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1. A pair of sub-Neptunes transiting the bright K-dwarf TOI-1064 characterized with CHEOPS

   https://doi.org/10.1093/mnras/stab3799  

   Wilson, Thomas G. ; Goffo, Elisa ; Alibert, Yann ; Gandolfi, Davide ; Bonfanti, Andrea ; Persson, Carina M. ; Collier Cameron, Andrew ; Fridlund, Malcolm ; Fossati, Luca ; Korth, Judith ; et al ( January 2022 , Monthly Notices of the Royal Astronomical Society)

   We report the discovery and characterization of a pair of sub-Neptunes transiting the bright K-dwarf TOI-1064 (TIC 79748331), initially detected in the Transiting Exoplanet Survey Satellite (TESS) photometry. To characterize the system, we performed and retrieved the CHaracterising ExOPlanets Satellite (CHEOPS), TESS, and ground-based photometry, the High Accuracy Radial velocity Planet Searcher (HARPS) high-resolution spectroscopy, and Gemini speckle imaging. We characterize the host star and determine $T_{\rm eff, \star }=4734\pm 67\,\mathrm{ K}$, $R_{\star }=0.726\pm 0.007\, \mathrm{ R}_{\odot }$, and $M_{\star }=0.748\pm 0.032\, \mathrm{ M}_{\odot }$. We present a novel detrending method based on point spread function shape-change modelling and demonstrate its suitability to correct flux variations in CHEOPS data. We confirm the planetary nature of both bodies and find that TOI-1064 b has an orbital period of Pb = 6.44387 ± 0.00003 d, a radius of Rb = 2.59 ± 0.04 R⊕, and a mass of $M_{\rm b} = 13.5_{-1.8}^{+1.7}$ M⊕, whilst TOI-1064 c has an orbital period of $P_{\rm c} = 12.22657^{+0.00005}_{-0.00004}$ d, a radius of Rc = 2.65 ± 0.04 R⊕, and a 3σ upper mass limit of 8.5 M⊕. From the high-precision photometry we obtain radius uncertainties of ∼1.6 per cent, allowing us to conduct internal structure and atmospheric escape modelling. TOI-1064 b is one of the densest, well-characterized sub-Neptunes, withmore »a tenuous atmosphere that can be explained by the loss of a primordial envelope following migration through the protoplanetary disc. It is likely that TOI-1064 c has an extended atmosphere due to the tentative low density, however further radial velocities are needed to confirm this scenario and the similar radii, different masses nature of this system. The high-precision data and modelling of TOI-1064 b are important for planets in this region of mass–radius space, and it allow us to identify a trend in bulk density–stellar metallicity for massive sub-Neptunes that may hint at the formation of this population of planets.

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2. TOI-1075 b: A Dense, Massive, Ultra-short-period Hot Super-Earth Straddling the Radius Gap

   https://doi.org/10.3847/1538-3881/ac9c5b  

   Essack, Zahra ; Shporer, Avi ; Burt, Jennifer A. ; Seager, Sara ; Cambioni, Saverio ; Lin, Zifan ; Collins, Karen A. ; Mamajek, Eric E. ; Stassun, Keivan G. ; Ricker, George R. ; et al ( January 2023 , The Astronomical Journal)

   Populating the exoplanet mass–radius diagram in order to identify the underlying relationship that governs planet composition is driving an interdisciplinary effort within the exoplanet community. The discovery of hot super-Earths—a high-temperature, short-period subset of the super-Earth planet population—has presented many unresolved questions concerning the formation, evolution, and composition of rocky planets. We report the discovery of a transiting, ultra-short-period hot super-Earth orbitingTOI-1075(TIC351601843), a nearby (d= 61.4 pc) late-K/early-M-dwarf star, using data from the Transiting Exoplanet Survey Satellite. The newly discovered planet has a radius of 1.791${}_{-0.081}^{+0.116}$R_⊕and an orbital period of 0.605 day (14.5 hr). We precisely measure the planet mass to be 9.95${}_{-1.30}^{+1.36}$M_⊕using radial velocity measurements obtained with the Planet Finder Spectrograph mounted on the Magellan II telescope. Our radial velocity data also show a long-term trend, suggesting an additional planet in the system. While TOI-1075 b is expected to have a substantial H/He atmosphere given its size relative to the radius gap, its high density (${9.32}_{-1.85}^{+2.05}$g cm⁻³) is likely inconsistent with this possibility. We explore TOI-1075 b’s location relative to the M-dwarf radius valley, evaluate the planet’s prospects for atmospheric characterization, andmore »discuss potential planet formation mechanisms. Studying the TOI-1075 system in the broader context of ultra-short-period planetary systems is necessary for testing planet formation and evolution theories and density-enhancing mechanisms and for future atmospheric and surface characterization studies via emission spectroscopy with the JWST.

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3. TESS-Keck Survey. IX. Masses of Three Sub-Neptunes Orbiting HD 191939 and the Discovery of a Warm Jovian plus a Distant Substellar Companion

   https://doi.org/10.3847/1538-3881/ac3d38  

   Lubin, Jack ; Van Zandt, Judah ; Holcomb, Rae ; Weiss, Lauren M. ; Petigura, Erik A. ; Robertson, Paul ; Akana Murphy, Joseph M. ; Scarsdale, Nicholas ; Batygin, Konstantin ; Polanski, Alex S. ; et al ( January 2022 , The Astronomical Journal)

   Exoplanet systems with multiple transiting planets are natural laboratories for testing planetary astrophysics. One such system is HD 191939 (TOI 1339), a bright (V= 9) and Sun-like (G9V) star, which TESS found to host three transiting planets (b, c, and d). The planets have periods of 9, 29, and 38 days each with similar sizes from 3 to 3.4R_⊕. To further characterize the system, we measured the radial velocity (RV) of HD 191939 over 415 days with Keck/HIRES and APF/Levy. We find thatM_b= 10.4 ± 0.9M_⊕andM_c= 7.2 ± 1.4M_⊕, which are low compared to most known planets of comparable radii. The RVs yield only an upper limit onM_d(<5.8M_⊕at 2σ). The RVs further reveal a fourth planet (e) with a minimum mass of 0.34 ± 0.01M_Jupand an orbital period of 101.4 ± 0.4 days. Despite its nontransiting geometry, secular interactions between planet e and the inner transiting planets indicate that planet e is coplanar with the transiting planets (Δi< 10°). We identify a second high-mass planet (f) with 95% confidence intervals on mass between 2 and 11M_Jupand period between 1700 and 7200 days, based on a joint analysis of RVs and astrometry from Gaia and Hipparcos. As a bright starmore »hosting multiple planets with well-measured masses, HD 191939 presents many options for comparative planetary astronomy, including characterization with JWST.

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4. TOI-5205b: A Short-period Jovian Planet Transiting a Mid-M Dwarf

   https://doi.org/10.3847/1538-3881/acabce  

   Kanodia, Shubham ; Mahadevan, Suvrath ; Libby-Roberts, Jessica ; Stefansson, Gudmundur ; Cañas, Caleb I. ; Piette, Anjali A. A. ; Boss, Alan ; Teske, Johanna ; Chambers, John ; Zeimann, Greg ; et al ( February 2023 , The Astronomical Journal)

   We present the discovery of TOI-5205b, a transiting Jovian planet orbiting a solar metallicity M4V star, which was discovered using Transiting Exoplanet Survey Satellite photometry and then confirmed using a combination of precise radial velocities, ground-based photometry, spectra, and speckle imaging. TOI-5205b has one of the highest mass ratios for M-dwarf planets, with a mass ratio of almost 0.3%, as it orbits a host star that is just 0.392 ± 0.015M_⊙. Its planetary radius is 1.03 ± 0.03R_J, while the mass is 1.08 ± 0.06M_J. Additionally, the large size of the planet orbiting a small star results in a transit depth of ∼7%, making it one of the deepest transits of a confirmed exoplanet orbiting a main-sequence star. The large transit depth makes TOI-5205b a compelling target to probe its atmospheric properties, as a means of tracing the potential formation pathways. While there have been radial-velocity-only discoveries of giant planets around mid-M dwarfs, this is the first transiting Jupiter with a mass measurement discovered around such a low-mass host star. The high mass of TOI-5205b stretches conventional theories of planet formation and disk scaling relations that cannot easily recreate the conditions required to form such planets.
5. The young HD 73583 (TOI-560) planetary system: two 10-M⊕ mini-Neptunes transiting a 500-Myr-old, bright, and active K dwarf

   https://doi.org/10.1093/mnras/stac638  

   Barragán, O. ; Armstrong, D. J. ; Gandolfi, D. ; Carleo, I. ; Vidotto, A. A. ; Villarreal D’Angelo, C. ; Oklopčić, A. ; Isaacson, H. ; Oddo, D. ; Collins, K. ; et al ( March 2022 , Monthly Notices of the Royal Astronomical Society)

   We present the discovery and characterization of two transiting planets observed by TESS in the light curves of the young and bright (V = 9.67) star HD73583 (TOI-560). We perform an intensive spectroscopic and photometric space- and ground-based follow-up in order to confirm and characterize the system. We found that HD73583 is a young (∼500 Myr) active star with a rotational period of 12.08 ± 0.11  d, and a mass and radius of 0.73 ± 0.02 M⊙ and 0.65 ± 0.02 R⊙, respectively. HD 73583 b (Pb = $6.3980420 _{ - 0.0000062 } ^ { + 0.0000067 }$ d) has a mass and radius of $10.2 _{ - 3.1 } ^ { + 3.4 }$ M⊕ and 2.79 ± 0.10 R⊕, respectively, which gives a density of $2.58 _{ - 0.81 } ^ { + 0.95 }$ ${\rm g\, cm^{-3}}$. HD 73583 c (Pc = $18.87974 _{ - 0.00074 } ^ { + 0.00086 }$ d) has a mass and radius of $9.7 _{ - 1.7 } ^ { + 1.8 }$ M⊕ and $2.39 _{ - 0.09 } ^ { + 0.10 }$ R⊕, respectively, which translates to a density of $3.88 _{ - 0.80 } ^ { + 0.91 }$ ${\rm g\, cm^{-3}}$. Both planets are consistent with worlds made of a solid core surrounded by a volatile envelope. Because of their youth andmore »host star brightness, they both are excellent candidates to perform transmission spectroscopy studies. We expect ongoing atmospheric mass-loss for both planets caused by stellar irradiation. We estimate that the detection of evaporating signatures on H and He would be challenging, but doable with present and future instruments.

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