Gregory P Laughlin. American Scientist. Volume 94, Issue 5. Sep/Oct 2006.
Christiaan Huygens carried out the first known search for extrasolar planets in the 160Os, but the next three centuries were filled only with false alarms, dashed hopes and nondetections. It took until 1988 before the first hints of progress began to emerge: Gordon A. H. Walker and his colleagues at the University of British Columbia reported evidence for unseen planetary-mass companions orbiting several nearby stars. These investigators were extremely cautious, however, and stated that orbiting planets were just one of several possible interpretations for their data. Consequently, few people took much notice.
The following year, David W. Latham of the Harvard-Smithsonian Center for Astrophysics and four colleagues reported strong evidence for what might be a planet orbiting an obscure star known as HD 114762. Because Latham’s planet has at least 10 times the mass of Jupiter, astronomers tended to assume that it was either a brown dwarf or a star of very low mass. So it, too, didn’t make headlines.
In 1992, Alexander Wolsczan of Penn State University and Dale A. Frail of the National Radio Astronomy Observatory used a highly accurate timing method to discover two very small planets in orbit around a pulsar. These utterly bizarre worlds apparently formed from a disk of radioactive debris left over after the supernova explosion that created the pulsar. This strange setting perhaps accounts for why few people felt that a true analog of our solar system had been found. Yet the detection gave the first hint that planet formation is a common and robust process.
Then in 1995, two Swiss astronomers, Michel Mayor and Didier Queloz of the Geneva Observatory, stunned the world when they detected a planet circling 51 Pegasi, a nearby star not all that different from the Sun. The planet, they claimed, is roughly 150 times more massive than Earth and travels in an orbit that takes only 4.2 days to complete. When the announcement was made at a scientific conference in Italy, the general reaction tended toward disbelief. A planet with such a short orbital period must lie extremely close to its parent star, about 5 percent of the distance between Earth and the Sun. Conventional wisdom in 1995 held that massive planets should be located much farther out. How could this newfound world (which was given the utilitarian name “51 Peg b”) even survive in its crazy orbit?
Within days, other astronomers had verified Mayor and Queloz’s observations, and several teams of astrophysicists had subjected computer models of this “hot Jupiter” to a variety of tests. To the surprise of many, the calculations showed that a planet such as 51 Peg b would, in fact, easily weather the intense radiation and would likely. lose only a negligible fraction of its mass during the billions of years it and its parent star would endure.
As yet, astronomers have no images of this far-off planet to examine. Still, it’s not too difficult to imagine what it would look like up close. The dayside of 51 Peg b would be roughly 400 times brighter than Earth’s desert sand dunes on a midsummer day. Even the nightside would glow red. To look at the illuminated face of the planet at all, you would need extremely dark sunglasses, or better yet, a welder’s mask. With the brilliance of the dayside reduced to a manageable level, you might be able to see swirling clouds made from droplets of sodium sulfide. The outer layers of such a planet’s atmosphere are probably a toxic brew of hydrogen, helium, steam, methane, carbon monoxide, cyanide, acetylene, hydrogen sulfide, soot and a host of other unpleasant compounds.
The discovery of this bizarre, completely unpredicted world spawned a revolutionary new astronomical field: the study of alien planetary systems. Astronomers have now found nearly 200 extrasolar worlds, which populate planetary systems of astonishing diversity. The sheer number shows that planet formation must be common, and the characteristics of these worlds and their orbits are helping specialists like myself model how planetary systems form and evolve.
A Star Is Born
Look up at the sky on a truly dark night, and you’ll have no trouble discerning the Milky Way: our own disk-shaped galaxy, seen from within. Floating between these many points of light are giant molecular clouds-great masses of molecular hydrogen and helium laced with dust that is slightly finer than the particles found in cigarette smoke. These dark monsters, which congregate in the spiral arms of the Milky Way, are frigid (with a temperature around 10 kelvins), and if you could watch a time-lapse movie of 10 million years compressed into a minute, you would see them billow and boil.
Within the cores of such clouds, the cold, dense gas is always poised on the verge of gravitational collapse. Disaster is staved off by the presence of charged particles, ions and free electrons, which are outnumbered by neutral atoms and molecules by a factor of 10 million or more. These charged particles are tied to magnetic-field lines. The motion of charges drags the field lines along and vice versa. The lines, however, don’t like being compressed or twisted and have a tendencyverging on insistence-to spring back into shape. This phenomenon prevents the ions and electrons from joining the gravitational collapse of the cloud. And because these charged particles bounce against the neutral particles around them, they counteract the great inward crush.
Most of the time, such outward forces keep giant molecular clouds inflated. As a result, they usually end up being torn apart by tidal forces before they can compress under their own weight. Occasionally, however, the ions and electrons are overwhelmed. Neutral gas slips past them and pools in a concentrated mass. As this process gains momentum, the ions and magnetic fields lose their grip, and vast portions of the cloud begin to collapse. Viewed from the center of action, the surroundings stars would quickly become blotted out. The scene there is utterly black and frigidly cold, but to ears pitched 52 octaves below middle C, it is not silent. The cloud rumbles and groans.
As the collapse proceeds, the concentrated gas in the center begins to form a star. Soon a new effect becomes apparent: Material that falls inward from large distances does not land on the central protostar; rather, it just misses it, forming a whirling platter of gas and dust. This motion comes about because the original molecular cloud harbored an ever-so-slight component of random rotation. And as with a figure skater pulling in his arms, the spinning quickens as mass is drawn inward.
The idea that the Sun and its planets arose from such a disk of gas and dust dates to the 18th century. The French naturalist Georges Louis Leclerc, Comte de Buffon, proposed that a celestial body had a close encounter with the Sun, throwing out the material that later condensed into the planets. This theory accounted for the fact that the planets all orbit in the same direction. The German philosopher Immanuel Kant postulated that the planets arose from a primordial cloud of spinning gas. This concept was more fully developed later, independently, in the 179Os by the French mathematician Pierre-Simon Laplace, who imagined that the disk contracted as it cooled, leaving behind a succession of rings that fragmented to form the planets. These 18th century cosmogonies were couched in quaint language and are not fully correct, but they nevertheless hit surprisingly close to the mark. Today’s computer simulations support the general picture they provide and fill in much of the missing detail.
One modern theory for how a planet, say a gas giant like Jupiter, forms from a protostellar disk of gas and dust hinges on gravitational instability. Simply put, as the density of the protostellar disk increases, it starts to clump here and there in response to self-gravitation. Simultaneously, the pressure of the gas in each mass concentration pushes back and partially offsets the tendency to collapse. In addition, the differential rotation of the disk (whereby material located closer to the star orbits faster) tries to shear each growing fragment apart. Differential rotation thus acts as a large-scale stabilizing influence against the gravitational nucleation of planets. The key question is whether gravity wins, allowing Jupiter-mass bodies to form, or shear and pressure win, keeping the disk free of planets.
The situation resembles what sometimes transpires when the members of a student rock band attempt to attract an audience. For that, they usually provide free beer, or more precisely, flyers posted all over campus advertising a party with music and free beer. These enticements act in a way analogous to the self-gravity of a protostellar disk. As with the astronomical case, such inducements can lead to instability. For example, dozens of thugs whom nobody has ever seen before (and whom nobody wants to see again) will likely descend on the hapless band’s house-party show. Amplifiers will be destroyed. Holes will be kicked in walls. Fights will erupt. When the cops arrive, the band will be sent home. (This outcome can be compared with a protostellar disk that undergoes unrestrained gravitational collapse into planetary-mass fragments.)
In practice, however, the police don’t always show up, and sometimes the band gets to play. This happier result might come about from two stabilizing influences. On small scales, the analogue of “gas pressure” in a protostellar disk might be provided by the combined effect of overcrowding and body odor. Long lines at the keg and the unpleasantness associated with a sweaty throng of fans will tend to drive thugs away. On the large scale, the band could create the equivalent of differential rotation by not indicating the precise time of the show on their flyers. Thugs will drift in and out from time to time, but they will never form a critical mass for mayhem. The police are never called, and the band can play its entire set to a grateful (if stinky) audience.
Despite its intuitive appeal, the weight of observational and theoretical evidence seems to be shifting against the gravitational-instability hypothesis. Computer simulations show that for fragments to form and persist as planets, the rate of cooling in the disk must be extremely efficient. (Rapid cooling robs nascent concentrations of mass of their ability to produce pressure and hence permits gravitational fragmentation.) Perhaps more important, these numerical experiments indicate that long before a disk attains sufficient mass for self-gravitation to take over, it will form waves that act to push gas out of the regions where the material is in the most danger of fragmenting into planets.
If giant planets do not condense directly out of these disks as the result of gravitational instability, how then do they form? The best guess currently is called core-accretion theory. The idea is that planets start small and grow through the agglomeration of dust. If you live in a house with hardwood floors, you can develop a handson sense of how this planet-forming mechanism operates by not vacuuming under the bed for a while. You’ll notice that the dust does not accumulate there in a uniformly thick layer. Rather, random air currents swirl the dust around, causing it to build up in dust bunnies. Each dust bunny-made of hair, dandruff, dust and countless unidentifiable strands of ticky-tacky-has an airy structure that takes up a large volume in comparison with its mass. This property makes it effective at scooping up more material. So once dust bunnies begin to form, their subsequent growth becomes relatively easy. A similar process may be at work in protostellar disks.
Even so, the initial growth of these dusty, icy objects is difficult for theoreticians to understand. The problem is that agglomerations of dust experience a “headwind” from the gas in the disk, which should cause them to spiral inward, eventually vaporizing as they get close to the central star. Some mechanism must concentrate the dusty debris and allow it to build in size quickly enough that it won’t be destroyed in this way. One possibility is the existence of vortices, hurricane-like flow patterns in the disk itself. Numerical simulations show that disk vortices, if they live long enough, can trap and concentrate solid particles in their centers.
Another possibility is that solid particles settle into a thin layer at the disk mid-plane. If this layer grows dense and massive enough, a form of gravitational instability can ensue whereby the dust, ice or gravel present can rapidly form larger and larger objects. Once these bodies attain a certain size, several kilometers, say, they become safe from the drag force exerted by the surrounding gas and do not spiral inward.
By such mechanisms, trillions of kilometer-size planetesimals might emerge in a protostellar disk 100,000 years or so after it forms. Trillions sounds like a lot, but the disk at that stage would not seem particularly crowded. The density of gas present would be millions of times less than the density of air at sea level, and the distance between kilometer-sized bodies would be measured in thousands of kilometers. If you could transport yourself to a random point in the middle of such a disk, there would seem to be only empty blackness around you: no view of the stars, no sign of the young sun forming nearby. Indeed, it might appear as though you were floating in a run-of-the-mill molecular cloud. A thermometer, however, would show a difference. Whereas a molecular cloud would be incredibly cold, 5 or 10 kelvins, the temperatures you’d encounter would be much warmer: ranging from hundreds or even thousands of kelvins very near the central star, down to several tens of kelvins in the farthest reaches of the disk.
In such a primitive planetary system, ice would form at the expense of water vapor wherever the temperature fell below about 150 kelvins. The 150-kelvin isotherm in a protostellar disk is located roughly at Jupiter’s current distance from the Sun, marking what astronomers like to call the “snowline.” Just beyond the snowline, the temperature is cold enough for ice to be stable, yet the distance from the center is small enough for the density of objects to be high. These properties allow icy planetesimals to bash into one another relatively frequently.
Such collisions, at least the ones that take place at low speeds, are sticky events. So as thousands of years pass, the planetesimals gradually become fewer in number and individually larger. Those that experienced a handful of extra collisions in the beginning are able to take advantage of their burgeoning self-gravity to collide more often and are thus able to grow faster than others. Inevitably, a few big winners emerge. These bodies, which measure thousands of kilometers across, are massive enough to haul in their neighboring kilometer-sized brethren. And the bigger each such mass gets, the more it wants, and the farther its reach extends.
When the growing body, now a full-fledged protoplanet, reaches 7 to 10 times the mass of Earth, its gravity begins pulling in appreciable quantities of gas. As a result, the atmosphere heats up and swells to a thickness of some hundreds of thousands of kilometers. Heated by gravitational compression, the gas glows fire-engine red and pours infrared light out into space. This radiation is accompanied by the slow cooling and contraction of the lower layers, providing room at the top for more gas to flow in.
Finally the growth spurt ends. The neighborhood still contains vast quantities of gas, but gravity is too weak to allow this material to settle quickly onto the bloated planet, which can only add new gas as fast as the older stuff is able to radiate away its heat. So for the next million years, the planet bulks up slowly.
The new world, although grown big and robust, still faces a significant threat to its existence: inward migration. The reason for this movement is rather subtle. As the planet acquires more and more mass, its gravitational pull on the gas in the disk becomes stronger and stronger. In particular, as the planet moves through the disk, it leaves a wake of concentrated gas behind it.
A boat traveling through still water creates a V-shaped wake. The gravity of a planet moving through still gas would produce a similar effect. In a disk, however, differential rotation acts to distort the gas in the planet’s wake. Curiously enough, the portion of the wake that is closer to the star runs ahead of the planet, whereas the outer wake trails behind. The upshot is that the planet creates a spiral wake (or spiral wave), which marks a region of increased gas density. The extra mass that runs ahead exerts a gravitational force that propels the planet along in its orbit. This forward pull causes the planet to speed up and as a consequence to drift outward through the disk. The portion of the spiral wave that sweeps backward has the opposite effect, draining orbital energy from the planet and causing it to shift inward.
In general, the structure of planet-forming disks seems to be such that the tendency to migrate inward is stronger than the tendency to move outward. The propensity for growing planets to spiral inward to destruction is known to astronomers as the Type I migration problem. At the moment, it is not clear how this fate is circumvented in nature. It could be the case that many protoplanetary cores are lost to inward death spirals. Alternatively, and this is perhaps more likely, the disk might exert countervailing forces on the nascent planet. Adriane Steinacker of NASA’s Ames Research Center, Fred C. Adams of the University of Michigan in Ann Arbor and I have recently shown, for example, that if the disk is turbulent, a growing planet’s migration can switch from an uninterrupted inward course to one that skitters randomly inward and outward. So perhaps this “problem” for growing planets is not really a problem at all.
A dozen years ago, astronomers had no way of knowing whether the conditions that led to the solar system were universal, reasonably common or unbelievably rare. Over the past decade, however, this ignorance has been rapidly evaporating. Astronomers now have a bewildering variety of other planetary systems with which we can compare our own. We can now talk about classes of planets, and we are beginning to make broad statements based on the statistics of the current planetary census.
The Catalog of Planets
The vast majority of the nearly 200 worlds uncovered so far are orbiting ordinary Sun-like stars in our immediate galactic neighborhood. A good way to begin making sense of this population is to plot them on what is known in the trade as a mass-period diagram, one in which the orbital period of a planet around its host star is plotted on the x-axis, and the estimated mass is plotted on the y-axis. A slight variation is first to translate the periods to distances from the central star.
Such a diagram shows several distinct populations of planets. The first group, the hot Jupiters, all resemble 51 Peg b, with orbital periods ranging from a fleet 29 hours to about a week. As these planets skim endlessly above the surfaces of their parent stars, they experience enormous tides, which inexorably lock their rotation periods to their orbital periods (as has happened to the Moon). The strong tides also act to mold their orbits into the energetically preferred circular shape.
A typical hot Jupiter orbits at a distance of roughly 10 times the radius of its parent star, which means that there is roughly a 10-percent chance that the planet can be observed to cross in front of this star when viewed from Earth. This phenomenon is known as a planetary transit, and when it occurs, the observations are of great scientific value. By measuring the fraction of starlight cut off, astronomers can work out the true size of the planet, which, when combined with the mass, yields a density, which in turn reveals general composition. To date, astronomers have found 10 transiting extrasolar planets.
On average, the radii of these planets are somewhat larger than the size of Jupiter. This observation demonstrates that these worlds are gas giants like Jupiter or Saturn, composed predominantly of hydrogen and helium. (If these planets were oversize analogues of Earth, fashioned primarily from dense rock and metal, they would be much smaller.) Detailed models also show that the radii of the known transiting planets are best explained if they contain fairly sizable cores of heavy elements. The likely presence of cores inside the transiting planets is a strong piece of evidence in favor of coreaccretion theory.
One transiting planet in particular, HD 149026 b, appears to have an extremely large core. Its radius is small enough to imply the presence of approximately 70 Earth masses of solid material buried within 40 or 50 Earth masses of gaseous and liquid hydrogen and helium. This composition is completely inconsistent with formation through gravitational instability. Indeed, it’s difficult to stretch the coreaccretion theory enough to explain the existence of this world. It seems that some still-mysterious process managed to feed HD149026 b a steady diet of rocks, metal and possibly ice, while keeping much of the gas around it at bay.
A second major class of planets, which I like to call the “Eccentric Giants,” can also be delineated in the mass-period diagram. These planets have longer orbital periods (ranging from a week to nearly a decade) and tend to have masses that are several times larger than that of the average hot Jupiter. They are located far enough from their parent stars that tidal circularization is ineffective. Hence many of these worlds trace out highly elliptical trajectories.
Because the hot Jupiters and Eccentric Giants have no counterpart in our own solar system, one might be tempted to jump to the conclusion that our situation is unusual. This view may wind up being correct, but at this point it’s a premature speculation. The makeup of the current mass-period diagram is heavily influenced by the fact that the planets included are the ones that are most readily observed. The methodology used to find these planets-looking for Doppler shifts created by the wobbles they induce in their parent stars-is most sensitive to planets with large masses (which make for bigger wobbles) and short periods (which allow astronomers to chart these oscillations relatively quickly).
Experience gained so far from surveys shows that at least one hot Jupiter or Eccentric Giant accompanies a bit less than 10 percent of the stars in the solar neighborhood. The $64,000 question is: What about the other 90 percent? Do they have Jupiter-mass bodies in big, long-period orbits and smaller rocky worlds circling farther in? Or are they largely devoid of planets? Perhaps their planetary arrangements (when astronomers manage to chart them) will come as a complete surprise, as has largely been the case with the planets discovered so far.
One thing is certain: The answer will demand patience. Astronomers now have close to 5,000 stars under surveillance for planets. Most of the stars that are in these surveys were added within the last few years, and many of them have only been observed a handful of times. So it will likely take another 5 to 10 years before astronomers are able to get a good statistical handle on whether our own system is unusual or run-of-the-mill.
Theorists like me thus have some breathing room. Although neither hot Jupiters nor Eccentric Giants were anticipated prior to their discovery, it is perhaps possible that we can now put together some predictions that won’t fall that far from the actual truth.
A crucial insight into the process of planetary formation comes not from the attributes of the planets themselves, but rather from the properties of their parent stars. Astronomers, including Debra A. Fischer of the University of California, Berkeley, and Jeff A. Valenti of the Space Telescope Science Institute as well as Nuno Santos of the University of Lisbon and his collaborators in Europe, have shown that the likelihood of there being a detectable planet depends on the fraction of heavy elements present in the host star-a quantity known in the trade as “metallicity.” The Sun, for example, is composed of somewhat less than 2 percent heavy elements by mass. Stars with similar metallicity to the Sun have a roughly 5 percent chance of harboring a readily detectable hot Jupiter or Eccentric Giant. When the stellar metallicity increases to twice the solar value, however, the rate of planet detection jumps by more than a factor of five. Likewise, when stars with metallicities that are less then half that of the Sun are monitored, the hit rate drops below 1 percent.
The metallicity connection provides an important clue that core accretion must have been responsible for the formation of the majority of exoplanets that have been found so far. Stars that form from metal-rich disks (which have lots of solid dust whirling around, rather than just gas) should get the core-accretion process going relatively rapidly. And when a core forms quickly, enough gas remains in the disk to allow the planet to grow substantially in mass. Were core accretion to take too long, the gas in the disk would dissipate before a gas giant could form. Such slow accretion would nevertheless allow Neptune-sized cores to build up, even if they never attract much gas. Stars that are less metal-rich than the Sun should thus have an abundance of such “failed” Jovian planets.
My prediction is that the solar system will turn out to have a moderately unusual configuration, in that we have a Jovian planet that managed to form, yet did not migrate inward significantly. I think that the most common planetary architecture in our galaxy will wind up being a configuration that has terrestrial-mass planets within the inner few astronomical units (one astronomical unit being about 150 million kilometers, the distance between Earth and the Sun) and Neptune-mass planets farther out. This guess is based on a series of numerical simulations I did in collaboration with Adams and my departmental colleague Peter H. Bodenheimer. Our results strongly suggest that a star with 40 percent of the Sun’s mass would require considerably more than 10 million years to “go Jovian.” After 10 million years, however, the gas in most protostellar disks is long gone. The core-accretion theory predicts, therefore, that low-mass red dwarf stars (the most numerous type of star in the galaxy, with masses less than about half a solar mass) should very often be accompanied by Neptune-mass planets but should almost never have Jupiter-mass companions.
So far, this assessment has turned out to be nearly spot-on. Surveys of stellar wobble have located a number of Neptune-mass planets orbiting the 150-odd red dwarfs that are near enough (and hence bright enough) to be studied in this way. And astronomers have made use of a phenomenon called gravitational lensing to discover several more. All in all, the observations have revealed only one red dwarf that harbors a system containing Jovian-mass planets.
The red dwarf that bucks the trend (the less-than-poetically named Gliese 876) lies a mere 15 light-years from Earth. Still, it is more than 100 times too faint to be seen with the naked eye. Gliese 876 boasts one of about 10 multiple-planet systems known so far. It has two massive planets in 30- and 60-day orbits, and a much smaller 7.5-Earth-mass planet in a 2-day orbit.
This curious system presents a wealth of insight about the process of planet formation. The two massive outer planets are trapped in what is known as a mean-motion resonance: On average, the outer planet makes exactly one orbit for every two orbits executed by the middle planet. This configuration is maintained by the mutual gravitational tugs between the two bodies. It appears that the outer planet must have migrated inward through the original disk with respect to the middle planet. When the planets drew close enough, they were “captured” into the resonance. Further migration then carried both planets inward in strict lock-step.
It is hard to see how a single gas giant, much less two, could form through the core-accretion process within the short lifetime of what was presumably a low-mass protostellar disk that accompanied the birth of Gliese 876. At the same time, it’s difficult to imagine this system achieving the conditions required for gravitational instability to take over and then having the dynamical activity calm down enough to allow capture into the resonance.
The Vast Unknown
Gliese 876 provides a warning that theories of planet formation are still lacking important elements. For one, it seems very likely that the runaway growth of a Jupiter-mass object influences the structure of the disk in a way that can promote the rapid development of additional giant planets. In our own solar system, for example, the presence of Jupiter may have controlled the growth of Saturn. Investigation of such mechanisms is an important area of current research.
Such work on the interaction between planets may help elucidate why the Eccentric Giants tend to be more massive than either our own Jupiter or the far-off hot Jupiters. I think this observation provides a hint that many of the Eccentric Giants arose in systems that originally contained several massive planets that, in a game of gravitational rub-a-dub-dub, altered one another’s orbits. Numerical simulations show that, over time, such interactions can lead to the ejection of the smaller planets from these systems, with the surviving massive planets left following eccentric orbits, much as is seen.
Clearly, the observations collected over the last decade have been enormously helpful in shaping theory, but can they sometimes be misleading? When a distant system contains more than one planet, astronomers have no trouble dividing the observed wobble signal into distinct components, in which case the individual orbits are easy to characterize. Many of the most interesting observations, however, such as that for a star called 55 Cancri (which has at least four planets), are quite tough to analyze. A great deal of finesse and insight are often necessary to guide the process, and when gravitational interactions between planets are important, finding acceptable orbital models can be a computational nightmare.
These and other thorny issues can make the interpretation of wobble measurements very tricky, which creates worry that some of the statistical conclusions drawn from them might be subtly biased-perhaps even wrong. To address this concern, I and colleagues at the University of California, Santa Cruz, and the Naval Observatory have been putting together an elaborate experiment. We’ve created a virtual galaxy of 100,000 stars, some of which have planetary systems. The set is designed to mimic (in a statistical sense) the characteristics of the planetary systems known to exist. From the sizes and orbits of these make-believe worlds, we can compute the wobbles in their parent stars that would be observed from Earth.
Our goal is to analyze this set of synthetic observations using the usual tools and techniques. By comparing the interpretations of synthetic wobble data with the planetary configurations that went into this virtual galaxy, we should be able to gauge our underlying biases and blind spots.
Because a certain amount of subjectivity and intuition is often part of the process of planet-finding, we want to involve as many people as possible (both professional astronomers and amateurs). To that end, we have created a user-friendly software tool for analyzing wobble data (either real or virtual) and finding the planets lurking within. It’s a Java applet called the “Systemic Console,” which we have made available to the public at www. oklo.org. The experiment is just in its beginning stages, but we hope that over the next year or two it will improve our ability to interpret the growing catalog of wobble data.
Although we might find some inadequacies in the way extrasolar worlds are now being characterized, we don’t expect to revise the fundamental conclusion of the last decade-that our galaxy alone contains many billions of planets. Could some of these distant worlds resemble own watery blue planet? Astronomers don’t yet know for sure, but literally everything that we’ve learned over the past decade suggests that terrestrial planets are common and easily formed, and that some would presumably be in a position to support living things. What’s more, astronomical observations should one day be sufficiently sensitive to detect the telltale signatures of photosynthesis or methanogenesis in the atmospheres of such distant planets, should these biological processes be taking place on a large scale.
Could there really be other worlds out there teeming with life? It’s thrilling to think that telescopes placed in space (beginning with the Kepler and Corot spacecraft, which will be launched within the next few years) might provide an affirmative answer within our lifetimes. I for one am very much looking forward to that day.