Multi-Wavelength High-Resolution Spectroscopy for Exoplanet Detection: Motivation, Instrumentation and First Results

Abstract

Exoplanet research has shown an incessant growth since the first claim of a hot giant planet around a solar-like star in the mid-1990s. Today, the new facilities are working to spot the first habitable rocky planets around low-mass stars as a forerunner for the detection of the long-awaited Sun-Earth analog system. All the achievements in this field would not have been possible without the constant development of the technology and of new methods to detect more and more challenging planets. After the consolidation of a top-level instrumentation for high-resolution spectroscopy in the visible wavelength range, a huge effort is now dedicated to reaching the same precision and accuracy in the near-infrared. Actually, observations in this range present several advantages in the search for exoplanets around M dwarfs, known to be the most favorable targets to detect possible habitable planets. They are also characterized by intense stellar activity, which hampers planet detection, but its impact on the radial velocity modulation is mitigated in the infrared. Simultaneous observations in the visible and near-infrared ranges appear to be an even more powerful technique since they provide combined and complementary information, also useful for many other exoplanetary science cases.Geosciences 2018, xx, x 2 of 34 measured, the real mass of the planet can be obtained. This is just an example of the synergy among the different planet detection techniques (e.g., [4]).The RV method is more sensitive to a large planetary mass and a short orbital period: actually, a Jupiter-like planet orbiting at 1 AU from its host star produces an RV semi-amplitude of about 30 m s −1 , while an Earth-like planet at the same distance induces an RV modulation of only 9 cm s −1 (see also Table 1 in [5] for an overview of the typical semi-amplitude values induced by different types of planets). An RV survey also requires an intense observational effort: the observing baseline should be long enough to sample at least one, up to many periods of the planetary companion.Current state-of-the-art spectrographs in the visible (VIS) range provide an RV accuracy of about 1 m s −1 (see Section 2.1), allowing one in principle to detect small-mass exoplanets around solar-type stars with relatively short orbital periods. Anyway, the search for low-mass planets is hampered by the presence of the stellar activity that induces an intrinsic RV variation with similar amplitude with respect to the amplitude of the Keplerian signal, or even larger. Recently, M dwarfs have been identified as the most interesting objects to investigate, because of the higher planet occurrence rate in their habitable zones (Section 2.2) and the more favorable ratio between planet and stellar masses. On the other hand, they are red stars, fainter in the VIS wavelength range. To overcome the limitations imposed by the stellar activity and to allow a proper measurement of their spectra, observations in the near-infrared (NIR) range have been proposed, since in this band, the RV modulations...

Spatially resolved spectroscopy across stellar surfaces

“…Several precision instruments realize λ/∆λ ∼100 000; resolutions of about 200 000 are possible with ESPRESSO at the ESO VLT 1 (Pepe et al 2014(Pepe et al , 2021González Hernández et al 2018) but extended spectral ranges approaching λ/∆λ ∼ 300 000 are currently only offered by PEPSI at the LBT 2 (Strassmeier et al 2015(Strassmeier et al , 2018b. Actual performance comparisons have been made by Adibekyan et al (2020), Benatti (2018), andCrossfield (2014). Despite the ample photon fluxes in future extremely large telescopes, conventional spectrometer designs will likely not reach equally high values, constrained by the difficulty to match diffraction grating sizes to the large image scales.…”

Spatially resolved spectroscopy across stellar surfaces. V. Observational prospects: Toward Earth-like exoplanet detection

“…Several precision instruments realize λ/∆λ ∼100,000; resolutions of about 200,000 are possible with ESPRESSO at the ESO VLT 1 (Pepe et al 2014(Pepe et al , 2021González Hernández et al 2018) but extended spectral ranges approaching λ/∆λ ∼300,000 are currently only offered by PEPSI at the LBT 2 (Strassmeier et al 2015(Strassmeier et al , 2018b. Actual performance comparisons have been made by Adibekyan et al (2020), Benatti (2018), andCrossfield (2014). Despite the ample photon fluxes in future extremely large telescopes, conventional spectrometer designs will likely not reach equally high values, constrained by the difficulty to match diffraction grating sizes to the large image scales.…”