Today, we know that our planet is just one of eight planets in our solar system, which consists of the Sun, the planets and their moons, and countless smaller objects including asteroids, comets, and specks of interplanetary dust. Our solar system , in turn, is just one of more than 100 billion star systems that make up the Milky Way Galaxy. And our galaxy is one of some 100 billion star systems that make up the Milky Way Galaxy. And our galaxy is one of some 100 billion galaxies in our universe.
Numbers like 100 billion are truly astronomical; it takes some effort to conceive of their size. Let's start by considering a galaxy of 100 billion stars. Imagine that, tonight, you are having difficulty falling asleep, perhaps because you are contemplating the vastness of the Milky Way Galaxy. Instead of counting sheep, you decide to count stars. If you count about one star each second, how long would it take to count 100 billion stars? Clearly, the answer is about 100 billion seconds. But how long is that? You can get the answer by dividing 100 billion seconds by 60 seconds per minute, 60 minutes per hour, 24 hours per day, and 365 years per year. If you do this calculation, you'll find that 100 billion seconds is more than 3,000 years. In other words, you would need thousands of years just to count the stars in the Milky Way Galaxy, let alone study them or search their planets for signs of life. And this assumes you never take a break--no sleeping, no eating, and absolutely no dying.
The number of stars in the universe is even more incredible. Just as it would take thousands of years to count the 100 billion (or more) stars in the Milky Way, it would take thousands of years to count the 100 billion galaxies in our universe. How can we conceive of the total number of stars in the universe? Visit a beach. Run your hands through the fine grained sand. Try to imagine counting every tiny grain of sand as it slips through your fingers. Then imagine counting to scoop up and count the grains until you have counted every grain of sand on the beach. Next think about visiting every beach on Earth and counting every grain of dry sand you can find. Of course, you could never actually complete this task. But if you could, you'd eventually know the total number of grains of sand on all the beaches on Earth. Incredibly, this number is roughly the same as the number of stars in our universe.
The total number of worlds--by which we mean any reasonably large bodies in space, such as planets, moons, or even large asteroids--may be even greater. If planetary systems are as common as recent discoveries suggest, many or even most stars may have at least a few planets or moons, some of which could potentially harbor life. Clearly, our universe contains worlds beyond imagination.
We have assumed that planets should be common around stars throughout the universe. However, all the extrasolar planets discovered to date are quite nearby on the scale of the Milky Way Galaxy. Why, then, do we believe that planets should be common everywhere? Part of the answer lies in the fact that, in science, we always assume that our location in the universe is typical of many other places, and not special in any way. In that case, the discovery of numerous extrasolar planets nearby must imply that there are many others waiting to be discovered at greater distances. In addition to this observational evidence, our understanding of how stars and planets are made gives us good reason to think that planets are made gives us good reason to think that planets must be common. To see why, we must look briefly at the history of matter in the universe.
Strong evidence now points to the idea that our universe was born somewhere between about 12 and 15 billion years ago, in an event we call the Big Bang . The Universe began in a state of extremely high density and temperature and has been expanding and cooling ever since. However, even as the universe as a whole expands, on smaller scales the force of gravity has drawn matter together to make galaxies. that is, galaxies represent localized places where gravity has won out against the overall expansion. Within galaxies, gravity drives the collapse of clouds of gas and dust to form stars (and their planets). Stars are certainly not living organisms, but they nonetheless go through "life cycles." After its birth in a giant cloud of gas and dust, a star shines for millions or billions of years by carrying out nuclear reactions in its core. A star dies when it exhausts its usable fuel. When a star dies, it blows much of its gas back out into space. The returned matter mixes with other interstellar matter, eventually forming new clouds of gas from which new generations of stars can be born. Thus, the Milky Way Galaxy is in many ways like a giant recycling plant, recycling matter from dead stars into new generations of living stars. Our own Sun is product of many generations of such recycling--the Milky Way Galaxy predates our Sun by at least 5 billion years.
Planets like Earth must be made from material that has been cycled through generations of stars. Based on evidence we'll discuss shortly, we have good reason to believe that the early universe contained only the simplest chemical elements: hydrogen and helium (and a trace amount of lithium). But we and the Earth are made primarily of "other" elements such as carbon, nitrogen, oxygen, and iron. Where did all these other elements come from? Remarkably, modern science tells us that all these elements were manufactured inside stars (or during stellar explosions that occur at the end of the lives of massive stars). This means that most of the atoms from which we and the Earth are made were manufactured inside stars that lived and died long ago. In other words of noted astronomer Carl Sagan (1934-1996), we are "star stuff".
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