In all of science fiction, written and televised, there is perhaps one paragraph which stands preeminent over all the rest, known verbatim by millions, repeated countless times around the world, even its grammar debated endlessly. I refer, of course, to the immortal words of James Tiberius Kirk:
These are the voyages of the starship Enterprise. Its five-year mission: To explore strange new worlds. To seek out new life and new civilizations. To boldly go where no man has gone before.
Those words reflect a presumption dating back to the dawn of the space age: that discovering new worlds would require boldly going to them, physically and in person. This near-universal assumption appears in story after story, from the Golden Age of the thirties through to the modern era of Babylon and Battlestars. Yet in the last decade, advances in astronomy have completely overturned that presumption, in a dramatic revolution of discoveries made from the comfort of home. In my prior column, we discussed the stars which make up the solar neighborhood. Now it's time to zoom in closer and take a look at the worlds around them.
The discovery of planets around other stars is now a routine occurrence. As I write this, 240 are known, but that total may well be higher by the time you read these words: on average, a new planet is found every two weeks. In fact, technology has advanced so far that detecting worlds around other stars is literally an undergraduate lab assignment these days. The techniques used—precision radial velocity measurements and transit searches—do have their limitations, being capable of finding only relatively large planets. But the meaning of "relatively large" is steadily shrinking, as improvements in technique have gradually pushed the detection threshold down from Jupiter-sized (about 300x more massive than Earth) all the way to Neptune-sized (a mere dozen times more massive).
Admittedly, we're still a long way from knowing whether any of these planets have blue-skinned women to rescue or alien masterminds who need to be punched. But with the information that we do have—their masses, distances from their stars, and orbital periods from radial velocity measurements, and diameters and a few tentative measurements of atmospheric composition from transits—we can assemble a rich picture of the solar systems around us. We may not be able to boldly go just yet, but that won't stop us from exploring strange new worlds.
Red Hot Jupiters and Eccentric Neighbors
Perhaps the biggest surprise of the exoplanet revolution has been how quickly they were found in the first place. With only our own solar system as a model, scientists naturally assumed that most solar systems would consist of small rocky planets near the star and massive gas giants at greater distances, which took decades to round the star in their ponderous orbits. Theory supported this view, predicting that giant planets could only form at large distances from a star, where temperatures were cold enough for gases to condense into ice and accrete to form giant planet cores. Since the radial velocity and transit search methods both require observing at least one complete orbit before a planet can be detected, planet hunters initially assumed that it would take at least a decade to detect any worlds around other stars. Thus the scientific community was stunned by the announcement in 1995 of the detection of a Jupiter-mass planet in a four-day orbit, whipping around at a rocket's pace just barely above the stellar surface. This planet—now called 51 Pegasi b—is at least half the mass of Jupiter, orbits ten times closer to its star than Mercury is to our own sun, and has an atmosphere at a roasting hot 1200 K. To the naked eye, it would glow like an ember, shrouded in clouds of molten metal and rock vapor. The surprising 51 Peg b proved to be just the first of many such "hot Jupiters," and today dozens and dozens of such worlds are known.
Hot Jupiters were so unexpected, and for a time came to so dominate the scientific debate and press coverage, that it would be easy to get the impression that most known exoplanets are hot Jupiters. Yet that's not actually the case. With increasing numbers of planets discovered, we've been able to assess the overall properties of the population. It's now apparent that the planet distribution has two peaks: there is a "pileup" of hot Jupiters close to stars (say, from 0.05-0.1 AU, in three- or four-day orbits), and then a relatively sparse population at intermediate distances (corresponding to the locations of Mercury and Venus in our own solar system), and finally an increasingly large population of giant planets out beyond 1 AU. Discovering these planets, with their multi-year orbits, is a game only for the patient, requiring painstaking observations year after year to measure even a single orbit. Only in the last few years have decade-old searches discovered planets in distant orbits like those of our own Jupiter. Yet now more and more are being found all the time. Hot Jupiters now seem to be the exception, not the rule, with the majority of giant planets being located several AU from their parent stars, just as theory predicted.
But there were other surprises in store, too. The planets in our own solar system all move in nearly circular orbits, with moons in tow on similarly circular paths. So it was natural to surmise that circular orbits were typical for planets. But nothing could be further from the truth. The vast majority of exoplanets have highly elliptical orbits. Wandering Pluto, with an orbital eccentricity of 0.25, the highest of any solar system planet (well, former planet), isn't even as lopsided as the average exoplanet. Some worlds swing from the Earth's orbital distance all the way in to hot Jupiter territory, and back out again, in an never-ending bake/freeze cycle. Others come in pairs locked in the gravitational dance called orbital resonance. Only a small handful of systems contain massive planets in circular orbits relatively far from their stars, like our own solar system.
Recent surveys have also taught us a lot about where planets aren't. Several thousand stars have been examined for radial velocity trends, and the vast majority show no sign of any orbiting giant planets. Within the last few months, these radial velocity nondetections have been joined by imaging nondetections: telescopes have at last become sensitive enough to directly image some extrasolar planets—the largest ones, that is, at the youngest ages and the greatest distances from the star. Yet so far, there has been not a single picture of anything convincingly planetlike. These nondetections let us put some tight limits on the fraction of stars that have solar systems. If we consider planets of Saturn mass or larger, no more than ten percent of stars have such planets in 5 AU or smaller orbits. No more than 19 percent have Saturn-or-larger planets within 20 AU orbits (that is, within the equivalent of Uranus's distance). Planets may be common, existing by the billions across the galaxy, but systems without gas giants vastly outnumber systems with. True, these apparently-empty systems could still possess medium-sized ice giants like Neptune, small rocky planets, or even vast swarms of asteroids, so they're not necessarily truly empty. But no more than one in five stars has planets as massive as those in our solar system, and of those, only the barest handful have circular orbits like our own.
However, if you're hoping for lots of planets to explore, all is not lost. The good news is that where one planet is found, more are almost certain to follow. A third of all known stars with planets have two or more, and this total is climbing rapidly as more and more double-, triple-, and even quadruple-planet systems are found. (And I've heard recent rumors of a quintuple system!)
Super Earths versus Mini Neptunes
Present planet-hunting methods are sensitive only to massive planets, approximately Neptune-sized and larger. (The mass threshold isn't a single number, but rather depends on stellar mass and planetary orbital distance. Still, "about Neptune sized" is a good rule of thumb in most cases today.) The masses of detected planets range upwards from that size, through Saturn and Jupiter masses, and beyond. The super massive ones, ten to fifteen times the mass of Jupiter, are exceedingly rare—only a bare few out of the 240 total known. Generalizing across the whole set, for masses twice as large, planets become twice as rare, and are vice versa twice as common at half the mass. Hence, there are many more "small" exoplanets known than truly huge ones (but remember, "small" of course still means "many times larger than Earth" in this context!).
We are at present utterly stymied by our inability to detect yet smaller planets. Extrapolating from what we know about the mass distribution for planets we can detect, there are probably far more rocky Earth- or Mars-sized planets waiting to be discovered than the entire known population of exo-gas-giants. But today we can say nothing at all for sure about their locations or orbits, or even their very existance.
Equally interesting is the question of whether there are classes of planets out there which are unrepresented in our solar system. Science fiction has frequently made use of "heavy worlds," large rocky super-Earths with strong gravity, for instance Niven's Jinx. Can such things exist? Planets with intermediate masses between Earth and Neptune are now being routinely found. For instance, the innermost planet of three known around the M star Gliese 876 has only seven times the Earth's mass, making it one of the smallest exoplanets known. Perhaps it is a midget ice giant, composed like Neptune of mostly water, methane, and ammonia compressed into solid "hot ice" by the tremendous pressures deep inside a giant planet, with a dusting of hydrogen and helium on top. Or else it might instead be a rocky world, with a dense iron-nickel core surrounded by silicates and other minerals; such a world would have about twice the surface gravity of Earth. It might even be a hybrid world, a metallic core surrounded with a hydrogen and helium envelope but lacking any ice or rock (although astronomers think such a world is highly improbable). But which is it?
Recently, for the first time a Neptune-mass planet was found that transits in front of the star it orbits, Gliese 436, enabling the first radius measurement for a Neptune-mass exoplanet. When combined with the measured mass, the radius lets astronomers calculate density, which is a strong indicator of composition. In the case of Gl 436 b, the measured density is around twice that of water, quite similar to Neptune's or Uranus's overall densities but far lower than that of rocky Earth. Said another way, the size of the shadow cast by Gl 436 b when it passes in front of its star indicates that it is much too big to be a rocky planet. In this case, at least, nature has come down on the side of mini-Neptunes. But elsewhere? The possibilities remain wide open.
Some believe that if super-Earths do exist, they might best be termed Waterworlds instead. Forgive me a brief digression into planetary ancient history: the very young Earth was hot and molten for megayears, with such a hellacious atmosphere of rock vapor and sulfurous fumes that scientists refer to this time as the Hadean Period. Any water present on Earth then would have been destroyed, its molecules ripped apart and its hydrogen lost to space. The water which today makes up our oceans and clouds, our pineapples and margaritas, and yes, you dear reader, is believed to have arrived slightly later, as the now-cool Earth gravitationally scavenged comets and icy asteroids from its part of the solar system. But then consider again our putative super-Earth. With its greater gravity, it cannot help but gather up more small icy bodies from around its orbit. Computer simulations have shown that most worlds more massive than our own probably gather up many, many oceans' worth of water. Our home is 70% covered in water; if the planet had been only a small amount larger, that fraction might have been much closer to 100%. I will be so bold as to issue a challenge: I want to see a story set on such a world, a place with twice the Earth's surface gravity, a world girdled entirely in water, with an ocean six times the area of the Pacific and a hundred miles deep. Now there's a place for sea monsters!
Earths Aplenty, Around the Corner
Science is often an incredibly painstaking undertaking, requiring obsessive attention to subtle details, careful calibrations of measurements, and patience measured in decades. Other times we just toss some points up on a plot, draw a straight line through them, and hope that back-of-the-envelope estimates might be worth something. If we take the latter approach, and plot the minimum detectable planet mass versus time (from Jupiters in 1995 to sub-Neptunes in the present) something remarkable becomes clear: if techniques continue to improve at the same pace, we will be able to detect Earth-sized worlds by 2010. Rough plans for such instruments are already on the drawing boards. Before today's newborns enter kindergarten, before the next Winter Olympics, even before the movie version of Harry Potter and the Deathly Hallows comes out, we may well have the first conclusive evidence about how common small rocky planets are. We may confirm the predictions that terrestrial planets are common. We may discover a new population of intermediate super-earth water worlds. We may find something completely unexpected and have to go back to the drawing board and rework all those theories that seem so convincing today, in the absence of most of the data.
So if you're casting about for a distant solar system in which to set a story, remember that places like home may well be rare. The universe is full of a diversity of exotica: glowing red hot Jupiters, Saturn-sized worlds looping around at Venusian distances, vast numbers of planets moving in eccentric orbits, giant planets a dozen times more massive than Jupiter careening around mini-Neptunes, and perhaps flooded super-Earths. Heck, have one of each: where there's one planet, there's often more. Planets do form in systems. Yet the majority of stars may well have no planets at all, or else only small ones undetectable today. Planet formation is a common process in our galaxy, it seems, but not an inevitable one. And when it does happen, the outcome in any given system seems to depend in large part upon chance, resulting in solar systems complex and diverse enough to keep astronomers—and fiction writers!—happily occupied for a long, long time.
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