Combining high-dispersion spectroscopy with high contrast imaging : Probing rocky planets around our nearest neighbors

By I. Snellen, Remco J. de Kok, J. Birkby, B. Brandl, M. Brogi, C. Keller, M. Kenworthy, H. Schwarz, R. Stuik. 2015

226 citations recorded by the source corpus.

Context. Ground-based high-dispersion (R 100,000) spectroscopy (HDS) is proving to be a powerful technique with which to characterize extrasolar planets. The planet signal is distilled from the bright starlight, combining spectral and time-di erential filtering techniques. In parallel, high-contrast imaging (HCI) is developing rapidly, aimed at spatially separating the planet from the star. While HDS is limited by the overwhelming noise from the host star, HCI is limited by residual quasi-static speckles. Both techniques currently reach planet-star contrast limits down to 10 5 , albeit for very di erent types of planetary systems. Aims. In this work, we discuss a way to combine HDS and HCI (HDS+HCI). For a planet located at a resolvable angular distance from its host star, the starlight can be reduced up to several orders of magnitude using adaptive optics and/or coronography. In addition, the remaining starlight can be filtered out using high-dispersion spectroscopy, utilizing the significantly di erent (or Doppler shifted) high-dispersion spectra of the planet and star. In this way, HDS+HCI can in principle reach contrast limits of 10 5 10 5 , although in practice this will be limited by photon noise and/or sky-background. In contrast to current direct imaging techniques, such as Angular Di erential Imaging and Spectral Di erential Imaging, it will work well at small working angles and is much less sensitive to speckle noise. For the discovery of previously unknown planets HDS+HCI requires a high-contrast adaptive optics system combined with a high-dispersion R 100,000 integral field spectrograph (IFS). This combination currently does not exist, but is planned for the European Extremely Large Telescope. Methods. We present simulations of HDS+HCI observations with the E-ELT, both probing thermal emission from a planet at infrared wavelengths, and starlight reflected o a planet atmosphere at optical wavelengths. For the infrared simulations we use the baseline parameters of the E-ELT and METIS instrument, with the latter combining extreme adaptive optics with an R=100,000 IFS. We include realistic models of the adaptive optics performance and atmospheric transmission and emission. For the optical simulation we also assume R=100,000 IFS with adaptive optics capabilities at the E-ELT. Results. One night of HDS+HCI observations with the E-ELT at 4.8 m ( = 0:07 m) can detect a planet orbiting Cen A with a radius of R=1.5 Rearth and a twin-Earth thermal spectrum of Teq=300 K at a signal-to-noise (S/N) of 5. In the optical, with a Strehl ratio performance of 0.3, reflected light from an Earth-size planet in the habitable zone of Proxima Centauri can be detected at a S/N of 10 in the same time frame. Recently, first HDS+HCI observations have shown the potential of this technique by determining the spin-rotation of the young massive exoplanet Pictoris b. Conclusions. The exploration of the planetary systems of our neighbor stars is of great scientific and philosophical value. The HDS+HCI technique has the potential to detect and characterize temperate rocky planets in their habitable zones. Exoplanet scientists should not shy away from claiming a significant fraction of the future ELTs to make such observations possible.

Subfields: Astronomy and Astrophysics, Atomic and Molecular Physics, and Optics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/249f33a8f0ee8411c480ef6177c8bc3312066cbd

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In Astronomy and Astrophysics

1. N. Madhusudhan
2. Nuno C. Santos
3. S. Udry
4. B. Bowler
5. X. Delfosse
6. C. Lovis
7. F. Selsis
8. F. Pepe

In Atomic and Molecular Physics, and Optics

1. N. Madhusudhan
2. B. Bowler
3. S. Udry
4. Nuno C. Santos
5. F. Pepe
6. C. Lovis
7. I. Snellen
8. T. Robinson

In Instrumentation

1. N. Madhusudhan
2. Nuno C. Santos
3. S. Udry
4. B. Bowler
5. I. Snellen
6. C. Lovis
7. F. Pepe
8. J. Lustig-Yaeger

Citations in the corpus

• Atmospheric characterization of Proxima b by coupling the Sphere high-contrast imager to the Espresso spectrograph (2016, methodology): “In the context of the HCHR technique, the signal-to-noise ratio on the planet spectrum can be expressed as (Snellen et al. 2015):”
• SPHERE: the exoplanet imager for the Very Large Telescope (2019, unknown): “– Earth-like planets (Snellen et al. 2015; Wang et al. 2017).”
• Exoplanetary Atmospheres: Key Insights, Challenges, and Prospects (2019, methodology): “best constraints possible. Similarly, the combination of the cross-correlation technique with high-contrast imaging provides new promise for medium-high resolution spectroscopy at extreme ux contrasts(248, 106). These advancements will naturally be further enhanced with the next generation of large ”
• Imaging Extrasolar Giant Planets (2016, background): “…in stellar age-dating at the individual and population levels, merging high-contrast imaging with high-resolution spectroscopy (Snellen et al. 2014; Snellen et al. 2015), surveying the companion mass function to sub-Jovian masses, polarimetric observations of photospheric clouds (e.g., Marley et…”
• Evolved Climates and Observational Discriminants for the TRAPPIST-1 Planetary System (2018, background): “…journal citation and DOI. and ground-based extremely large telescopes (ELTs), may be capable of probing Mdwarf planetary atmospheres, including a handful of HZ planets (Rodler & López-Morales 2014; Cowan et al. 2015; Quanz et al. 2015; Snellen et al. 2015; Greene et al. 2016; Lovis et al. 2017).”
• Spectrally resolved detection of sodium in the atmosphere of HD 189733b with the HARPS spectrograph (2015, background): “This is also a motivation for building a high-resolution spectrograph on the European Extremely-LargeTelescope such as HiReS and METIS (Udry et al. 2014; Brandl et al. 2014; Snellen et al. 2015).”
• The habitability of Proxima Centauri b II. Possible climates and Observability (2016, methodology): “Snellen et al. (2015) estimated that it could be doable in 10 hours with the instrument IFU/E-ELT.”
• Exoplanet Biosignatures: A Review of Remotely Detectable Signs of Life (2017, background): “Ground-based observers are also devising instrumentation and techniques for current and future observatories that will have the capacity to image Earth-sized planets around nearby stars (Kawahara et al., 2012; Snellen et al., 2013, 2015; Lovis et al., 2016).”