Distant planetisimals may have similar sun-distances and temperatures to Earth.
It was a particularly clear night atop a Chilean mountain, as University of Arizona astronomers gazed 430 light-years away. What they imaged was a mysteriously hot, infrared glow of stardust. Could it be that in the Southern constellations between Scorpio and Centaurus, what they saw were precursors to Earth-like planets?
At the 199th National Meeting of the American Astronomical Society in Washington, D.C., Michael Meyer of the University of Arizona and his colleagues announced indeed, that in the direction of the constellation Centaurus, the star classified as HD 113766A, is likely hatching planetary embryos. To make matters more intriguing, their stellar candidate shows evidence that orbiting around a Sun-like star are young planets, or planetisimals, which may have similar sun-distances and temperatures to Earth.
But the parent star called HD 113766A is no ordinary Sun-like candidate. The number 113766 is its unique listing in Henry Draper's classic, century-old star catalog. The letter 'A' represents one of a binary star pair. Compared to the Sun, both stars in the binary pair are slightly hotter, bigger and brighter.
At a distance of 430 light-years these stars are not visible without a powerful telescope. Even a state-of-the-art infrared telescope must look hard for this particular binary star pairing in the Southern Hemisphere, pointing towards the Scorpio-Centaurus part of the night sky. As astronomer Dana Backman of Franklin and Marshall College and second author on the planetisimal article notes, these kind of observations across 430 light years "require the largest telescopes and best equipment astronomers have available."
Indeed, to image such a distant binary star, the research team relied on the Magellan I 6.5-meter (21 foot diameter) telescope at Las Campanas in Chile, an advanced infrared camera system. Because the binary twins are so close together, the team needed the telescope's large aperture and infrared camera, along with excellent viewing conditions in Chile during August 2001.
These types of stars, with candidate planets orbiting in similar conditions to Earth-distances and temperatures, have come to be called "Vega-like" stars. Vega is relatively nearby on an astronomical scale, and in the case of the most recent findings for HD 113766A, shows excess heat emission from the dust of planetary disk formation.
Pinpointing a relatively dark, Earth-like planet orbiting around a star likely to be millions to billions of times brighter remains one of the great challenges in modern astronomy. Secondary signatures like changes in the star's orbit due to a massive outer planet like Jupiter or in this case, infrared dust glow, present the best opportunities for inferring the presence of planets.
The temperatures in the planet-forming band around the star HD 113766A range between 805 and 195 K (990 and minus 110 F), with the hottest material closer to the star. For these temperatures, the dust bands spread over a distance comparable in our own solar system between the orbits of Mercury and Jupiter, or between 0.35 and 5.8 times Earth-distance from the Sun.
What the infrared astronomers found in the hot debris disk around HD 113766A proved surprising. Only the first of the binary star pairing (A) had a hot dust cloud, a remnant of planetary formation, and the radiation temperatures of the cloud were similar or slightly hotter than those expected for Earth-like planets. Looking for such infrared planetary dust around such star candidates "is by far the easiest way to identify these systems", according to Meyer.
Backman said that "it is a mystery why two nearby fraternal twin stars, with identical ages and nearly identical properties, would have such different amounts of circumstellar material. One could say there is some 'hidden variable' that makes the two superficially similar stars' histories so different, but we don't know what that might be."
Based on models for how planets aggregate and collide with surrounding dust, the estimated age for the planetisimals is relatively young, around 10 million years.
For comparison, in the first 10 million years of our own solar system's formation, only Jupiter and Saturn had formed. Hot gaseous disks, the precursors to the current inner planets like Earth and Venus, had only just begun to appear. At this early stage in their evolution, these systems are called inner debris disk systems. According to Backman, the astronomers are able to take advantage of such large disperse dust clouds to image younger planets, a task made easier by their large surface area. The infrared "glow from dust belts as planets form should be much easier to detect than the same glow from completed planets."
The inner debris disk around HD 113766A has other properties that surprised the scientists as well. Notable was the dust density around HD 113766A did not decay or diminish as they looked further out from the star, meaning a source of dust replenishment must be present. This constant dust density is usually attributed to particle collisions and points the astronomical detectives towards new candidates for active planet formation. Without such replenishment, the strong gravity of the parent star would have captured the dust in only a few thousand years, not the 10 million or so years to the present. In our own solar system, the asteroid belt provides such a constant dust-per-area profile of collisions, called the zodiacal dust cloud.
But for HD 113766A, the scale of debris is much bigger, equaling nearly one-tenth the mass of the Earth in dust alone, requiring around a 200 times larger mass than our own asteroid belt and a much higher density, around a 250,000 times thicker dust cloud.
According to Meyer, "What we have uncovered here - utilizing the new generation of high resolution mid-IR cameras on large ground-based telescopes - is the third example of an inner debris disk with dust comparable to that generated through collisions in our asteroid belt without an attendant massive outer disk." The lack of an outer disk becomes key, because that points to collisions and active planet formation across the band of Earth-like inner planets. "We expect that surveys soon to be undertaken with NASA's Space Infrared Telescope Facility (SIRTF) will tell us whether such systems are common or very rare indeed."
While a direct photograph of a giant planet around a distant star may soon be within reach of very large, ground-based telescopes, going to space remains the most promising way to image the much smaller Earth-like planets directly.
Even with the current generation of 6-10 meter telescopes, if using adaptive optics to adjust for viewing distortions, "direct detection of extrasolar giant planets may soon be possible," says Meyer. But "the detection of Earth-mass planets around solar type stars is the goal of future space missions and is currently beyond the limits of observation."
Future space-based infrared telescopes, such as NASA's Space Infrared Telescope Facility (SIRTF) will greatly increase the chances of finding other stellar candidates like HD 113766A, as well as ruling out alternative explanations that can account for infrared dust glow other than the formation of planetary bands. Meyer concludes: "We have not yet been able to conduct the systematic survey of solar type stars needed to understand which systems are the exception rather than the rule. Our SIRTF Legacy Science Program - the Formation and Evolution of Planetary Systems (http://feps.as.arizona.edu) - aims to do just that."
Collaborators on the study included: Eric Mamajek, Philip Hinz, and William Hoffman of the University of Arizona (Tucson, AZ), Dana Backman and Victor Herrera of Franklin and Marshall College (Lancaster, PA), John Carpenter and Sebastian Wolf of Caltech (Pasadena, CA), and Joseph Hora of the Harvard/Smithsonian Center for Astrophysics (Cambridge, MA).