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1. The Aligned Orbit of a Hot Jupiter around the M Dwarf TOI-4201

   https://doi.org/10.3847/2041-8213/ad5967  

   Gan, Tianjun; Wang, Sharon X; Dai, Fei; Winn, Joshua N; Mao, Shude; Xu, Siyi; Pallé, Enric; Bean, Jacob L; Brady, Madison; Brown, Nina; et al (July 2024, The Astrophysical Journal Letters)

   Abstract Measuring the obliquities of stars hosting giant planets may shed light on the dynamical history of planetary systems. Significant efforts have been made to measure the obliquities of FGK stars with hot Jupiters, mainly based on observations of the Rossiter–McLaughlin effect. In contrast, M dwarfs with hot Jupiters have hardly been explored because such systems are rare and often not favorable for such precise observations. Here, we report the first detection of the Rossiter–McLaughlin effect for an M dwarf with a hot Jupiter, TOI-4201, using the Gemini-North/MAROON-X spectrograph. We find TOI-4201 to be well aligned with its giant planet, with a sky-projected obliquity of $\lambda =-{3.0}_{-3.2}^{+3.7}°$and a true obliquity of $\psi ={21.3}_{-12.8}^{+12.5}°$with an upper limit of 40^◦at a 95% confidence level. The result agrees with dynamically quiet formation or tidal obliquity damping that realigned the system. As the first hot Jupiter around an M dwarf with its obliquity measured, TOI-4201b joins the group of aligned giant planets around cool stars (T_eff< 6250 K), as well as the small but growing sample of planets with relatively high planet-to-star mass ratio (M_p/M_*≳ 3 × 10⁻³) that also appear to be mostly aligned. 

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2. The Warm Neptune GJ 3470b Has a Polar Orbit

   https://doi.org/10.3847/2041-8213/ac6e3c  

   Stefànsson, Guđmundur; Mahadevan, Suvrath; Petrovich, Cristobal; Winn, Joshua N.; Kanodia, Shubham; Millholland, Sarah C.; Maney, Marissa; Cañas, Caleb I.; Wisniewski, John; Robertson, Paul; et al (May 2022, The Astrophysical Journal Letters)

   Abstract The warm Neptune GJ 3470b transits a nearby (d= 29 pc) bright slowly rotating M1.5-dwarf star. Using spectroscopic observations during two transits with the newly commissioned NEID spectrometer on the WIYN 3.5 m Telescope at Kitt Peak Observatory, we model the classical Rossiter–McLaughlin effect, yielding a sky-projected obliquity of $\lambda ={98}_{-12}^{+15◦}$and a $v\sin i={0.85}_{-0.33}^{+0.27}\mathrm{km}{\mathrm{s}}^{-1}$. Leveraging information about the rotation period and size of the host star, our analysis yields a true obliquity of $\psi ={95}_{-8}^{+9◦}$, revealing that GJ 3470b is on a polar orbit. Using radial velocities from HIRES, HARPS, and the Habitable-zone Planet Finder, we show that the data are compatible with a long-term radial velocity (RV) slope of $\dot{\gamma }=-0.0022\pm 0.0011\mathrm{m}{\mathrm{s}}^{-1}{\mathrm{day}}^{-1}$over a baseline of 12.9 yr. If the RV slope is due to acceleration from another companion in the system, we show that such a companion is capable of explaining the polar and mildly eccentric orbit of GJ 3470b using two different secular excitation models. The existence of an outer companion can be further constrained with additional RV observations, Gaia astrometry, and future high-contrast imaging observations. Lastly, we show that tidal heating from GJ 3470b’s mild eccentricity has most likely inflated the radius of GJ 3470b by a factor of ∼1.5–1.7, which could help account for its evaporating atmosphere. 

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3. A Small Brown Dwarf in an Aligned Orbit Around a Young, Fully Convective M Star

   https://doi.org/10.3847/1538-3881/ad9c66  

   Brady, Madison; Bean, Jacob_L; Stefánsson, Gumundur; Brown, Nina; Seifahrt, Andreas; Basant, Ritvik; Das, Tanya; Luque, Rafael; Stürmer, Julian (January 2025, The Astronomical Journal)

   Abstract A star's spin–orbit angle can give us insight into a system's formation and dynamical history. In this paper, we use MAROON-X observations of the Rossiter–McLaughlin effect to measure the projected obliquity of the LP 261-75 (also known as TOI-1779) system, focusing on the fully convective M dwarf LP 261-75A and the transiting brown dwarf LP 261-75C. This is the first obliquity constraint of a brown dwarf orbiting an M dwarf and the seventh obliquity constraint of a brown dwarf overall. We measure a projected obliquity of $5_{-10}^{+11}$degrees and a true obliquity of $14_{-7}^{+8}$degrees for the system, meaning that the system is well aligned and that the star is rotating very nearly edge-on, with an inclination of 90° ±  11°. The system thus follows along with the trends observed in transiting brown dwarfs around hotter stars, which typically have low obliquities. The tendency for brown dwarfs to be aligned may point to some enhanced obliquity damping in brown dwarf systems, but there is also a possibility that the LP 261-75 system was simply formed aligned. In addition, we note that the brown dwarf's radius (R_C =  0.9R_J) is not consistent with the youth of the system or radius trends observed in other brown dwarfs, indicating that LP 261-75C may have an unusual formation history. 

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4. The Super-puff WASP-193 b is on a Well-aligned Orbit*

   https://doi.org/10.3847/1538-3881/adba5f  

   Yee, Samuel W; Stefánsson, Gudmundur; Thorngren, Daniel; Monson, Andy; Hartman, Joel D; Charbonneau, David B; Teske, Johanna K; Butler, R Paul; Crane, Jeffrey D; Osip, David; et al (March 2025, The Astronomical Journal)

   Abstract The “super-puffs” are a population of planets that have masses comparable to that of Neptune but radii similar to Jupiter, leading to extremely low bulk densities (ρ_p ≲ 0.2 g cm⁻³) that are not easily explained by standard core accretion models. Interestingly, several of these super-puffs are found in orbits significantly misaligned with their host stars’ spin axes, indicating past dynamical excitation that may be connected to their low densities. Here, we present new Magellan/Planet Finder Spectrograph radial velocity measurements of WASP-193, a late F star hosting one of the least dense transiting planets known to date ( $M_p=0.112_{-0.034}^{+0.029}{M}_{\mathrm{J}}$, $R_p=1.319_{-0.048}^{+0.056}{R}_{\mathrm{J}}$,ρ_p = 0.060 ± 0.019 g cm⁻³). We refine the bulk properties of WASP-193 b and use interior structure models to determine that the planet can be explained if it consists of roughly equal amounts of metals and H/He, with a metal fraction ofZ= 0.42. The planet is likely substantially reinflated due to its host star’s evolution, and expected to be actively undergoing mass loss. We also measure the projected stellar obliquity using the Rossiter–McLaughlin effect, finding that WASP-193 b is on an orbit well aligned with the stellar equator, with $\lambda =16_{-15}^{+16}$degrees. WASP-193 b is the first Jupiter-sized super-puff on a relatively well-aligned orbit, suggesting a diversity of formation pathways for this population of planets. 

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5. TOI-2015 b: A Warm Neptune with Transit Timing Variations Orbiting an Active Mid-type M Dwarf

   https://doi.org/10.3847/1538-3881/ad55ea  

   Jones, Sinclaire_E; Stefánsson, Guđmundur; Masuda, Kento; Libby-Roberts, Jessica_E; Gardner, Cristilyn_N; Holcomb, Rae; Beard, Corey; Robertson, Paul; Cañas, Caleb_I; Mahadevan, Suvrath; et al (July 2024, The Astronomical Journal)

   Abstract We report the discovery of a close-in (P_orb= 3.349 days) warm Neptune with clear transit timing variations (TTVs) orbiting the nearby (d= 47.3 pc) active M4 star, TOI-2015. We characterize the planet's properties using Transiting Exoplanet Survey Satellite (TESS) photometry, precise near-infrared radial velocities (RVs) with the Habitable-zone Planet Finder Spectrograph, ground-based photometry, and high-contrast imaging. A joint photometry and RV fit yields a radius $R_p={3.37}_{-0.20}^{+0.15}{R}_{\oplus }$, mass $m_p={16.4}_{-4.1}^{+4.1}{M}_{\oplus }$, and density ${\rho }_p={2.32}_{-0.37}^{+0.38}\mathrm{g}{\mathrm{cm}}^{-3}$for TOI-2015 b, suggesting a likely volatile-rich planet. The young, active host star has a rotation period ofP_rot= 8.7 ± 0.9 days and associated rotation-based age estimate of 1.1 ± 0.1 Gyr. Though no other transiting planets are seen in the TESS data, the system shows clear TTVs of super-period $P_{\sup }\approx 430\mathrm{days}$and amplitude ∼100 minutes. After considering multiple likely period-ratio models, we show an outer planet candidate near a 2:1 resonance can explain the observed TTVs while offering a dynamically stable solution. However, other possible two-planet solutions—including 3:2 and 4:3 resonances—cannot be conclusively excluded without further observations. Assuming a 2:1 resonance in the joint TTV-RV modeling suggests a mass of $m_b={13.3}_{-4.5}^{+4.7}{M}_{\oplus }$for TOI-2015 b and $m_c={6.8}_{-2.3}^{+3.5}{M}_{\oplus }$for the outer candidate. Additional transit and RV observations will be beneficial to explicitly identify the resonance and further characterize the properties of the system. 

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