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Project Progress

In this project, we have concentrated on gathering and providing spectroscopic information required for modeling extrasolar planets. The two current components of this effort are 1) providing high-resolution, full-wavelength range spectra for stars of different spectral type, and 2) collating and providing a comprehensive database of the spectral line lists and wavelength-dependent surface albedos required by the VPL radiative transfer models to produce synthetic spectra of planetary atmospheres and surfaces.

Stellar Spectra: Cohen has worked to provide broad-wavelength spectra for stars of different spectral type, and to look at the range of ultraviolet (UV) properties of G star “solar analogs”. We have extracted short- and long-wavelength UV spectra of 18 G-dwarf stars, and the K2V star epsilon Eri from NASA’s International Ultraviolet Explorer (IUE) Final Archive and have recalibrated them. To these UV spectra we have appended Kurucz photospheric spectra covering the visible and infrared (IR). These full-wavelength range spectra were used by VPL team members (led by Kasting) to model the atmospheres and appearance of Earth-like planets around stars of different spectral type.

To explore G star UV properties and their effects on habitability, Cohen compared the empirical UV and modeled-photospheric spectra for two solar-analog G0V stars, HD206860 (active) and HD 114710 (inactive) and also compared them to the Sun. From this analysis we conclude that the solar-analog assumption based on classification by spectral type alone is a poor way to establish stars with potentially habitable planets, as both the active and inactive G0V stars are far more UV-luminous than the Sun, and provide very different UV fluxes incident on an associated planet.

Line List Database: Brown and Butler have worked to collect line lists and absorption cross-sections for 45 molecules of interest to planetary modelers. These data have been collated and can be “ordered” by the team via a web-page interface that provides spectra of the line-list data in both wavenumbers and microns. Each molecule in the database may have available data from several different sources, including infrared absorption cross-sections from Pacific Northwest National Lab (PNNL), infrared line lists from the HITRAN database, UV cross-sections from personal collections of VPL personnel, or journal articles. Although the database is currently only accessible by VPL members, we plan to make it available to the NAI community later this year.