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Title: Measured spin–orbit alignment of ultra-short-period super-Earth 55 Cancri e
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.  more » « less
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Nature Astronomy
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National Science Foundation
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