IU astronomer develops new theory of planet formation

At the Kirkwood Observatory IU astronomy students can view solar phenomena and celestial bodies Wednesday nights. For some students, simply viewing planets might be fulfilling; however, for those students and scientists interested in the science behind the planets' formations, there are now three theories to turn to, thanks to studies by IU astronomy professor Richard Durisen. His work is published in the February issue of the journal Icarus.\nPreviously, there were two main existing theories of how planets such as Earth and gas giants like Jupiter came to be in the universe -- the core creation theory and the gravitational instability theory. Currently, debate exists in the planetary science and astrophysics community between which theory is likely to be more correct. Durisen's new theory is a hybridization of the former two.\nIn the early stages of galaxy development, objects, called protoplanetary disks, rotate like liquid around young stars. These disks are mostly gas with bits of dust, Durisen said. Scientists believe that if these disks are large and cold enough, they will create gravitational instabilities within them. These instabilities are the key aspects that drive Durisen's theory to planet development.\nWhen the gravity of the disk becomes unstable, it overwhelms gas pressure. When this happens, part of the disk clumps together and forms dense structures. It is these masses, Durisen said, that will later go on to form planets.\nTo illustrate his theory, Durisen uses computer simulation models with the use of computer graphics and hydrodynamics. \nDuring these simulations, green disks of gas swirl around a central star. Regions where the gas becomes denser show up as yellow spiraling arms inside of the protoplanetary disk. High-density red regions appear as faint blobs that become slowly more stable. It is the red regions that will eventually grow in size and develop gas planets.\nCurrently, scientists know of 100 planets that circle other stars. Nearly all of them are at least the size of Jupiter. Planets this massive are known as "gas giants." New studies have suggested that these giants form more quickly than smaller planets to consume the gas before it disappears. This idea of planets forming quickly is the main deficiency with the older core creation theory. \n"In the core accretion theory, gas giants form slowly," Durisen said. "First, smaller solid particles accrete to make a 'core.' Then this solid core attracts a huge gaseous envelope from the gas disk once the core is large enough."\nThe core creation theory postulates that gas planets begin their formation with solid masses colliding with each other and forming larger masses that increase in mass up to 10 times Earth's mass, resulting in a massive Jupiter-like planet, Durisen explained.\nThe downfall of this theory is the formation of planets that exceed the mass of Jupiter would take between 10 and 100 million years in the formation of a solid core, according to Spaceflight Now, an online organization of space information. In light of new research, this amount of time is not feasible for gas giants to form. \nAlternatively, the gravitational instability theory explains the formation of planets in terms of "disk instability." This theory, first purposed 50 years ago, suggests that spiral arms form inside the protoplanetary disk. These break into clumps that have different orbits. As a result, these would-be planets have a high risk of colliding or breaking off into the central star.\n"In order for planets to form in unstable disks, there must be a more protective environment for them to so without them becoming torn apart," Durisen said.\nDurisen's theory might have discovered the way that these solid clumps can find stable places within the protoplanetary disk to form planets by combining both of the old theories.\nOn the edge of the disk, stable and dense rings of matter form. This is the first time a simulation has been able to show the steady growth of rings in a protoplanetary disk. According to the new theory, if potential planets move to this ring, they have more of a chance to quickly develop a core and survive.\n"If the ring grows out of this gravitational instability, it will help the core grow faster," said Kai Cai, the second author of the paper and an astronomy graduate student at IU. "Contrary to the gravitational instability theory, we say (planets form) not directly from the instability of the disk. Our point is that we see these rings from gravitational instabilities that will speed up and help planet formation."\nThis gravitational instability depends on the temperature of the disk. It appears that stable spirals ideal for planet formation develop when the protoplanetary disk is cooling. However, the hotter the disk gets, the more likely it is to become violently unstable and eliminate the possibility of planet formation. \n"The idea is very interesting and plausible," Cai said. "Right now, we think these rings are a natural outcome of these instabilities, We are planning on doing more simulation to see if the disks always conceive these rings. If these rings are a natural outcome of gravitational instability, it will be a natural thing to occur."\nUnderstanding this phenomenon is essential to future studies of Durisen's theory and the understanding of how massive planets form.\n"Gas giant planet formation is one of the key fundamental questions concerning our own human origin in the universe, with implications for the existence of other life-friendly worlds," Durisen said.\n-- Contact Staff Writer Fatimah Scott at fascott@indiana.edu.