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

An Author Correction to this article was published on 11 January 2023

This article has been updated


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 planet’s orbital axis and its host star’s spin axis) to be \(\lambda =10\begin{array}{c}+1{7}^{\circ }\\ -20^{\circ }\end{array}\) with an unprojected angle of \(\psi =23\begin{array}{c}+1{4}^{\circ }\\ -12^{\circ }\end{array}\). The best-fit Rossiter–McLaughlin model to the Extreme Precision Spectrograph data has a radial velocity semi-amplitude of just \(0.41\begin{array}{c}+0.09\\ -0.10\end{array}\) m s−1. The spin–orbit alignment of 55 Cnc e 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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Fig. 1: RM model fit to two nights of EXPRES RV data.
Fig. 2: A visual representation of the sky-projected alignment between 55 Cnc e’s stellar spin axis and planetary orbit axis.

Data availability

The EXPRES radial velocities used in this study are published as part of the Supplementary Information. The TESS data used in this study are publicly available and can be obtained from the Mikulski Archive for Space Telescopes (MAST;

Code availability

The code associated with this work used only open source software. This research made use of SciPy56, NumPy57,58, Astropy59,60, lightkurve61, starry62, emcee48, celerite35,36, ellc47 and LDTk37. This research also made use of exoplanet31 and its dependencies32,33,59,60,62,63,64.

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These results made use of data provided by the EXPRES team using the Extreme Precision Spectrograph at the Lowell Discovery Telescope, Lowell Observatory. Lowell is a private, non-profit institution dedicated to astrophysical research and public appreciation of astronomy and operates the LDT in partnership with Boston University, the University of Maryland, the University of Toledo, Northern Arizona University and Yale University. EXPRES was designed and built at Yale with financial support from grants NSF MRI-1429365 and NSF ATI-1509436 and Yale University. Research with EXPRES is possible thanks to the generous support from grants NSF AST-2009528, NSF 1616086 and NASA 80NSSC18K0443, the Heising-Simons Foundation and an anonymous donor in the Yale alumni community. This paper includes data collected with the TESS mission, obtained from the MAST data archive at the Space Telescope Science Institute (STScI). Funding for the TESS mission is provided by the NASA Explorer Program. STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract no. NAS 5-26555. V.K. and J.L. acknowledge support from NSF award nos. AST-2009501 and AST-2009343. J.M.B. acknowledges support from NASA grant nos. 80NSSC21K0009 and 80NSSC21K0571. R.M.R. acknowledges support from the Heising-Simons 51 Pegasi b Postdoctoral Fellowship.

Author information

Authors and Affiliations



L.L.Z. designed the project and drafted the manuscript. V.K. led the analysis and drafted the manuscript. S.C.M. contributed to the scientific interpretation. C.H. processed the TESS data. L.L.Z., J.M.B., J.L., A.E.S., R.M.R., S.H.C.C., S.A.W. and D.A.F. are members of the EXPRES team that built and commissioned EXPRES, maintain the instrument for high precision work and supervise the data reduction pipeline. D.A.F. is the PI of the EXPRES Team and derived the EXPRES RVs. J.M.B. ran the stellar parameter analysis. L.L.Z., V.K., J.M.B., J.L., S.H.C.C., S.A.W. and D.A.F. contributed to the EXPRES observations.

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Correspondence to Lily L. Zhao.

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Nature Astronomy thanks Teruyuki Hirano and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary information

Supplementary Information

Supplementary Figs. 1–2 and Tables 1–4.

Supplementary Data

EXPRES RVs used in this work.

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Zhao, L.L., Kunovac, V., Brewer, J.M. et al. Measured spin–orbit alignment of ultra-short-period super-Earth 55 Cancri e. Nat Astron (2022).

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