Alternatively, free-floating organic molecules may have been generated before or during the formation of our solar system. Organic molecules have been detected spectroscopically in interstellar space for some time. Perhaps the organic seeds of life are ubiquitous in the galaxy—in effect, waiting for the right conditions to settle down.
Though Mars may once have been hospitable to life, it does not appear to be now. However, at about the same time as the announcement of the putative Martian fossil life-forms, NASA released pictures taken by the Galileo spacecraft during a close flyby of Jupiter’s moon Europa. The surface of Europa is clearly frozen, but the markings thereon indicate significant disturbance either from internal energy sources or the gravitational tidal stress induced by Jupiter. What appear to be ice floes and evidence of geyser-like activity suggest that liquid water may well have existed, and may still exist, beneath the moon’s frozen crust. And just as on comet Hale-Bopp, perhaps a slew of organic molecules exists there, too. Given the discoveries of life in unlikely places on Earth, it is even conceivable that self-reproducing life might exist in a hidden Europan ocean. Indeed, the numerous small moons of the outer planets appear to offer more potential niches for the development of life than their planets do.
Of course, as exciting as it would be to find life on Europa or on Saturn’s Titan, say, it’s clear that there is no intelligence outside Earth in our own solar system. If we want to find kindred spirits in the universe, we have to look beyond our Sun. While the preceding chapters suggest that the likelihood of doing so in spacecraft is remote, a number of new discoveries suggest that we may eventually be able to directly detect Class M planets—Star Trek’s term for Earth-like systems—orbiting other stars.
Until a few years ago, while astronomers had long argued that a significant fraction of stars probably possess solar systems, skeptics countered that though there were perhaps as many as 400 billion stars in our galaxy, there were still only 9 known planets. Well, that isn’t the case anymore. We have discovered a handful of planets orbiting Sun-like stars, the closest of them tens of light-years away. The evidence suggests that planetary formation is a rather common occurrence and not at all the rarity it once appeared to be. The first claimed observation of a planet orbiting a Sun-like star other than our own was made in 1995 by Michel Mayer and his group at the Geneva Observatory. Most of the new data, however, and certainly the most convincing results obtained thus far, have been amassed by a group centered at the University of San Francisco, under the direction of Geoff Marcy, which had been carefully tooling up for this task over the past decade.
The search for planets outside our solar system has been undertaken by making use of the following idea: Although we customarily think of the Copernican Revolution as the discovery that our planet orbits the Sun rather than vice versa, this is not strictly correct. Gravity is a two-way street. The same Newtonian law that tells us that hovering giant flying saucers will crush us implies that as planets orbit the Sun, the Sun moves in response. While we tend to idealize planetary orbits by imagining them around a fixed Sun, in fact both planet and Sun orbit a point located between them, called the center of mass of the system. Because the planets are much lighter than the Sun, this point is located close to the center of the Sun, so that the Sun actually orbits a point just slightly outside its surface.
Thus (as the Catholic Church maintained steadfastly for the nearly 400 years it refused to reassess its condemnation of Galileo) the Sun orbits in the solar system! But not much. In fact, we can estimate how much by recognizing that since Jupiter is by far the most massive planet, its gravitational pull dominates the calculations. Since Jupiter orbits the Sun once every 11.86 years, this means that the Sun orbits the center of mass of the Sun/Jupiter system—situated just outside the Sun’s surface, at a distance of about 800,000 kilometers from the Sun’s center—once every 11.86 years. If you then work out the velocity of the Sun in this orbit, you find that it is moving about 10 meters per second, or at about the same speed reached by Olympic sprinters. For a human being, this is pretty fast; for an astrophysical object like the Sun, it is almost unimaginably slow.
A sensible person—say, a Star Trek writer—might dismiss the possibility of measuring motions this small in distant stars; however, one of the most fascinating things about modern experimental science, at least to me, is that precisions which once seemed fantastical are now routinely achieved. The key is not unique to planetary searches: it is the workhorse of modern astronomy—the Doppler effect. (For those whose only association with this effect is high school physics, it may lack poetry, but poetry is in the ear of the hearer. I have a cartoon in my office by the great science cartoonist Sid Harris; it shows two cowboys on the plain at sunset, looking at a distant train. One cowboy says to the other, “I love to listen to the lonesome wail of a train whistle as the magnitude of the frequency of the wave shifts due to the Doppler effect.”) The well-known fact that sirens are pitched higher as they approach than they are after they’ve passed has been used by astronomers for most of the past century to learn about the universe. The siren sounds higher because the sound waves coming at you are of shorter wavelength, which produces a higher pitch. The same phenomenon obtains for light; when light is emitted by an object moving toward you, the waves you receive are compressed, making the light look bluish. If it’s moving away from you, the light shifts to the red. The American astronomer Edwin Hubble became famous in the late 1920s for his demonstration that light frequencies emitted by distant galaxies showed that these galaxies, on average, were moving away from us, and that their velocity was proportional to their distance. In this way, we discovered that the universe was expanding.
In a similar way, by observing the frequency shift in the light from one side of a galaxy and comparing it with that on the other, astronomers can infer the galactic rotation rate. In the 1970s, Vera Rubin and her colleagues were able to show that this rotation was anomalous—that is, the galactic motions appeared to be due to the gravitational pull of a great deal more matter than was visible in the galaxies themselves. Thus was “dark matter” discovered. It turns out that over 90 percent of the mass in the observable universe is in the form of stuff that doesn’t shine, and its nature is one of the outstanding puzzles of modern astrophysics and cosmology.
Clearly, the simple Doppler effect can be pretty powerful, and in 1995 and 1996, Mayer, Marcy, and their colleagues were finally able to use it to measure the wobbles of nearby stars and thus discover a new kind of invisible matter: Jupiter-size planets. In such investigations, one has to make very precise measurements not just of a star’s wobbling velocity but of the period of the variations in its velocity in order to determine the characteristics of the orbiting planet. With these two measurements, the mass of the planet can be unambiguously determined.
Indeed, what is most remarkable is that some of these newly discovered giant planets, up to nearly 5 times the mass of Jupiter, seem to be in orbits closer to their host stars than Mercury is to the Sun. One of them—the first to be found—has an orbital period, or “year,” of only about 4 days! Not long before these observations were made, theoretical predictions had suggested that giant planets could not form in orbits that close to their Sun because of tidal stresses. The new observations suggest that planet formation may be not only easier than previously thought but also much more varied. Perhaps our solar system is not particularly typical. With a new set of possibilities for planetary formation, new possibilities emerge for the origin of life.
It is important to stress that the planetary systems observed to date appear unable to support Earth-like, advanced life-forms. The conditions of extreme heat and very high surface gravity are unlikely to allow the evolution of such life. One of the newly discovered planets, however, is far enough from its host star so that liquid water might exist on or near its surface. As we have learned from recent discoveries on Earth, this and a little heat may be all it takes to support primitive life.
I wan
t to emphasize how astonishing the discovery of these Jupiter-like planets really is. To infer the existence of these objects, stellar motion on the order of tens of meters per second must be observed by means of Doppler shifts. Such motions produce frequency shifts of the observed light of less than 1 part per million. Not only do such small frequency shifts have to be resolved, but they have to be carefully monitored over days, weeks, and months to demonstrate convincingly that their regularity is indicative of an orbiting planet and not, say, the ordered pulsations of the stellar surface. Because of their perseverance and technical mastery, a small group of dedicated observers has brought us one step closer to the stars.
However, as agents Scully and Mulder would probably tell you, sifting through indirect hints of alien intelligence is interesting but only enough to begin to get the blood going. Whereas coming face-to-face, or at least body part to body part, with an alien—now, that’s what it’s all about! No matter how many exotic metallic objects the X-Files team extracted from the nasal passages of alien abductees, it would probably take the discovery of a bona fide alien body—one that didn’t keep inconveniently disappearing—to persuade their superiors (or at least the ones who aren’t part of an evil government conspiracy) to pay attention. Sometimes only seeing is believing, even on The X-Files.
Similarly, as exciting as the discovery of extra-solar-system planets is, it’s worth emphasizing that we still have not yet seen one directly. Moreover, the velocity kick given to a Sun-like star by an Earth-like planet at an Earth-like distance is only about 10 centimeters per second, and even indirect detection of such an object is no small task. To resolve such velocities would require a frequency resolution and stability of better than 1 part in a billion a result unlikely to be obtained in the foreseeable future. Even if it were, so many other sources of astronomical “noise” might be picked up at this level that the signal would be hopelessly buried.
A technique that might allow us to infer the existence of smaller planets at Earth-like distances from their stars involves measuring not the velocity of motion of the star in response to the planet’s orbit but the change in the star’s position on the sky. This technique was developed more than 100 years ago by the first American Nobel laureate in physics, Albert A. Michelson, of the Physics Department at my home institution, Case Western Reserve University (then Case Institute of Technology). It is called optical interferometry. A distant light source is simultaneously observed by two neighboring telescopes, so that the troughs and peaks of the light’s wavelength can be compared. Since the wavelength of visible light is so small, even a small change in the position of the star on the sky will produce a measurable change in how these peaks and troughs line up at the two telescopes. This allows one to obtain a high resolution of the star’s motion on the sky. A new binocular device atop Mt. Palomar has a resolution on the sky in principle of about a 100-millionth of a degree. This is of an order I would have labeled science fiction just a little while ago—it’s like resolving from a vantage point on Earth whether I’m holding up one finger or two while I’m standing on the Moon!
You might suspect that if we can achieve this level of resolution, we should be able to directly “see” the planets orbiting nearby stars. From there, we are just one step away from taking out our tricorders and scanning for life-forms, as Dr. McCoy or Dr. Crusher might do. Well, there’s still a problem to be overcome. While in principle one can easily resolve the distance between a planet and a star if the planet is the same distance away from its star as the Earth is from the Sun, and if the system under observation is within, say, 100 light-years from us, the problem is that stars are very bright, while their planets, which merely reflect the light, are much darker. On top of this, there is a competing problem. As the light from cosmic objects passes through our atmosphere, it bends to and fro because of variations in air density, motion, and so on; as a result, the signal from a point source is spread over a disk-like region. This “seeing disk” for a typical terrestrial observatory is such that the light from a nearby star would easily envelop the space containing its planets.
One of the few examples of something useful produced by work on the Strategic Defense Initiative is a technique known as adaptive optics, which has allowed astronomers to circumvent this last problem, in principle. Thankfully, now that SDI is defunct, this once-classified research is being put to good use. The idea is simple: If one has a reference object whose original light profile is known or can at least be closely approximated, then by observing this object through the atmosphere and seeing how its light is spread out, one can subtract the effects of the atmosphere at any given instant. If there is another object close on the sky to the reference object, one can use this subtraction technique to resolve the second object with a greater degree of accuracy. But what if there is no reference star close to the one you want to observe? Well, at Lawrence Livermore National Laboratory, one of the original homes of SDI research, a group has come up with a novel solution. If you don’t have a star nearby, then why not just make one?
This sounds even more ambitious than something Geordi LaForge or Data would suggest to Captain Picard—or something only a research group made giddy by a surfeit of Defense Department dollars would undertake. But from an operational standpoint, a star is simply a point of light in the sky—something a whole lot easier to create than an actual star. Lawrence Livermore scientists have done it using a powerful laser based on light emitted by sodium atoms. The laser beam is powerful enough to make it up through the atmosphere as a thin column of light. About 30 kilometers above the Earth’s surface, sodium atoms in the rarefied upper atmosphere absorb this laser light and reradiate it. Voila!—a glowing point of light in the sky! It is amazing to see photographs of these artificial stars high above the lights of Livermore, California, at night. One can see the narrow, powerful laser beam rising into the sky; then its light fades as the atmosphere off which it reflects becomes thinner; then, high above the ground, in the region where sodium is present to absorb and reradiate the light, is a single yellowish-red “star.”
Since one knows very well what the initial profile of the laser beam is and exactly where it is pointed, one can use the observed characteristics of these “guide stars” to subtract the effects of the atmosphere with great precision. And since one can shine the laser in any direction, one can place the guide star as close as one wants to the star one wants to observe. It is thus possible to model the scattered light from the real star, allowing one to probe for faint objects in its vicinity. More important, one will localize the faint light from any orbiting planet (which is also spread out by atmospheric effects) amid the smooth background of noise—the scattered light from the nearby star. As difficult as this sounds, some astronomers believe that within a decade—if the Keck 10-meter telescopes in Hawaii, the largest telescopes in the world, are fitted with a laser guide-star apparatus—it will be possible to directly observe the dim light of Jupiter-like planets. One of my colleagues at Case Western Reserve University, Glenn Starkman, has added a new wrinkle to this scheme. He proposes sending up a satellite that will release a large balloon, which can then be maneuvered to occult the ambient starlight and thus aid in the planetary search.
Once the prospect of directly observing planets becomes possible, the idea of scanning for life does not seem all that far-fetched. Of course, one would not look for life-forms directly; however, by observing the color of light reflected by a planet, one can learn a great deal about its atmosphere and the characteristics of its surface. NASA has proposed direct observation of extra-solar system planets as one of the agency’s goals for the next century. The next generation of telescopes in space will build on the incredible success of the Hubble Space Telescope, surpassing any observations we can now make from Earth, and I am prepared to believe that within the next century we may well directly detect the existence of an organic, water-filled world elsewhere in the cosmos.
CHAPTER SEVEN
GAMBLING ON THE GALAXY
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He is glorified not in one, but in countless suns; not in a single earth, a single world, but in a thousand thousand, I say in an infinity of worlds.
—Giordano Bruno
Thankfully, only 400 years after Bruno was burned at the stake for this claim, screenwriters are relatively free to let their imaginations run wild. I have always been impressed with the ingenuity of Hollywood science fiction writers when it comes to the creation of alien beings. Yet if there is one place where literary and back-lot imagination probably fall short of the mark, it is in conjuring up the possible variety and quantity of life in the universe. Even when you put together the silicon-based Horta, the insect-like Harada, and the cyber-based Borg; the Wooky, the Sand Worms, Yoda, and Jabba the Hut; little ET, and the slimy beasts of Independence Day and the Alien series; and all the creatures in Men in Black, you barely scratch the surface of what may be possible.
Consider the following. In DNA-based self-replicating lifeforms, there are 4 different genetic “letters,” and approximately 1,000 of these letters, in various combinations, make up a gene; you therefore end up with approximately 10600 possible variants. Even if nature somehow produced a new gene combination once every second in each cell on Earth throughout Earth’s history, the total number of combinations thus produced would have been only about 1047.
Now, many of the individual letters in a gene may be irrelevant, but even so, if 99.999999999999999999999999999999999999999999999 percent of all the possible gene combinations lead to junk genes, the total number of different life-forms which could have appeared on Earth this way would still be smaller in relation to the number of viable possibilities than one atom is compared to the total number of atoms in the universe!
And that’s just DNA. We have no idea whether other self-replicating organic, or inorganic, combinations might also be able to exist—in which case, the above estimate of the possible varieties of life in the universe could be too small by many orders of magnitude. Not only are the possibilities virtually endless, but a host of exciting discoveries in recent years have caused us to readjust upward our estimates of how likely life might be to evolve elsewhere in our galaxy. If there is to be a Year of the Alien, this past year is one of the best candidates so far. Every indirect indication we have suggests, now more strongly than ever, that life is ubiquitous. We once had no notion at all of how the building blocks of life might have formed on Earth; now we have a variety of compelling competing theories. Moreover, as I noted in the last chapter, life has been discovered in all the wrong places. Nothing is more exciting for a scientist than when things turn out as we didn’t expect, amid a wealth of new data.
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