CARMENES-PLUS Project

• CARMENES-PLUS is a comprehensive upgrade of the original survey, enhancing the detection of low-mass exoplanets around M dwarfs via improved radial velocity precision.
• The project employs advanced technical methods such as continuous cryogenic cooling, automatic vacuum systems, and dual-channel calibration to minimize instrumental noise.
• Expanded high-cadence, multi-year observations and refined calibration strategies enable robust statistical analyses and detailed characterization of exoplanet populations.

The CARMENES-PLUS Project is an advanced instrumental and scientific upgrade building upon the legacy of the original CARMENES survey at the 3.5-m Calar Alto telescope. It is focused on improving the detection and characterization of low-mass exoplanets around nearby M dwarfs through significant enhancements in spectrograph stability, calibration systems, data processing, and survey methodology. CARMENES-PLUS intensifies the pursuit of precise radial velocity (RV) measurements via innovative upgrades—particularly to the near-infrared (NIR) channel—and extends the collaborative scientific effort to maximize the discovery and physical characterization of exoplanets in the solar neighborhood.

1. Technical Advancements and Instrument Upgrades

CARMENES-PLUS implements key engineering upgrades, most notably to the NIR channel, targeting millikelvin-level temperature stability to reduce RV drifts. The main technical improvements include:

• Continuous Flow Cryogenic Cooling System: Upgrade from a discontinuous (on/off) to a continuously regulated (proportional) liquid nitrogen (LN₂) flow, governed by a PID control algorithm of the form $$u(t) = K_P e(t) + K_I \int e(t) dt + K_D \frac{de(t)}{dt}$$, where $e(t)$ is the temperature error. This decreases radiation shield temperature fluctuations from $\Delta T \sim 0.09$ K to $\Delta T \sim 0.002$ K (Varas et al., 22 Sep 2025).
• Automatic Vacuum System: Continuous maintenance of the vacuum in the cryogenic transfer lines to $<10^{-5}$ mbar, minimizing conduction losses and preventing frost formation affecting NIR stability.
• Pressure Control Unit and Fixed-Dewar: Automated PID feedback to stabilize the dewar pressure to $\sim0.421$–$0.426$ bar, eliminating thermal instabilities arising from daily manual dewar changes.
• Removal of Mechanical Bottlenecks: Elimination of a check valve in the feed line, thus preventing intermittent flow interruptions and ensuring smooth coolant delivery.

Collectively these measures reduce NIR intra-night RV scatter (from $\sim2.62$ m/s to $\sim0.67$ m/s in Fabry-Perot calibrations), lower the nightly zero-point scatter, and improve the median on-sky RV precision for slowly rotating M dwarfs from $8.8$ m/s to $6.7$ m/s (Varas et al., 22 Sep 2025).

2. Instrument Configuration and Calibration

The upgraded CARMENES-PLUS spectrograph retains the dual-channel, fiber-fed echelle design with simultaneous VIS and NIR operation ($0.52$–$1.71$ μm). Both channels are housed in thermally isolated, vacuum tanks to ensure ultra-stable conditions.

• Calibration System Enhancements: Replacement of individual Fabry–Pérot etalons with a single cryogenically-stabilized etalon for simultaneous VIS/NIR calibration, providing greater wavelength stability across the entire spectral range (Caballero et al., 7 Mar 2025).
• Customized Fiber Switches: Introduction of a switch system for calibration fiber injection, enabling real-time calibration spectra acquisition from multiple sources, thus enhancing RV stability.
• Intrinsic RV Precision: Improvements bring NIR precision close to VIS channel performance (VIS channel typical scatter at $\sim4.3$ m/s), expanding the parameter space for exoplanet detection around faint, red M dwarfs (essential for habitable-zone planets).

3. Survey Goals and Methodology

CARMENES-PLUS builds upon the existing science objectives of detecting and characterizing Earth and super-Earth planets around a sample of $\sim300$–$400$ nearby M dwarfs via high-cadence, high-precision RV monitoring (Amado et al., 2012, Ribas et al., 2023).

• Expanded Sample and Temporal Baseline: The survey continues to update and expand high-cadence, multi-year observations, ensuring high sensitivity to both short- and long-period planetary signals, as well as secular and dynamical effects within planetary systems (Ribas et al., 2023, Stauffenberg et al., 16 Jul 2024).
• Simultaneous Stellar Activity Characterization: Broad wavelength coverage supports the measurement of chromatic RV indices, activity diagnostics (e.g., H$\alpha$, Ca II IRT, Paschen lines), and sophisticated modeling (including Gaussian processes) to disentangle stellar noise from Doppler shifts induced by planets (Jeffers et al., 2022, Fuhrmeister et al., 2023).

4. Scientific Impact and Results

CARMENES-PLUS has already demonstrated advancements in both instrumentation and scientific output:

• Exoplanet Yield and Population Studies: Detection and confirmation of dozens of planetary systems, including super-Earths, temperate rocky planets, and Saturn-mass objects, with an overall application to the statistical occurrence rates of exoplanets around low-mass stars. The planet occurrence rate is found to be $1.44 \pm 0.20$ planets per M dwarf for $1\, M_\oplus < M \sin i < 1000\, M_\oplus$ and $1 < P_\mathrm{orb} < 1000$ days (Ribas et al., 2023).
• High-Quality Spectral Library: Creation of a public, telluric-corrected spectral library (TDTM), spanning $382$ M dwarfs and $\sim40,000$ spectra, with $R > 80,000$ and high S/N across $0.52$–$1.71$ μm, supporting RV extraction, photospheric modeling, chemical abundance determination, and atmospheric transmission/occultation studies (Nagel et al., 2023).
• Improved NIR RV Sensitivity: Post-upgrade NIR channel enables robust detection of Earth-like planets exploiting the NIR flux peak of M dwarfs and diminishes the gap in sensitivity between red-optical and NIR, a key advance for studies of late-M dwarfs and activity-sensitive targets (Varas et al., 22 Sep 2025).

5. Collaboration, Consortium, and Data Distribution

The project is managed by a German–Spanish consortium comprising eleven core institutions, which ensure parity in both technical and scientific leadership (Caballero et al., 7 Mar 2025).

• Operational Model: The consortium coordinates guaranteed and legacy time observations, while fostering a broader collaborative network with projects such as TESS, PLATO, MAROON-X, and ESPRESSO (Caballero et al., 7 Mar 2025).
• Data Release and Community Resource: Legacy data releases (e.g., Data Release 1) provide calibrated spectra, processed RVs, and auxiliary data streams (stellar activity indicators, bisectors, etc.), catalyzing broader research in low-mass star science (Ribas et al., 2023, Nagel et al., 2023).

6. Future Directions and Ancillary Science

CARMENES-PLUS is the platform for continuing upgrades and scientific exploitation:

• Enhanced Calibration and Fiber System: Further development of calibration etalons and fiber switches will permit simultaneous multi-source calibration and active monitoring of instrument drift, anticipated to push RV measurements to the sub-m/s regime in both VIS and NIR (Caballero et al., 7 Mar 2025).
• Ancillary Science Surveys: The expanded infrastructure supports auxiliary programs (e.g., the KOBE survey for K dwarfs, CAVITY for galaxy studies), demonstrating the versatility of the instrument suite.
• Synergy with Contemporary Facilities: Strategic coordination with upcoming space missions and ground-based facilities will deepen the characterization of planetary atmospheres, test occurrence rate variations with metallicity and host properties, and enable detection of smaller, more temperate exoplanets.

7. Significance for Exoplanet Research

By achieving high thermal and instrumental stability in the NIR, improving calibration strategies, and expanding the survey’s temporal and spectral reach, CARMENES-PLUS enables:

• Detection of low-mass (Earth and sub-Earth) planets in the habitable zones of M dwarfs, reducing stellar noise contributions and identifying signals previously inaccessible due to instrument limitations.
• High-fidelity atmospheric studies through transit and occultation spectroscopy in both optical and NIR, offering constraints on molecular signatures and planetary dynamics.
• A comprehensive legacy of high-resolution, multi-epoch M-dwarf spectra, essential for both planetary science and detailed stellar astrophysics.

This suite of upgrades positions CARMENES-PLUS as a leading facility in the field of exoplanetary research, particularly for the characterization of planetary systems around low-mass stars, and as a model for future ultra-stable, high-resolution spectrographs in the optical and NIR domains (Varas et al., 22 Sep 2025, Nagel et al., 2023, Caballero et al., 7 Mar 2025).