was, they realized after much deliberation and data diving, a planet or three.
Holy shit, how did these things survive the explosion of the star, and/or did they reform from the debris? Tarter recalls thinking.
But the first planet that reminded us—a tiny bit—of our own came only two years later. At the time, Tarter and the SETI team had just returned from searching for signals from intelligent extraterrestrials with the private Phoenix Project in Australia for six months. On an October day in 1995, Seth Shostak came in to their newly reoccupied office at the SETI Institute and said, breathless, “They have a planet.”
It was a massive world in a 4-day orbit around its sun-like star.
“It was such a startling result,” says Tarter. The world needed to expand its vision of what a planetary system could be—making this the best kind of scientific discovery, one that reveals something utterly unexpected.
The very next day, Tarter received an email from astronomer Phil Morrison, who had written that very first paper, from decades earlier, about how extraterrestrials might communicate. He and his wife were in the Southern Hemisphere at the time, teaching in Africa. But even there—and before the real Internet—word of this new world had gotten to them.
Since those heady early days, scientists have found thousands more planets, many of them with the Kepler Space Telescope, an observatory that launched beyond our atmosphere in 2009 and has shown us, in the eight ensuing years, that planets are more common than stars.
Kepler ended up being a success. But for a long time, NASA deemed it a telescope that should not exist. When Bill Borucki first brought up the idea of seeing such tiny planets passing in front of such big stars, people believed the project was impossible. “The scientific community felt it couldn’t work,” says Borucki. “No one had ever been able to build a photometer with the kind of precision we’re talking about.”
Undeterred, Borucki began working on it and first proposed a version of Kepler in 1992. “We had to get the answer,” says Borucki. “Are there small planets, or not?” Back then, the mission Borucki was proposing was called Frequency of Earth-Sized Inner Planets (FRESIP). Borucki and his team proposed the mission four more times before NASA finally approved in 2000. Along the way, Tarter had a hand in helping the project move ahead. In 1995, after Borucki’s proposal to NASA had once more been turned down with expressions of doubt over the ability to achieve necessary precision, David Morrison, then the director of space science at Ames, asked Tarter to review the proposed technology. When the group of experts she assembled agreed that the CCD precision would be just good enough, Tarter wrote a letter report to Morrison, urging him to find internal funding to keep the technology development going. In that same letter, she also advised Morrison to change the name from FRESIP to basically anything else, arguing that other planetary systems might not follow our template. With continued support up to the proposal’s acceptance, Kepler launched in 2009, and it soon became a colossus of exoplanet studies.
In their first major announcement, in 2010, the Kepler team announced 306 potential planets. Since then, the telescope’s data has revealed nearly 5,000 planet candidates and more than 2,000 confirmed ones. In our galaxy, based on what Kepler has found, about 20 percent of sun-like stars have rocky planets in their “habitable zones,” where water can stay liquid. Scientist Natalie Batalha, a principal investigator on the Kepler project, doesn’t just see stars anymore where she looks up at the night sky. “I see solar systems,” she says.
One of Kepler’s stated goals is to find terrestrial worlds. And in talk among scientists and in the headlines that splash across homepages, that often translates to excitement about finding “another Earth.” That’s the wrong way to think about it, says astrobiologist Margaret Turnbull, who studies the habitability of planets. “There’s never going to be another Earth,” she says. “We live on a planet that has a particular history . . . The Earth did not look like the Earth a billion years ago or two billion years ago. So no, there will never be another Earth. But that doesn’t mean that life is rare or that there aren’t habitable abodes all over the galaxy. We have to liberate ourselves from the idea of these identical habitable worlds.”
And when we do that, it’s easier to see that planets somewhat like ours are probably everywhere—and that’s meaningful, good news for SETI. “I think that helped the SETI program, the fact—the proof, not the guess—that most stars have planets,” says Borucki. “We now know the size distribution of planets. We have information, hard scientific program, to get those answers.”
But despite the headlines, we don’t actually know whether any worlds are like Earth or whether they are habitable. “We astronomers are going into hypeland,” says Tarter. “It’s going to be a long while—a disappointingly long time, I’m afraid—before we can say for certain what a habitable planet looks like and how many of them there are in the Milky Way.”
And the distance between “habitable” and “inhabited” is a giant leap. Our only example of a for-sure habitable world is Earth, and our only examples for-sure inhabitation are us and our extremophile friends. We have an n of 1—one solar system with one Earth with one set of DNA-based life—and we base our predictions on that one. “If you only have an example of one, your models are probably going to be pretty biased and not very reliable,” says Tarter.
But more examples should be forthcoming, revealing what “habitable” really means in this weird universe, and what might take advantage of those conditions.
“Exoplanets are real, and now we need to make SETI real,” Tarter says now.
The SETI Institute’s real search for aliens has recently taken a turn, looking for life around a type of star that—like black smokers and nuclear cooling tanks—scientists once thought antithetical to life. For decades, scientists thought that extraterrestrials could not survive around most common stars in the universe—the smallest ones, called M dwarfs, which far outnumber stars like our sun. These miniatures flared too much. Their planets would be locked, like our moon to Earth, to the star. One side would perpetually bake, while the other would stay frozen. Plus, these worlds couldn’t hang on to their water. But new research has begun to suggest they might actually make okay homes, with regions of habitability and suitable wetness.
Or not. It’s a debate that’s ongoing in the scientific world at large and within the walls of the SETI Institute. In 2016, scientists discovered that the nearest star to Earth—a dwarf star called Proxima Centauri—has a planet at least 1.3 times Earth’s mass within the habitable zone. With so many open M dwarf questions, having nature provide an exemplar so close to us is like winning the Daily Double. In the not too distant future, theory and pontification will yield to actual observation, and we will have a much better understanding of how viable these star systems are as life homes. For now, at least, they are back in the SETI game.
When Tarter was at the head of the SETI portion of the SETI Institute, the Allen Telescope Array was tasked with scanning all of the exoplanetary systems that the Kepler Space Telescope and ground
-based searches have found, as well as star systems in the HabCat. This habitability catalog, which astronomer Margaret Turnbull created with Tarter, identifies star systems that stay, give off the right kind of light for life, have enough heavy elements to make rocky planets, and are old enough that some advanced life form could have evolved on them.
But the ATA strategy began to change in 2012, when the CEO of the institute—Tom Pierson, who’d first taught the early SETIites to fundraise—offered Tarter a buyout. If she left, her salary would allow other members of the SETI team to keep their jobs.
She took it.
Because this was executed as a salary savings, the institute did not look for anyone external to take her job—leading the search for alien life being a hard sell with nebulous qualification requirements. So they looked inside their own office, and there they found Seth Shostak and Gerry Harp. Together, they took over the job that Tarter had done alone.
Pierson died unexpectedly two years later, after a quick bout with a bad illness, and the institute shuddered without his leadership. To fill the void, they hired David Black, briefly, as the CEO. But less than a year in, Black crashed his bicycle on one of his regular rides around the hillsides of Silicon Valley. A concussion left him in a coma, and when he came out of it, he needed more time to gain the strength needed to shoulder the stress of running the institute. In addition, board members believed they needed a new leader focused on fundraising—someone who knew the business end of a business meeting—and they pushed for a replacement who was not a science specialist but a person of the world.
In searching for Black’s successor, says Tarter, “members of our board of trustees guided us away from equating our leadership with scientific aplomb and instead urged us to select a successful Silicon Valley corporate leader who understood science.”
Enter Bill Diamond, a tech guy who’d worked at start-ups and Fortune 100 companies, who’d raised millions in venture capital in his past lives. The board believed he, still in the post today, had the entrepreneurial mind needed to lead an ailing institute, strapped for cash and not as connected to the Valley as it was in the Hewlett-Packard heyday, into the future.