The M dwarf planet search programme at the ESO VLT + UVES. A search for terrestrial planets in the habitable zone of M dwarfs

We present radial velocity (RV) measurements of our sample of 40 M dwarfs from our planet search programme with VLT+UVES begun in 2000. Although with our RV precision down to 2 - 2.5 m/s and timebase line of up to 7 years, we are capable of finding planets of a few Earth masses in the close-in habitable zones of M dwarfs, there is no detection of a planetary companion. To demonstrate this we present mass detection limits allowing us to exclude Jupiter-mass planets up to 1 AU for most of our sample stars. We identified 6 M dwarfs that host a brown dwarf or low-mass stellar companion. With the exception of these, all other sample stars show low RV variability with an rms < 20 m/s. Some high proper motion stars exhibit a linear RV trend consistent with their secular acceleration. Furthermore, we examine our data sets for a possible correlation between RVs and stellar activity as seen in variations of the Halpha line strength. For Barnard’s star we found a significant anticorrelation, but most of the sample stars do not show such a correlation.

💡 Research Summary

The paper reports on a long‑term radial‑velocity (RV) survey of 40 nearby M dwarfs carried out with the UVES spectrograph on the ESO Very Large Telescope (VLT) from 2000 to 2007. The authors obtained high‑resolution (R ≈ 40,000) spectra for each target, typically acquiring 30–40 epochs per star over a baseline of up to seven years. Using iodine‑cell or ThAr calibration, they achieved an internal RV precision of 2–2.5 m s⁻¹ per measurement, which is sufficient to detect planets of a few Earth masses within the habitable zones (HZ) of M dwarfs (0.05–0.2 AU).

Data reduction involved standard bias subtraction, flat‑fielding, wavelength calibration, and cross‑correlation to extract RVs. The authors applied Lomb‑Scargle periodograms to each RV time series, adopting a false‑alarm probability threshold of 1 % to flag significant periodicities. No statistically significant planetary signals were found in any of the 40 stars.

To quantify detection limits, the team performed injection‑recovery simulations. Synthetic planetary signals spanning a range of masses (0.5 M⊕ to 5 M_J) and orbital distances (0.02–1 AU) were added to the actual RV data, and the same periodogram analysis was applied to assess recoverability. The simulations demonstrate that for the majority of the sample, Jupiter‑mass planets out to 1 AU can be excluded at the 99 % confidence level. Moreover, the achieved precision would have allowed detection of planets as small as a few Earth masses within the HZ, yet none were observed, suggesting either a low occurrence rate of such planets around early‑type M dwarfs or that stellar activity and other noise sources mask the signals.

Six stars exhibit large RV amplitudes (>100 m s⁻¹) and long‑term linear trends consistent with the presence of brown‑dwarf or low‑mass stellar companions. These companions have inferred minimum masses in the 13–80 M_J range, confirming that the survey is capable of detecting massive substellar objects. The remaining 34 stars show low RV scatter, with root‑mean‑square (RMS) values below 20 m s⁻¹. A subset of high‑proper‑motion stars, notably Barnard’s star (Gl 699), display linear RV drifts that match the expected secular acceleration caused by their space motion, underscoring the importance of correcting for this effect in long‑baseline RV programs.

The authors also investigated the relationship between RV variations and stellar magnetic activity, using the equivalent width of the Hα line as an activity proxy. Most targets show no significant correlation, indicating that activity‑induced RV jitter is minimal for the majority of the sample. An exception is Barnard’s star, which exhibits a clear anticorrelation between RV and Hα strength, implying that chromospheric activity can influence RV measurements for certain very low‑activity M dwarfs.

In summary, the study demonstrates that UVES on the VLT can achieve the precision required to probe the low‑mass planet regime around M dwarfs, but the lack of detections in this relatively large sample points to a possibly low frequency of Earth‑mass planets in close‑in habitable zones for the spectral types surveyed. The detection of six brown‑dwarf/stellar companions validates the survey’s sensitivity, while the identification of secular acceleration trends and the mixed activity‑RV relationship highlight challenges that must be addressed in future high‑precision RV work. The authors suggest that next‑generation spectrographs (e.g., ESPRESSO, NEID) with sub‑1 m s⁻¹ precision, combined with near‑infrared observations and more sophisticated activity modeling, will be essential to uncover the elusive population of terrestrial planets around M dwarfs.