Origins of Planetary Systems and the Development of Prebiotic Conditions

Research by M. Velli, (PI), M. Allen, G. Bryden, K. Grogan, R. Kidd, G. Orton, N. Turner, K. Willacy 1 S3263, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, 2 S3222 IBID, 3 S312A IBID, 4 S3226 IBID

Research Objectives:

Scientists at NASA's JPL brought together expertise in planet formation and prebiotic chemistry to develop new interdisciplinary tools for interpreting observations of protostellar disks returned by space telescopes such as Spitzer and Hubble. From these observations the scientists are extracting key signatures of planet formation to provide clues to the physical processes at work. There are four main areas of study the scientists chose to begin creating the planetary formation model:

Transport of Gas and Dust

The panels above show a selected volume of the disk a few AU in size, and the evolution of the dust and gas over time given by MHD simulations.

An outstanding issue in protostellar disk evolution is the observed variability of accretion onto the central star. The variations could be produced by collisions between solid bodies within the disk. Dust released after the collisions can spread through the turbulent gas, soaking up ions and electrons and reducing the conductivity so the magnetic fields that drive the accretion are no longer coupled to the gas. The accretion is switched off until the dust settles or coagulates.

Disk Morphology in the Presence of Planets

Sub-millimeter observations of Vega above left show evidence of a face-on disk around 100 AU in radius, with a central clearing.

Both ground and space based observations show evidence for interesting morphological structure in disks. For example, some disks seem to display inner holes, typically assumed to be due to clearing processes invoked by the gravitational influence of an embedded planet. In order to quantify these processes in greater detail, the scientists used used dust orbital evolution codes to follow the paths of particles in the presence of a planet. They found strong variations in the spatial distributions of dust as a function of planet mass, which has direct relevance to future planet finding missions such as SIM PlanetQuest and TPF.

Chemical Composition of Planet Forming Disks

This figure shows early results of linking chemical and protostellar disk models with radiative transfer codes. It also shows the distribution of CO and resulting spectra from a planetary disk viewed head-on.

Observations of molecules in young disks provide valuable information about the chemical composition and physical conditions. Understanding how molecules form in the disk, the likelihood that they can survive as the disk evolves and planets form, and their transport around the disk is fundamental to understanding the origin and evolution of the chemical and biological precursors for life. The scientists engaged in modeling the evolution of molecules and their distribution in protoplanetary disks. They have begun to use these results with a radiative transfer code to produce simulated line spectra that provide the observational signature that would be expected to be seen for a given molecule. The figure above illustrates their work. Shown is the calculated CO abundance distribution across a young disk (red is high abundance, green/blue is low abundance) and the simulated spectra such a distribution would produce.

Photometric Variability due to Embedded Planets

Model of shadowing across a protostellar disk caused by a Neptune mass planet at 4 AU. As the planet orbits the star the shadow varies in position causing significant variability of the system photo center.

Embedded planets are expected to cast a shadow across the disk, resulting in significant photometric variability over an orbital period of the planet. The above figure is a snapshot from a movie showing our model disk, with a Neptune mass planet at 4AU. The red cross to the lower right of the figure represents the photocenter of the system. As the orbital period progresses the photocenter moves to an extent greater than the SIM PlanetQuest accuracy (1 microarcsecond), represented by the size of the cross itself. This technique demonstrates how SIM photometric observations can be used for planet finding, and related to the mass and orbital characteristic of the disk.

Implications

This study brings together the expertise and theoretical models in a range of scientific fields bearing on the structure of proto-stellar disks, the formation of planets within the disks, and the development of pre-biotic chemistry in the material incorporated into the planets.

Significance to Solar System Exploration

This research will allow NASA missions to better detect and characterize extra-solar planets, including observations from the Herschel Space Observatory, SIM PlanetQuest, the Terrestrial Planet Finder Interferometer, and the Terrestrial Planet Finder Coronagraph. The research project has brought together scientists from a variety of disciplines to more comprehensively assess the evolution of circumstellar disks and the origins of planets.

Written by Samantha Harvey

For more information about NASA Science Highlights and information on publication, please contact Samantha Harvey, Samantha.K.Harvey@jpl.nasa.gov.

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