More Like this

1. Stellar Activity Manifesting at a One-year Alias Explains Barnard b as a False Positive

   https://doi.org/10.3847/1538-3881/ac0057  

   Lubin, Jack; Robertson, Paul; Stefansson, Gudmundur; Ninan, Joe; Mahadevan, Suvrath; Endl, Michael; Ford, Eric; Wright, Jason T.; Beard, Corey; Bender, Chad; et al (July 2021, The Astronomical Journal)

   Abstract Barnard’s star is among the most studied stars given its proximity to the Sun. It is often considered the radial velocity (RV) standard for fully convective stars due to its RV stability and equatorial decl. Recently, an M sin i = 3.3 M ⊕ super-Earth planet candidate with a 233 day orbital period was announced by Ribas et al. New observations from the near-infrared Habitable-zone Planet Finder (HPF) Doppler spectrometer do not show this planetary signal. We ran a suite of experiments on both the original data and a combined original + HPF data set. These experiments include model comparisons, periodogram analyses, and sampling sensitivity, all of which show the signal at the proposed period of 233 days is transitory in nature. The power in the signal is largely contained within 211 RVs that were taken within a 1000 day span of observing. Our preferred model of the system is one that features stellar activity without a planet. We propose that the candidate planetary signal is an alias of the 145 day rotation period. This result highlights the challenge of analyzing long-term, quasi-periodic activity signals over multiyear and multi-instrument observing campaigns. 

   more » « less
2. Into the Depths: A New Activity Metric for High-precision Radial Velocity Measurements Based on Line Depth Variations

   https://doi.org/10.3847/1538-3881/ac609a  

   Siegel, Jared C.; Rubenzahl, Ryan A.; Halverson, Samuel; Howard, Andrew W. (May 2022, The Astronomical Journal)

   Abstract The discovery and characterization of extrasolar planets using radial velocity (RV) measurements is limited by noise sources from the surfaces of host stars. Current techniques to suppress stellar magnetic activity rely on decorrelation using an activity indicator (e.g., strength of the Ca ii lines, width of the cross-correlation function, broadband photometry) or measurement of the RVs using only a subset of spectral lines that have been shown to be insensitive to activity. Here, we combine the above techniques by constructing a high-signal-to-noise activity indicator, the depth metric  ( t ) , from the most activity-sensitive spectral lines using the “line-by-line” method of Dumusque (2018). Analogous to photometric decorrelation of RVs or Gaussian progress regression modeling of activity indices, time series modeling of  ( t ) reduces the amplitude of magnetic activity in RV measurements; in an α CenB RV time series from HARPS, the RV rms was reduced from 2.67 to 1.02 m s −1 .  ( t ) modeling enabled us to characterize injected planetary signals as small as 1 m s −1 . In terms of noise reduction and injected signal recovery,  ( t ) modeling outperforms activity mitigation via the selection of activity-insensitive spectral lines. For Sun-like stars with activity signals on the m s −1 level, the depth metric independently tracks rotationally modulated and multiyear stellar activity with a level of quality similar to that of the FWHM of the CCF and log R HK ′ . The depth metric and its elaborations will be a powerful tool in the mitigation of stellar magnetic activity, particularly as a means of connecting stellar activity to physical processes within host stars. 

   more » « less
3. High-pass Filtering and Gaussian Process Regularization: Stellar Activity Characterization Techniques Applied to the 55 Cancri Planetary System

   https://doi.org/10.3847/1538-3881/ae04ea  

   Harrell, Justin; Dodson-Robinson, Sarah E (October 2025, The Astronomical Journal)

   Abstract Doppler planet searches are complicated by stellar activity, through which cyclical changes in the host star’s photosphere and chromosphere can mask or mimic planetary signals. A popular technique for modeling stellar activity is to apply a quasiperiodic Gaussian process (GP) kernel, which provides a flexible model with rigorous error propagation. However, observers must guard against overfitting, as a GP may be flexible enough to subsume other signals besides the one it is intended to model. To counteract overfitting, we introduce a curvature-penalizing objective function for fitting GP models to long-term magnetic activity cycles. We also demonstrate that a Gaussian filter can be an effective method of detrending radial velocities (RVs) so that shorter-period signals can be extracted even in the absence of a mathematical model of the long-term trend. We apply our methods to the heavily studied 55 Cancri system, fitting Keplerian orbits plus the GP activity-cycle model. We show that a four-Keplerian model that includes planets b, c, e, and f combined with a GP for the activity cycle performs at least as well as the widely agreed upon five-planet system with its own GP activity model. Our results suggest that the existence of planet d cannot be established from the RVs alone; additional data are required for confirmation. 

   more » « less
4. Searching for Low-mass Exoplanets amid Stellar Variability with a Fixed Effects Linear Model of Line-by-line Shape Changes

   https://doi.org/10.3847/1538-3881/adf29d  

   Salzer, Joseph; Cisewski-Kehe, Jessi; Ford, Eric B; Zhao, Lily L (August 2025, The Astronomical Journal)

   Abstract 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 is fast and flexible, allowing us to use cross validation to evaluate model performance on unseen data. 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 rms errors by approximately 76% (from 1.722 to 0.403 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 cross-correlation function-based RV estimators and comparable to other state-of-the-art methods for mitigating stellar variability. 

   more » « less
5. The CARMENES search for exoplanets around M dwarfs: Wolf 1069 b: Earth-mass planet in the habitable zone of a nearby, very low-mass star

   https://doi.org/10.1051/0004-6361/202245322  

   Kossakowski, D.; Kürster, M.; Trifonov, T.; Henning, Th.; Kemmer, J.; Caballero, J. A.; Burn, R.; Sabotta, S.; Crouse, J. S.; Fauchez, T. J.; et al (February 2023, Astronomy & Astrophysics)

   We present the discovery of an Earth-mass planet (M_bsini= 1.26 ± 0.21M_⊕) on a 15.6 d orbit of a relatively nearby (d ~9.6 pc) and low-mass (0.167 ± 0.011M_⊙) M5.0 V star, Wolf 1069. Sitting at a separation of 0.0672 ± 0.0014 au away from the host star puts Wolf 1069 b in the habitable zone (HZ), receiving an incident flux ofS= 0.652 ± 0.029S_⊕. The planetary signal was detected using telluric-corrected radial-velocity (RV) data from the CARMENES spectrograph, amounting to a total of 262 spectroscopic observations covering almost four years. There are additional long-period signals in the RVs, one of which we attribute to the stellar rotation period. This is possible thanks to our photometric analysis including new, well-sampled monitoring campaigns undergone with the OSN and TJO facilities that supplement archival photometry (i.e., from MEarth and SuperWASP), and this yielded an updated rotational period range ofP_rot= 150–170 d, with a likely value at 169.3_−3.6^+3.7. The stellar activity indicators provided by the CARMENES spectra likewise demonstrate evidence for the slow rotation period, though not as accurately due to possible factors such as signal aliasing or spot evolution. Our detectability limits indicate that additional planets more massive than one Earth mass with orbital periods of less than 10 days can be ruled out, suggesting that perhaps Wolf 1069 b had a violent formation history. This planet is also the sixth closest Earth-mass planet situated in the conservative HZ, after Proxima Centauri b, GJ 1061 d, Teegarden’s Star c, and GJ 1002 b and c. Despite not transiting, Wolf 1069 b is nonetheless a very promising target for future three-dimensional climate models to investigate various habitability cases as well as for sub-m s⁻¹RV campaigns to search for potential inner sub-Earth-mass planets in order to test planet formation theories. 

   more » « less