A combined transmission spectrum of the Earth-sized exoplanets TRAPPIST-1 b and c (1606.01103v1)

Abstract: Three Earth-sized exoplanets were recently discovered close to the habitable zone of the nearby ultracool dwarf star TRAPPIST-1. The nature of these planets has yet to be determined, since their masses remain unmeasured and no observational constraint is available for the planetary population surrounding ultracool dwarfs, of which the TRAPPIST-1 planets are the first transiting example. Theoretical predictions span the entire atmospheric range from depleted to extended hydrogen-dominated atmospheres. Here, we report a space-based measurement of the combined transmission spectrum of the two inner planets made possible by a favorable alignment resulting in their simultaneous transits on 04 May 2016. The lack of features in the combined spectrum rules out cloud-free hydrogen-dominated atmospheres for each planet at 10-$\sigma$ levels; TRAPPIST-1 b and c are hence unlikely to harbor an extended gas envelope as they lie in a region of parameter space where high-altitude cloud/haze formation is not expected to be significant for hydrogen-dominated atmospheres. Many denser atmospheres remain consistent with the featureless transmission spectrum---from a cloud-free water vapour atmosphere to a Venus-like atmosphere.

• The paper demonstrates a combined transmission spectrum analysis that rules out cloud-free hydrogen-dominated atmospheres for TRAPPIST-1 b and c at 12- and 10-sigma levels.
• The study employs HST’s WFC3 in the 1.1–1.7 μm range with rigorous data reduction achieving a precision of 650 ppm per 112-second exposure.
• The findings pave the way for future high-resolution observations with JWST to better constrain dense or hazy atmospheric compositions of Earth-sized exoplanets.

Combined Transmission Spectrum of TRAPPIST-1 b and c

The paper investigates the atmospheric properties of the two inner Earth-sized exoplanets, TRAPPIST-1 b and TRAPPIST-1 c, through a combined transmission spectrum analysis. Utilizing observations from the Hubble Space Telescope (HST) on May 4, 2016, during a rare simultaneous transit event, the authors provide significant constraints on the atmospheric composition of these exoplanets.

Methodology and Observations

The observations were conducted using the Wide Field Camera 3 (WFC3) aboard HST, focusing on the near-infrared wavelengths between 1.1 and 1.7 micrometers. The transmission spectroscopic data were collected by observing the changes in starlight as the planets transited their host star. Comprehensive data reduction techniques were employed to minimize the systematics inherent in HST data, achieving a standard deviation of normalized residuals (SDNR) of 650 parts per million (ppm) per 112-second exposure.

Results and Analysis

The analysis primarily rules out the possibility of TRAPPIST-1 b and c having cloud-free hydrogen-dominated atmospheres. This conclusion is based on the lack of detectable spectral features, which were ruled out at significance levels of 12- and 10-sigma for TRAPPIST-1 b and c, respectively. Consequently, the notion of an extended gas envelope is contraindicated, aligning with predictions of atmospheres characterized by higher molecular weights.

Alternative atmospheric compositions, including dense atmospheres such as water vapor or Venus-like compositions with high-altitude hazes, remain plausible. The featureless spectrum suggests the presence of clouds or hazes, possibly lagging at lower pressures than would create significant spectral variations detectable at the measured precision.

Implications and Future Directions

The findings contribute to the growing field of exoplanet atmospheric characterization by demonstrating the efficacy of transmission spectroscopy in constraining atmospheric compositions, even at the challenges posed by ultracool dwarf star systems such as TRAPPIST-1. These results guide future observational campaigns aimed at the intricate analysis required to discern among plausible atmospheric scenarios.

Prospective developments in observatories, such as the James Webb Space Telescope, will build on the foundational constraints provided herein. Enhanced spectral resolutions and coverage will be imperative in elucidating the atmospheric composition, dynamics, and potential habitability characteristics further for Earth-sized exoplanets in similar regimes.

Conclusion

This paper marks a critical step towards refining our understanding of the atmospheric properties of Earth-sized exoplanets, particularly those in close orbits around ultracool dwarf stars. The effective utilization of double transit observations and the subsequent ruling out of hydrogen-dominated atmospheres without significant clouds or hazes delineate clear pathways for future investigations in planetary science and exoplanet habitability.