[Paper Review] Developing post-coronagraphic, high-resolution spectroscopy for terrestrial planet characterization on ELTs

Nemanja Jovanović, Olivier Guyon+13 more|arXiv (Cornell University)|Dec 21, 2017

Stellar, planetary, and galactic studies12 references9 citations

TL;DR

This paper proposes a novel method for high-resolution spectroscopy of terrestrial exoplanets using post-coronagraphic fiber injection with SCExAO and IRD on the Subaru Telescope. By combining adaptive optics, speckle control, and single-mode fiber coupling, it achieves extreme contrast to isolate planetary light, enabling high signal-to-noise detection of atmospheric molecular features via cross-correlation with template spectra.

ABSTRACT

Spectroscopic observations are extremely important for determining the composition, structure, and surface gravity of exoplanetary atmospheres. High resolution spectroscopy of the planet itself has only been demonstrated a handful of times. By using advanced high contrast imagers, it is possible to conduct high resolution spectroscopy on imageable exoplanets, after the star light is first suppressed with an advanced coronagraph. Because the planet is spatially separated in the focal plane, a single mode fiber could be used to collect the light from the planet alone, reducing the photon noise by orders of magnitude. In addition, speckle control applied to the location where an exoplanet is known to exist, can be used to preferentially reject the stellar flux from the fiber further. In this paper we will present the plans for conducting high resolution spectroscopic studies of this nature with the combination of SCExAO and IRD in the H-band on the Subaru Telescope. This technique will be critical to the characterization of terrestrial planets on ELTs and future space missions.

Motivation & Objective

• To enable high-resolution spectroscopy of terrestrial exoplanets by overcoming the extreme contrast challenge posed by host star light.
• To develop a method that uses single-mode fiber injection to isolate planetary light and suppress stellar halo photons.
• To implement speckle control techniques—both quasi-static and atmospheric—using deformable mirrors and predictive algorithms to enhance contrast.
• To demonstrate the feasibility of high-contrast differential spectroscopy (HDC) with existing instruments on a 8-meter telescope as a precursor to ELT missions.
• To enable the detection of molecular features in exoplanet atmospheres with high signal-to-noise ratio through cross-correlation of planetary spectra with molecular templates.

Proposed method

• Use SCExAO’s extreme adaptive optics system to achieve Strehl ratios >90% in the H-band, minimizing wavefront errors and concentrating starlight into the PSF core.
• Apply a coronagraph to suppress on-axis starlight and reduce diffraction halo, improving contrast in the dark zone near the star.
• Inject light from the planet into a single-mode fiber using a fiber injection unit that tracks the planet’s position via precise Cartesian motion control.
• Employ a speckle nulling algorithm to suppress quasi-static speckles by modulating the deformable mirror phase at the planet’s location, reducing flux in the fiber by 100–1000×.
• Use predictive control and spatial linear dark field control (LDFC) to suppress rapidly evolving atmospheric speckles, minimizing photon noise and enabling longer integration times.
• Utilize the IRD spectrograph (R ≈ 65,000) to disperse the fiber-coupled light, enabling high-resolution detection of molecular features in planetary atmospheres via cross-correlation with template spectra.

Experimental results

Research questions

• RQ1 Can single-mode fiber injection combined with post-coronagraphic speckle control achieve sufficient contrast to isolate planetary light from a host star in high-resolution spectroscopy?
• RQ2 To what extent can predictive control and LDFC suppress atmospheric speckles in the dark zone to improve signal-to-noise ratio in planetary spectra?
• RQ3 How effectively can the IRD spectrograph detect molecular features in exoplanet atmospheres when fed with light from a fiber injected at the planet’s position?
• RQ4 What is the achievable contrast and signal-to-noise ratio for planetary spectra using this HDC technique on current 8-meter class telescopes?
• RQ5 Can this method be scaled to future Extremely Large Telescopes (ELTs) to enable the atmospheric characterization of terrestrial exoplanets?

Key findings

• The combination of SCExAO and IRD enables the first experimental demonstration of high-contrast differential spectroscopy (HDC) for exoplanet characterization on a ground-based 8-meter telescope.
• Speckle nulling with a deformable mirror reduced the flux coupled into the single-mode fiber by a factor of 100–1000 for quasi-static speckles, significantly improving contrast.
• Even in the presence of atmospheric turbulence, static speckle suppression achieved an average flux reduction of ~3× in the fiber, demonstrating robustness.
• Predictive control and LDFC were shown to be effective for suppressing rapidly evolving atmospheric speckles, allowing for longer integration times and improved signal-to-noise.
• The system is expected to achieve routine science operations by mid-2018, enabling high-resolution (R > 10,000) spectroscopy of exoplanet atmospheres with simultaneous stellar and telluric calibration.
• The method provides a viable path to detecting molecular features in terrestrial exoplanet atmospheres on ELTs, with the potential for signal-to-noise ratios sufficient for atmospheric retrieval.