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Alignment of the stellar spin with the orbits of a three-planet system

Abstract

The Sun’s equator and the planets’ orbital planes are nearly aligned, which is presumably a consequence of their formation from a single spinning gaseous disk. For exoplanetary systems this well-aligned configuration is not guaranteed: dynamical interactions may tilt planetary orbits, or stars may be misaligned with the protoplanetary disk through chaotic accretion1 , magnetic interactions2 or torques from neighbouring stars. Indeed, isolated ‘hot Jupiters’ are often misaligned and even orbiting retrograde3,4. Here we report an analysis of transits of planets over starspots5,6,7 on the Sun-like star Kepler-30 (ref. 8), and show that the orbits of its three planets are aligned with the stellar equator. Furthermore, the orbits are aligned with one another to within a few degrees. This configuration is similar to that of our Solar System, and contrasts with the isolated hot Jupiters. The orderly alignment seen in the Kepler-30 system suggests that high obliquities are confined to systems that experienced disruptive dynamical interactions. Should this be corroborated by observations of other coplanar multi-planet systems, then star–disk misalignments would be ruled out as the explanation for the high obliquities of hot Jupiters, and dynamical interactions would be implicated as the origin of hot Jupiters.

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Figure 1: Evidence for a low obliquity based on transits over several starspots at differing stellar longitudes.
Figure 2: Evidence for a low obliquity based on a consecutive pair of transits over a single starspot.
Figure 3: Evidence for coplanar planetary orbits based on an analysis of transit times and durations.

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Acknowledgements

Kepler was competitively selected as the tenth Discovery mission. Funding for this mission was provided by NASA’s Science Mission Directorate. The data presented in this Letter were obtained from the Mikulski Archive for Space Telescopes (MAST). STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Support for MAST for non-HST data is provided by the NASA Office of Space Science via grant NNX09AF08G and by other grants and contracts. S. Albrecht, E. Agol, J. A. Carter, L. Doyle and A. Shporer provided comments on the manuscript. D.C.F. acknowledges NASA support through Hubble Fellowship grant HF-51272.01-A, awarded by STScI. D.R. acknowledges the Harvard Institute for Theory and Computation. E.B.F., M.J.H. and J.N.W. acknowledge NASA support through the Kepler Participating Scientist programme.

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Contributions

R.S.-O. performed the spot analysis and wrote the first draft of the paper. D.C.F. performed the dynamical analysis, contributed to the spot modelling, and helped write the paper. J.N.W. contributed to the modelling and helped write the paper. The remaining authors listed below contributed equally. T.B., B.D.C., F.M., S.E.S., M.S. and S.E.T. worked on the data collection, processing and review that yielded the time-series photometry. E.B.F. and M.J.H. provided feedback on the text and interpretation. J.J.F. and J.J.L. worked on elucidating the structure and radii of the planets. J.C.G. worked on the development of Kepler spacecraft photometer electronics, is a builder of Keplercam for the Kepler Input Catalog and follow-up spectral typing. A.W.H. contributed the Keck-HIRES spectra from which the stellar properties were derived. J.M.J. is the Co-Investigator for Data Analysis and designed and built the pipeline that produced the light curves on which this paper is based. D.K. contributed to the concept, design, development, testing and commissioning of the Kepler Mission. G.W.M. contributed the Keck spectroscopy and helped with the imaging, and with some other parts of the original follow-up observations. D.R. studied the possibility of mutual transits and provided feedback on the technical details of the analysis. All authors discussed the results and commented on the manuscript.

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Correspondence to Roberto Sanchis-Ojeda or Daniel C. Fabrycky.

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The authors declare no competing financial interests.

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This file contains Supplementary Text and Data, additional references, Supplementary Tables 1-4 and Supplementary Figures 1-2. (PDF 767 kb)

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Sanchis-Ojeda, R., Fabrycky, D., Winn, J. et al. Alignment of the stellar spin with the orbits of a three-planet system. Nature 487, 449–453 (2012). https://doi.org/10.1038/nature11301

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