Sibling Sub-Neptunes Around Sibling M Dwarfs: TOI-521 and TOI-912

Abstract

Sub-Neptunes are absent in the Solar System, yet they are commonly found in our Galaxy. They challenge the internal structure models and prompt investigation on their formation, evolution, and atmospheres. We report the characterisation of new sub-Neptunes orbiting two similar M dwarfs, TOI-521 (T_eff=3544 K), and TOI-912 (T_eff=3572 K). Both stars host a candidate identified by TESS and are part of the THIRSTEE follow-up program, which aims at understanding the sub-Neptune population through precise characterisation studies on a population level. We analysed light curves, ground-based photometry and ESPRESSO, HARPS and IRD RVs to infer precise orbital and physical parameters. The two stars host nearly identical planets in terms of mass and radius. TOI-521 b is a transiting sub-Neptune in a 1.5-d orbit with radius and mass of R=1.98+/-0.14 R_e and M=5.3+/-1.0 M_e respectively. Moreover, we identified an additional candidate at 20.3 d, with a minimum mass of Msini=10.7+/-2.4 M_e currently not detected to transit. Similarly, TOI-912 b is a 4.7-d sub-Neptune with R=1.93+/-0.13 R_e and M=5.1+/-0.5 M_e. Interestingly, TOI-912 b likely has an unusually high eccentricity (e=0.58+/-0.02), and it is probably undergoing strong tidal dissipation. If such eccentricity is confirmed, it would make it one of the most eccentric sub-Neptunes known to date. TOI-521 b and TOI-912 b have very similar densities (4 g/cm^3) and they lie in the degenerate region of the mass-radius diagram where different compositions are plausible, including a volatile-rich composition, or a rocky core surrounded by a H-He envelope. Our sample supports the division of sub-Neptunes into two distinct populations divided by a density gap. Both planets are interesting targets for atmospheric follow-up in the context of understanding the temperature-atmospheric feature trend that starts to emerge thanks to JWST observations.

22 pages. 23 Figures. Accepted from publication in A&A

References in corpus (50)

• Gaia Data Release 3: Summary of the content and survey properties
• The Zwicky Transient Facility: System Overview, Performance, and First Results
• The generalised Lomb-Scargle periodogram. A new formalism for the floating-mean and Keplerian periodograms
• Improved Age Estimation for Solar-Type Dwarfs Using Activity-Rotation Diagnostics
• The Zwicky Transient Facility: Data Processing, Products, and Archive
• The All-Sky Automated Survey for Supernovae (ASAS-SN) Light Curve Server v1.0
• EXOFAST: A fast exoplanetary fitting suite in IDL
• Growth Model Interpretation of Planet Size Distribution
• ESPRESSO@VLT -- On-sky performance and first results
• Spectrum radial velocity analyser (SERVAL). High-precision radial velocities and two alternative spectral indicators
• Density, not radius, separates rocky and water-rich small planets orbiting M dwarf stars
• Magnetic Cycles and Rotation Periods of Late Type Stars from photometric time series
• The New Generation Planetary Population Synthesis (NGPPS). IV. Planetary systems around low-mass stars
• The CARMENES search for exoplanets around M dwarfs: Different roads to radii and masses of the target stars
• Calibrating Gyrochronology using Kepler Asteroseismic targets
• Long-term tidal evolution of short-period planets with companions
• CARMENES input catalogue of M dwarfs V. Luminosities, colours, and spectral energy distributions
• Stellar chromospheric activity of 1,674 FGK stars from the AMBRE-HARPS sample I. A catalogue of homogeneous chromospheric activity
• Mass-radius relationships for irradiated ocean planets
• The CARMENES search for exoplanets around M dwarfs. Stellar atmospheric parameters of target stars with SteParSyn
• Conclusive evidence for a population of water-worlds around M-dwarfs remains elusive
• The Monitor project: Data processing and lightcurve production
• JWST/NIRISS reveals the water-rich "steam world" atmosphere of GJ 9827 d
• Rocky super-Earths or waterworlds: the interplay of planet migration, pebble accretion and disc evolution
• The Exoplanet Radius Valley from Gas-driven Planet Migration and Breaking of Resonant Chains
• Majority of water hides deep in the interiors of exoplanets
• The Densities of Planets in Multiple Stellar Systems
• From super-Earths to sub-Neptunes: Observational constraints and connections to theoretical models
• Precision radial velocity measurements by the forward-modeling technique in the near-infrared
• A fading radius valley towards M-dwarfs, a persistent density valley across stellar types
• SteParSyn: A Bayesian code to infer stellar atmospheric parameters using spectral synthesis
• GJ 273: On the formation, dynamical evolution and habitability of a planetary system hosted by an M dwarf at 3.75 parsec
• Unresolved Binary Exoplanet Host Stars Fit as Single Stars: Effects on the Stellar Parameters
• Atmospheres as windows into sub-Neptune interiors: coupled chemistry and structure of hydrogen-silane-water envelopes
• TOI-1235 b: a keystone super-Earth for testing radius valley emergence models around early M dwarfs
• ExoMDN: Rapid characterization of exoplanet interior structures with Mixture Density Networks
• TOI-269 b: An eccentric sub-Neptune transiting a M2 dwarf revisited with ExTrA
• Elemental Abundances of M Dwarfs Based on High-Resolution Near-Infrared Spectra: Verification by Binary Systems
• A super-Earth and a mini-Neptune near the 2:1 MMR straddling the radius valley around the nearby mid-M dwarf TOI-2096
• Elemental abundances of nearby M dwarfs based on high-resolution near-infrared spectra obtained by the Subaru/IRD survey: Proof of concept
• Revisiting K2-233 spectroscopic time-series with multidimensional Gaussian Processes
• Planets larger than Neptune have elevated eccentricities
• Resonant sub-Neptunes are puffier
• Gliese 12 b: A temperate Earth-sized planet at 12 pc ideal for atmospheric transmission spectroscopy
• The SHERLOCK pipeline: new exoplanet candidates in the WASP-16, HAT-P-27, HAT-P-26, and TOI-2411 systems
• A Super-Earth Orbiting Near the Inner Edge of the Habitable Zone around the M4.5-dwarf Ross 508
• Characterisation of TOI-406 as showcase of the THIRSTEE program: A 2-planet system straddling the M-dwarf density gap
• A Gap in the Densities of Small Planets Orbiting M Dwarfs: Rigorous Statistical Confirmation Using the Open-source Code RhoPop
• The Hot-Neptune Initiative (HONEI) I. Two hot sub-Neptunes on a close-in, eccentric orbit (TOI-5800 b) and a farther-out, circular orbit (TOI-5817 b)
• A transiting rocky super-Earth and a non-transiting sub-Neptune orbiting the M dwarf TOI-771