Research Article

Constraining Exoplanet Mass from Transmission Spectroscopy

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Science  20 Dec 2013:
Vol. 342, Issue 6165, pp. 1473-1477
DOI: 10.1126/science.1245450

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  1. Fig. 1 Transit-depth variations, Embedded Image, induced by the wavelength-dependent opacity of a transiting planet atmosphere.

    The stellar disk and the planet are not resolved; the flux variation of a point source is observed.

  2. Fig. 2 Basics of a planet’s transmission spectrum (planetary atmosphere scaled up to enhance visibility).

    (A) In-transit geometry as viewed by an observer presenting the areas of the atmospheric annulii affecting the transmission spectrum. (B) Side view showing the flux transmitted through an atmospheric annulus of radius r. (C) Transmittance as a function of the radius at wavelengths with high and low atmospheric absorption: λ1 (solid lines) and λ2 (dash-dotted lines), respectively. Due to higher atmospheric absorption at λ1, the planet will appear larger than it does at λ2 because of the more-extended opaque atmospheric annulus [heff1) > heff2)] that translates into an additional flux drop (14).

  3. Fig. 3 The boundaries of MassSpec’s application domain for 200 hours of in-transit observations.

    Using JWST, MassSpec could yield the mass of super-Earth and Earth-sized planets up to the distance shown by the black dashed and dotted lines, respectively. Similarly, the maximum distance to Earth for MassSpec’s application based on EChO’s observations of a mini-Neptune, a super-Earth, and an Earth-sized planet are shown by the blue solid, dashed, and dotted lines, respectively. The green dotted line refers to the case of an Earth-sized planet observed with a 20-m space telescope. The gray area shows the stars too bright for JWST/NIRSpec in the R = 1000 mode (J-band magnitude ≲7).

  4. Fig. 4 MassSpec’s application to the synthetic transmission spectrum of a water-dominated super-Earth transiting an M1V star at 15 pc as observed with JWST for a total of 200 hours in-transit.

    (A) Synthetic data and the best fit, together with the individual contributions of the atmospheric species. (B) Normalized posterior probability distribution (PPD) of the atmospheric species number densities at the reference radius. (C) Normalized PPD for the scale height. (D) Normalized PPD for the pressure at the deepest atmospheric level probed by transmission spectroscopy. (E) Normalized PPD for the temperature. (F) Normalized PPD for the exoplanet mass. The diamonds indicate the values of atmospheric parameters used to simulate the input spectrum, and the asterisks in (A) indicate molecules that are not used to simulate the input spectrum. The atmospheric properties (number densities, scale height, and temperature) are retrieved with significance, yielding a mass measurement with a relative uncertainty of ∼10%.

  5. Fig. 5 MassSpec’s application to the synthetic transmission spectrum of an Earth-like exoplanet transiting an M7V star at 15 pc as observed with JWST for a total of 200 hours in-transit.

    (A to F) show the same quantities as on Fig. 4. The atmospheric properties (number densities, scale height, and temperature) are retrieved with significance, yielding a mass measurement with a relative uncertainty of ∼8%.

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