Research ArticleASTRONOMY

The origin of RNA precursors on exoplanets

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Science Advances  01 Aug 2018:
Vol. 4, no. 8, eaar3302
DOI: 10.1126/sciadv.aar3302
  • Fig. 1 Reaction scheme.

    The scheme of light and dark reactions we explore, considering two steps along the path to forming pyrimidines (RNA precursors): one with bisulfite Embedded Image as the electron donor and the other with hydrogen sulfide (H2S and HS) as the electron donor.

  • Fig. 2 Surface actinic fluxes.

    The surface actinic flux Fλ as a function of the wavelength λ for planets within the habitable zones of five stars (with spectral type given in the legend): early Earth in the case of the young Sun and Proxima b in the case of Proxima Centauri. For GJ832, ∈ Eri, and AD Leo, these are hypothetical planets at the innermost edge of the liquid water habitable zone to maximize the surface flux. The yellow shaded region (the width of which accounts for the errors; see Materials and Methods, especially Eqs. 27 to 30) shows the average flux needed between 200 and 280 nm for the light chemistry and dark chemistry to proceed both at the same rate at 0°C. The color-shaded horizontal lines show the average fluxes of the respective stars. The surface actinic flux of Earth today is also included. Its 200- to 280-nm flux is strongly attenuated by atmospheric ozone.

  • Fig. 3 Reaction yields.

    Contour plot of (A) the yield after one reaction and (B) the final concentration of the pyrimidines after seven consecutive reactions (three of which involve two products of a previous reaction) with initial reactants all at 1 M concentrations, as a function of temperature T (K) and the integrated surface UV flux ∫Fλ dλ (cm2 s−1) from 200 to 280 nm, assuming that all photochemical reactions have similar cross sections and rate constants to the Embedded Image reactions we measured, by multiplying the yield with itself 10 times, which accounts for the arithmetic demon for the case of 10 consecutive reactions and matches with the pseudo-equilibrium results if rate equations were applied. Comparable surface fluxes are listed on the left.

  • Fig. 4 Abiogenesis zone.

    A period-effective temperature diagram of confirmed exoplanets within the liquid water habitable zone (and Earth), taken from a catalog (1, 42, 43), along with the TRAPPIST-1 planets (3) and LHS 1140b (4). The “abiogenesis zone” indicates where the stellar UV flux is large enough to result in a 50% yield of the photochemical product. The red region shows the propagated experimental error. The liquid water habitable zone [from (44, 45)] is also shown.

  • Fig. 5 Flare frequencies.

    The power-law fits for the cumulative flare frequencies above a particular U-band energy, log (EU/1 erg), given for a variety of Kepler stars from Davenport (17). The grayscale indicates stellar mass, the red line indicates the average stellar flare frequency for stars later than K5, and the blue dashed line indicates the lower limit of flaring needed: If the star’s power-law flare frequency intersects the dotted blue line, then the star is active enough for its planets to be considered within the abiogenesis zone. Flares with EU > 1035 are often extrapolations of flare frequencies beyond what has been observed, and flare frequencies may not follow the same power-law at these high energies (17).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/8/eaar3302/DC1

    Fig. S1. The UV reactor.

    Fig. S2. Reactor emission plot of the measured (solid line) emission from the Hg lamps over the 200- to 400-nm spectral range.

    Fig. S3. Stellar spectra.

    Fig. S4. Surface spectral irradiance at Cambridge.

    Fig. S5. Arrhenius plots.

    Fig. S6. Flare spectra.

    Table S1. HCN + Formula.

    Table S2. Glycolonitrile + HS.

    Table S3. Percentages of stars in different spectral type ranges that meet the criterion for activity given by Eq. 38 (defined here as “Active” stars).

    Section S1. General procedures for reactions of KCN with Na2SO3 in the dark.

    Section S2. General procedures for reactions of KCN with Na2SO3 under UV light.

    Section S3. General procedures for reactions of glycolonitrile with NaHS in the dark.

    Section S4. General procedures for reactions of glycolonitrile with NaHS under UV light.

    References (4648)

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. The UV reactor.
    • Fig. S2. Reactor emission plot of the measured (solid line) emission from the Hg lamps over the 200- to 400-nm spectral range.
    • Fig. S3. Stellar spectra.
    • Fig. S4. Surface spectral irradiance at Cambridge.
    • Fig. S5. Arrhenius plots.
    • Fig. S6. Flare spectra.
    • Table S1. HCN + SO32.
    • Table S2. Glycolonitrile + HS.
    • Table S3. Percentages of stars in different spectral type ranges that meet the criterion for activity given by Eq. 38 (defined here as “Active” stars).
    • Section S1. General procedures for reactions of KCN with Na2SO3 in the dark.
    • Section S2. General procedures for reactions of KCN with Na2SO3 under UV light.
    • Section S3. General procedures for reactions of glycolonitrile with NaHS in the dark.
    • Section S4. General procedures for reactions of glycolonitrile with NaHS under UV light.
    • References (4648)

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