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2013 Annual Science Report

VPL at University of Washington Reporting  |  SEP 2012 – AUG 2013

Exoplanet Detection and Characterization: Techniques, Retrieval and Analysis

Project Summary

In this task VPL team members use theoretical modeling and analysis to develop new techniques for detecting and characterizing extrasolar planets. These include developing new techniques for detecting exoplanets in transit data, especially those planets with unusual orbital properties. Team members also work to provide the underlying theory required to develop new remote-sensing techniques for probing planetary atmospheres. They also work on techniques and tests for the retrieval of information about planetary environments from exoplanet photometry and spectra.

4 Institutions
3 Teams
5 Publications
0 Field Sites
Field Sites

Project Progress

In this task, VPL team members develop new techniques for detecting exoplanets in astronomical data, as well as new remote-sensing techniques for retrieving information about planetary environments from exoplanet photometry and spectra.

We have improved algorithms and software for the detection of exoplanets. Agol and colleagues published a new technique for detecting quasi-periodic transiting planets (Carter and Agol, 2013). This new technique will increase our ability to find planets whose orbits are very strongly perturbed by their planetary companions, that would normally be missed by classical search algorithms. Agol and colleagues also developed a software tool to allow rapid analysis of exoplanet transit data (Eastman et al., 2013).

In the area of predictions, retrieval and analysis for exoplanet characterization Agol and Dobbs-Dixon used 3-D hydrodynamical simulations of highly irradiated Jovian planets to predict primary and secondary eclipse timing offsets as a function of wavelength (Dobbs-Dixon and Agol). These predictions may help improve observations of these targets as observed by the James Webb Space Telescope. Line, Crisp, Yung, and colleagues developed, tested, and published the relative performance of three commonly used remote sensing retrieval algorithms (optimal estimation, Markov-Chain Monte Carlo, and Bootstrap Monte Carlo) for interpreting realistic, synthetic spectra of exoplanets (Line et al., 2013). Ongoing work includes the retrieval of chemical disequilibrium states from exoplanet spectra.

VPL team members also developed a new technique for probing the atmospheric pressure of a terrestrial exoplanet, a planetary characteristic that is very challenging to measure. Misra, Meadows, Claire and Crisp used SMART to assess the use of dimers, temporary associations of two atmospheric molecules, to retrieve surface pressures on exoplanets. Absorption from dimer molecules (e.g. O4) is more strongly dependent on pressure than that of monomer molecules (e.g. O2). We found that by comparing the absorption strengths of O2 monomer and dimer features it may be possible to estimate the surface or cloud-top pressure on some exoplanets (Misra et al., in press). In ongoing work, Misra, Crisp and Meadows, completed modification of the VPL’s line-by-line radiative transfer model (SMART)to generate transit transmission spectra (Misra et al., in prep). The model includes gas absorption, cloud and aerosol extinction, refraction, and the effects of stellar limb darkening. We find that the inclusion of refraction decreases the predicted detectability of spectral absorption features in transit transmission (Misra et al., in prep).

The VPL transit-transmission model was used to calculate the realistic detectability of planetary spectral features with refraction and viewing geometry included. The plot shows the fractional increase in integration time required to achieve the same SNR for absorption features in a predicted spectrum that includes refraction - compared with a non-refracted spectrum - as a function of the angular diameter of the star from the Earth analog’s perspective. The stellar masses given on the top x-axis are the mass of the star for a given angular diameter assuming the Earth-analog is orbiting at a flux-equivalent distance, or the distance where the top of atmosphere incident flux is 1373 W/m2. Molecules that are concentrated at the lowest altitudes, such as water vapor and dimer molecules, are the most affected by the cutoff set by refraction, as are planets orbiting larger mass stars like our Sun (Misra et al., submitted).
Transmission spectra of an Earth-like atmosphere with different total atmospheric pressures. (a) Difference in flux from the stellar flux. (b) Percent of stellar flux absorbed by the atmosphere. Features from both the oxygen monomer (O2) and the oxygen dimer (O4) can be seen. The 1.06 μm dimer feature is strong only in the spectra corresponding to atmospheric pressures greater than ∼0.5 bar. The spectra for atmospheres with pressure ≥1 bar are nearly identical except for an offset because there is a fundamental limit on which heights in an atmosphere can be probed using transmission spectroscopy. Measurements of the monomer and dimer strengths can be used as an atmospheric pressure probe. The strong dimer bands may also be the most detectable biosignature in Earth-like planetary atmospheres (Misra et al., 2013).

    Eric Agol
    Project Investigator

    Victoria Meadows
    Project Investigator

    Mark Claire

    David Crisp

    Michael Line

    Amit Misra

    Yuk Yung

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

    Objective 7.2
    Biosignatures to be sought in nearby planetary systems