One crisp summer day in upstate New York, a restless crowd wove its way into a small conference room in the Bausch & Lomb physics hall at the University of Rochester. Outside, the sun sliced through a flawless azure sky, and a warm breeze danced off the gentle waves on Lake Ontario. Inside, I squeezed in among the two dozen professors and students and waited for the presentation to begin. Then the lights dimmed, and any longing for the lovely day outside was instantly displaced by the far more powerful allure of new worlds coming into view, worlds located many light-years away.
A computer projector snapped on and began displaying data from the Spitzer Space Telescope, a powerful companion to the famed Hubble Space Telescope. Spitzer was launched the previous summer, and my colleagues and I were witnessing some of its earliest results: evidence of planets being born around nearby stars. The actual data did not look like much, mostly line graphs showing the intensity of radiation emitted by the stars at various wavelengths, but the meaning hidden behind those numbers had us talking all at once, lost in the fever of discovery.
Spitzer was designed to pick up infrared rays that, unlike visible light, can penetrate thick dust and probe the dense interstellar clouds where stars and planets form. After just a few months of operation, the telescope already exceeded its creators’ optimistic expectations. Not only had it clearly identified evidence of newly formed planets, it had demonstrated that the planet-building process is far wilder, messier, and more varied than anyone expected.
The truth is that astronomers still do not know much about the origin of planets, but they are learning quickly. For a long time the only solar system they were able to study was our own, which formed a long 4.6 billion years ago. Over the past decade, the discovery of planets around other stars and the development of intricate computer simulations have suggested that our solar system is something of an oddball. Planet building seemed to favor giant worlds careering around their stars in extreme orbits. Most of the worlds we have found seem unlikely to support life.
Spitzer’s findings suggest that nature is far more interesting than that. Planets seem to form in all kinds of orbits and in all kinds of distances from their stars. They also form through processes that do not clearly fit into any of the standard theoretical models. Small, rocky bodies may aggregate gradually over hundreds of millions of years, while nearby—maybe even around the same star—Jupiter-size objects pull together in just a few hundred years.
All this variety probably includes countless bizarre worlds, but also many that are similar to Earth. That is why the mood in that conference room was electric. Spitzer was unmasking the secrets of planet building, the process that created our own solar system and that may, at this very moment, be creating new habitable worlds around other stars.
Astronomers deduced the basic recipe for making planets a long time ago, and it could hardly be simpler. Just take an interstellar molecular cloud—in essence, a big bag of cold gas and dust—shake it lightly and allow the ingredients to settle.
When the gas and dust begin to collapse under their own weight, the bulk of the material falls to the center, giving rise to a protostar. Meanwhile, any slight rotation in the original cloud is enormously amplified as it contracts. The spinning motion flattens the material into a round disk of gas and dust that spirals inward and rains down onto the natal star for about a million years. Planets begin to form in the plane of the disk from leftover scraps. This process tidily explains why all the planets in our solar system orbit in the same direction and in nearly the same plane.
So far, so good, but trying to figure out the details of how planets emerge from stellar leftovers is confusing. Theorists such as Alan Boss of the Carnegie Institution of Washington, Douglas Lin of the University of California at Santa Cruz, and Jack Lissaur of NASA’s Ames Research Center have developed sophisticated models of planet formation, but the process is furiously complicated. The models rely on a long list of assumptions, making it difficult to know which one of them (if any) corresponds to the real world. “There are a lot of detailed theories about how planets form,” says astronomer George Rieke of the University of Arizona, the lead researcher on one of Spitzer’s three primary instruments. “What we really need are constraints, something that can make the theories more than fantasy.”
Those theories got a jolt 10 years ago, when astronomers first began discovering planets outside our solar system orbiting other stars. To everyone’s surprise, the alien solar systems looked nothing like ours. Many are home to so-called hot Jupiters, massive gas balls orbiting extremely close to their parent stars and roasting at temperatures in excess of 1,000 degrees Fahrenheit. Many of the newfound worlds follow highly elliptical paths that take them close to and then far away from their star, quite unlike the nearly circular orbits typical in our solar system.
Researchers used to think that planets stayed where they initially formed, but the existence of hot Jupiters suggests that orbits often shift radically during the early life of a planet. Massive planets might originate far out and then spiral inward due to gravitational interactions with their disk. Such orbital migration would destroy any smaller, Earth-like planets that had formed, as an inward-moving giant would scatter smaller planets the way a bowling ball would blast through a pile of marbles. That is why the timing and the scale of planet formation—fast or slow, big or small—is a critical issue.
For gas giants like Jupiter, formation theories come in two flavors. The planet can form quickly when a large chunk of the disk becomes gravitationally unstable and collapses on itself. In some models, such instabilities can produce a planet in a few hundred years or less, possibly even in a single human lifetime, but only if the disk is very dense and cold. Alternately, the planet can accumulate mass slowly as bits of dust collide and become pebbles, which collide to become boulders, which collide to become asteroids, and so on, until a rocky planetary core develops. When the core has enough gravity to attract gases in the disk, it begins to accumulate an atmosphere. This process, sometimes called core accretion, is rife with uncertainties. Models do not reveal exactly how long it takes to make a space mountain out of a space molehill. The timescale usually associated with core accretion is tens of millions of years.
Computer simulations suggest that smaller, terrestrial worlds probably arise slowly, through the core-accretion process. Then again, all of these theories and their impressive-looking conclusions are only as good as the assumptions built into them. “So how long does it all take—a thousand years or 10 million years?” Rieke asks. “The theorists can argue about it forever. What is needed is real data from real planet-forming systems.” That is exactly what is coming every day from Spitzer.
NASA’s new infrared eye
Space Telescope and the Chandra X-ray Observatory. Any warm object emits infrared rays, so Spitzer is cooled to just 10 degrees Fahrenheit above absolute zero to detect even faint heat. A large shield blocks out solar radiation, and the telescope orbits millions of miles from Earth’s heat. The result is an incredibly sensitive telescope that provides insights into many different astronomical objects, not just newly forming stars and planets. Among its discoveries:
•Supermassive black holes looming within baby galaxies near the edge of the visible universe. (Spitzer teamed with Hubble and Chandra to zero in on these distant objects.)
•A previously unknown globular cluster—a vast ball of ancient stars—orbiting our galaxy.
•Enormous, enigmatic blobs of gas that may arise when one galaxy plows headlong into another galaxy.
•An entire population of monstrously bright galaxies, unseen until now because they are completely cloaked in dust.
•Two mature planets circling other stars, detected directly for the first time. Spitzer showed that these worlds are fiercely hot, more than 1,300 degrees F.