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2013 Annual Science Report

NASA Goddard Space Flight Center Reporting  |  SEP 2012 – AUG 2013

Evolution of Protoplanetary Disks and Preparations for Future Observations of Habitable Worlds

Project Summary

The evolution of protoplanetary disks tells the story of the birth of planets and the formation of habitable environments. Microscopic interstellar materials are built up into larger and larger bodies, eventually forming planetesimals that are the building blocks of terrestrial planets and their atmospheres. With the advent of ALMA, we are poised to break open the study of young exoplanetesimals, probing their organic content with detailed observations comparable to those obtained for Solar System bodies. Furthermore, studies of planetesimal debris around nearby mature stars are paving the way for future NASA missions to directly observe potentially habitable exoplanets.

4 Institutions
3 Teams
11 Publications
2 Field Sites
Field Sites

Project Progress

Debris disks are relatively gaspoor circumstellar disks around young main sequence stars (~ 10 – 100s of million years old). The younger debris disks are likely in the late stages of terrestrial planet formation, while the older ones correspond to the “clearing out” phase of planetary system evolution, where most leftover planetesimals are removed from the system by collisions and ejection. Some older debris disks with anomalously high dust abundances may currently be experiencing a dramatic rearrangement of their planetary systems, like the one that occurred during the Solar System’s Late Heavy Bombardment Period and explained by the so-called “Nice Model.”

While it has long been known that these disks are composed of the destruction products of young extrasolar planetesimals, detailed studies of their composition have been hard to come by. However, such studies would provide radical new information on the bulk organic content of young planetesimals formed in environments both similar to and different from the solar nebula, illuminating unobserved processes that occurred in the early Solar System and providing context for Solar System planetesimal compositions. Roberge is a participant in a new ALMA study that has obtained the first detection of sub-mm CO emission from the famous Beta Pictoris debris disk (Figure 1). We propose that the clumps of CO gas seen are coming from extrasolar comets trapped into resonance with an unseen giant exoplanet (Dent et al., submitted to Science). Roberge is PI or Co-I on several follow up ALMA proposals to further probe carbonaceous gases in Beta Pic and other debris disks.

Roberge led a study of the unusual 49 Ceti debris disk using farinfrared data from the Herschel Space Observatory, which showed a surprising amount of carbon gas emission that may arise from destruction of exocomets (Roberge et al. 2013). She is leading a follow up study of 49 Cet: a sensitive probe of the disk gas elemental composition using farultraviolet spectroscopy from the Hubble Space Telescope. Collaborator C. Grady is a participant in this work.

Furthermore, warm debris dust coming from planetesimals in extrasolar planetary systems (aka. exozodiacal dust) is likely to be the largest source of noise for future direct observations of habitable planets (further details in Roberge et al. 2012). Unfortunately, our current knowledge about exozodiacal dust is far from adequate to robustly plan or prepare for a future exoplanet mission aimed at observations of exoEarths (i.e. a New Worlds Mission as envisaged in the Astro2010 Decadal Survey). Co-I Roberge is addressing this problem through her membership in the LBTI Key Science Team. LBTI is a NASA-funded instrument for the Large Binocular Telescope, with the primary science aim of probing for exozodiacal dust down to levels relevant for a New Worlds Mission. Instrument commissioning and science planning for this survey are in progress. Science observations will begin in a few months and will extend over the next 4 years.

Figure 1.
ALMA observed CO emission from the Beta Pic debris disk. This figure shows the de-projected face-on distribution of CO, assuming circular orbital motion. The CO clumps are likely coming from swarms of comets trapped into mean motion resonances with an unseen giant exoplanet. Image credit: Figure 3A, Dent et al., submitted.

    Aki Roberge
    Project Investigator

    Carol Grady

    Objective 1.1
    Formation and evolution of habitable planets.

    Objective 1.2
    Indirect and direct astronomical observations of extrasolar habitable planets.

    Objective 3.1
    Sources of prebiotic materials and catalysts

    Objective 4.3
    Effects of extraterrestrial events upon the biosphere

    Objective 7.2
    Biosignatures to be sought in nearby planetary systems