Statistical Methods for Exoplanet Detection with Radial Velocities

Hara, N.

doi:10.1146/annurev-statistics-033021-012225

Cited by 7 publications

References 235 publications

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Solar photospheric spectrum microvariability

Dravins,

Ludwig

2023

A&A

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Context. Extreme precision radial-velocity spectrometers enable extreme precision in stellar spectroscopy. Searches for low-mass exoplanets around solar-type stars are limited by various types of physical variability in stellar spectra, such as the short-term jittering of apparent radial velocities on levels of ∼2 m s−1. Aims. To understand the physical origins of radial-velocity jittering, the solar spectrum is assembled, as far as possible, from basic principles. Solar surface convection is modeled with time-dependent 3D hydrodynamics, followed by the computation of high-resolution spectra during numerous instances of the simulation sequence. The behavior of different classes of photospheric spectral lines is monitored throughout the simulations to identify commonalities or differences between different classes of lines: weak or strong, neutral or ionized, high or low excitation, atomic or molecular. Methods. Synthetic spectra were examined. With a wavelength sampling λ/Δλ ∼ 1 000 000, the changing shapes and wavelength shifts of unblended and representative Fe I and Fe II lines were followed during the simulation sequences. The radial-velocity jittering over the small simulation area typically amounts to ±150 m s−1, scaling to ∼2 m s−1 for the full solar disk. Flickering within the G-band region and in photometric indices of the Strömgren uvby system were also measured, and synthetic G-band spectra from magnetic regions are discussed. Results. Most photospheric lines vary in phase, but with different amplitudes among different classes of lines. Amplitudes of radial-velocity excursions are greater for stronger and for ionized lines, decreasing at longer wavelengths. Matching precisely measured radial velocities to such characteristic patterns should enable us to remove a significant component of the stellar noise originating in granulation. Conclusions. The granulation-induced amplitudes in full-disk sunlight amount to ∼2 m s−1; the differences between various line groups are an order of magnitude less. To mitigate this jittering, a matched filter must recognize dissimilar lineshifts among classes of diverse spectral lines with a precision of ∼10 cm s−1 for each line group. To verify the modeling toward the filter, predictions of center-to-limb dependences of jittering amplitudes for different classes of lines are presented, testable with spatially resolving solar telescopes connected to existing radial-velocity instruments.

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Solar photospheric spectrum microvariability

Dravins,

Ludwig

2023

A&A

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A linearized approach to radial velocity extraction

Shahaf,

Zackay

2023

Monthly Notices of the Royal Astronomical Society

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High-precision radial velocity (RV) measurements are crucial for exoplanet detection and characterization. Efforts to achieve ∼10 cm s–1 precision have been made over the recent decades, with significant advancements in instrumentation, data reduction techniques, and statistical inference methods. However, despite these efforts, RV precision is currently limited to ∼50 cm s–1. This value exceeds state-of-the-art spectrographs’ expected instrumental noise floor and is mainly attributed to RV signals induced by stellar variability. In this work, we propose a factorization method to overcome this limitation. The factorization is particularly suitable for controlling the effect of localized changes in the stellar emission profile, assuming some smooth function of a few astrophysical parameters governs them. We use short-time Fourier transforms (STFTs) to infer the RV in a procedure equivalent to least-squares minimization in the wavelength domain and demonstrate the effectiveness of our method in treating arbitrary temperature fluctuations on the star’s surface. The proposed prescription can be naturally generalized to account for other effects, either intrinsic to the star, such as magnetic fields, or extrinsic to it, such as telluric contamination. As a proof-of-concept, we empirically derive a set of factorization terms describing the solar centre-to-limb variation and apply them to a set of realistic SOAP-GPU spectral simulations. We discuss the method’s capability to mitigate variability-induced RV signals and its potential extensions to serve as a tomographic tool.

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A non-zero Doppler amplitude is not enough: revisiting the putative radial velocity detection of sub-Venus exoplanet L 98-59b

Rajpaul,

Barragán,

Zicher

2024

Monthly Notices of the Royal Astronomical Society

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L 98-59b is a transiting exoplanet with radius ${0.85^{+0.061}_{-0.047}}$ R⊕ and orbital period ${2.2531136^{+1.2e-06}_{-1.5e-06}}$ d. In 2021, a Doppler detection of L 98-59b was announced, with radial velocity (RV) semi-amplitude $K_b=0.46^{+0.20}_{-0.17}$ m s−1 inferred from 164 HARPS and 66 ESPRESSO RVs. This translated into a mass $M_b=0.40^{+0.16}_{-0.15}$ M⊕, or half the mass of Venus, making L 98-59b then (by far) the lowest-mass planet with a putative RV detection. To illuminate the difficulties in securely detecting <1 m s−1signals, we argue here that the published RV detection of L 98-59b is not supported by sufficient statistical evidence. We show, under the published modelling assumptions, that Bayesian model comparison (BMC) suggests a $\sim 50~{{\%}}$ false-detection probability. We also often infer ∼40 cm s−1 semi-amplitudes for Keplerians with periods not corresponding to any known planet; importantly, though, BMC rejects all these ‘detections’. By implementing a more sophisticated stellar activity model and more realistic parameter priors, we infer a mass Mb = 0.47 ± 0.14 M⊕ from semi-amplitude Kb = 0.56 ± 0.16 m s−1, with BMC now indicating substantial yet still not decisive evidence for the detection. Definitive detection and characterisation may require many additional RVs, alongside careful modelling and stringent statistical analysis. Our case study has implications for current extreme-precision RV campaigns and the longer-term quest to discover the first Earth-twins.

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Statistical Methods for Exoplanet Detection with Radial Velocities

Cited by 7 publications

References 235 publications

Solar photospheric spectrum microvariability

Solar photospheric spectrum microvariability

A linearized approach to radial velocity extraction

A non-zero Doppler amplitude is not enough: revisiting the putative radial velocity detection of sub-Venus exoplanet L 98-59b

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