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PLANETARY ENVIRONMENTS
Sir, it's quite possible this asteroid is not entirely stable.—C-3PO, The Empire Strikes Back
It comes into view as a small, pale dot against the blackness of space. Dim, inconsequential beside the brilliance of a star. Yet for us, it is a safe haven in the endless vacuum of space. Only here, on this fragile bit of rock or others like it, can life develop and survive. It formed billions of years ago, the right elements combining in the right proportions at the right distance from its sun to bring it to dynamic life. Volcanoes breathed out an atmosphere. Life-giving rains fell, the bit of rock evolved.
As it grows closer, the dot gains color and definition. Major features are revealed: rock, water, ice, clouds. Within the atmosphere, that protective, nurturing envelope, more details become apparent. Only on the surface, though, does the unique character of the planet become clear: the shapes and colors of the topography, the peculiar quality of the star's light scattered through the atmosphere, the composition and scents of the air, the strength of the gravity, the texture of the ground beneath our feet, the bizarre life forms that are another expression of the growth and development of the planet.
We have visited many such balls of life-giving elements. Each landscape is committed to memory. A flat plain of sand broken only by harsh, jagged rocks. A vast, snow-covered waste. A fog-shrouded swamp chattering with life. An ancient forest stretching high into the sky. A planet-sized city of level upon level. Some seem mysterious; others feel almost like home. We've seen planets and moons; we've even traveled through an asteroid field. Each has unique characteristics. Anakin's and Luke's home world, Tatooine, is part of a binary star system. Naboo has a bizarre internal structure. The Ewok moon circles the gas giant Endor.
In Star Wars, we're swept up in events that take us to a wide array of strange and intriguing planets. They present an exciting picture of the universe as we'd like it to be: filled with exotic yet welcoming worlds. These planets are generally friendly to human life—which is why the human characters have traveled to them. In addition, though, they have indigenous life of their own, in a variety that keeps us surprised and delighted. But how realistic is this view of the universe, based on what we know today? Are Earth-type planets like those we see in Star Wars likely to exist? And will so many of them be home to alien life?
YOU CAN'T HAVE AN EMPIRE WITHOUT REAL ESTATE
To have a universe like that in Star Wars, the first thing we need is planets, and lots of them. If our solar system is a fluke, and we happen to orbit the only sun in the universe that has planets, then we'll never be able to pop across the galaxy for some Jedi training, set up a hidden base in another solar system, or get into bar fights with intelligent alien life.
How numerous are planets in our universe? Let's first look at how planets form, and what ingredients are necessary in their formation. To form rocky planets like Earth, we need heavy elements like iron, carbon, nitrogen, and oxygen. Unfortunately, they are rare. The two lightest elements, hydrogen and helium, currently comprise 99.8 percent of the atoms in the universe. Hydrogen and helium are great for making stars, but not for creating Earthlike planets or complex life-forms. The heavier elements did not even exist at the beginning of the universe, so stars formed in those early days could not have Earthlike planets orbiting them. Since then, however, stars have been steadily producing heavier elements through the nuclear fusion reactions that power their brilliant light.
In fusion, energy is produced when lighter elements are combined to make heavier ones. When a star exhausts its fuel and dies, it releases these heavy elements into space by exploding or by ejecting its outer layers. A supernova explosion, through its incredible energy, creates even more heavy elements.
If the star lives in a massive enough galaxy, like our Milky Way, then these new heavy elements are held within the galaxy by gravity. They combine with other debris into a cloud of gas and dust, and may eventually form into new stars and planets. These new, younger stars can potentially have Earthlike planets, since the heavy elements necessary have been thoughtfully provided by the older generation.
Considering that Star Wars is set "a long time ago," is it too long ago to allow for Earthlike planets? While the universe formed about fifteen billion years ago, it wasn't until ten billion years ago that enough heavy elements had been created to form a planet like Earth. Dr. Bruce Jakosky, professor of geology at the Lab for Atmospheric and Space Physics at the University of Colorado at Boulder, concludes that " ‘A long time ago' is fine if we're talking a few billion years, but a dozen billion years—that's too long ago." So we've narrowed things down . . . a bit.
Once we have the heavy elements required as raw materials, how do the planets actually form? According to current theory, this debris forms a rotating cloud. Just as a ball of pizza dough, when you toss and spin it, will flatten into a thin crust, so the rotating cloud will collapse into a thin, spinning disk of material. This disk is made up of gas, dust, and frozen chemicals. The dense, inner section of the disk coalesces first into a star. At this point the disk looks like a rotating Frisbee with a hole in the center, the star in the middle of the hole. Dr. Jakosky notes that these disks that form the birthplace of planets seem fairly common. "Between one-quarter and one-half of all stars, when they form, seem to leave behind these disks."
The solid particles in the disk stick together to form large grains of dust. These grains collide with each other and form larger grains, eventually growing into small bodies called planetesimals. A planetesimal may be only a few inches across, or it may be the size of the Moon. Some planetesimals remain small, becoming asteroids or comets. Others, though, as they rotate around the sun, continue to collide and merge with each other, in a sense sweeping up all the material at the same orbital distance from the sun. As a planetesimal collects all the material in a band around the sun, it becomes a planet. The closer the band is to the star, the smaller the band's circumference is, and so the less material there is to create a planet. That's why, so the theory goes, smaller planets tend to form closer to stars and larger planets farther away.
In addition to affecting planet size, the distance from the star also affects planetary composition. Closer to the star, the disk is very hot, and only materials with high melting temperatures, like iron and rock, are solid. Thus those elements make up the majority of the planetesimals, and the planets. In our own solar system, the four planets closest to the sun—Mercury, Venus, Earth, and Mars—are made up mainly of dense rock and iron. Farther from the sun, where the temperature is lower, additional materials solidify, such as water, methane, and ammonia, and become part of the core of the outer planets. These larger planets have stronger gravitational fields, and can attract huge amounts of light gases, such as hydrogen, to surround their cores as massive atmospheres. This process creates distant gas giants like Jupiter and Saturn. Jupiter, for example, has a core ten times the mass of Earth, which is impressive, but including its thick hydrogen-helium atmosphere, Jupiter's mass totals 318 times Earth's. Each planet, then, is a product of the unique conditions of its formation.
If this theory is true, then planetary formation is a natural part of stellar formation, and there should be a lot of planets out there. Our current theory certainly does a fairly good job of explaining the features we observe in our own solar system. But until recently, we've had no other solar systems to test it against.
In the last eight years, however, a string of discoveries has thrown the theory of planetary formation into doubt. Planets seem more common than ever, which supports our theory. Yet the planets we've been discovering around other stars are quite different than those our local system led us to expect. Dr. Jakosky explains, "A lot of the planets we're finding are oddballs." In an attempt to explain the presence of these oddballs, many new theories are being suggested. While most still start with a disk of material orbiting a forming star, many suggest ways in which solar systems much different than our own might result. Why? Because what we're learning is that the universe is a much stranger and more varied place than we imagined.
A PLANET A DAY KEEPS THE EMPIRE AWAY
While science fiction has long posited the existence of other planets, up until recently, we could only guess whether there might be planets orbiting other stars in the universe. False reports of the discovery of planets outside our solar system, called extra-solar planets, have arisen since the 1940s, but only recently have we obtained convincing evidence that such planets do indeed exist.
Planets are very difficult to detect because they're much smaller than stars and they shine only by catching and reflecting a small portion of their star's light. Our sun, for example, is one billion times brighter than the planets that orbit it. If we look at a star through a telescope, the light from the star completely overwhelms that from any planets. As an example of how hard it is to find planets, consider that it took us until 1930 to find Pluto, a planet in our very own solar system. The nearest star, Proxima Centauri, is ten thousand times farther away from us than Pluto. These great distances make seeing planets through telescopes nearly impossible.
Instead of trying to see and photograph extra-solar planets, astronomers instead look for indirect signs of their presence. A wobble in the normally straight path of a star could reveal a star being tugged gravitationally back and forth as a planet orbits it.
We usually think of a planet circling about a stationary star. But the truth is both the planet and the star move, orbiting around their center of gravity. Imagine two children of approximately equal weight—say the twins Luke and Leia at age seven. They face each other, hold each other's hands, and begin to spin around. Since they are of equal mass, their center of gravity will be the point exactly halfway between them, and they will each circle around that point. Their footsteps will trace out a common circle with a common diameter. Now imagine daddy Vader arrives on the scene. He breaks up the circle, turns Luke around to face him, takes Luke's hands in his, and they begin to spin around. Since Vader is much more massive than Luke, the center of gravity will be much closer to Vader. While Vader will not exactly pivot on a single point, he will move off that point by only a small amount, his footsteps tracing out a circle of tiny diameter, while Luke is whipped around in a wide circle.
Just as Vader is not entirely stationary, a star is not completely stationary as a planet orbits it. The planet's gravity affects the star the same way the star's gravity affects the planet. Thus the star will move in a small, cyclical orbit. Our sun's small orbit is generated mainly by Jupiter, its most massive planet. Since Jupiter is one-thousandth the mass of the sun, the sun's orbit is one-thousandth the size of Jupiter's orbit. The sun revolves around a center of gravity just beyond its surface.
Such movements of stars are quite small, so they are very difficult to detect. Yet observing stars has one important advantage over observing planets: stars radiate light that allows us to see them easily. That's why astronomers are searching for planets by looking at stars.
Astronomers have focused on two main techniques for detecting these cyclical movements in a star's course. One is to visually look for tiny wobbles back and forth and measure the extent of these wobbles. This is very difficult, since the wobbles are very small. Let's say the star is the size of our sun and is ten light-years away, and the planet orbiting it is the size of Jupiter. How small would the star's wobble be? Imagine Princess Leia standing two miles away across the flat desert of Tatooine. She plucks a hair out of one of her buns and holds it up. The width of her hair, as it appears from two miles away, is the size of the wobble we're looking for. Not surprisingly, a number of scientists have reported discoveries of planets only to later learn the tiny wobbles they detected were simply observational errors.
A more successful technique has been to search for a cyclical Doppler shift in the light coming from a star. Instead of looking for a wobble back and forth across our field of vision, scientists study the light from a star to see if it is moving toward us and away from us in a cyclical manner. This type of movement causes a shift in the frequency of light coming from the star. Most of us have experienced Doppler shifts—not in light waves, but in sound waves. Imagine a train coming toward you and blowing its whistle in a long, sustained blast. Sound waves will propagate out from the whistle in all directions. Those waves coming toward you, traveling in the same direction as the train, are crunched together by the movement of the train and its whistle. This crunching-up process increases the frequency of the sound waves, making the tone of the whistle sound higher. The train now passes you and starts moving away, still blowing its whistle. The sound waves coming toward you are now traveling in the direction opposite the train, so the sound waves are in essence stretched out. The tone will now sound lower, its frequency decreased.
The same thing happens with light waves emitted by a star. If the star is moving toward us, the frequency of the light increases; if the star is moving away from us, the frequency of the light decreases. Again, these shifts are very small, only one part in ten million if the star has a Jupiter-type planet, and detecting them requires high precision. Yet scientists have reached greater levels of accuracy in measuring Doppler shifts than in measuring visual wobbles. Astronomers can actually measure the velocity of a star toward or away from us down to an accuracy of seven miles per hour. I'm not sure that state troopers are so accurate.
This level of precision means the Doppler technique allows us to find Jupiter-sized gas-giant planets, but not Earth-sized planets, which would cause an even smaller shift. This technique is also much better at finding stars that are moving toward us and away from us at high velocities, when the Doppler shift is greatest. These high velocities are most likely to occur when planets are close to a star. Planets in close orbit revolve around the star faster than planets farther away, forcing the star to also revolve faster. Both the wobble technique and the Doppler technique are most successful at finding large planets around relatively small stars, meaning systems more like Leia and Luke than Vader and Luke, since those solar systems will have the greatest amount of stellar movement.
The first extra-solar planet orbiting a sunlike star was discovered in 1995. Two Swiss astronomers using the Doppler technique found that the star 51 Pegasi moves forward and back every 4.2 days. This means that a planet revolves around the star every 4.2 days: that is the length of a year for anything living on the planet. Since we know that the closer a planet is to a star, the faster it orbits, we know that this planet is very close to 51 Pegasi. In our solar system, the planet closest to the sun, with the shortest orbital period or year, is Mercury. Yet Mercury's year is a leisurely 88 days. The newly discovered planet orbits at only one-eighth the distance from Mercury to the sun. This close, the star would heat the planet to a blistering 1,900 degrees. Not very friendly for life. From our theory of planetary formation, we would expect a planet so close to its star to be small and rocky. Yet the 51 Pegasi planet is a huge gas giant, half the size of Jupiter.
We've now confirmed the discoveries of about fifteen planets around other stars. Most of these are more massive than Jupiter, and most orbit their stars more closely than Mercury. Remember, one reason we've found these "oddball" planets is that they're the easiest to detect. Yet their existence calls into question whether our own solar system is the exception or the rule, and how planets really form. Scientists are struggling to understand how these massive gas giants could have formed so close to their stars, or could have migrated there after their formation.
Not all the planets we've detected are oddballs, though. We have also discovered Jupiter-sized planets farther away from their stars, at an orbital distance comparable to Jupiter, which suggests to some astronomers that Earthlike planets may also exist in these systems. Although we can't yet detect Earth-sized planets around sunlike stars, scientists believe they too may be common. This makes the many Earthlike planets we see in Star Wars seem fairly reasonable.
Even with our limited ability to find planets, about one out of every twenty stars we've studied thus far has a planet we can detect. Scientists now estimate that perhaps 10 percent of all stars have planets. That would mean our galaxy alone would be home to twenty billion solar systems. As for how many planets might be Earthlike, we can only make a very rough estimate. But scientists now believe perhaps two billion of these solar systems may have Earthlike planets.
Earthlike, though, doesn't mean that a planet will look like northern California. It means only that a planet will have a rocky composition and size similar to Earth. Other than that, it may have very little in common with our planet. Mercury, Venus, and Mars are considered Earthlike, but that doesn't mean alien life exists on those planets, or that human life could survive there.
Now that we know planets are plentiful, we need three more ingredients to create the Star Wars universe. First, planets that can give rise to their own life. Second, planets that, having the potential to give rise to life, do so. Third, planets that can support human life.
What qualities must planets have to fulfill these needs? Let's consider our first need first.
TWIN SUNS
Luke Skywalker stares off across the Tatooine desert at the dramatic sunset. Two suns, close beside each other, make their way toward the horizon.
Binary star systems, in which two stars orbit around a common center of gravity, are fairly common. Yet scientists think planets around binaries are unlikely, because the gravity of one star may prevent planets from developing around the other. As two stars of different masses orbit about each other, the surrounding gravitational field would shift, setting up potential instabilities in the orbits of any planets.
Even stable orbits would most likely have complex trajectories and variable climates. For example, as a planet orbits past the larger, hotter star, the strong gravitational field would draw the planet close, initiating a period of searing heat. Then as the planet approaches the smaller, cooler star, the weaker gravitational field would allow the planet to swing out to a great distance, sending the planet into a long period of frigid temperatures. In addition, such a planet could have a complex, shifting cycle of sunrise and sunset. This would add to climatic instability.
But astronomers do envision two possible situations in which planets might form in binary star systems, and might even support life. If the two stars are very far from each other—for example billions of miles apart—then planets might be able to orbit one of the stars with minimal influence from the other. For example, Proxima Centauri, the star closest to our sun, is part of a trinary star system. Proxima is one trillion miles from its two sisters, though, 250 times the distance from the Sun to Pluto. Many astronomers believe Proxima could have planets of its own, only minimally affected by its far-off sisters. From the surface of one of these planets, the two sisters would appear only as bright stars in the sky.
The other possibility is that the two stars could be so close together—only a few million miles apart—that to a planet orbiting far enough away, the gravitational field of the two stars would seem almost like that of one. Dr. Jakosky estimates, "If the distance between the stars is only one-tenth the distance to the planet, that would probably be stable." In this situation, the orbit of the planet might be close to circular, and the temperature might remain relatively stable. At dawn two suns would rise, and at dusk two suns would set, just as we see on Tatooine.
Thus, while planets in binary star systems may be rare, Tatooine seems to be an example of one specific situation in which a planet can have a stable orbit. Such planets may well exist, and may even support life. And they'll have some pretty spectacular sunrises too.
ARE STAR SYSTEMS SLIPPING THROUGH YOUR FINGERS?
"A galaxy far, far away" teems with life. On every planet, over every snowbank, hidden in every cave, submerged in every garbage masher, life abounds. This is one of the qualities that makes Star Wars seem so real and so fully imagined. But how common is life in the universe? In a few thousand years, might our descendents be walking into a cantina populated with an incredibly bizarre range of life-forms, a real-life "wretched hive of scum and villainy"?
Scientists believe a wide variety of factors affect a planet's ability to develop life. Many of the necessary characteristics depend not on the planet itself, but on conditions within its solar system, and on the planet's position within that solar system. Here are a few of the key factors.