Within a week we theorists found that a planet responsible for the hole did not need to be as massive as Jupiter. A body twice the size of Neptune and about one-tenth as heavy as Jupiter (but still about 30 times the mass of Earth) would have sufficed. This lower limit was intriguing because the outer edge of the hole is about as far from its star as the giant planets in our solar system are from the sun. Unlike the many bizarre, star-hugging worlds discovered by other means, the planet that seems to be orbiting Cohen-Kuhi Tau/4 looks reassuringly familiar. Spitzer seemed to have uncovered a new planetary system built on the same overall engineering plan as our own.

We also found that the alien planet must have formed no more than a few hundred thousand years ago. If it were any older, the gravitational interaction between the disk and the planet would have forced it to spiral inward, perhaps even to be swallowed by its star. That timescale supported rapid planet-formation models.

In other ways, however, Cohen-Kuhi Tau/4 didn’t fit the models at all. The disk around the star appeared too small and low mass to support the leading rapid-formation theory, the gravitational instability model. In fact, the Spitzer results did not fit well with any existing theory of how planets form. The new data and our new calculations were forcing us to rethink our assumptions.


HOW TO SPOT A PLANET

Spitzer identifies young planets by how they affect infrared radiation from stellar disks. A naked star produces a simple, sloping spectrum (longer wavelengths indicate cooler temperatures). A star with a surrounding disk displays an additional glow from warm dust in the disk. A planet in the disk sweeps up some of the dust, creating a dip in the infrared spectrum; the shape of the dip reveals the planet’s distance from the star.

Graphic courtesy of NASA/JPL-Caltech/D.Watson (University of Rochester)

We soon realized that the discovery of the hole in the disk around Cohen-Kuhi Tau/4 was only the beginning of a story. Since then, scientists have discovered similar partly empty disks around many other young stars.

“We have dozens of sources with their inner disks cleared out to at least the size of the orbit of the Earth. All these systems have lost their inner disks early, in less than a million years,” Rieke says. “Spitzer allowed us to see really faint objects so that we could do a census of all the star-forming regions out to 3,000 light-years. With enough data you can tell pretty accurately how long the average disk exists. We now have a clear enough picture of the data to tell how long planet-building disks last around young stars.”

The strange answer is that planet formation is not fast or slow; it is fast and slow. On average, the disks studied by Spitzer turn out to be 100 million years old, one hundred times the duration of planet formation implied by the Cohen-Kuhi Tau/4 results. This finding startled astronomers who had just absorbed Spitzer’s evidence that planets may form extremely rapidly. They had assumed that fast planet building would deplete the surrounding material and make the disks disappear quickly. Instead, Spitzer was showing that disks, and perhaps subsequent rounds of planet building, could go on far longer than even the old core-accretion models implied.

Courtesy of NASA/JPL-Caltech/G. Melnick (Harvard-Smithsonian CFA)

The stellar nursery called Sharpless 140 contains brilliant protostars still surrounded by the dusty clouds from which they emerged. Jets of gas emitted by the stars are stirring up the surrounding nebula. Such jets form when magnetic fields fling material from the disks around the stars.
For instance, the bright star Vega—shining high overhead on summer nights in the Northern Hemisphere—is 100 million years old, yet it still has a disk. The observable material is a so-called debris disk consisting entirely of dust and large rocks, some of them possibly as large as planets. Such disks have lost all of their gas and are far less dense than the ones around younger stars.

That dust cannot be primordial material because it would have fallen into the star long ago. “With Spitzer we found clear evidence that Vega and other stars have recently had their debris disks resupplied with dust,” Rieke says. “The only way to produce as much dust as we are seeing in these older stars is through huge collisions.” Larger bodies, such as giant comets, asteroids, and protoplanets, must have coalesced from the original disk and then crashed into one another.

Such impacts are a key feature of the core-accretion model, yet nobody expected to find the process continuing around a star as old as Vega. Evidently the building blocks of planets keep colliding, merging, shattering, and pulverizing one another long after the first large bodies come together. “It’s a mess out there,” Rieke says. “We are seeing that planets have a long, rocky road to go down before they become full grown. The kinds of processes we associate with planet building—big collisions—are still going on, even though these systems are so old.” Those long timescales may bode well for the existence of Earth-like planets.

Rieke notes that our solar system contains a faint debris disk of its own—micrometer-size dust particles slowly spiraling in toward the sun. We see this disk as a dim glowing band, called the zodiacal light, running along the plane of the planets. Under clear skies, it shows up as a diaphanous cone of light hanging in the west after sunset. “The zodiacal light comes from sunlight reflected off dust grains blown off comets or from asteroid collisions,” Rieke says. “If you could look at our solar system 5 billion years ago, it would probably look similar to what we are seeing in Vega.” Put another way, the dust around Vega is a reassuring sign that many stars form planetary systems broadly similar to our own.

Whenever a major new instrument switches on, the dance between theory and observation in science always turns from a stately waltz into a chaotic jitterbug. After a little more than a year in orbit, the Spitzer Space Telescope has sped the tempo of discovery in planet formation studies so much that scientists are overwhelmed. The telescope contains enough liquid helium coolant to keep going at this pace for another three years.

The cosmos seen by Spitzer seems strangely disconnected from the one predicted by our meticulous models. “We thought that young stars, about 1 million years old, would have larger, brighter disks and that older stars from 10 million to 100 million years old would have fainter ones,” Rieke says. “Instead we found some young stars missing disks and some old stars with massive disks.” The most recent findings from Spitzer continue this confounding pattern. Some mature stars that are known to have planets are still surrounded by debris disks 100 times as thick as the dust in our solar system, for unknown reasons. One middle-aged star, known as HD 69830, appears to be surrounded by an asteroid belt that is 25 times as dense as the one in our solar system, possibly the remnants of a rocky planet that never formed.

All these findings carry a deep relevance to humans because they will tell us whether our solar system and our Earth are flukes or the routine results of a ubiquitous process. “Years ago Frank Drake wrote down an equation for the number of intelligent civilizations in the galaxy,” Rieke says. “One of the first factors in that equation is the number of stars with planets. The next is the number of planets that can support life. The question of where, when, and how planets form touches on both these factors.”

Spitzer is exposing how much astronomers don’t yet know about the new worlds forming out there—and about the old worlds, possibly even habitable ones, that are still undetected. For scientists, being placed in a position of ignorance is good. That’s when the real work begins.

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