The boxing analogy starts to fail when we consider that, for exoplanets, what's just as important, if not more important, than overall mass is what they're made of. For most exoplanets, this is extrapolated directly from their mass. Thus the giant planets are, like Jupiter and Saturn, thought to be primarily vast orbs of hydrogen and helium. The midsized planets, such as Neptune and Uranus, form from ices dominated by water, methane, and ammonia. The smallest planets are the ones dominated by rock and metal—planets such as Earth, Venus, Mars, and Mercury. For this solar system vernacular, exoplanet explorers have had to develop a new lexicon to describe the types of planets and planetary behaviors alien to our Solar System. Thus we have “hot Jupiters” (Jupiter-sized planets close to their stars); one astronomer has suggested classifying all such close-in planets as “roasters”; and there is also the possibility of a new class of water-world planets—those whose entire surface is covered in liquid water.
It's still very much a developing and contested vernacular, in which the differences in language reflect competing views of what's out there—nowhere more so than when regarding the concept of super-Earths and mini-Neptunes. The discovery of a continuous range of planetary masses between those of Earth and Neptune—a class that doesn't exist in our Solar System—has been one of the great discoveries of the early exoplanet era. The big question is whether an exoplanet two and a half times Earth's mass should be dubbed a super-Earth or a mini-Neptune. The problem is that usually no one knows what the exoplanet is made of: Is it rocky, like Earth, and thus more conceivably habitable; or is it gassy, like Neptune? Some exoplanet astronomers believe that for exoplanets found to date, the term “super-Earth” is largely a sexed-up word used more to garner public attention than to accurately describe distant worlds that could just as well be, and probably are, Neptune-like.
However, we've yet to extend familiar-sounding names, like those of ancient Greek and Roman deities, to exoplanets. For now, they're named as stellar or telescopic accessories. Thus, the first exoplanet around an already cataloged star gets the star's name followed by a b. If the star is uncataloged, the exo-world gets the moniker of the discovering telescope—a number indicating the order of discovery by the telescope and a letter designating its order of discovery in an alien solar system; thus Kepler-10b is the name of the planet discovered by the Kepler Space Telescope.
On a more detailed level, what's emerging from the current exoplanet census taking is that the overall frequency of exoplanet types is quite different from that in our Solar System. It turns out that while our Solar System is dominated by mighty Jupiter's tug, planets this big are relatively rare. With the discovery of 51 Peg b, the first exoplanet, hot Jupiters appeared to be major players. The cosmic irony is that they've been like a sizzling movie trailer that belies the nature of the full-length movie. Based on the current exoplanet census, fewer than one in a hundred solar systems with Sun-like stars include red-hot giants. Similarly, only a tenth of all solar systems include a Jupiter-sized planet at all.
While Jupiter-sized planets appear to be relatively rare, it's the smaller planets that are ratcheting up the exoplanet tally. It may be that in the Milky Way overall, the number of planets increases with decreasing mass; in other words, there are far more small planets than big ones. One major survey of 166 Sun-like stars found that about 6 percent have a Neptune-sized planet. What's most striking is that the Milky Way is resplendent with an abundance of a planet weight class that is absent in our Solar System: the super-Earths, planets between three and ten times the Earth's mass. In our Solar System, we jump from Earth mass to Neptune mass, but the two largest exoplanet surveys to date—the European HARPS survey led by Michel Mayor and NASA's Kepler mission—indicate that between a third and a half of all Sun-like stars have super-Earth-sized planets with close-in orbits of less than fifty days.
Exoplanet scientists come in two basic flavors: those who like finding these alien worlds, and those who like thinking about how these worlds got to be the way they are. These are the astronomy equivalents of experimentalists and theorists. And when it comes to exoplanets, both groups in Jackson Hole were enthralled with what had been found out there. Exoplanets don't just come in a wild variety of sizes and orbital periods; they've also broken open theoretical thinking on how planets form and what planets can do. We've found not just different types of planets but also previously unimagined types of solar-system behaviors. Like in a family, it appears that solar systems emerge into stability only after a period of enormous sibling rivalry, great gravitational tussles in which young planets can be consumed by their stars or can be sling-shot out to spend billions of years wandering the cold, dark interstellar spaces of the Milky Way.
The biggest behavioral shocker for planetary theorists is the fact that giant Jupiter-like planets don't stay put where they're formed but often migrate enormous distances toward and away from their star. The logic goes like this: gas giants must form in the protoplanetary disk where there's lots of gas rather than dust, which would form rocky planets—that section of the disk is relatively far from the star, where Jupiter is in our Solar System. So what explains hot Jupiters? The thinking is that soon after birth, these giant planets spiral toward their star, drawn in by the gravitational and frictional dynamics of the depleting protoplanetary disk. Some of these migrating giants brake at orbital distances of only days; what causes this last-minute parking is still a mystery. Theorists now imagine that some doomed migrating planets do indeed continue on a death spiral, to be finally consumed and to disappear in the fiery maw of their star.
While some young planets get closer to their star, many others appear to be abandoned forever. In 2011, a joint Japan-New Zealand team released the results of a unique survey: in a search for exoplanets, they'd scanned the space not immediately around stars but in the vast reaches of interstellar space between them. The survey used a technique called gravitational lensing, which is based on Einstein's concept that gravity bends light. For an observer, bend is another way of saying focus. So the Japan-New Zealand team scanned a plethora of distant stars, watching for momentary intense brightenings—a gravitational lensing event when an object passes between the viewer and the star, gravitationally bending, or focusing, and thus causing the distant star to appear to momentarily shine more brightly. From the amount of brightening, astronomers can estimate the mass of the intervening object, whether it is another star or something else. The Japan-New Zealand team monitored about fifty million Milky Way stars every hour for almost two years. What it found came as a shock: ten free-floating, Jupiter-mass planets in interstellar space. So was born a new type of planetary category: the orphan planet. This initial sample led the astronomers to infer that there could be twice as many orphan planets as stars in the Milky Way. These dark bodies are orbiting like stars in the twirling stream of the galaxy. Where do all these lonesome planets come from? The astronomers in this study believe that the evidence points to planets that were gravitationally ejected from the stellar nest by close gravitational interactions with their star or other planets. Subsequently, other researchers have suggested that many solar systems end up adopting interstellar orphans as their own.
Given all these possibly eaten or orphaned planets, no single finding more impressed exoplanet theorists, experimentalists, and Star Wars fans than the discovery, announced in Jackson Hole, of Kepler-16b—the first circumbinary planet (a planet that orbits two stars). As occurs on Luke Skywalker's fictional home planet of Tatooine, on Kepler-16b, each day involves a double sunset. The cold, gaseous planet orbits a pair of stars, both smaller than the Sun, that in turn orbit each other. For planet hunters, this discovery opens yet another frontier. Until the discovery of Kepler-16b, astronomers didn't know if planets could form, or survive, in double-star systems. Kepler-16b isn't just a single new planet; it's also the first in a new class of planets that greatly broadens the possibilities for exoplanet exploration.
Rather than our own Solar System informing us about what
we've found, the age of exoplanets has given us a new perspective on our home planetary system. Without doubt, the biggest local impact of exoplanets was Pluto's demotion to dwarf-planet status. When there were only nine planets in the entire cosmos, no one thought of ditching one, even if it was no bigger than distant asteroids. But a growing crowd of exoplanets, coupled with the discovery of other Solar System objects of equal or bigger size, resulted in Pluto becoming the first victim of planetary downsizing. Similarly, the realization that planets can migrate enormous distances from where they were formed, producing hot Jupiters, has inspired astronomers to take another look at Jupiter's history and possible past wanderings. It's now clear that solar systems are much more chaotic and interactive families than was thought before, with sibling gravitational interactions as planets grow from the protoplanetary disk having an enormous impact on any one planet's future.
The broader perspective provided by exoplanet behaviors has encouraged the view that Jupiter, as an infant, might have been like a sailboat caught in a gravitational current that took a grand tack from near its current distance from the Sun and moved to within a little more than Earth's distance, before being heaved back by Saturn's gravitational tug. The wanderings of this exoplanet giant would have set the rough outer limit for the formation of the rocky planets that appeared slightly later. An even more dramatic rewriting of our Solar System's early days is the possibility that somewhere out in the depths of interstellar space, Earth has a lost giant planetary sibling. One computational model of the early Solar System formation found that the current planetary alignments make sense only with the addition of an original fifth giant planet, one that was gravitationally ejected from the natal crib by a jumpy Jupiter.
But for many exoplanet searchers, the wonderful menagerie of known exoplanets is but the lead-up to the most sought-after type of exoplanet—a living one.
ALIEN EARTH
Amid all the excitement and anticipation over the extent and pace of exoplanet discovery, William (Bill) Borucki sat quietly on the podium beside two other panelists at Jackson Hole for an informal evening of exoplanet discussion. One front-row audience member stood up and, in a playful, congratulatory tone, said, “Bill, you always look so worried. You're already a hero. It's a revolution in planetary astronomy.”
Both observations were true. As the persevering brainchild behind NASA's $650 million Kepler Space Telescope, the seventy-two-year-old Borucki, dressed modestly in beige slacks and a tan windbreaker, had turned the search for exoplanets from individual finds into a tsunami of discoveries. In just eighteen months since opening its electronic eye, Kepler had detected thousands of probable exoplanets and had shown that in time it will reap thousands more. But sitting there that evening, Borucki wasn't content. Many of the astronomers and astrophysicists searching for and studying exoplanets are primarily driven by the technical and scientific challenge; enamored with the motion of celestial bodies and solving complex multi-body motion problems; and building the complex mathematical, computational, and astronomical tools needed to do this. At this level, the exoplanet search is an extension of one branch of astrophysics that concerns itself primarily with celestial mechanics.
Borucki is as energized by the technical challenge as the next astrophysicist, intently discussing the nuances of the latest speaker's data during each coffee break. Yet that isn't what has driven him for the past three decades. He is part of an exoplanet researcher subclan for whom the search for exoplanets isn't ultimately about physics but about biology, about life. Borucki doesn't just want to find more planets; he wants to find the ultimate goal: another living planet, an alien Earth. The Kepler Space Telescope has detected thousands of exoplanets, but that's not why Borucki spent fifteen years doggedly proposing and developing the technology for the now-heralded mission at a time when, at best, he'd be dismissed, and, at worst, he'd be laughed off as a science-fiction-infected dreamer. Kepler's done a lot, but it has yet to achieve Borucki's lifelong goal of finding alien life or, in this case, a planet that ET might call home. Seated on the stage, Borucki knew that for all his decades of work, hope, and dreaming, it might not ever achieve this goal.
Borucki embodies the entire NASA vision and half-century history that encompasses both the dream of space travel and the way it has grown to merge with the search for extraterrestrial life and our astrophysical origins. In the mid-1950s, as a teenager in Delavan, Wisconsin—circus capital of the world and home to the original P. T. Barnum Circus—Borucki didn't find high school at all amusing. The school principal suggested a science club to keep him engaged, and the science teacher suggested projects—which Borucki promptly rejected in favor of his own: optical communication with UFOs. He led his 1950s crew-cut classmates in building a combination of ultraviolet, visual light, and infrared optical transmitters to signal any flying saucers passing over the US Midwest. They didn't make contact—didn't even complete the devices—but Borucki had found his passion: a way to turn the science fiction he loved reading into the tentative truths of science fact (his favorite is still Ray Bradbury's 1950 short-story collection The Martian Chronicles). In the process, he also learned a critical lesson that would guide the rest of his life: he might fail, but it was technically and scientifically possible to search for alien life.
If that was his first lesson, the next was on-the-job training in NASA's can-do culture. In 1962, the year after President John F. Kennedy committed Americans to go to the Moon within the decade, Borucki was one of the legions of young, talented engineers and scientists hired as part of NASA's Apollo program. The following year, his local draft board called him up for service in Vietnam, but, as Borucki puts it, NASA overrode them, saying his services were needed for another Cold War battle: to beat the Commies not in Vietnam but to the Moon. Fresh from college, Borucki was hired by NASA to help develop the Apollo reentry vehicle's heat shield. It was an awesome responsibility, at the very edge of engineering. Astronauts might make it to the Moon, but if the heat shield didn't have the right stuff, the crew would, like a burning meteor, be incinerated as the reentry vehicle plunged into the Earth's upper atmosphere. “It was like dipping the heat shield into the Sun for several minutes,” says Borucki.
Borucki became part of a NASA team that worked around the clock, in shifts, seven days a week, to develop the heat-shield technology. The Apollo crew would reenter the Earth's atmosphere, traveling at a blistering six miles a second. To simulate the pressure and temperatures at this speed, Borucki and colleagues used two artillery cannons from a mothballed US Navy battleship. When the cannons were fired, the blast was so powerful it lifted the roof of the building in which they were housed. For all this brute force, Borucki's job focused on understanding the atomic minutiae at the interface between the atmosphere and the heat shield, where a superheated plasma formed—atoms shorn of electrons and emitting intense radiation. To do this, he built a state-of-the-art spectroscope, using lenses developed for US Air Force spy cameras, to study the light emitted by the plasma. Analyzing the nature of this light was critical to knowing the types of radiation and radiation flux the heat shield needed to endure. In the end, the team developed a graphite-epoxy-based shielding that brought successive Apollo astronauts safely home. Borucki learned about the enormous value of what could be learned from the study of light, and that with determination almost anything was possible.
When the Apollo program ended, Borucki began his own space research missions aimed at the search for alien life. First, he spent a decade extending his research in plasmas by exploring lightning on other Solar System planets. When an electrical charge passes through the atmosphere, it creates superheated plasmas, which Borucki and others thought might be the spark for forging prebiotic molecules, the building blocks of life. But in the early 1980s, when astronomers like Bruce Campbell were talking up the search for planets around distant stars, Borucki came across a quixotic scientific paper by Cornell University computer scientist Frank Rosenblatt that would change both Borucki's life
and the search for alien worlds.
Rosenblatt's paper is one of the great, surprising gems of the Stardust Revolution. In the 1950s and ’60s, Frank Rosenblatt was a computer scientist at Cornell University, where he developed the Perceptron, the first computer that could learn how to use a neural network that simulated human thought processes. But Rosenblatt was a prodigious polymath who combined his computer science research with interests in the psychology of perception—doing research in how air force pilots perceive depth and distance when landing on aircraft carriers—and astronomy, with a homemade observatory at his home outside Ithaca, New York. He talked about the stars and the search for alien life with leading Cornell astronomers, including Carl Sagan and Edwin Salpeter, who'd contributed to figuring out how stars make the elements.
While most Americans were focused on the Moon landings, Rosenblatt was already imagining exploring for more distant cosmic terrain. His 1971 paper, “A Two-Color Photometric Method for Detection of Extra-Solar Planetary Systems,” proposed a two-pronged new way to search for these systems. First, Rosenblatt imagined that when a “dark companion” passes in front of its star, as seen from Earth, this transit, or mini-eclipse, would be visible as a tiny, transient dip in the star's light. In essence, the star would make the subtlest of winks. This stellar dimming could be detected by a telescope equipped with a photometer, a device for measuring the intensity of the light. Second, Rosenblatt noted that you didn't need an army of astronomers to keep watch for these transits; instead, he outlined a computerized system that could automatically watch stars and periodically record their brightness. It was the only astronomy paper Rosenblatt ever wrote. Six months after the paper was published, he drowned in a boating accident.
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