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1. GJ 3929: High-precision Photometric and Doppler Characterization of an Exo-Venus and Its Hot, Mini-Neptune-mass Companion

   https://doi.org/10.3847/1538-4357/ac8480  

   Beard, Corey ; Robertson, Paul ; Kanodia, Shubham ; Lubin, Jack ; Cañas, Caleb I. ; Gupta, Arvind F. ; Holcomb, Rae ; Jones, Sinclaire ; Libby-Roberts, Jessica E. ; Lin, Andrea S. J. ; et al ( August 2022 , The Astrophysical Journal)

   We detail the follow-up and characterization of a transiting exo-Venus identified by TESS, GJ 3929b (TOI-2013b), and its nontransiting companion planet, GJ 3929c (TOI-2013c). GJ 3929b is an Earth-sized exoplanet in its star’s Venus zone (P_b= 2.616272 ± 0.000005 days; S_b=${17.3}_{-0.7}^{+0.8}$S_⊕) orbiting a nearby M dwarf. GJ 3929c is most likely a nontransiting sub-Neptune. Using the new, ultraprecise NEID spectrometer on the WIYN 3.5 m Telescope at Kitt Peak National Observatory, we are able to modify the mass constraints of planet b reported in previous works and consequently improve the significance of the mass measurement to almost 4σconfidence (M_b= 1.75 ± 0.45M_⊕). We further adjust the orbital period of planet c from its alias at 14.30 ± 0.03 days to the likely true period of 15.04 ± 0.03 days, and we adjust its minimum mass to$m\sin i$= 5.71 ± 0.92M_⊕. Using the diffuser-assisted ARCTIC imager on the ARC 3.5 m telescope at Apache Point Observatory, in addition to publicly available TESS and LCOGT photometry, we are able to constrain the radius of planet b toR_p= 1.09 ± 0.04R_⊕. GJ 3929b is a top candidate for transmission spectroscopy in its size regime (TSM = 14 ± 4), and future atmospheric studies of GJ 3929b stand to shed light on the nature of small planets orbiting M dwarfs.

    

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2. The LHS 1678 System: Two Earth-sized Transiting Planets and an Astrometric Companion Orbiting an M Dwarf Near the Convective Boundary at 20 pc

   https://doi.org/10.3847/1538-3881/ac32e3  

   Silverstein, Michele L. ; Schlieder, Joshua E. ; Barclay, Thomas ; Hord, Benjamin J. ; Jao, Wei-Chun ; Vrijmoet, Eliot Halley ; Henry, Todd J. ; Cloutier, Ryan ; Kostov, Veselin B. ; Kruse, Ethan ; et al ( March 2022 , The Astronomical Journal)

   Abstract We present the Transiting Exoplanet Survey Satellite (TESS) discovery of the LHS 1678 (TOI-696) exoplanet system, comprised of two approximately Earth-sized transiting planets and a likely astrometric brown dwarf orbiting a bright ( V J = 12.5, K s = 8.3) M2 dwarf at 19.9 pc. The two TESS-detected planets are of radius 0.70 ± 0.04 R ⊕ and 0.98 ± 0.06 R ⊕ in 0.86 day and 3.69 day orbits, respectively. Both planets are validated and characterized via ground-based follow-up observations. High Accuracy Radial Velocity Planet Searcher RV monitoring yields 97.7 percentile mass upper limits of 0.35 M ⊕ and 1.4 M ⊕ for planets b and c, respectively. The astrometric companion detected by the Cerro Tololo Inter-American Observatory/Small and Moderate Aperture Telescope System 0.9 m has an orbital period on the order of decades and is undetected by other means. Additional ground-based observations constrain the companion to being a high-mass brown dwarf or smaller. Each planet is of unique interest; the inner planet has an ultra-short period, and the outer planet is in the Venus zone. Both are promising targets for atmospheric characterization with the James Webb Space Telescope and mass measurements via extreme-precision radial velocity. A third planet candidate of radius 0.9 ± 0.1 R ⊕ in a 4.97 day orbit is also identified in multicycle TESS data for validation in future work. The host star is associated with an observed gap in the lower main sequence of the Hertzsprung–Russell diagram. This gap is tied to the transition from partially to fully convective interiors in M dwarfs, and the effect of the associated stellar astrophysics on exoplanet evolution is currently unknown. The culmination of these system properties makes LHS 1678 a unique, compelling playground for comparative exoplanet science and understanding the formation and evolution of small, short-period exoplanets orbiting low-mass stars. 

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3. TESS-Keck Survey. XIV. Two Giant Exoplanets from the Distant Giants Survey

   https://doi.org/10.3847/1538-3881/aca6ef  

   Van Zandt, Judah ; Petigura, Erik A. ; MacDougall, Mason ; Gilbert, Gregory J. ; Lubin, Jack ; Barclay, Thomas ; Batalha, Natalie M. ; Crossfield, Ian J. M. ; Dressing, Courtney ; Fulton, Benjamin ; et al ( January 2023 , The Astronomical Journal)

   We present the Distant Giants Survey, a three-year radial velocity campaign to measure P(DG∣CS), the conditional occurrence of distant giant planets (DG;M_p∼ 0.3–13M_J,P> 1 yr) in systems hosting a close-in small planet (CS;R_p< 10R_⊕). For the past two years, we have monitored 47 Sun-like stars hosting small transiting planets detected by TESS. We present the selection criteria used to assemble our sample and report the discovery of two distant giant planets, TOI-1669 b and TOI-1694 c. For TOI-1669 b we find that$M\sin i=0.573\pm 0.074M_{\mathrm{J}}$,P= 502 ± 16 days, ande< 0.27, while for TOI-1694 c,$M\sin i=1.05\pm 0.05M_{\mathrm{J}}$,P= 389.2 ± 3.9 days, ande= 0.18 ± 0.05. We also confirmed the 3.8 days transiting planet TOI-1694 b by measuring a true mass ofM= 26.1 ± 2.2M_⊕. At the end of the Distant Giants Survey, we will incorporate TOI-1669 b and TOI-1694 c into our calculation of P(DG∣CS), a crucial statistic for understanding the relationship between outer giants and small inner companions.

    

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4. A Gap in the Densities of Small Planets Orbiting M Dwarfs: Rigorous Statistical Confirmation Using the Open-source Code RhoPop

   https://doi.org/10.3847/PSJ/ad26f5  

   Schulze, J. G. ; Wang 王, Ji 吉. ; Johnson, J. A. ; Gaudi, B. S. ; Rodriguez Martinez, R. ; Unterborn, C. T. ; Panero, W. R. ( March 2024 , The Planetary Science Journal)

   Using mass–radius composition models, small planets (R≲ 2R_⊕) are typically classified into three types: iron-rich, nominally Earth-like, and those with solid/liquid water and/or atmosphere. These classes are generally expected to be variations within a compositional continuum. Recently, however, Luque & Pallé observed that potentially Earth-like planets around M dwarfs are separated from a lower-density population by a density gap. Meanwhile, the results of Adibekyan et al. hint that iron-rich planets around FGK stars are also a distinct population. It therefore remains unclear whether small planets represent a continuum or multiple distinct populations. Differentiating the nature of these populations will help constrain potential formation mechanisms. We present theRhoPopsoftware for identifying small-planet populations.RhoPopemploys mixture models in a hierarchical framework and a nested sampler for parameter and evidence estimates. UsingRhoPop, we confirm the two populations of Luque & Pallé with >4σsignificance. The intrinsic scatter in the Earth-like subpopulation is roughly half that expected based on stellar abundance variations in local FGK stars, perhaps implying M dwarfs have a smaller spread in the major rock-building elements (Fe, Mg, Si) than FGK stars. We applyRhoPopto the Adibekyan et al. sample and find no evidence of more than one population. We estimate the sample size required to resolve a population of planets with Mercury-like compositions from those with Earth-like compositions for various mass–radius precisions. Only 16 planets are needed when${\sigma }_{M_p}=5\%$and${\sigma }_{R_p}=1\%$. At${\sigma }_{M_p}=10\%$and${\sigma }_{R_p}=2.5\%$, however, over 154 planets are needed, an order of magnitude increase.

    

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5. TESS Hunt for Young and Maturing Exoplanets (THYME). VI. An 11 Myr Giant Planet Transiting a Very-low-mass Star in Lower Centaurus Crux

   https://doi.org/10.3847/1538-3881/ac511d  

   Mann, Andrew W. ; Wood, Mackenna L. ; Schmidt, Stephen P. ; Barber, Madyson G. ; Owen, James E. ; Tofflemire, Benjamin M. ; Newton, Elisabeth R. ; Mamajek, Eric E. ; Bush, Jonathan L. ; Mace, Gregory N. ; et al ( March 2022 , The Astronomical Journal)

   Mature super-Earths and sub-Neptunes are predicted to be ≃ Jovian radius when younger than 10 Myr. Thus, we expect to find 5–15R_⊕planets around young stars even if their older counterparts harbor none. We report the discovery and validation of TOI 1227b, a 0.85 ± 0.05R_J(9.5R_⊕) planet transiting a very-low-mass star (0.170 ± 0.015M_⊙) every 27.4 days. TOI 1227's kinematics and strong lithium absorption confirm that it is a member of a previously discovered subgroup in the Lower Centaurus Crux OB association, which we designate the Musca group. We derive an age of 11 ± 2 Myr for Musca, based on lithium, rotation, and the color–magnitude diagram of Musca members. The TESS data and ground-based follow-up show a deep (2.5%) transit. We use multiwavelength transit observations and radial velocities from the IGRINS spectrograph to validate the signal as planetary in nature, and we obtain an upper limit on the planet mass of ≃0.5M_J. Because such large planets are exceptionally rare around mature low-mass stars, we suggest that TOI 1227b is still contracting and will eventually turn into one of the more common <5R_⊕planets.

    

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