Living next to a ringed planet in a flat solar system in a spiral galaxy may make you think there are a lot of disk-shapes in space. And, indeed, there are. A January 2005 issue of the journal Science contains a special section featuring the roles disks play in the universe.
NASA Astrobiology Institute investigators are studying circumstellar disks, the gasses and dust surrounding stars, to discover how planets are made and whether Earth-like planets may exist beyond our solar system.
Stars form from condensing gas and dust. After a star is born, the remaining particles swirl around the dense body forming the circumstellar disk. The conservation of angular momentum coupled with gravity pulling particles together causes flattening of the particles into a disk shape, similar to a tossed ball of pizza dough bulging around the sides and flattening in the center along the axis of rotation.
While it is too late to watch our solar system form, the circumstellar disks of neighboring stars may help determine how it formed. Studying young stars with their disks may be like looking back in time at our Sun's youth.
Most circumstellar disks cannot be studied by simply collecting light through a telescope. The star's light far outshines the light reflected by surrounding dust like the light of our own Sun preventing Mars or Venus being seen during the day. By using cameras that observe infrared wavelengths of light, researchers are able to observe the dust surrounding stars by detecting the emitted heat radiation.
NAI investigator Alycia Weinberger of the Carnegie Institution of Washington, D.C. takes infrared images of young stars with the Hubble Space Telescope. From the color and morphology of the circumstellar disks, she is able to study the composition and evolution of planetary systems. She looks for disk distortions such as rings, gaps, and warps, which may disclose the presence of a planet.
NAI Investigator Joan Najita of the University of Arizona also images disks using infrared spectroscopy. By quantifying the gas content of circumstellar disks, she studies how the conditions around young stars may lead to the rich diversity of planetary systems seen.
While it is known that planets are formed in this circumstellar disk, the process of formation is debated. In the classic Core Accretion model, a solid planetary core forms from collisions between small chunks of ice and rock, eventually reaching a size that allows a gaseous atmosphere to accrue. Because the millions of years required for this process is thought to be too slow, a conflicting model of star formation is gaining support. The Disk Instability model predicts gravitational instabilities in the disk, causing the fast collapse of clumps that contract to form planet cores. This process would take just thousands of years. NAI scientist Alan Boss of the Carnegie Institute of Washington, D.C., a proponent of the Disk Instability theory, is tackling the computationally heavy computer simulations of star formation within circumstellar disks.
The young planets attain positions and orbits based on their interaction with the other debris in the circumstellar disks. NAI investigator Brad Hansen of the University of California, Los Angeles, models the evolution of this process in both single and multiple planet systems. He and his collaborators hope to understand the unusual orbits of planets seen around other stars.
Understanding the process of planet formation and characterizing young stars will help answer the question of how common Earth-like planets are, uncovering another clue in the mystery of whether there is other life in the universe.