The benefits of comparative planetology discussed in the previous section can sometimes be overdone—hyped beyond its ability to deliver. A few decades ago, the Solar System was judged a simple place: Commonalities among bodies prevailed—and great promise, too, for us to learn more about global warming on Earth by deciphering the chemistry and climate of Venus, or to better forecast weather on Earth by monitoring and modeling Jupiter’s great red spot, or generally to know more about Earth by examining other planets. Alas, the answers aren't so simple.

Although some common features do pervade the many planets and moons of our Solar System, spacecraft exploration of the past few decades has shown, if anything, the great diversity of objects in our celestial neighborhood. Our expectations have lowered somewhat, as the focus has shifted from similarities to differences among alien venues: Mars is locked in a permanently frozen ice age; Venus is a hellhole wrapped in an immobile shell; the Jovian planets and their moons are a panoply of bizarre balls of matter that resemble neither Earth nor Moon. Much as with highly diverse life forms on our planet, we now ponder the staggering variety of material worlds beyond our own. Simple classification and great insight sought using comparative planetology has partly receded, given the rich detail of recent observations of the planets and their moons. What we need now are better statistics for whole families of planets and that means finding more of them—which is exactly what has happened in recent years as astronomers have begun to discover scores of alien worlds beyond our Solar System.

The condensation model holds that the events that produced our home in space are not at all unique. Even considering the role of chance in the mix of changes that occurred long ago, astronomers can still clearly identify an underlying, deterministic sequence of events that led naturally to the birth of our Sun and its family of planets. Furthermore, we have no reason to expect that similar events, in general, would not have happened elsewhere. Many stars are expected to have planetary systems of some sort, and if we could find and map them then growing statistics would bolster the subject of comparative planetology. Even if only 1% of all the stars in the Milky Way Galaxy have planetary systems, that still leaves billions of stars with planets. And each star, of course, would likely have more than a single planet orbiting about it.

Theory is one thing, yet observation quite another. Until the mid-1990s, astronomers had no reliable evidence of planets orbiting other stars. The scientific literature of the last many decades is littered with claims for “extrasolar planets” (called “exoplanets” for short) circling stars beyond our Sun. But none of those early claims could be confirmed and most were eventually retracted. Despite a strong desire and effort to prove that our planetary system is not alone in space, astronomers were unsure until recently about the plurality of planets elsewhere. All that quickly changed as new telescope technology and powerful computers made it feasible to detect the presence of planets around some nearby stars. Early in the 21^st century, we now know of >900 such exoplanets (and another several thousand unconfirmed candidates) orbiting stars beyond our own—that’s at least a hundred times the number of planets in our own Solar System. And although the alien planets discovered to date seem to have properties quite different from those in our own system—few are even remotely Earthlike—they do provide examples of such worldly systems against which to test our ideas about the origin and evolution of planets in general.

No extrasolar planets have yet been unambiguously imaged directly around main-sequence stars. Their presence to date is based only on indirect inference. Astronomers have no clear photographs of them as yet; most reports claiming direct imaging have been challenged and some refuted. Faint and fuzzy planets orbiting other stars are too dim and too close to their bright parent stars for today’s telescopes to resolve them. Instead, the techniques used to find distant planets are based on studies of emitted light from their parent star, not of reflected light from the planets themselves.

Hot Jupiters As a planet orbits a star, gravitationally pulling first one way and then the other, the central star tends to “wobble” slightly. The higher the planet’s mass, or the lower the star’s mass, the greater the induced motion. However, even this wobble cannot be seen directly in the movement of stars across the sky. Instead, the presence of the new planets is inferred, as are their masses, by careful observations with moderate-sized telescopes equipped to monitor the spectral shift in the light of a star while it moves back and forth along our line of sight. This is again the famous Doppler effect, much like that used to track the recession of the galaxies or to catch a speeder on the highway. Here, light emitted by a star in motion has its wavelength shortened or lengthened, owing to motion toward or away from us, thus betraying the presence of unseen objects orbiting about the star.

The first such star system where a planet was found and confirmed in this way, 51 Pegasi, is a near-twin to our Sun yet ~50 light-years away and barely visible with the naked eye just outside the great square of the Pegasus constellation. Figure 4.30 traces the measured Doppler shifts toward this star, and Figure 4.31 is an artist’s conception of what it might look like. Analysis of this star’s radiation implied a planet having ~0.5 of Jupiter’s mass and orbiting with a period of only ~4 days. That’s an extremely short period, meaning, according to Newton’s law of gravity, that this foreign planet must be very near its parent star, in fact well inside the equivalent orbit of Mercury which takes 88 days to orbit the Sun. Thus, the first such exoplanet discovered was odd to say the least and quite unlike anything in our Solar System—a surprisingly massive planet in a highly eccentric orbit and very close (~0.05 A.U.) to its parent star. Even more surprising, this trend has continued in the past few years as more planets have been found around other stars: “hot Jupiters,” as they are called, are massive planets orbiting in close proximity to their central stars. Inward orbital migrations, especially for the giant planets whose large tidal interactions might cause them to spiral toward their parent stars, might be a common dynamical feature in all planetary systems and may have already occurred in our own Solar System.

FIGURE 4.30 — This graph shows the radial velocity (Doppler) shift of the light from the nearby star 51 Pegasi, the inference being a to and fro motion of the star along the line of sight that is presumably caused by an unseen planet—though it would have to be a massive (nearly Jupiter-sized) one in a very tight orbit having a period of only ~4 days and thus quite unlike anything known in our own Solar System.

FIGURE 4.31 – Artist’s conception of the star 51 Pegasus and its very nearby, virtually roasting planet as yet unseen but discovered by indirect means. (Dana Berry)

One of the most interesting extrasolar systems discovered to date is Upsilon Andromedae, whose planets' orbits are sketched in Figure 4.32. Here, a triple-planet system orbits a single star much like our Sun, just 44 light-years away. All 3 of the suspected planets have Jupiter-sized masses, and all 3 are well inside the equivalent orbit of “our” Jupiter. Clearly, this family of planets doesn’t resemble our own Solar System much at all. But which is the “normal” system, them or us? Virtually all the recently found planets seem highly peculiar by our home standards, yet who has the right to claim our planetary system as the standard?

About 10% of all stars surveyed to date show evidence for exoplanets and most of those stars are within a relatively nearby 500 light-years, so the planetary properties of the vast majority of stars remain a mystery. Hardly anything is known about these planets' internal structure or composition.

FIGURE 4.32 – A sketch of the inferred orbits of three planets of the Upsilon Andromedae system (in orange), with the orbits of the four terrestrial planets of our own Solar System superposed for comparison (in white).

These new planetary findings, however strange and unexpected they seem, almost surely suffer from observational bias. The techniques used to detect extrasolar planets—including careful monitoring of distant stars whose brightness slightly diminishes as unseen planets transit across the face of those stars—are most sensitive to massive planets, so it's not too surprising that the earliest results favor the gas-giant planets. Only recently have those observational methods become accurate enough to have found smaller, less massive planets. To date, a few hundred Neptune-sized (~0.1 Jupiter-masses) have been discovered, as well as ~75 "super-Earths" with masses roughly 2-10 times that of Earth; only a handful of "exo-Earths" (comparable to Earth's mass) have been spotted, most of them close to their parent stars, hellishly hot, and unlike anything we might regard as hospitable. Almost surely, smaller planets will be increasingly found as observations improve in the years ahead.

What remains puzzling is that so many of the new Jovian-sized planets are so nearby their parent stars. Not inconceivably, some of them might eventually prove to be brown dwarfs, or “failed stars,” and not genuine planets at all. That is, some could be double-star systems, their current status as planetary systems misidentified. While the dividing line between a planet and a Sun-like star is ~70 Jupiter masses, that between a planet and a dwarf star may be as little as ~12 Jupiter masses—and some of the newly found objects are close to that latter mass value. Even so, the consensus in the astronomical community today is that not all the new objects are likely to be brown dwarfs. At least some of them must be bona fide planets, quite possibly most of them. The unassailable and important take-home message is this: Our Solar System is not alone in space!

Alien Earths Jovian- and Neptunian-sized exoplanets are intriguing, but we naturally (and chauvinistically) wonder: Are other genuine “Earths” out there? How unique is our home planet? Alas, that question remains as difficult as ever to answer. As noted in the previous STELLAR EPOCH, astronomers haven’t yet invented the equipment needed to inventory small, compact, dark bodies residing in even nearby space. To stress an annoying limitation as ironic as it is: We can detect objects as large as stars, for they glow of their own accord, making themselves visible. And we can detect objects as small as atoms, largely by means of spectral radiation they emit and absorb. But we have a hard time detecting anything midway between these two extremes, unless they are very nearby, like asteroids. Faraway objects having sizes between stars and atoms go mostly unseen, and unless they tug gravitationally on some other nearby object or just happen to transit a companion star they’re virtually invisible—which is the case so far, as Earth-sized planets are usually too small for us to detect them.

It does seem likely that small, rocky planets are mostly absent from those several hundred stars where the massive exoplanets have been found to date. In our own Solar System, the presence of Jupiter in a nearly circular orbit well out from the Sun is judged a stabilizing influence. Jupiter helps to dynamically regulate the orbits of Earth and the other Terrestrial Planets; its gravitational tides tend to damp large-scale orbital eccentricities, causing planetary paths to more readily circularize. Jupiter also helps to protect the inner parts of our Solar System from huge rocks wending their way toward the Sun. This big, outer planet literally acts like a vacuum cleaner, using its ample gravity and large crossection to sweep our planetary system relatively clean, and thereby prevent too many impacts from badly whacking the inner, terrestrial planets. By contrast, for the newly discovered extrasolar systems, having inwardly migrating Jupiter-sized planets plowing through their inner parts on elliptical orbits means that any small, Earth-like planets were likely destroyed in place or ejected from these systems long ago.

The upshot is that astronomers are now finding undeniable evidence for planetary systems beyond our own. That’s the good news, for it does bolster the idea that planets form everywhere as natural by-products of star formation. But the new results are also unsettling, since they don’t even remotely resemble our own Solar System. Perhaps it was silly to think, even with the condensation scenario operating in many nooks and crannies of our Galaxy, that all such alien systems would look like ours—though that’s what the theorists thought just a decade ago. Now, armed with actual data, astronomers are rapidly changing preconceived ideas, fine-tuning their models to match the real worlds recently discovered.

Despite the avalanche of incoming data in this fast-breaking field, no one is about to abandon the intricate scenario that explains so well so many of the gross features of our Solar System—not yet anyway. The condensation model remains the most viable explanation for our home planet’s origin. That said, some of the latest computer models suggest that planetary systems perhaps ought to look more like those implied by the new extrasolar data, with big Jupiter-sized planets in tight, eccentric orbits, presumably the result of frequent and dramatic migrations inbound and outbound from their central stars. Maybe our Solar System is the unusual case after all. This new field is data driven; only more observations will tell for sure.

We are left with the notion that, if Earthlike planets in stable orbits do form by condensing out of cooling gas and dust, then space should be teeming with them, just as it bristles with stars and galaxies beyond. But the feeling is an uneasy one, for we don’t yet know for sure. Just how common or rare—or special—are Earthlike planets in the Universe?