Exoplanetology: Exoplanets & Exomoons

Searching for Low-Mass Exoplanets Amid Stellar Variability with a Fixed Effects Linear Model of Line-by-Line Shape Changes

By Keith Cowing

Status Report

astro-ph.EP

February 18, 2025

Filed under astro-ph.EP, Astrometry, astronomy, Doppler spectroscopy, Earth-like Planet, exoplanet, NEID, radial velocity, Stellar Cartography

Searching for Low-Mass Exoplanets Amid Stellar Variability with a Fixed Effects Linear Model of Line-by-Line Shape Changes — Heatmaps of the NEID Solar RV measurements over time. The x-axis represents the 330 days with measurements (in order, but not scaled by time). The black vertical line is at the day when the detector was shut off due to a forest fire and subsequently brought back online. The top subplot displays the unadjusted distribution of RVs for each line (note that around 4% of the observations fall outside the limits of this plot). The bottom subplot displays the RVs that have been centered by line and by the temporal group (note that around 2% of the observations fall outside the limits of this plot). The colors indicate the number of RV measurements from absorption lines within each bin (orange = fewer lines, blue = more lines). Bins are vertical so that there is no overlap between days. — astro-ph.EP

The radial velocity (RV) method, also known as Doppler spectroscopy, is a powerful technique for exoplanet discovery and characterization. In recent years, progress has been made thanks to the improvements in the quality of spectra from new extreme precision RV spectrometers.

However, detecting the RV signals of Earth-like exoplanets remains challenging, as the spectroscopic signatures of low-mass planets can be obscured or confused with intrinsic stellar variability. Changes in the shapes of spectral lines across time can provide valuable information for disentangling stellar activity from true Doppler shifts caused by low-mass exoplanets.

In this work, we present a fixed effects linear model to estimate RV signals that controls for changes in line shapes by aggregating information from hundreds of spectral lines. Our methodology incorporates a wild-bootstrap approach for modeling uncertainty and cross-validation to control for overfitting.

We evaluate the model’s ability to remove stellar activity using solar observations from the NEID spectrograph, as the sun’s true center-of-mass motion is precisely known. Including line shape-change covariates reduces the RV root-mean-square errors by approximately 70% (from 1.919 m s−1 to 0.575 m s−1) relative to using only the line-by-line Doppler shifts.

The magnitude of the residuals is significantly less than that from traditional CCF-based RV estimators and comparable to other state-of-the-art methods for mitigating stellar variability.

Joseph Salzer, Jessi Cisewski-Kehe, Eric B. Ford, Lily L. Zhao

Comments: Submitted to AAS Journals. 20 pages, 5 figures, 2 tables
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM); Solar and Stellar Astrophysics (astro-ph.SR); Applications (stat.AP)
Cite as: arXiv:2502.11930 [astro-ph.EP] (or arXiv:2502.11930v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2502.11930
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Submission history
From: Joseph Salzer
[v1] Mon, 17 Feb 2025 15:41:40 UTC (644 KB)
https://arxiv.org/abs/2502.11930
Astrobiology,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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