Eyes on K2-3: A system of three likely sub-Neptunes characterized with HARPS-N and HARPS

Context. M-dwarf stars are promising targets for identifying and characterizing potentially habitable planets. K2-3 is a nearby (45 pc), early-type M dwarf hosting three small transiting planets, the outermost of which orbits close to the inner edge of the stellar (optimistic) habitable zone. The K2-3 system is well suited for follow-up characterization studies aimed at determining accurate masses and bulk densities of the three planets. Aims. Using a total of 329 radial velocity measurements collected over 2.5 years with the HARPS-N and HARPS spectrographs and a proper treatment of the stellar activity signal, we aim to improve measurements of the masses and bulk densities of the K2-3 planets. We use our results to investigate the physical structure of the planets. Methods. We analysed radial velocity time series extracted with two independent pipelines using Gaussian process regression. We adopted a quasi-periodic kernel to model the stellar magnetic activity jointly with the planetary signals. We used Monte Carlo simulations to investigate the robustness of our mass measurements of K2-3 c and K2-3 d, and to explore how additional high-cadence radial velocity observations might improve these values. Results. Even though the stellar activity component is the strongest signal present in the radial velocity time series, we are able to derive masses for both planet b (Mb = 6.6 ± 1.1 M⊕) and planet c (Mc = 3.1−1.2+1.3 M⊕). The Doppler signal from K2-3 d remains undetected, likely because of its low amplitude compared to the radial velocity signal induced by the stellar activity. The closeness of the orbital period of K2-3 d to the stellar rotation period could also make the detection of the planetary signal complicated. Based on our ability to recover injected signals in simulated data, we tentatively estimate the mass of K2-3 d to be Md = 2.7−0.8+1.2 M⊕ M⊕. These mass measurements imply that the bulk densities and therefore the interior structures of the three planets may be similar. In particular, the planets may either have small H/He envelopes (<1%) or massive water layers, with a water content ≥50% of their total mass, on top of rocky cores. Placing further constraints on the bulk densities of K2-3 c and d is difficult; in particular, we would not have been able to detect the Doppler signal of K2-3 d even by adopting a semester of intense, high-cadence radial velocity observations with HARPS-N and HARPS.

[1]  A. Walsh,et al.  The Astrophysical Journal Supplement , 2020 .

[2]  C. Moutou,et al.  The HARPS search for southern extra-solar planets , 2004, Astronomy & Astrophysics.

[3]  Xavier Bonfils,et al.  A temperate exo-Earth around a quiet M dwarf at 3.4 parsec , 2017, 1711.06177.

[4]  D. Sasselov,et al.  Exoplanet Radius Gap Dependence on Host Star Type , 2017, 1712.05458.

[5]  A.H.M.J.Triaud,et al.  Temporal Evolution of the High-energy Irradiation and Water Content of TRAPPIST-1 Exoplanets , 2017, 1708.09484.

[6]  A. Cameron,et al.  A Kepler study of starspot lifetimes with respect to light-curve amplitude and spectral type , 2017, 1707.08583.

[7]  X. Delfosse,et al.  Characterization of the K2-18 multi-planetary system with HARPS: A habitable zone super-Earth and discovery of a second, warm super-Earth on a non-coplanar orbit , 2017, 1707.04292.

[8]  Christoph Mordasini,et al.  Compositional Imprints in Density–Distance–Time: A Rocky Composition for Close-in Low-mass Exoplanets from the Location of the Valley of Evaporation , 2017, 1706.00251.

[9]  James E. Owen,et al.  The Evaporation Valley in the Kepler Planets , 2017, 1705.10810.

[10]  Joseph E. Rodriguez,et al.  A temperate rocky super-Earth transiting a nearby cool star , 2017, Nature.

[11]  E. Quintana,et al.  Plausible Compositions of the Seven TRAPPIST-1 Planets Using Long-term Dynamical Simulations , 2017, 1704.02261.

[12]  Howard Isaacson,et al.  The California-Kepler Survey. III. A Gap in the Radius Distribution of Small Planets , 2017, 1703.10375.

[13]  D. Ciardi,et al.  Characterizing K2 Candidate Planetary Systems Orbiting Low-mass Stars. II. Planetary Systems Observed During Campaigns 1–7 , 2017, 1703.07416.

[14]  F. Bouchy,et al.  The Kepler-19 System: A Thick-envelope Super-Earth with Two Neptune-mass Companions Characterized Using Radial Velocities and Transit Timing Variations , 2017, 1703.06885.

[15]  C. S. Fernandes,et al.  Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1 , 2017, Nature.

[16]  D. Charbonneau,et al.  Characterizing K2 Candidate Planetary Systems Orbiting Low-mass Stars. I. Classifying Low-mass Host Stars Observed during Campaigns 1–7 , 2017, 1701.00586.

[17]  E. Lopez Born dry in the photoevaporation desert: Kepler's ultra-short-period planets formed water-poor , 2016, 1610.01170.

[18]  M. Damasso,et al.  Proxima Centauri reloaded: Unravelling the stellar noise in radial velocities , 2016, 1612.03786.

[19]  Antonino Francesco Lanza,et al.  The HADES RV Programme with HARPS-N@TNG II. Data treatment and simulations , 2016, 1610.08698.

[20]  Aomawa L. Shields,et al.  The Habitability of Planets Orbiting M-dwarf Stars , 2016, 1610.05765.

[21]  T. Hirano,et al.  GROUND-BASED TRANSIT OBSERVATION OF THE HABITABLE-ZONE SUPER-EARTH K2-3D , 2016, 1610.01333.

[22]  A. Bonomo,et al.  KEPLER-21b: A ROCKY PLANET AROUND A V = 8.25 mag STAR , 2016, 1609.07617.

[23]  S. Aigrain,et al.  Radial-velocity fitting challenge - II. First results of the analysis of the data set , 2016, 1609.03674.

[24]  L. F. Sarmiento,et al.  A terrestrial planet candidate in a temperate orbit around Proxima Centauri , 2016, Nature.

[25]  R. Rebolo,et al.  The HADES RV Programme with HARPS-N@TNG - GJ 3998: An early M-dwarf hosting a system of Super-Earths , 2016, 1607.03632.

[26]  D. Ciardi,et al.  Radial Velocity Planet Detection Biases at the Stellar Rotational Period , 2016, 1604.03143.

[27]  Fei Dai,et al.  DOPPLER MONITORING OF FIVE K2 TRANSITING PLANETARY SYSTEMS , 2016, 1604.01413.

[28]  Erik A. Petigura,et al.  SPITZER OBSERVATIONS OF EXOPLANETS DISCOVERED WITH THE KEPLER K2 MISSION , 2016, 1603.01934.

[29]  Dimitar Sasselov,et al.  MASS–RADIUS RELATION FOR ROCKY PLANETS BASED ON PREM , 2015, 1512.08827.

[30]  S. Ginzburg,et al.  SUPER-EARTH ATMOSPHERES: SELF-CONSISTENT GAS ACCRETION AND RETENTION , 2015, 1512.07925.

[31]  Antonio Manescau,et al.  HARPS Gets New Fibres After 12 Years of Operations , 2015 .

[32]  Howard Isaacson,et al.  ELEVEN MULTIPLANET SYSTEMS FROM K2 CAMPAIGNS 1 AND 2 AND THE MASSES OF TWO HOT SUPER-EARTHS , 2015, 1511.09213.

[33]  P. Berlind,et al.  PLANETARY CANDIDATES FROM THE FIRST YEAR OF THE K2 MISSION , 2015, 1511.07820.

[34]  E. Chiang,et al.  BREEDING SUPER-EARTHS AND BIRTHING SUPER-PUFFS IN TRANSITIONAL DISKS , 2015, 1510.08855.

[35]  A. Santerne,et al.  A HARPS view on K2-3 , 2015, 1509.02917.

[36]  E. Chiang,et al.  TO COOL IS TO ACCRETE: ANALYTIC SCALINGS FOR NEBULAR ACCRETION OF PLANETARY ATMOSPHERES , 2015, 1508.05096.

[37]  M. Esposito,et al.  Stellar parameters of early M dwarfs from ratios of spectral features at optical wavelengths , 2015, 1503.03010.

[38]  B. J. Fulton,et al.  A NEARBY M STAR WITH THREE TRANSITING SUPER-EARTHS DISCOVERED BY K2 , 2015, 1501.03798.

[39]  D. Charbonneau,et al.  THE OCCURRENCE OF POTENTIALLY HABITABLE PLANETS ORBITING M DWARFS ESTIMATED FROM THE FULL KEPLER DATASET AND AN EMPIRICAL MEASUREMENT OF THE DETECTION SENSITIVITY , 2015, 1501.01623.

[40]  D. Queloz,et al.  The HARPS search for southern extra-solar planets - XXXVI. Planetary systems and stellar activity of the M dwarfs GJ 3293, GJ 3341, and GJ 3543 , 2014, 1411.7048.

[41]  Gautam Vasisht,et al.  Observations of Transiting Exoplanets with the James Webb Space Telescope (JWST) , 2014, 1411.1754.

[42]  A. Vanderburg,et al.  A Technique for Extracting Highly Precise Photometry for the Two-Wheeled Kepler Mission , 2014, 1408.3853.

[43]  L. Rogers MOST 1.6 EARTH-RADIUS PLANETS ARE NOT ROCKY , 2014, 1407.4457.

[44]  Simon Thibault,et al.  SPIRou: the near-infrared spectropolarimeter/high-precision velocimeter for the Canada-France-Hawaii telescope , 2014, Astronomical Telescopes and Instrumentation.

[45]  Shawn Domagal-Goldman,et al.  HABITABLE ZONES AROUND MAIN-SEQUENCE STARS: DEPENDENCE ON PLANETARY MASS , 2014, 1404.5292.

[46]  M. Tuomi,et al.  Bayesian search for low-mass planets around nearby M dwarfs – estimates for occurrence rate based on global detectability statistics , 2014, 1403.0430.

[47]  F. Mullally,et al.  The K2 Mission: Characterization and Early Results , 2014, 1402.5163.

[48]  M. Riva,et al.  ESPRESSO: The next European exoplanet hunter , 2014, 1401.5918.

[49]  J. Fortney,et al.  UNDERSTANDING THE MASS–RADIUS RELATION FOR SUB-NEPTUNES: RADIUS AS A PROXY FOR COMPOSITION , 2013, 1311.0329.

[50]  F. Feroz,et al.  Bayesian analysis of radial velocity data of GJ667C with correlated noise: evidence for only two planets , 2013, 1307.6984.

[51]  P. Giommi,et al.  The PLATO 2.0 mission , 2013, 1310.0696.

[52]  R. P. Butler,et al.  A dynamically-packed planetary system around GJ 667C with three super-Earths in its habitable zone , 2013, 1306.6074.

[53]  A. Collier Cameron,et al.  Planets and Stellar Activity: Hide and Seek in the CoRoT-7 system , 2013, Proceedings of the International Astronomical Union.

[54]  M. G. Lattanzi,et al.  The APACHE Project , 2013, 1303.1275.

[55]  D. Charbonneau,et al.  THE OCCURRENCE RATE OF SMALL PLANETS AROUND SMALL STARS , 2013, 1302.1647.

[56]  R. Deshpande,et al.  HABITABLE ZONES AROUND MAIN-SEQUENCE STARS: NEW ESTIMATES , 2013, 1301.6674.

[57]  Cambridge,et al.  A Detailed Model Grid for Solid Planets from 0.1 through 100 Earth Masses , 2013, 1301.0818.

[58]  Paul Robertson,et al.  Hα ACTIVITY OF OLD M DWARFS: STELLAR CYCLES AND MEAN ACTIVITY LEVELS FOR 93 LOW-MASS STARS IN THE SOLAR NEIGHBORHOOD , 2012, 1211.6091.

[59]  B. Scott Gaudi,et al.  EXOFAST: A Fast Exoplanetary Fitting Suite in IDL , 2012, 1206.5798.

[60]  F. Bouchy,et al.  The HARPS search for southern extra-solar planets XXXV. Super-Earths around the M-dwarf neighbors Gl433 and Gl667C , 2012, 1202.2467.

[61]  I. Savanov Activity cycles of M dwarfs , 2012 .

[62]  R. Paul Butler,et al.  THE HARPS-TERRA PROJECT. I. DESCRIPTION OF THE ALGORITHMS, PERFORMANCE, AND NEW MEASUREMENTS ON A FEW REMARKABLE STARS OBSERVED BY HARPS , 2012, 1202.2570.

[63]  F. Bouchy,et al.  The HARPS search for southern extra-solar planets - XXXI. The M-dwarf sample , 2011, 1111.5019.

[64]  X. Delfosse,et al.  Long-term magnetic activity of a sample of M-dwarf stars from the HARPS program I. Comparison of activity indices , 2011, 1109.0321.

[65]  Jan Swevers,et al.  Ground-based and airborne instrumentation for astronomy , 2010 .

[66]  Michael Wegner,et al.  Ground-based and Airborne Instrumentation for Astronomy III , 2010 .

[67]  M. Zechmeister,et al.  The generalised Lomb-Scargle periodogram. A new formalism for the floating-mean and Keplerian periodograms , 2009, 0901.2573.

[68]  Daniel R. Coulter,et al.  Techniques and Instrumentation for Detection of Exoplanets III , 2007 .

[69]  F. Pepe,et al.  A new list of thorium and argon spectral lines in the visible , 2007, astro-ph/0703412.

[70]  C. Sotin,et al.  Could we identify hot ocean-planets with CoRoT, Kepler and Doppler velocimetry? , 2007, astro-ph/0701608.