For many years scientists wondered if the Spitzer telescope would ever make it from the drawing pad to the launchpad. The concept started life in the late 1970s as an infrared observatory that could fly short stints aboard the space shuttle. (Its name pays homage to an earlier era: the 1940s, when the influential astronomer Lyman Spitzer Jr. made the first serious proposals for building large observatories in space.) During the 1980s the Spitzer expanded in scope to a free-flying observatory. In the 1990s, the project shrank again in response to budget pressures.

THE POWER OF

INFRARED VISION


Two views of the Trifid nebula, a giant cloud of gas and dust located 2,700 light-years from Earth, illustrate how the Spitzer Space Telescope exposes the secrets of star and planet formation. A conventional optical photograph (top) shows visible light emitted by the Trifid, highlighting the dusty clouds that divide the nebula and obscure what is happening in the densest, most interesting regions. A recent Spitzer image (bottom) shows the same nebula seen in the infrared part of the spectrum, displaying penetrating rays whose waves are much longer than those of red light. Spitzer’s infrared vision pierces the gloom to reveal a cosmic nursery containing dozens of previously unseen embryonic stars, many still ensconced in the dark clouds from which they formed. By measuring the infrared brightness of such protostars, astronomers have been able to measure how quickly the objects are growing. These observations help define where and how new planets are created.

Images courtesy of NASA/JPL-Caltech/J. Rho (SSC/Caltech) 
Through all these ups and downs, the essential mission of Spitzer remained constant: to learn more about the universe by detecting infrared rays, electromagnetic waves slightly longer than visible light. Such rays are ideal for studying cool objects—those that don’t attain the blazing heat of the sun and other stars—which give off little light but glow prominently in the infrared. More crucial, infrared rays penetrate effectively through dust clouds, making it possible to peer into otherwise obscured regions of space.

What changed during the redesigns was the telescope’s specific capabilities. “Spitzer was supposed to be the Swiss Army knife of infrared space telescopes,” says astrophysicist Dan Watson of the University of Rochester, who has participated in the project since its inception. “But as time went on the project got de-scoped. We had to circle the wagons around a few projects that could still be done.” Studying the birth of planets in the cool, dusty cocoons around infant stars turned out to be the perfect task for the downsized and reconfigured observatory.

Finally, on August 25, 2003, the $700 million Spitzer Space Telescope blasted into space inside the fairing of a Delta II-H rocket. When it safely reached its unusual orbit, trailing Earth in its path around the sun, astronomers, many of whom had staked 23 years of their careers on the project, breathed a sigh of relief. They then began the months-long process of calibrating Spitzer’s systems. “Everything functioned beautifully,” says astronomer Bill Forrest of the University of Rochester. “We were really happy.”

The telescope is built around a 34-inch-wide beryllium mirror, cooled by liquid helium to –450°F to eliminate the infrared noise that every warm object emits. Infrared rays collected by the mirror bounce to one of three instruments: an imaging camera, a spectrograph that breaks up infrared light into its constituent wavelengths (creating an infrared rainbow), and a combined camera-spectrograph that studies a somewhat different part of the infrared spectrum than the other two. Many teams across the country oversaw the design and construction of each instrument; researchers in my group at the University of Rochester collaborated on both the camera and the spectrograph.

Once Forrest and his colleagues were satisfied that the instruments were working correctly, the telescope’s real scientific observations began beaming down to a data center on the Caltech campus and then across the Internet to researchers’ computer hard drives. Within weeks came the discovery that drew the crowd into Bausch & Lomb Physics Hall on that sunny summer day. “My student Joel Green was reducing some of the new data one morning, and I was looking over his shoulder,” says Dan Watson. “The data came in short-wavelength and long-wavelength modules, and we needed to glue them together to create an entire spectrum. What we saw when we joined them was unexpected and really exciting.”

Spitzer’s spectrograph had split up infrared light from the baby star Cohen-Kuhi Tau/4, located 420 light-years away in the constellation Taurus, and spread that light out by wavelength. The spectrum showed how much energy the star emits at various wavelengths, each of which corresponds to a temperature. A naked star produces a single-humped spectrum, with the bulk of the energy concentrated at short, hot wavelengths. The cold, dusty disks around a star emit copious long infrared waves, producing a second hump in the spectrum. What Watson and his student saw did not fit either pattern. Clearly there was a young star, and clearly there was a cold disk, but something had taken a big bite out of the disk’s infrared signature.

“The light from the inner part of the disk was totally missing,” Watson says. “I knew immediately what it meant.” He cleaned up the data and passed it along to Bill Forrest, his collaborator. “Right away, I knew we had found a planet,” Forrest says.

If part of a disk is missing, something must have cleared it out. Soon after a giant planet forms, its gravity sweeps out a ring-shaped gap in the disk. In time, that gap expands into a hole as the remaining inner parts of the disk drain onto the star. The gap around Cohen-Kuhi Tau/4 most likely arose this way.

Until Spitzer, the best that infrared telescopes could do was find hints of such gaps around a few relatively close, bright young stars. “Spitzer’s instruments are far more sensitive than anything before it,” Rieke says. “That means we can look at regions of the galaxy where stars are forming that are much farther away than we could before. By seeing so many star-forming regions, we can catch the star- and planet-creation process happening at different stages.” Infant stars in the Cohen-Kuhi region, for example, were too distant and dim to study using earlier telescopes.

Cohen-Kuhi Tau/4 is at an early stage of evolution, one that nobody had examined in detail before. After a thorough study of its brightness and temperature, an international team that included Watson and Forrest concluded that the star and its disk are about a million years old, roughly a tenth the age of any stellar system that has shown a sizable hole. For the first time, astronomers have compelling evidence that planet formation can be a fast process. Here it had to happen in less than a million years.

All through the summer of 2004, the University of Rochester astronomy group gathered at lunchtime on Tuesdays to review, discuss, debate, and generally let our jaws drop over the new Spitzer data. One week we contemplated images of newly formed star clusters; another week we examined beautiful spectra of ice grains swirling around an infant star. On the day that the observers presented the Cohen-Kuhi Tau/4 results, there was a long moment of quiet. Every person in the room, from the most senior professor to the greenest graduate student, knew this was the kind of moment that justified decades of work. Then the buzz began.

The theorists in the group, led by Alice Quillen and Eric Blackman, stepped up to the whiteboard in the lunchroom and began sketching the links between planets and holes in a protostellar disk. Then the questions started to come into focus. What would the mass of the planet have to be? What mechanisms could form a planet that quickly? Could other planets survive in the disk? Everyone started talking at once, and the conversations quickly splintered. By the end of the lunch hour, factions had reconvened, and the outlines of a paper were already taking shape.

MAKING PLANETS THE FAST WAY

Gas in the dusty disk around a young star might become massive enough to pull more material directly from the disk, giving rise to a giant planet in a few centuries or less. Spitzer confirms that giant planets can form quickly and sweep out gaps in their disks—but some key observations do not fit this model.
STEP 1

A disk of gas and dust forms around a young, recently collapsed star.