Published: October 2008
Search for Origins
The farthest horizons
By Robert Irion

Has astronomy lost its romance? When I visit professional astronomers at work, it often seems that way. Instead of peering through telescopes, they sit in cozy control rooms—sometimes miles away from the mountaintop observatories—and stare at digital re-creations of stars and galaxies on computer screens. Some graduate students now earn Ph.D.s in astronomy just by analyzing reams of data. It's a far cry from the astronomer's life I imagined growing up in Vermont, where the crisp night skies were rich with constellations, the northern lights, the clouds of our Milky Way galaxy, and dreams.

But if part of romance is the thrill of not knowing what comes next, then astronomy has rarely held more allure. The questions we now face lie at the heart of understanding our place in the cosmos. We soon will know whether planets like Earth are rare or common in the Milky Way. Not long thereafter, we will plumb the atmospheres of some of those new worlds to search for faint signals of life. We're already exploring the birth pangs of stars hidden within warm knots of dust, like the nursery that sheltered our sun nearly five billion years ago, to learn how our family of planets arose.

When we probe beyond our own galaxy, we see cataclysmic explosions that scatter the ingredients of new stars and planets into space. We trace the origins of galaxies by finding their ancestors, ragged masses of stars near the limits of the visible universe. And in an unexpected twist, we wonder why most of the universe consists of stuff we cannot see—including an utterly unknown form of energy that compels space to expand faster and faster as time goes on.

Just 15 years ago, these mysteries were beyond reach. We simply didn't have the right tools. Since then, astronomers have built major telescopes to gather the barest flickers of starlight, designed powerful electronic detectors to analyze the light, and launched satellites tuned to scour the sky in x-rays, infrared light, and other radiation. Together, this equipment has parted the curtains that shrouded much of our galaxy and the distant universe from view. It's a time of genuine revelation, a time when technology has extended our reach into the deep.

Some of this research has irresistible "gee whiz" appeal. One of my favorite advances teamed a laser beam and a high-tech mirror to help expose the wild physics near the monster black hole at the center of the Milky Way. Stars trapped in orbit around the black hole dive close to the center and then race back out again, like comets dashing around our sun. But to gain a clear view of stars so far away at the galaxy's core, astronomers must erase the blurring effects of Earth's atmosphere. They aim a laser high in the sky, its point of light jitter�ing from distortion caused by air currents overhead. A special flexible mirror at the base of their telescope counteracts the light's motion, allowing the telescope's instruments to record a sharp image. Such engineering wizardry is not your grandparents' astronomy.

The shapes and speeds of the stars' orbits show that the black hole tips the scales at four million times the mass of our sun. A few years ago, one of the stars passed so close to the center (but not close enough to plunge into the maw) that the black hole's gravity boosted the star's speed from 170 miles a second at the far point of its orbit to 5,200 miles a second—a remarkable 3 percent the speed of light. Other stars skirt past the black hole so perilously that gravity slings them around and expels them from the galaxy, a cosmic version of crack-the-whip.

We've also found that the cosmos is a pyromaniac's delight: Something's always blowing up. The main attractions are gamma-ray bursts, fierce explosions that momentarily outshine the stars in a billion galaxies combined. NASA's Swift satellite finds the bursts and sends their locations to telescopes around the world within seconds. Astronomers think most gamma-ray bursts herald the deaths of massive stars in a particularly violent breed of supernova—one that creates a black hole at its core.

These bursts and their less violent cousins, ordinary supernovae, spread heavy elements throughout their host galaxies, including carbon, oxygen, silicon, magnesium, and iron—the stuff of new planets. In the Milky Way, it's clear that past generations of dying stars seeded our galactic neighborhood with these raw materials. Astronomers detected the first planets circling other stars like our sun in 1995. Now there are more than 300, and that number is soaring. Most of the alien worlds spotted by telescopes so far are bloated balls of gas like Jupiter. But as time goes on, we're finding smaller ones. The smallest to date, dubbed super Earths, are about five times the mass of our planet. They probably have solid surfaces, and some may have the right temperatures for water to flow. Many extrasolar planets come in family sets; the star 55 Cancri, in the constellation Cancer, hosts at least five sibling planets.

If this sounds comforting and familiar, it should. Planetary scientists think it's only a matter of time before we find a solar system resembling our own. The Hubble Space Telescope has spied disks of gas and dust around many young stars. The images suggest that planets are assembling within the disks, clearing out gaps as their gravity gathers more material. The Spitzer Space Telescope, which uses infrared light to look deeply into clouds of dust where stars form, also sees evidence of planetary embryos nearly everywhere it looks. Star birth and planetary formation apparently go hand in hand.

We don't yet know whether planets like ours are common denizens of the galaxy. We can't actually see planets around other stars, at least not the kinds of planets we're familiar with. In the glare of a star's light, they're far too faint. Rather, scientists infer the presence of each planet, either by the slight back and forth gravitational tug it exerts on its star as it orbits, or by a tiny eclipse in the star's light each time the planet crosses in front of it. Because an Earth-size world is so small, these signals are tough to discern. NASA will launch a mission in 2009 called Kepler to monitor nearly 200,000 stars for the shadows of planets the size of Earth.

Let's say we find dozens or hundreds of Earths, or even thousands, as some astronomers predict. What then? We surely won't visit any of them in the foreseeable future. We'll listen for signs of intelligent life with our radio telescopes, but it's far more likely that we'll first find signs of slime. Bacteria, algae, and other microorganisms alter the atmosphere of a planet by using carbon dioxide and producing unstable gases like oxygen and methane. These gases leave distinct imprints in the heat radiated by the planet—imprints that a special orbiting observatory could detect. NASA had long planned to build such a mission, called Terrestrial Planet Finder. But it's complex and so costly that NASA now has committed only to developing the technology in modest steps.

Meanwhile, astronomers are confronting another vexing challenge, and it's a whopper. Put simply, we don't know what 96 percent of the universe is made of. All of the galaxies, stars, gas, planets, and people combined make up just 4 percent of the contents of the cosmos. We've given the rest of the recipe the simple but sinister names of dark matter (25 percent) and dark energy (71 percent). Together they present the biggest mysteries in astronomy and physics, perhaps in all of science.

Dark matter is somewhat easier to picture. The motions of galaxies, both the way they revolve and the way they cluster in groups, imply that a major hidden source of gravity binds everything together. Astronomers believe every galaxy sits within a cocoon of dark matter that is far larger and heavier than the galaxy itself. Computer models of the universe predict a vast cosmic web of dark matter, a network of filaments within which galaxies are embedded like fireflies in a spider's lair.

This web does some odd things. For instance, a ray of light traveling through pockets of dark matter gets bent to and fro along the way. When a telescope takes images of remote galaxies, their shapes and positions are distorted— usually just slightly, but sometimes severely—by this effect. It's like looking through the wavy glass of the door on a shower stall. Nothing in the distant universe is quite as it appears.

Scientists think dark matter is made of unidentified particles that rarely interact with ordinary matter. They've built experiments to try to catch such particles, but so far dark matter has defied detection.

When it comes to dark energy, scientists don't have a clue. It's some substance or property of space itself that endows space with a negative pressure. It acts in a sense like antigravity, accelerating the growth of the cosmos with increasing urgency. As the entire universe expands—a process set in motion by the big bang 13.7 billion years ago—the influence of dark energy grows ever larger. It's disquieting, for scientists and astronomy fans alike.

Astrophysicists first theorized the existence of dark energy a decade ago after observations showed that distant supernovae—bright signposts that illuminate how quickly the universe grows—were farther away from Earth than predicted. Since then they confirmed it using a sensitive satellite, the Wilkinson Microwave Anisotropy Probe, to measure the heat left over from the big bang. The universe's temperature has faded from trillions of degrees then to mere wisps of warmth above absolute zero today. Fluctuations just a few hundred-thousandths of a degree in different spots in the sky preserve a record of the exact proportions of dark energy, dark matter, and ordinary matter soon after the big bang. It was a triumph of modern cosmology to measure and interpret this, but the quest won't be over until we know what dark energy is and why it even exists.

These mysteries shouldn't overshadow the remarkable threshold astronomy is nearing: We're able to see almost everything that shines. When I was growing up, I was convinced the universe was infinite and ungraspable, capable of hiding things we could never hope to detect. But today we can sense its margins and feel the faint glow of its creation. We soon will reach further into the epoch when the first stars ignited and the first shreds of galaxies coalesced. The James Webb Space Telescope, successor to Hubble, will take us there by the middle of the next decade. Shortly after that, a new generation of telescopes on the ground—giants with mirrors 80 feet across, or more—will expand our cosmic vision to yet fainter things. We'll never see every last star or nebula. But we will take the measure of enough of them to say, with confidence, how galaxies arose and evolved, how stars live and die, and how planets fit into these grand cycles. It will be a portrait of our origins.

Technology and computers drive astronomy today, but it remains a personal science. Those who devote their lives to it cherish their moments of reflection. Recently, a celebrated astronomer told me it took her more than three decades to truly see our galaxy. It happened in Chile, when she ventured outside a high observatory dome. Under some of the clearest skies on Earth, she let her eyes adapt to the night. Soon, she perceived the spiral arm of the Milky Way closest to us, and the dark clouds of dust around the center of the galaxy far beyond. Instead of a flat band across the sky, she saw the depths of the Milky Way, a 3-D whirlpool of stars. Vertigo swept over her. "The sensation of being on Earth faded away," she recalled. "I was a citizen of the galaxy."

Romantic? Perhaps. But that's the power of astronomy.