ReviewSPACE SCIENCES

The search for signs of life on exoplanets at the interface of chemistry and planetary science

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Science Advances  06 Mar 2015:
Vol. 1, no. 2, e1500047
DOI: 10.1126/sciadv.1500047

Figures

  • Fig. 1 Exoplanet discovery space as of 2014.

    Color coded according to the planet discovery technique. (Left) Plotted as mass versus orbital period and not including Kepler discoveries. (Right) Plotted as radius versus orbital period (and using a simplified mass-radius relationship to transform planet mass to radius) and shows just how many exoplanets have been discovered, most by the Kepler space telescope. The paucity of planets of Earth’s size or mass and orbit emphasizes the challenge of exoEarth discovery with any planet-discovery technique. Figure from (10).

  • Fig. 2 Simulated spectra of small exoplanet atmospheres.

    Reflected light spectra are presented in units of planet-to-star flux ratio and are of the spectral resolution anticipated for exoEarths with future space-based starlight-suppression capable telescopes. The Earth spectrum is a model developed to match Earth observations from the EPOXI mission (105), whereas the super-Earth is that model scaled by (1.5 R/1 R)2. The Venus spectrum is a model from the Virtual Planet Laboratory (VPL; http://depts.washington.edu/naivpl/). The Archean Earth spectrum is a model of the inhabited Earth before the rise of oxygen in its atmosphere (87). The sub-Neptune (“mini-Neptune”) model is a 2.5 R Neptune-like planet at 2 AU from a solar-twin star (R. Hu, personal communication). The spectra have been convolved to R = 70 spectral resolution and binned with two pixels per resolution element (Nyquist sampling). Figure courtesy of A. Roberge.

  • Fig. 3 Schematic of a transiting exoplanet.

    When the planet goes in front of the star as seen from a telescope (a “transit” or “primary eclipse”), the starlight drops in brightness by the planet-to-star area ratio. In addition, the starlight passes through the planet atmosphere and planet atmosphere spectral features are imprinted on the stellar spectrum. This is called transmission spectroscopy. When the planet goes behind the star, the planet disappears and reappears, adding either reflected light or thermal emission to the combined planet-star radiation. This is referred to as secondary eclipse photometry or spectroscopy. Dozens of transiting exoplanet atmospheres have been studied, taking advantage of the fact that the planet and star do not need to be spatially separated as projected on the sky.

  • Fig. 4 Schematic of a transmission spectrum.

    Transmission spectra are created by planet atmosphere absorption imprinted on starlight passing through the planet atmosphere. (Top) Shown are a variety of different cases: deep and wide spectral features are expected from a low–molecular weight atmosphere, which presents a relatively large volume to be sampled. (Middle) In contrast, only narrow, shallow spectral features are expected from a high–molecular weight atmosphere, which results in a limited atmosphere to be sampled by the starlight. (Bottom) Clouds block starlight and may entirely prevent sampling of the dense part of the atmosphere where spectral lines form. Other situations are also possible. Many exoplanet spectra including (22) find no spectral features, consistent with clouds. Figure credit: S. Seager and D. Beckner.

  • Fig. 5 Transmission spectra of GJ 1214b.

    Models are represented by solid curves. No spectral features are detected in the most recent data set, because a straight line can be fit to the data (bottom panel). Figure credit and more details in (22).

  • Fig. 6 Schematic illustration of microbial-used chemical potential energy gradients.

    Redox half-reactions are shown in order of their electrode potential (in volts) at pH 7.0, calculated from the standard electrode potentials for the reactions (106). Each reaction is shown as an oxidation half-reaction (left side) and as a reduction half-reaction (right side). If the oxidation of molecule A in the left column is above reduction of a molecule in the right column, then the oxidation of A can be coupled to the reduction of B yielding energy. Thus, arrows drawn between half-reactions yield energy if they run downward from left to right, and the length of the arrow indicates the amount of energy released. Coupled reactions noted in color are as follows: (1) nitrogen reduction (also nitrogen fixation or ammonia synthesis); (2) sulfate reduction (also anaerobic biomass oxidation); (3) anaerobic sulfide oxidation; (4) aerobic biomass oxidation/oxidation of organic matter/oxidation of carbohydrate; (5) aerobic sulfide oxidation; (6) aerobic ammonia oxidation; (7) aerobic iron oxidation.

  • Fig. 7 Schematic for the concept of considering all small molecules in the search for biosignature gases.

    The goal is to start with chemistry and generate a list of all small molecules and filter them for the set that is stable and volatile in temperature and pressure conditions relevant for exoEarth planetary atmospheres. Further investigation relates to the detectability: the sources and sinks that ultimately control the molecules’ accumulation in a planetary atmosphere of specific conditions as well as its spectral line characteristics. Geophysically or otherwise generated false positives must also be considered. In the ideal situation, this overall conceptual process would lead to a finite but comprehensive list of molecules that could be considered in the search for exoplanet biosignature gases. Figure credit: S. Seager and D. Beckner.

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