Last week's discovery of 715 planets orbiting other stars was more than just a remarkable piece of astronomical detective work. It was also a bold confirmation that we have entered a new era of cosmic exploration. Sara Seager of MIT, one of the scientists leading the search for other Earths, beautifully expressed this sentiment to me in a recent interview: "For exoplanets, I see ourselves like the generation of Christopher Columbus. We are leaving a legacy in terms of us as a generation, and as a society."

The multitude of worlds found by the Kepler space telescope. (Conceptual illustration by NASA)

Often it is hard to recognize a revolution while you are right in the middle of it, but that is what I believe is happening right now. When future generations look back, as Seager suggests, I think they will recognize this time as the fourth great era of exploration, comparable to...well, let's go back and look at the previous three great eras of exploration for context.

Era One: The discovery of boundless space. When Galileo Galilei turned his spyglass skyward in 1609, he witnessed a universe that is much messier and more complicated than anyone had realized. He observed spots on the sun and craters on the moon. He also famously delivered the death blow to the old Aristotelian idea that everything revolves around the Earth. Galileo saw that Venus has phases like the moon, something that is possible only if it orbits the sun and not the Earth. Equally significant, he observed four "stars" circling Jupiter (what we now know as the Galilean satellites: Io, Europa, Ganymede, and Callisto). Not only wasn't Earth the center of the universe, but Earth was not even unique among the planets in having secondary bodies going around it.

A draft letter by Galileo contains, at bottom, some of his first notes on the discovery of four moons around Jupiter. (Credit: University of Michigan Special Collections Library)

If you are familiar with the history of astronomy you probably know the general outline of this. But Galileo had another, deeper legacy. When he looked at the misty light of the Milky Way he resolved it into innumerable faint stars. In displacing Earth from the center of the universe, he set all in motion.

Put the two together and Galileo finally and completely demolished another aspect of Aristotle's cosmology: the idea that the universe was finite, tidy, and bounded, with the stars embedded in an outermost shell. Galileo's discoveries ushered in the idea of boundless, perhaps infinite space. It created room for Isaac Newton's theory of universal gravitation, and introduced the idea that astronomical exploration without limits.

Era Two: Bringing the heavens down to Earth. Everyone knows Galileo's name. The second great upheaval was ushered in by two scientists who are decidedly less familiar: Robert Bunsen and Gustav Kirchoff. Today's students recognize Bunsen's name mainly from the Bunsen burner widely used in high school and university labs. That is an awfully modest memorial to a man who did so much to expand the range of the human intellect.

Together, Bunsen and Kirchoff established the principles of spectroscopy starting in the 1850s. They showed that each chemical element has a distinct fingerprint in the way it emits or absorbs light. Pass the light of a distant object through a prism, spread it out into a rainbow, look for a characteristic pattern of absorption lines, and you can tell what the object is made of. In 1835, the renowned French philosopher August Comte wrote that "all physical, chemical, physiological, and social researches, for which our powers fit us on our own earth, are out of the question in regard to the planets." Less than 30 years later, Bunsen and Kirchoff proved him utterly wrong.

A detailed solar spectrum shows many dark lines--the chemical signature of elements in the sun's atmosphere. (Credit: AURA/NOAO)

Spectroscopy is a somewhat obscure tool, but there is nothing arcane about its importance. It proved that the rest of the universe is built from the same kinds of atoms as we are. It allowed astronomers to measure the composition of any bright object in space, whether here in our solar system or out at the edge of the visible universe. There was also an explosive corollary: If an object is moving relative to us, that motion shifts the appearance of the lines in its spectrum. That shift makes it possible to deduce the rotations of stars, the gravitational pull of unseen planets, the movements of galaxies, even the expansion of the universe. Which brings us to:

Era Three: The discovery of the universe. Edwin Powell Hubble (no relation!) did not single-handedly discover the universe beyond our galaxy, but he sure did make the crucial breakthroughs. Up until the 1920s, nobody was sure that other galaxies existed. Astronomers knew about intriguing spirals and smears of light in the sky, and many speculated that those intriguing blurs were other galaxies like our own. But many others thought the "spiral nebulae" were gas clouds within the Milky Way, and that our galaxy was alone--that our galaxy and the universe were one and the same.

Edwin Hubble's original diagram establishing the expansion of the universe. (Credit: NASA)

Late in 1923, Hubble found a specific type of star, called a Cepheid variable, in what was then known as the Andromeda Nebula. The star's pattern of variation allowed Hubble to derive its distance and prove that it, and the "nebula" around it, was far beyond the edge of the Milky Way. Overnight, the Andromeda Nebula became the Andromeda Galaxy, and our home galaxy became just one of a multitude.

A scant 6 years later, Hubble measured the motions of those other galaxies and discovered that they were systematically moving away from us, with their speed directly proportional to their distance. This was the discovery of the expanding universe, which led to the idea of the Big Bang, galaxy evolution, dark energy, and all the other wild concepts of modern cosmology. I still find it mind-boggling that less than a century ago nobody even knew whether other galaxies existed. The pace of astronomical discovery is truly shocking when you step back and look at it.

Era Four: The discovery of endless worlds. That brings us to the present era, and the discovery of planets around other stars. It is a different kind of breakthrough than the others. Smaller, in one sense, since it does not fundamentally shift the scale of our cosmic perception. But greater, in another sense, because it is the most personal of the four astronomical revolutions.

If life exists anywhere else in the universe, it almost surely sits on the surface of a planet. Those are the safe havens in space; those are the places where complex chemistry can happen, where biology can take hold, where--in at least one instance--inert molecules can come together to create consciousness, intellect, and passion. Just as Galileo brought us other stars and Hubble brought us other galaxies, people like Seager and Geoff Marcy and Bill Borucki are bringing us other worlds. And again, the pace of discovery is hard to fathom until you take a couple steps back.

Until 1992, astronomers did not know of a single planet outside our solar system--not one. Every idea, every discussion, lived in the realm of speculation and science fiction. Now we have charts containing thousands of planets. As has happened so many times before, scientists are finding that their imaginations paled in comparison to the creativity of Nature. The diversity of worlds out there is far richer than anyone envisioned. If we find alien life, it too may well turn out to be something entirely unexpected. Now at last we are securely on the path to finding out.

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