[Paper Review] Characterizing Earth-like Planets Using a Combination of High-Dispersion Spectroscopy and High-Contrast Instruments: Doppler-shifted Water and Oxygen Lines

Hajime Kawahara, Teruyuki Hirano|arXiv (Cornell University)|Sep 19, 2014

Spectroscopy and Laser Applications3 references3 citations

TL;DR

This paper proposes using high-dispersion spectroscopy combined with high-contrast imaging to detect Doppler-shifted water vapor and oxygen lines in Earth-like planets around M dwarfs, enabling atmospheric characterization of nontransiting exoplanets. With a 10-day exposure on a 30-meter telescope, raw contrasts of $10^{-4}$ and $10^{-5}$ enable 3σ and 16σ water detection, respectively, while $10^{-5}$ contrast allows 6σ oxygen detection at 1.27 μm.

ABSTRACT

Future radial velocity, astrometric, and direct-imaging surveys will find nearby Earth-sized planets within the habitable zone in the near future. How can we search for water and oxygen in those nontransiting planets? We show that a combination of high-dispersion spectroscopic and coronagraphic techniques is a promising technique to detect molecular lines imprinted in the scattered light of Earth-like planets (ELPs). In this method, the planetary signals are spectroscopically separated from telluric absorption by using the Doppler shift. Assuming a long observing campaign (a 10-day exposure) using a high-dispersion spectrometer (R=50,000) with speckle suppression on a 30-m telescope, we simulate the spectra from ELPs around M dwarfs (whose stellar effective temperature is 2750-3750 K) at 5 pc. Performing a cross-correlation analysis with the spectral template of the molecular lines, we find that raw contrasts of $10^{-4}$ and $10^{-5}$ (using Y, J, and H bands) are required to detect water vapor at the 3 $σ$ and 16 $σ$ levels, respectively, for $T_\star$=3000 K. The raw contrast of $10^{-5}$ is required for a 6 $σ$ detection of the oxygen 1.27 $μ$m band. We also examine possible systematics, incomplete speckle subtraction, and the correction for telluric lines. When those are not perfect, a telluric water signal appears in the cross-correlation function. However, we find the planetary signal is separated from that resulting from the velocity difference. We also find that the intrinsic water lines in the Phoenix spectra are too weak to affect the results for water detection. We conclude that a combination of high-dispersion spectroscopy and high-contrast instruments can be a powerful means to characterize ELPs in the extremely large telescope era.

Motivation & Objective

• To develop a method for detecting atmospheric water vapor and oxygen in nontransiting Earth-like planets around M dwarfs.
• To overcome challenges in low-resolution spectroscopy, such as telluric contamination and complex spectral modeling.
• To demonstrate that high-dispersion spectroscopy can robustly identify molecular lines without additional post-processing.
• To assess the required raw contrast levels for detecting water and oxygen at specific signal-to-noise levels.
• To evaluate the impact of systematics like incomplete speckle subtraction and telluric line correction on detection reliability.

Proposed method

• Simulate high-dispersion spectra (R = 50,000) of Earth-like planets around M dwarfs at 5 pc using a radiative transfer model of Earth's atmosphere.
• Apply cross-correlation analysis with molecular templates for water and oxygen to detect Doppler-shifted planetary signals.
• Use a 10-day total exposure time to integrate planetary signal and improve detection sensitivity.
• Model high-contrast imaging performance with speckle suppression to achieve raw contrasts of $10^{-4}$ and $10^{-5}$ at 15 mas separation.
• Assess systematics by simulating imperfect speckle subtraction and telluric line correction effects on the cross-correlation function.
• Focus on Y, J, and H bands, with particular emphasis on the J band due to strong water and oxygen features and better ExAO performance.

Experimental results

Research questions

• RQ1 Can high-dispersion spectroscopy combined with high-contrast imaging detect water vapor in nontransiting Earth-like planets around M dwarfs?
• RQ2 What raw contrast levels are required to achieve 3σ and 6σ detection of water vapor and oxygen at 1.27 μm?
• RQ3 How do systematics such as incomplete speckle subtraction and telluric line correction affect the reliability of molecular detection?
• RQ4 Is the planetary signal distinguishable from telluric contamination due to Doppler shift differences?
• RQ5 Can this method avoid the need for complex post-processing and be less sensitive to telluric noise than low-resolution spectroscopy?

Key findings

• A raw contrast of $10^{-4}$ enables a 3σ detection of water vapor in Earth-like planets around M dwarfs with $T_{\star} = 3000$ K.
• A raw contrast of $10^{-5}$ enables a 16σ detection of water vapor, and a 6σ detection of the oxygen 1.27 μm band.
• The J band provides the strongest water signal and is optimal for detection due to favorable spectral features and ExAO performance.
• The planetary signal is robustly separated from telluric contamination due to its Doppler shift, even when systematics are present.
• The method is less sensitive to telluric noise and does not require additional post-processing beyond standard cross-correlation.
• Intrinsic water lines in the Phoenix spectrum are too weak to interfere with the detection of planetary water vapor.