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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. Measured spin–orbit alignment of ultra-short-period super-Earth 55 Cancri e

   https://doi.org/10.1038/s41550-022-01837-2  

   Zhao, Lily L.; Kunovac, Vedad; Brewer, John M.; Llama, Joe; Millholland, Sarah C.; Hedges, Christina; Szymkowiak, Andrew E.; Roettenbacher, Rachael M.; Cabot, Samuel H.; Weiss, Sam A.; et al (December 2022, Nature Astronomy)

   A planet’s orbital alignment places important constraints on how a planet formed and consequently evolved. The dominant formation pathway of ultra-short-period planets (P < 1 day) is particularly mysterious as such planets most likely formed further out, and it is not well understood what drove their migration inwards to their current positions. Measuring the orbital alignment is difficult for smaller super-Earth/sub-Neptune planets, which give rise to smaller amplitude signals. Here we present radial velocities across two transits of 55 Cancri (Cnc) e, an ultra-short-period super-Earth, observed with the Extreme Precision Spectrograph. Using the classical Rossiter–McLaughlin method, we measure 55 Cnc e’s sky-projected stellar spin–orbit alignment (that is, the projected angle between the The star 55 Cancri (Cnc) A hosts five known exoplanets with minimum mass estimates ranging from approximately 8M⊕ to 3MJup and periods less than one day to nearly 20 years1–4. Of particular interest has been 55 Cnc e, one of the most massive known ultra-short-period planets (USPs) and the only planet around 55 Cnc found to transit5,6. It has an star’s spin axis and the planet’s orbit normal—will shed light on the formation and evolution of USPs, especially in the case of compact, multiplanet systems. It has been shown that USPs form a statistically distinct popula- tion of planets9 that tend to be misaligned with other planetary orbits in their system10. This suggests that USPs experience a unique migra- tion pathway that brings them close in to their host stars. This inward migration is most likely driven by dissipation due to star–planet tidal interactions that result from either non-zero eccentricities11,12 or plan- etary spin-axis tilts13. orbital period of 0.7365474 +1.3 × 10−6 days, a mass of 7.99 ± 0.33M −1.4 × 10−6 ⊕ and a radius of 1.853 +0.026 R⊕ (refs. 7,8). A precise measure of the −0.027 stellar spin–orbit alignment of 55 Cnc e—the angle between the host planet’s orbital axis and its host star’s spin axis) to be λ = 10 +17∘ with an +14∘ −20∘ unprojected angle of ψ = 23 −12∘. The best-fit Rossiter–McLaughlin model to the Extreme Precision Spectrograph data has a radial velocity semi- amplitude of just 0.41 +0.09 m s−1. The spin–orbit alignment of 55 Cnc e −0.10 favours dynamically gentle migration theories for ultra-short-period planets, namely tidal dissipation through low-eccentricity planet–planet interactions and/or planetary obliquity tides. 

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3. The First Spin-Orbit Obliquity of an M dwarf/brown dwarf system: an eccentric and aligned TOI-2119 b

   https://doi.org/10.1093/mnras/stae2819  

   Doyle, Lauren; Cañas, Caleb I; Libby-Roberts, Jessica E; Cegla, Heather M; Stefánsson, Guðmundur K; Anderson, David; Armstrong, David J; Bender, Chad; Bayliss, Daniel; Carmichael, Theron W; et al (January 2025, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We report the first instance of an M dwarf/brown dwarf obliquity measurement for the TOI-2119 system using the Rossiter–McLaughlin effect. TOI-2119 b is a transiting brown dwarf orbiting a young, active early M dwarf ($$T_{\rm {eff}}$$ = 3553 K). It has a mass of 64.4 M$$_{\rm {J}}$$ and radius of 1.08 R$$_{\rm {J}}$$, with an eccentric orbit (e = 0.3) at a period of 7.2 d. For this analysis, we utilize NEID spectroscopic transit observations and ground-based simultaneous transit photometry from the Astrophysical Research Consortium and the Las Campanas Remote Observatory. We fit all available data of TOI-2119 b to refine the brown dwarf parameters and update the ephemeris. The classical Rossiter–McLaughlin technique yields a projected star–planet obliquity of $$\lambda =-0.8\pm 1.1^\circ$$ and a three-dimensional obliquity of $$\psi =15.7\pm 5.5^\circ$$. Additionally, we spatially resolve the stellar surface of TOI-2119 utilizing the Reloaded Rossiter–McLaughlin technique to determine the projected star–planet obliquity as $$\lambda =1.26 \pm 1.3^{\circ }$$. Both of these results agree within $$2\sigma$$ and confirm the system is aligned, where TOI-2119 b joins an emerging group of aligned brown dwarf obliquities. We also probe stellar surface activity on the surface of TOI-2119 in the form of centre-to-limb variations as well as the potential for differential rotation. Overall, we find tentative evidence for centre-to-limb variations on the star but do not detect evidence of differential rotation. 

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4. An Aligned Sub-Neptune Revealed with MAROON-X and a Tendency Toward Alignment for Small Planets

   https://doi.org/10.3847/1538-3881/adf070  

   Polanski, Alex S; Crossfield, Ian_J M; Seifahrt, Andreas; Bean, Jacob L; Brande, Jonathan; Collins, Karen A; Coria, David R; Fukui, Akihiko; Narita, Norio; Stürmer, Julian; et al (August 2025, The Astronomical Journal)

   Abstract We present the Rossiter–McLaughlin measurement of the sub-Neptune TOI-1759A b with MAROON-X. A joint analysis with MuSCAT3 photometry and nine additional TESS transits produces a sky-projected obliquity of ∣λ∣ = 4° ± 18°. We also derive a true obliquity ofψ= 24° ± 12° making this planet consistent with full alignment albeit to <1σ. With a period of 18.85 days and ana/R_*of 40, TOI-1759A b is the longest period single sub-Neptune to have a measured obliquity. It joins a growing number of smaller planets which have had this measurement made and, along with K2-25 b, is the only single, aligned sub-Neptune known to date. We also provide an overview of the emerging distribution of obliquity measurements for planets withR< 8R_⊕. We find that these types of planets tend toward alignment, especially the sub-Neptunes and super-Earths, implying a dynamically cool formation history. The majority of misaligned planets in this category have 4 <R≤ 8R_⊕and are more likely to be isolated than planets rather than in compact systems. We find this result to be significant at the 3σlevel, consistent with previous studies. In addition, we conduct injection and recovery testing on available archival radial velocity data to put limits on the presence of massive companions in these systems. Current archival data is insufficient for most systems to have detected a giant planet. 

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5. Teegarden’s Star revisited: A nearby planetary system with at least three planets

   https://doi.org/10.1051/0004-6361/202348033  

   Dreizler, S; Luque, R; Ribas, I; Koseleva, V; Ruh, H L; Nagel, E; Pozuelos, F J; Zechmeister, M; Reiners, A; Caballero, J A; et al (April 2024, Astronomy & Astrophysics)

   The two known planets in the planetary system of Teegarden’s Star are among the most Earth-like exoplanets currently known. Revisiting this nearby planetary system with two planets in the habitable zone aims at a more complete census of planets around very low-mass stars. A significant number of new radial velocity measurements from CARMENES, ESPRESSO, MAROON-X, and HPF, as well as photometry from TESS motivated a deeper search for additional planets. We confirm and refine the orbital parameters of the two know planets Teegarden’s Star b and c. We also report the detection of a third planet d with an orbital period of 26.13 ± 0.04 days and a minimum mass of 0.82 ± 0.17M_⊕. A signal at 96 days is attributed to the stellar rotation period. The interpretation of a signal at 172 days remains open. The TESS data exclude transiting short-period planets down to about half an Earth radius. We compare the planetary system architecture of very low-mass stars. In the currently known configuration, the planetary system of Teegarden’s star is dynamically quite different from that of TRAPPIST-1, which is more compact, but dynamically similar to others such as GJ 1002. 

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