Abstract
Detecting and confirming terrestrial planets is incredibly difficult due to their tiny size and mass relative to Sun-like host stars. However, recent instrumental advancements are making the detection of Earth-like exoplanets technologically feasible. For example, Kepler and TESS photometric precision means we can identify Earth-sized candidates (and PLATO in the future will add many long-period candidates to the list), while spectrographs such as ESPRESSO and EXPRES (with an aimed radial velocity precision [RV] near 10 cm s − 1 ) mean we will soon reach the instrumental precision required to confirm Earth-mass planets in the habitable zones of Sun-like stars. However, many astrophysical phenomena on the surfaces of these host stars can imprint signatures on the stellar absorption lines used to detect the Doppler wobble induced by planetary companions. The result is stellar-induced spurious RV shifts that can mask or mimic planet signals. This review provides a brief overview of how stellar surface magnetoconvection and oscillations can impact low-mass planet confirmation and the best-tested strategies to overcome this astrophysical noise. These noise reduction strategies originate from a combination of empirical motivation and a theoretical understanding of the underlying physics. The most recent predications indicate that stellar oscillations for Sun-like stars may be averaged out with tailored exposure times, while granulation may need to be disentangled by inspecting its imprint on the stellar line profile shapes. Overall, the literature suggests that Earth-analog detection should be possible, with the correct observing strategy and sufficient data collection.
Highlights
The confirmation and detailed characterization of exoplanets necessitates a measurement of the planetary mass
The most tried and tested technique to obtain such a mass measurement comes from analyzing the planetary-induced Doppler wobble of the host star
For a true Earth-analog, the planet-induced radial velocity (RV) on the host star is a minuscule ∼9 cm s−1 —making this feat extremely challenging; why such a confirmation still eludes us to date
Summary
The confirmation and detailed characterization of exoplanets necessitates a measurement of the planetary mass. Recent advancements in instrumentation promise to make this challenge technologically feasible Both the ESPRESSO [1,2]. Geosciences 2019, 9, 114 spectrographs, these RVs are determined by observing the stellar absorption lines in a relatively large passband, usually several thousand angstroms wide, and cross-correlating them with a template mask. The presence of a planetary companion induces wholesale Doppler shifts of the individual lines, where the net influence can be determined from the center of the cross-correlation function (CCF) [5]. The focus of this paper is to briefly discuss the physics driving convection and oscillation-induced RV variability (Section 2), review the recent works in the literature that aim to reduce these stellar noise sources in high precision RVs (Section 3), and comment on the prospects for the future confirmation of habitable worlds (Section 4)
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