The Science of Star Wars
By Jeanne Cavelos
St. Martin's Press Copyright © 2000 Jeanne Cavelos
All rights reserved.
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. (Continues...)
Excerpted from The Science of Star Wars by Jeanne Cavelos. Copyright © 2000 Jeanne Cavelos. Excerpted by permission of St. Martin's Press.
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