M-dwarfs have proven to be ideal targets for planetary radial velocity (RV) searches due to their higher planet-star mass contrast, which favors the detection of low-mass planets. The abundance of super-Earth and Earth-like planets detected around this type of stars encourages us to continue this search on hosts without reported planetary companions. The HADES and CARMENES programs aim to carry out extensive searches of exoplanetary systems around M-type stars in the northern hemisphere, allowing us to address statistically the properties of the planets orbiting these objects. In this work, we performed a spectroscopic and photometric study on one of the program stars (GJ 740), which exhibits a short-period RV signal compatible with a planetary companion. We carried out a spectroscopic analysis based on 129 HARPS-N spectra taken over a time-span of 6 yr combined with 57 HARPS spectra taken over 4yr, and 32 CARMENES spectra taken during more than 1 yr, resulting in a dataset with a time coverage of 10yr. We also relied on 459 measurements from the public ASAS survey with a time-coverage of 8yr along with 5yr of photometric magnitudes from the EXORAP project taken in the V, B, R and I filters to carry out a photometric study. Both analyses were made using Markov Chain Monte Carlo (MCMC) simulations and Gaussian Process regression to model the activity of the star. We present the discovery of a short-period super-Earth with an orbital period of 2.37756^+0.00013^_-0.00011_d and a minimum mass of 2.96^+0.50^_-0.48_Me. We update the previously reported characterization of the magnetic cycle and rotation period of the star, obtaining values of Prot=35.563+/-0.071d and Pcycle=2800+/-150d. The RV time-series exhibits a possibly periodic long-term signal which might be related to a Saturn-mass planet of ~100Me.
Surveys have shown that super-Earth and Neptune-mass exoplanets are more frequent than gas giants around low-mass stars, as predicted by the core accretion theory of planet formation. We report the discovery of a giant planet around the very-low-mass star GJ 3512, as determined by optical and near-infrared radial-velocity observations. The planet has a minimum mass of 0.46 Jupiter masses, very high for such a small host star, and an eccentric 204-day orbit. Dynamical models show that the high eccentricity is most likely due to planet-planet interactions. We use simulations to demonstrate that the GJ 3512 planetary system challenges generally accepted formation theories, and that it puts constraints on the planet accretion and migration rates. Disk instabilities may be more efficient in forming planets than previously thought.
We report on the detection of GJ3634b, a super-Earth of mass msini=7.0+/-0.9 Mearth and period P=2.64561+/-0.00066-day. Its host star is a M2.5 dwarf, has a mass of 0.45+/-0.05M_{sun}_, a radius of 0.43+/-0.03R_{sun}_ and lies 19.8+/-0.6pc away from our Sun. The planet is detected after a radial-velocity campaign using the ESO/Harps spectrograph. GJ3634b had an a priori geometric probability to undergo transit of ~7% and, if telluric in composition, a non-grazing transit would produce a photometric dip of <~0.1%. We therefore followed-up upon the RV detection with photometric observations using the 4.5-um band of the IRAC imager onboard Spitzer. Our six-hour long light curve excludes that a transit occurs for 2sigma of the probable transit window, decreasing the probability that GJ3634b undergoes transit to ~0.5%.
The interaction between Earth-like exoplanets and the magnetic field of low-mass host stars are considered to produce weak emission signals at radio frequencies. A study using LOFAR data announced the detection of radio emission from the mid M-type dwarf GJ 1151 that could potentially arise from a close-in terrestrial planet. Recently, the presence of a 2.5-M_{earth}_ planet orbiting GJ 1151 with a 2-day period has been claimed using 69 radial velocities (RVs) from the HARPS-N and HPF instruments. We have obtained 70 new high-precision RV measurements in the framework of the CARMENES M-dwarf survey and use these data to confirm the presence of the claimed planet and to place limits on possible planetary companions in the GJ 1151 system. We analysed the periodicities present in the combined RV data sets from all three instruments and calculated the detection limits for potential planets in short-period orbits. We cannot confirm the recently announced candidate planet and conclude that the 2-day signal in the HARPS-N and HPF data sets is most probably produced by a long-term RV variability, possibly arising from an outer planetary companion that has yet to be constrained. We calculate a 99.9% significance detection limit of 1.50m/s in the RV semi-amplitude, which places upper limits of 0.7M_{earth}_ and 1.2M_{earth}_ on the minimum masses of potential exoplanets with orbital periods of 1 and 5 days, respectively.
We report the detection of a Neptune-mass exoplanet around the M4.0 dwarf GJ 4276 (G 232-070) based on radial velocity (RV) observations obtained with the CARMENES spectrograph. The RV variations of GJ 4276 are best explained by the presence of a planetary companion with a minimum mass of m_b_sini~16M_{sun}_ on a P_b_=13.35-day orbit. The analysis of the activity indicators and spectral diagnostics exclude stellar induced RV perturbations and prove the planetary interpretation of the RV signal. We show that a circular single-planet solution can be excluded by means of a likelihood ratio test. Instead, we find that the RV variations can either be explained by an eccentric orbit or interpreted as a pair of planets on circular orbits near a period ratio of 2:1. Although the eccentric single-planet solution is slightly preferred, our statistical analysis indicates that none of these two scenarios can be rejected with high confidence using the RV time series obtained so far. Based on the eccentric interpretation, we find that GJ 4276 b is the most eccentric (e_b_=0.37) exoplanet around an M dwarf with such a short orbital period known today.
The GJ 581 planetary system is already known to harbour three planets, including two super-Earth planets that straddle its habitable zone. We report the detection of an additional planet - GJ 581e - with a minimum mass of 1.9M_{earth}_. With a period of 3.15 days, it is the innermost planet of the system and has a ~5% transit probability. We also correct our previous confusion about the orbital period of GJ 581d (the outermost planet) with a one-year alias, benefitting from an extended time span and many more measurements. The revised period is 66.8 days, and positions the semi-major axis inside the habitable zone of the low mass star. The dynamical stability of the 4-planet system imposes an upper bound on the orbital plane inclination. The planets cannot be more massive than approximately 1.6 times their minimum mass.
M dwarfs are considered ideal targets for Doppler radial velocity searches. Nonetheless, the statistics of frequency of low-mass planets hosted by low mass stars remains poorly constrained. Our M-dwarf radial velocity monitoring with HARPS-N can provide a major contribution to the widening of the current statistics through the in-depth analysis of accurate radial velocity observations in a narrow range of spectral sub-types (79 stars, between dM0 to dM3). Spectral accuracy will enable us to reach the precision needed to detect small planets with a few earth masses. Our survey will bring a contribute to the surveys devoted to the search for planets around M-dwarfs, mainly focused on the M-dwarf population of the northern hemisphere, for which we will provide an estimate of the planet occurence. We present here a long duration radial velocity monitoring of the M1 dwarf star GJ 3998 with HARPS-N to identify periodic signals in the data. Almost simultaneous photometric observations were carried out within the APACHE and EXORAP programs to characterize the stellar activity and to distinguish from the periodic signals those due to activity and to the presence of planetary companions. The radial velocities have a dispersion in excess of their internal errors due to at least four superimposed signals, with periods of 30.7, 13.7, 42.5 and 2.65-days. The analysis of spectral indices based on Ca II H & K and H{alpha} lines demonstrates that the periods of 30.7 and 42.5-days are due to chromospheric inhomogeneities modulated by stellar rotation and differential rotation. The shorter periods of 13.74+/-0.02d and 2.6498+/-0.0008d are well explained with the presence of two planets, with minimum masses of 6.26+/-0.79M_Earth_ and 2.47+/-0.27M_Earth_ and distances of 0.089AU and 0.029AU from the host, respectively.
We present the confirmation and characterisation of GJ 3473 b (G 50-16, TOI-488.01), a hot Earth-sized planet orbiting an M4 dwarf star, whose transiting signal (P=1.1980035+/-0.0000018d) was first detected by the Transiting Exoplanet Survey Satellite (TESS). Through a joint modelling of follow-up radial velocity observations with CARMENES, IRD, and HARPS together with extensive ground-based photometric follow-up observations with LCOGT, MuSCAT, and MuSCAT2, we determined a precise planetary mass, M_b_=1.86+/-0.30M_{sun}_, and radius, R_b_=1.264+/-0.050R_{sun}_. Additionally, we report the discovery of a second, temperate, non-transiting planet in the system, GJ 3473 c, which has a minimum mass, M_c_sini=7.41+/-0.91M_{sun}_ and orbital period, P_c_=15.509+/-0.033d. The inner planet of the system, GJ 3473 b, is one of the hottest transiting Earth- sized planets known thus far, accompanied by a dynamical mass measurement, which makes it a particularly attractive target for thermal emission spectroscopy.
High-precision eclipse spectrophotometry of transiting terrestrial exoplanets represents a promising path for the first atmospheric characterizations of habitable worlds and the search for life outside our solar system. The detection of terrestrial planets transiting nearby late-type M-dwarfs could make this approach applicable within the next decade, with soon-to-come general facilities. In this context, we previously identified GJ 1214 as a high-priority target for a transit search, as the transit probability of a habitable planet orbiting this nearby M4.5 dwarf would be significantly enhanced by the transiting nature of GJ 1214 b, the super-Earth already known to orbit the star. Based on this observation, we have set up an ambitious high-precision photometric monitoring of GJ 1214 with the Spitzer Space Telescope to probe the inner part of its habitable zone in search of a transiting planet as small as Mars. We present here the results of this transit search.
We report the detection of two exoplanets and a further tentative candidate around the M-dwarf stars Gl96 and Gl617A, based on radial velocity measurements obtained with the SOPHIE spectrograph at the Observatoire de Haute-Provence. Both stars were observed in the context of the SOPHIE exoplanet consortium's dedicated M-dwarf subprogramme, which aims to detect exoplanets around nearby M-dwarf stars through a systematic survey. For Gl96 we present the discovery of a new exoplanet at 73.9d with a minimum mass of 19.66 earth masses. Gl96b has an eccentricity of 0.44, placing it among the most eccentric planets orbiting M stars. For Gl617A we independently confirm a recently reported exoplanet at 86.7d with a minimum mass of 31.29 earth masses. Both Gl96 b and Gl617Ab are potentially within the habitable zone, though the high eccentricity of Gl96 b may take it too close to the star at periapsis.