So Many Suns So Many Worlds

What a wonderful and amazing scheme have we here of the magnificent vastness of the Universe! So many Suns, so many Earths . . . !


New Conjectures Concerning the Planetary Worlds, Their Inhabitants and Productions (ca. 1670)

In December 1995, an entry probe, detached from the Galileo Jupiter orbiter, entered the turbulent, roiling atmosphere of Jupiter and sank to a fiery death. Along the way it radioed back information on what it found. Four previous spacecraft had examined Jupiter as they raced by. The planet has also been studied by ground-based and space telescopes. Unlike the Earth, which is made mainly of rock and metal, Jupiter is made mostly of hydrogen and helium. It is so big that a thousand Earths could fit inside. At depth, its atmospheric pressure gets so large that electrons are squeezed off atoms and the hydrogen becomes a hot metal. This state of affairs is thought to be the reason that twice as much energy comes pouring out of Jupiter than Jupiter gets from the Sun. The winds that buffeted the Galileo probe at its deepest entry point probably arise not from sunlight but from the energy originating in the deep interior. At the very core of Jupiter there seems to be a rocky and iron world many times the mass of the Earth, surmounted by the immense ocean of hydrogen and helium. Visiting the metallic hydrogen—much less the rocky core—is beyond human abilities for at least centuries or millennia to come.

The pressures are so great in the interior of Jupiter that it is hard to imagine life there—even life very different from our own. A few scientists, myself among them, have tried, just for fun, to imagine an ecology that might evolve in the atmosphere of a Jupiter-like planet, somewhat like the microbes and fish in the Earth's oceans. The origin of life might be difficult in such an environment, but we now know that asteroidal and cometary impacts transfer surface material from world to world, and it is even possible that impacts in the early history of the Earth transferred primitive life from our planet to Jupiter. This, though, is the merest speculation.

Jupiter is about 5 astronomical units from the Sun. An astronomical unit (abbreviated AU) is the distance of the Earth from the Sun, about 93 million miles, or 150 million kilometers. If not for the interior heat and the greenhouse effect in Jupiter's immense atmosphere, the temperatures there would be about 160 degrees below zero Celsius. That's roughly the temperature on the surface of Jupiter's moons—much too cold for life.

Jupiter and most of the other planets in our Solar System orbit the Sun in the same plane, as if they were confined to separate grooves on a phonograph record or a compact disc. Why should this be? Why shouldn't the orbital planes be tilted at all angles? Isaac Newton, the mathematical genius who first understood how gravity makes the planets move, was puzzled by the absence of much tilt in the orbital planes of the planets, and deduced that, at the beginning of the Solar System, God must have started the planets out all orbiting in the same plane. But the mathematician Pierre Simon, the Marquis de Laplace, and later the celebrated philosopher Immanuel Kant, discovered how it could have happened without recourse to divine intervention. Ironically, they relied on the very laws of physics that Newton had discovered. A brief rendition of the Kant-Laplace hypothesis goes as follows: Imagine an irregular, slowly rotating cloud of gas and dust sitting between the stars. There are many such clouds. If its density is sufficiently high, the gravitational attraction of the various parts of the cloud for each other will overwhelm the internal random motion, and the cloud will start contracting. As it does so, it will spin faster, like a pirouetting ice skater bringing in her arms. The spin won't retard the collapse of the cloud along the axis of rotation, but it will slow the contraction down in the plane of rotation. The initially irregular cloud converts itself into a flat disk. So planets that accrete or condense out of this disk will all be orbiting pretty much in the same plane. The laws of physics suffice, without supernatural intervention.

But predicting that such a disklike cloud existed before the planets formed is one thing; confirming the prediction by actually seeing such disks around other stars is quite another. When other spiral galaxies like the Milky Way were discovered, Kant thought that these were the predicted preplanetary disks, and that the "nebular hypothesis" of the origin of planets had been confirmed. (Nebula comes from the Greek word for cloud.) But these spiral forms proved to be distant star-studded galaxies and not nearby birthing grounds of stars and planets. Circumstellar disks proved hard to find.

It was not until more than a century later, using equipment including orbiting observatories, that the nebular hypothesis was confirmed. When we look at young Sun-like stars, like our Sun of four or five billion years ago, we find that more than half of them are surrounded by flat disks of dust and gas. In many cases the parts close to the star seem to be empty of dust and gas, as if planets had already formed there, gobbling up the interplanetary matter. It is not definitive evidence, but it strongly suggests that stars like our own frequently, if not invariably, are accompanied by planets. Such discoveries expand the likely number of planets in the Milky Way Galaxy at least into the billions.

But what about actually detecting other planets? Granted, the stars are very far away—the nearest almost a million AU distant—and in visible light they shine only in reflection. But our technology is improving by leaps and bounds. Shouldn't we be able to detect at least large cousins of Jupiter around nearby stars, perhaps in infrared if not visible light?

In the last few years we have entered into a new era in human history, where we are able to detect the planets of other stars. The first planetary system reliably discovered accompanies a most unlikely star: B 1257 + 12 is a rapidly rotating neutron star, the remnant of a star once more massive than the Sun that blew itself up in a colossal supernova explosion. The magnetic field of this neutron star captures electrons and constrains them to move in such paths that, like a lighthouse, they shine a beam of radio light across interstellar space. By chance, the beam intercepts the Earth—once every 0.0062185319388187 seconds. This is why B 1257 + 12 is called a pulsar. The constancy of its period of rotation is astonishing. Because of the high precision of the measurements, Alex Wolszczan, now at Penn State University, was able to find "glitches"—irregularities in the last few decimal places. What causes them? Starquakes or other phenomena on the neutron star itself? Over the years, they have varied in precisely the way expected were there planets going around B 1257 + 12, tugging slightly, first this way and then that. The quantitative agreement is so exact that the conclusion is compelling: Wolszczan has discovered the first planets known beyond the Sun. What's more, they're not big Jupiter-sized planets. Two of them are probably only a little more massive than the Earth, and orbit their star at distances not too different from the Earth's distance from the Sun, 1 AU. Might we expect life on these planets? Unfortunately, there is a gale of charged particles hurtling out of the neutron star, which will raise the temperature of its Earth-like planets far above the boiling point of water. At 1,300 light-years away we will not be traveling to this system soon. It is a current mystery whether these planets survived the supernova explosion that made the pulsar, or were formed from the debris of the supernova explosion.

Shortly after Wolszczan's epochal discovery, several more objects of planetary mass were discovered (mainly by Geoff Marcy and Paul Butler of San Francisco State University) going around other stars—in this case, ordinary Sun-like stars. The technique used was different and much more difficult to apply. These planets were found by conventional optical telescopes monitoring the periodic changes in the spectra of nearby stars. Sometimes a star may be moving for a while toward us and then away from us, as determined by the changes in wavelength of its spectral lines, the Doppler Effect—akin to the changes in frequency of a car's horn as it drives toward or away from us. Some invisible body is tugging at the star. Again, an unseen world is discovered by a quantitative agreement—between the observed slight periodic motions of the star and what you would expect if the star had a nearby planet.

The planets responsible go around the stars 51 Pegasi, 70 Vir-ginis, and 47 Ursae Majoris, respectively in the constellations Pegasus, Virgo, and Ursa Major, the Big Dipper. In 1996, such planets were also found orbiting the star 55 Cancri in the constellation Cancer, the Crab: Tau Bootis and Upsilon Androme-dae. Both 47 Ursae Majoris and 70 Virginis can be seen with the naked eye in the spring evening sky. They are very near as stars go. The masses of these planets seem to range from a little less than Jupiter to several times more than Jupiter. What is most surprising about them is how close to their stars they are, from 0.05 AU for 51 Pegasi, to a little more than 2 AU for Ursae Majoris. These systems may also contain smaller Earth-like planets, not yet discovered, but their layout is not like ours. In our Solar System, we have the small Earth-like planets on the inside and the large Jupiter-like planets on the outside. For these four stars, the Jupiter-mass planets seem to be on the inside. How that could be, no one now understands. We do not even know that these are truly Jupiter-like planets, with immense atmospheres of hydrogen and helium, metallic hydrogen down deep and an Earth-like core still deeper. But we do know that the atmospheres of Jupiter-like planets at such close distances to their stars will not evaporate away. It seems implausible that they formed in the periphery of their solar systems and somehow wandered much closer to their stars. But maybe some early massive planets could have been slowed by the nebular gas and spi-raled in. Most experts hold that a Jupiter could not be formed so close to the star.

Why not? Our standard understanding of the origin of Jupiter is something like this: In the outer parts of the nebular disk, where the temperatures were very low, worldlets of ice and rock condensed out, something like the comets and icy moons in the outer parts of our Solar System. These frigid worldlets collided at low speeds, stuck together, and gradually became large enough to gravitationally attract the prevalent hydrogen and helium gases from the nebula, forming a Jupiter from the inside out. In contrast, nearer to the star, it is thought, the nebular temperatures were too high for ice to condense in the first place, and the whole process is short-circuited. But I wonder if some nebular disks were below the freezing point of water even very close to the local star.

In any case, with Earth-mass planets around a pulsar and four new Jupiter-mass planets about Sun-like stars, it follows that our kind of solar system may hardly be typical. This is key if we have any hopes of constructing a general theory of the origin of planetary systems: It now must encompass a diversity of planetary systems.

Still more recently, a technique called astrometry has been used to detect two and possibly three Earthlike planets around a star very near to our Sun, Lalande 21185. Here the precise motion of the star is monitored over many years, and the recoil due to any planets in orbit about it is carefully watched. Departures from circular or elliptical orbits by Lalande 21185 permit us to detect the presence of planets. So here we have a familiar, or at least a somewhat familiar, planetary system to our own. There seem to be at least two and maybe more categories of planetary systems in adjacent interstellar space. As for life on these Jupiter-like worlds, it is no more likely than on our own Jupiter. But what is probable is that these other Jupiters have moons, like the 16 that circle our Jupiter. Because these moons, like the giant worlds they orbit, are close to the local star, their temperatures, especially for 70 Virginis, might be clement for life. At 35 to 40 light-years away, these worlds are close enough for us to begin to dream of one day sending very fast spacecraft to visit them, the data to be received by our descendants. Meanwhile, a range of other techniques are coming along. Besides pulsar timing glitches and Doppler measurements of the radial velocities of stars, interferometers on the ground or, better, in space; ground-based telescopes that cancel out the turbulence of the Earth's atmosphere; ground-based observations using the gravitational lens effect of distant massive objects; and very accurate space-borne measurements of the dimming of a star when one of its planets passes in front of it aU"seem ready in the next few years to yield significant results. We are now on the verge of trolling through thousands of nearby stars, searching for their companions. To me it seems likely that in the coming decades we will have information on at least hundreds of other planetary systems close to us in the vast Milky Way Galaxy— and perhaps even a few small blue worlds graced with water oceans, oxygen atmospheres, and the telltale signs of wondrous life.


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