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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. 

Abstract Giant exoplanets orbiting close to their host stars are unlikely to have formed in their present configurations¹. These ‘hot Jupiter’ planets are instead thought to have migrated inward from beyond the ice line and several viable migration channels have been proposed, including eccentricity excitation through angular-momentum exchange with a third body followed by tidally driven orbital circularization^2,3. The discovery of the extremely eccentric (e = 0.93) giant exoplanet HD 80606 b (ref. ⁴) provided observational evidence that hot Jupiters may have formed through this high-eccentricity tidal-migration pathway⁵. However, no similar hot-Jupiter progenitors have been found and simulations predict that one factor affecting the efficacy of this mechanism is exoplanet mass, as low-mass planets are more likely to be tidally disrupted during periastron passage^6–8. Here we present spectroscopic and photometric observations of TIC 241249530 b, a high-mass, transiting warm Jupiter with an extreme orbital eccentricity ofe = 0.94. The orbit of TIC 241249530 b is consistent with a history of eccentricity oscillations and a future tidal circularization trajectory. Our analysis of the mass and eccentricity distributions of the transiting-warm-Jupiter population further reveals a correlation between high mass and high eccentricity. 

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.