Celestial Birthing Grounds: Wild Places
A mammoth sky survey led by University of Florida astronomers has uncovered seven planet-forming disks in clusters of young stars, doubling the number of such disks discovered and expanding the territory that might yield new planets.
The disks, composed of giant clouds of gas and dust that surround infant stars, are around 1,000 light years away, about four times farther away than most disks seen previously. They also are by far the biggest yet observed – which suggests that planets, known to coalesce in such disks as they rotate, may exist at much greater distances from stars than any yet discovered. This observation could lead astronomers to expand the areas in which they search for new planets, a search that has so far been confined to stars’ immediate vicinities.
|Artist’s conception of a gas giant planet orbiting a nearby star.
Credit: NASA and G. Bacon (STScI)
"You might be able to look much farther out than people have been looking and find planets," said Richard Elston, a UF professor of astronomy who conducted the survey with his colleague and wife, UF astronomy Professor Elizabeth Lada. Elston and Lada presented their findings during a news conference at the American Astronomical Society meeting in Nashville, Tenn.
Lada also showed that planets may come together and form in the disks in far less time than currently believed. Her findings suggest planets may form in the first 3 million years of a star’s life, much earlier than the 10-12 million years thought previously. Although 3 million years may seem like a long time, it is actually brief for stars, which can live tens of billions of years. "If you think of our Sun as a middle-age star and that middle-age people are about 36, it would seem planet formation occurs within 1 week of stellar birth," Lada said.
As a cloud of molecular gas collapses under the pull of gravity to form a star, it rotates and the dust, gas and debris gradually gel in the shape of a two-dimensional disk. The material in these disks both feeds into the forming star and steadily coagulates into bigger and bigger chunks, which eventually form planets. The remnants of this process are visible in our own solar system, where all the planets line up, more or less, along the same two-dimensional plane.
|Our Milky Way galaxy is packed with 400 billion stars and perhaps even more planets.
Elston and Lada found seven such disks as part of a major survey for newborn celestial objects in "giant molecular clouds" in the constellations Orion and Perseus. These clouds, which contain the raw material for stars and planets, are the largest features of our galaxy, stretching hundreds of light years across. The clouds in this study are located approximately 1,000 light years away.
The astronomers worked at the National Science Foundation’s 2-meter, or 88-inch, telescope at the Kitt Peak National Observatory in Arizona, using a UF-developed near-infrared spectrometer and imager. The Florida Multi-object Imaging Grism Spectrometer, or FLAMINGOS, can image tens of thousands of stars in a cloud in the near-infrared each night – many more than could have been examined without the instrument. Such observations must be made in the near-infrared because visible light from young stars is nearly completely absorbed by dust in the molecular clouds, rendering the forming stars invisible to the human eye.
|By combining the high sensitivity of space telescopes with the sharply detailed pictures from an interferometer, TPF will be able to reduce the glare of parent stars to see planetary systems as far away as 50 light years.
Computers can encode the infrared light in optical wavelengths, creating visible images. Most of these "snapshots" revealed only mature stars, and the resulting images appear similar to the sky on a clear night. However, the survey also revealed in several positions, in Elston’s words, "wild places" – startling clusters of infant stars in varied stages of formation. These stars appear in the image as colorful balls of light, with each of the seven disks resembling a dark swath surrounding each star.
Due to the blinding glare created by forming stars, such disks are not normally detectable from Earth. However, when the disk lies between Earth and a star, it masks some of the light from the star. When rendered as visible images, such nearly "edge on" disks resemble a pair of butterfly wings with a dark lane down the middle and are often called "silhouette disks."
Astronomers used to think that stars formed in relative isolation. But over the past two decades, research by Lada and others has shown that stars usually form in clusters – celestial birthing grounds. Lada and her colleagues have shown that the majority of stars in such clusters are formed with circumstellar disks. So while it wasn’t unusual to find the disks, the surprise was that they ranged in size from 10 to 100 times larger than any of the handful of similar disks yet seen and imaged – with each disk stretching thousands of astronomical units in diameter.
One astronomical unit, the distance from the sun to Earth, measures 93 million miles. The diameter of our solar system is approximately 60 astronomical units. The fact that these disks extend many times farther than that suggests that planets, too, could extend well beyond the relatively close proximity observed in our solar system and elsewhere. That would be good news for astronomers, because the further planets are from stars, the easier they are to detect, Elston said.
For most planet finders, the real challenge is to identify faint planets in the glare of their much brighter parent stars. To overcome the distortion of how our own atmosphere may further obscure this detection, both large land-based telescopes and space missions will likely combine in the future to complete the picture.
Both NASA and the European Space Agency (ESA) propose space missions to look for Earth-like planets in the infrared. NASA is developing the Terrestrial Planet Finder (TPF) project, part of the Jet Propulsion Laboratory Navigator Program, and ESA is developing its DARWIN project. The European Southern Observatory is exploring the possibility of ground-based searches using the future Overwhelmingly Large Telescope (OWL) project.
Related Web Pages
The University of California Planet Search Project
Astrobiology Magazine New Planets
Extrasolar Planets Encyclopedia
Planet Quest (JPL)
Space Interferometry Mission