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2014 Annual Science Report

NASA Goddard Space Flight Center Reporting  |  SEP 2013 – DEC 2014

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
7 Publications
2 Field Sites
Field Sites

Project Progress

Debris disks are relatively gas-poor 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.

A. Roberge is the lead on a new Hubble Space Telescope ultraviolet absorption study of gas in the unusual 49 Ceti debris disk system (Roberge et al. 2014). Collaborator C. Grady was a participant in this work. Analysis of these data showed that 1) this disk contains an unusually large amount of atomic gas and 2) the gas is extraordinarily carbon-rich (C/Fe ~ 24,000). The gas in the famous Beta Pictoris debris disk has a similar carbon overabundance (Roberge et al. 2006), which has been shown to play a vital dynamical role in preventing all the circumstellar gas from being rapidly blown out by stellar radiation pressure (Fernandez et al. 2006). Furthermore, there is tentative evidence that the carbon overabundance has its origin in the parent material of the gas, which may be overabundant in volatile material compared to solar abundances (Xie et al. 2014). It appears likely that a similar scenario is playing out in the slightly older 49 Ceti debris disk.

The HST UV data also showed transits of stargrazing exocomets around 49 Cet, through the detection of time-variable gas falling onto the star at high velocity (Figure 1; Miles et al., in preparation). Analysis of the infalling gas coming directly from comet vaporization by 2014 summer intern B. Miles (UCLA) indicates that it too is carbon-rich. A paper on these features is in preparation. A. Roberge also wrote a Nature News & Views article (Roberge 2014) about a new paper on discovery of two dynamical families in the exocomets around Beta Pic (Kiefer et al. 2014).

A. Roberge is Co-I on an approved ALMA Cycle 1 project (PI: M. Hughes, Wesleyan University) to map 49 Cet’s CO content and PI of an approved ALMA Cycle 2 project to map the neutral carbon content. The Cycle 1 data are in hand; preliminary analysis shows an intriguing asymmetry in the CO map that might be the sign of a gas clump. Together, these ALMA data will illuminate the relationship between the molecular and atomic gas in this important debris disk.

Finally, 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 (e.g. Weinberger et al. 2015, Kennedy et al. 2015). Early science observations have begun (Defrere et al. 2015); the full survey will extend over the next 4 years.

This figure shows ionized carbon absorption in two epochs of UV spectra of the star separated by 5 days. In the second visit, redshifted CII absorption is seen, indicating variable gas falling onto the star at high velocity. Similar features are seen in the famous Beta Pictoris debris disk and are attributed to evaporating exocomets passing through our line of sight to the central star. Image credit: Miles et al., in preparation.

    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