Panning Distant Dust

The hunt for extrasolar planets gets dirty

Mike Meyer was just 28, finishing up his Ph.D. thesis on baby stars, when his career turned to dust. Astronomers announced the discovery of a planet orbiting a sunlike star beyond the solar system. With planet hunting becoming the hottest topic in astronomy, Meyer, then at Amherst (Mass.) College, realized that he was well positioned to jump into the fray. Researchers had suggested since the 1980s that dust disks around stars were signposts of planets, and Meyer had just agreed to join a team in Heidelberg, Germany, that was studying such disks with a recently launched observatory.

WARM AND COOL PORTRAITS. Two views of the wide debris disk around the nearby star Vega, as seen by the infrared Spitzer Space Telescope. Left image shows warmer dust, which lies closer to the star, recorded at an infrared wavelength of 24 micrometers. At right, Spitzer images cooler dust, farther from the star, at an infrared wavelength of 70 micrometers. The radius of the disk is more than 20 times as great as the distance between the sun and Pluto. K. Su/Univ. of Arizona; NASA, JPL, Caltech
OFF CENTER. Ring of dust around the star Fomalhaut, imaged by the Hubble Space Telescope in visible light. The center of the ring (asterisk) lies 2.24 billion kilometers to one side of the star. Astronomers suggest that one or more unseen planets interior to the ring has pulled it off center. Kalas et al., NASA, STScI
FIRST LOOK. Infrared images of six old stars known to have planets for the first time reveal that these systems also have debris disks—the yellow, fuzzy blobs that surround each star. Beichman/Caltech, NASA/JPL

That was 10 years ago. The observatory is now defunct, but Meyer is a lead investigator with NASA’s Spitzer Space Telescope, the premier observatory for studying dust. Currently based at the University of Arizona in Tucson, Meyer and his colleagues have discovered hundreds of debris disks since Spitzer’s launch in 2003. Some circle sunlike stars only a few million years old, others orbit senior citizens that have been shining for several billion years.

From the disks now on record, Meyer and other scientists are mapping out the most likely locations for planets around sunlike stars. The dust is suggesting, for instance, that the majority of extrasolar planets lie in distant orbits, farther from their parent star than Pluto is from the sun. Such fringe orbits have yet to be explored by most planet-hunting techniques.

The multitude of disks is also helping answer a pressing question in extrasolar planetary science, says Meyer. Compared with other planetary systems in the Milky Way, is the solar system an oddball?

By the end of the summer, Meyer adds, “we should have a preliminary answer to this question. We’re literally just cranking toward the finish line.”

Disk diversity

Disks around stars come in two types. Primordial disks form during the initial star-making process, swaddling newborn stars and providing the material from which planets coalesce. Debris disks develop later, triggered by the interaction between planets and remnants of the planet-making process.

Gas, dust, and ice within the primordial disks gather and form larger and larger bodies, from boulders to kilometer-wide objects to planets. New studies using Spitzer confirm that the inner portion of primordial disks usually dissipates 2 million to 3 million years after the star starts shining. That estimate agrees with approximations that Jupiter and the inner planets formed during the first few million years of the sun’s history.

Around the same time, debris disks begin appearing. These reservoirs of dust are generated when the tugs of planets disturb the orderly orbits of asteroids, comets, and other detritus left over from the planet-making era. The gravitational disturbance causes these leftovers to slam into each other at high speeds, generating bursts of dust.

Soaking up visible and ultraviolet starlight, the dust shines at infrared wavelengths. It’s these debris disks that have grabbed the attention of Meyer, Lynne Hillenbrand of the California Institute of Technology (Caltech) in Pasadena, and a slew of other scientists.

Hillenbrand notes that the two bands of debris in our solar system—the rocky asteroid belt between Mars and Jupiter and the icy Kuiper belt, located beyond the orbit of Pluto—are continually replenished as the gravity of the solar system’s giant planets causes debris within each of these reservoirs to tumble together and generate fresh dust.

Without such continual replenishment, the debris disks in the solar system, as well those around other stars, would rapidly vanish. In less than a few thousand years, a star’s radiation and gravity either forces the fine debris out of the system or causes it to spiral inward.

The gravity of the planets sculpts as well as replenishes the disks. It draws away material, thereby keeping a disk narrow and creating gaps within it.

The long view

Although Spitzer is exquisitely sensitive to infrared-emitting dust from distant stars, its 0.85-meter-diameter mirror—roughly one-third the diameter of Hubble’s—isn’t big enough to image most dust disks. Even so, notes Hillenbrand, researchers can use the Spitzer data to map the locations of dust within a disk. That’s because each infrared wavelength that Spitzer detects from a patch of star-circling dust corresponds to a specific temperature. The temperature, in turn, indicates how far the dust lies from the star. The cooler the material, the farther away it lies.

After surveying hundreds of sunlike stars, researchers have found an intriguing trend, Hillenbrand notes. Cool, more-distant dust disks, akin to the Kuiper belt, are much more common than those that lie closer in, such as our asteroid belt. Of the stars examined by Spitzer, 20 to 30 percent have cool, distant disks. Warmer, closer-in disks circle only about 5 percent of the stars.

The larger fraction of stars with cool disks suggests that outlying planets might be much more numerous than those close to their parent stars. If that proves to be the case, as planet hunters refine their techniques and begin detecting planets in wider orbits about stars, they may be in for a bonanza of discoveries.

“Many more planets will be discovered as surveys push out to larger radii,” Hillenbrand predicts.

The relative lack of stars with the closer-in belts may also hint at something fundamental about planetary architecture, says Alycia Weinberger of the Carnegie Institution of Washington (D.C.). She notes that most extrasolar planets have elongated, rather than circular, orbits, so that their distance from their stars varies widely over a single orbit.

Planets on such elongated orbits can more easily stir up material within a debris disk, thereby hastening collisions and pulverizing material more quickly. This could clear out inner debris disks much quicker than Jupiter, a giant planet with a nearly circular orbit, sculpts the solar system’s asteroid belt.

Weinberger cautions, however, that such interpretations remain highly speculative “until we know the location of [many] planets around stars with known debris disks. Right now, we have this ground truth for only one system—our own.”

Case study

A ring of dust surrounding the star Fomalhaut, just 25 light-years away, provides strong evidence that debris disks indicate the presence of one or more unseen planets. The dust band around this 200-million-year-old star is one of the oldest, as well as the nearest, disks known.

The Hubble Space Telescope recently recorded the sharpest visible-light image ever taken of this dust band. The image confirms that the band is off center: Hubble found that the center of the ring is displaced 2.24 billion kilometers, or 15 times the Earth-sun distance, from the star. The most likely explanation for the offset, as well as for the ring’s sharp inner edge, is that at least one planet in a careening, elongated orbit is tugging on the ring, sweeping away material, and reshaping the ring, says Mark Clampin of NASA’s Goddard Space Flight Center in Greenbelt, Md.

The lopsided ring appears similar to the Kuiper belt but the diameter of its orbit is four times as great as that of the belt. The proposed planet would lie inside the ring, but still farther from its star than Pluto lies from the sun. Clampin, along with Paul Kalas and James Graham of the University of California, Berkeley describe their findings in the June 23 Nature.

Dust storms

Analyzing debris disks is a messy business, Spitzer researchers have been finding. The disks don’t develop in a steady, predictable way.

For example, several recent surveys have shown that some stars as old as a billion years are surrounded by large amounts of dust. According to theory, most of the dust generated by collisions between planets and their leftovers should have died down long before that time.

In another recent finding, Charles Beichman of Caltech used Spitzer to find evidence of a thick, warm disk of dust around a star similar in mass and age to the 4.5-billion-year-old sun (SN: 4/23/05, p. 259: Available to subscribers at Distant Dust: Asteroid belt or boiling comet?). The material’s warm temperature indicates that it’s about the same distance from its star as Venus is from the sun. Beichman suggests that the warm dust marks the location of an asteroid belt surrounding the star. He proposes that the belt is generated from asteroids banging together or from collisions between the debris and a dust-shedding comet, if one recently invaded the belt.

In either scenario, the collisions must be unusually intense because this proposed belt contains about 25 times as much dust as is contained in our asteroid belt.

In a related study, researchers examining dust around the star HIP 8920 reported that they had found the best evidence yet for a massive debris disk located close to a sunlike star. The belt’s location, similar to that of the asteroid belt in the solar system, suggests that a planet in an Earthlike orbit is tugging on it.

Weinberger’s team used the Gemini North and Keck telescopes atop Hawaii’s Mauna Kea to study the star HIP 8920, which is slightly more massive than the sun and lies 300 light-years away. Weinberger and her colleagues, including Inseok Song of the Gemini North Observatory, relied on an infrared spectrometer to examine the dust circling the star. The new data indicate that the disk, apparently made almost entirely of micrometer-size grains, lies at about the same distance from its star as Earth does from the sun.

The astronomers suggest that the dust constitutes an unusually massive asteroid belt surrounding the star, about 10,000 times as massive as the solar system’s belt. “The disk has got so much dust that it’s unlike any other system that we’ve seen before,” says Weinberger. She reported the findings last month in Minneapolis at a meeting of the American Astronomical Society.

To get so much dust, “you would have to completely pulverize a 200-kilometer-long asteroid into micrometer-size grains,” Weinberger says. “It’s a challenge to figure out how that happens.” It’s possible that a planet at an Earthlike distance from the star might have triggered the collision, she adds.

Moreover, because small dust grains don’t hang around for long, the collision had to have happened “effectively yesterday,” just a few thousand years before the observations were taken, says Weinberger.

She agrees with other astronomers, including George Rieke of the University of Arizona, that old stars can harbor such large dust disks only if they’ve been recently resupplied. Either another large body suddenly clobbered a planet or the migration of planets may have disturbed a previously quiescent band of debris.

Analogs to such cataclysmic events probably occurred in our solar system, says Meyer. For instance, simulations indicate that the moon arose when a Mars-size body plowed into Earth some 20 million years after its formation. Debris from such a collision would have briefly generated a bonanza of dust.

In May, researchers proposed that changes in the orbits of the solar system’s four outermost planets nearly 4.5 billion years ago triggered a shower of debris that bombarded the inner planets. In this scenario, Uranus and Neptune were knocked into the Kuiper belt, scattering dust all around the solar system (SN: 5/28/05, p. 340: Roaming Giants: Did migrating planets shape the solar system?). Some of the material might have triggered the period of late-heavy bombardment, an era about 3.8 billion years ago in which asteroids and comets pounded the inner solar system, perhaps ferrying in compounds essential to life.

“This was a crazy time” in the solar system, Meyer says, and other planetary systems may sometimes undergo similarly chaotic episodes.

Among these systems, adds Weinberger, is the star that her team has examined. HIP 8920 might have recently undergone its own late-heavy bombardment “with a couple of big chunks of planetary debris still scurrying around the inner part of [its] system and smashing into each other,” she speculates.

In another set of studies, Rieke and his colleagues aimed Spitzer at several A stars, which are a few times as massive as the sun is. The disks he found around these stars ranged from several hundred thousand to several million years old. But even among disks of the same age, the amount of dust varied considerably.

The variability suggests that disks evolve “spasmodically,” all but vanishing and then reappearing after a major collision produces a fresh dust supply, says study collaborator Karl R. Stapelfeldt of NASA’s Jet Propulsion Laboratory in Pasadena, Calif.

Each of these violent episodes may reset the clock for planet and dust-disk evolution, adds Hillenbrand.

Image search

Using disks identified by Spitzer as a guide, Meyer and his colleagues have set themselves an audacious goal: to capture the first image of an extrasolar planet orbiting a sunlike star.

The most popular method of detecting extrasolar planets relies on measuring the wobble that they induce in the motion of their parent stars. That technique works best for planets that lie close to, and therefore tug strongly on, their parent stars. However, their very proximity makes such planets difficult to capture on film.

In contrast, the dust disks found by Spitzer and other telescopes typically lie much farther from their stars. Astronomers have a better chance of capturing on camera planets that reside there.

Using the MMT Telescope near Tucson, Meyer’s team recently completed its first search for planets among the dust disks. They haven’t found any planets yet, but Meyer notes that his team has many more stars to examine.

Other researchers have set their sights on the same prize. This month, Clampin and his colleagues are using the Keck Observatory to hunt for planets near the lopsided ring around Fomalhaut.

Says Meyer: “The race is on.”

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