AU Microscopii (AU Mic) is the second closest pre-main-sequence star, at a distance of 9.79 parsecs and with an age of 22 million years1. AU Mic possesses a relatively rare2 and spatially resolved3 edge-on debris disk extending from about 35 to 210 astronomical units from the star4, and with clumps exhibiting non-Keplerian motion5,6,7. Detection of newly formed planets around such a star is challenged by the presence of spots, plage, flares and other manifestations of magnetic ‘activity’ on the star8,9. Here we report observations of a planet transiting AU Mic. The transiting planet, AU Mic b, has an orbital period of 8.46 days, an orbital distance of 0.07 astronomical units, a radius of 0.4 Jupiter radii, and a mass of less than 0.18 Jupiter masses at 3σ confidence. Our observations of a planet co-existing with a debris disk offer the opportunity to test the predictions of current models of planet formation and evolution.
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In addition to the figure data available, all raw spectroscopic data are available either in the associated observatory archive or upon request from the corresponding author. The TESS light curve is available at the MAST archive, and the SuperWASP light curve is available at the NASA Exoplanet Archive. Source data are provided with this paper.
All code that is not readily available on GitHub is available upon request.
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This work was supported by grants to P.P. from NASA (award 16-APROBES16-0020 and support from the Exoplanet Exploration Program) and the National Science Foundation (Astronomy and Astrophysics grant 1716202), the Mount Cuba Astronomical Foundation and George Mason University start-up funds.The NASA Infrared Telescope Facility is operated by the University of Hawaii under contract NNH14CK55B with NASA. Funding for the TESS mission is provided by NASA’s Science Mission directorate. Some of the data presented here were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and NASA. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. This research has made use of the NASA Exoplanet Archive, which is operated by the California Institute of Technology, under contract with NASA under the Exoplanet Exploration Program. Some of the data presented in this paper were obtained from the Mikulski Archive for Space Telescopes (MAST). The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. This research has made use of the services of the ESO Science Archive Facility, based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere with the HARPS spectrometer. This work has made use of data from the European Space Agency (ESA) mission Gaia, processed by the Gaia Data Processing and Analysis Consortium (DPAC). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. MINERVA-Australis is supported by Australian Research Council LIEF Grant LE160100001, Discovery Grant DP180100972, Mount Cuba Astronomical Foundation, and institutional partners University of Southern Queensland, MIT, Nanjing University, George Mason University, University of Louisville, University of California Riverside, University of Florida and University of Texas at Austin. This work was partly supported by JSPS KAKENHI grant numbers JP18H01265 and 18H05439, JST PRESTO grant number JPMJPR1775, NSFC grant number 11673011 and MINECO grant ESP2016-80435-C2-2-R. D.D. acknowledges support for this work provided by NASA through Hubble Fellowship grant HST-HF2-51372.001-A awarded by the Space Telescope Science Institute. B.P.B. acknowledges support from National Science Foundation grant AST-1909209. J.W. and P.G. acknowledge support from the Heising-Simons Foundation 51 Pegasi b fellowship.
The authors declare no competing interests.
Peer review information Nature thanks Suzanne Aigrain and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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Extended data figures and tables
Extended Data Fig. 1 TESS and Spitzer light curves for AU Mic centred on four transit events.
a, b, Two TESS transits (respectively 1 and 2) for AU Mic b, with the model components plotted as indicated in the key. A flare is present during the egress of the first transit of AU Mic b, and a flare is present just after the ingress during the second transit of AU Mic b. Although this is unfortunate timing, flares of this amplitude are pervasive throughout the TESS light curve for AU Mic, and complicate the recovery of these events from automated transit search algorithms. c, The Spitzer transit observation of AU Mic b. The deviations in transit are not instrumental and will be the subject of a future paper, and are likely to be related to the planet crossing large active regions on the stellar surface (key from a and b applies here). d, The ~1 p.p.t. candidate single transit event seen in the TESS light curve. For all panels, 1σ measurement uncertainties are suppressed for visual clarity and are <1 p.p.t. 1σ model uncertainties in transit are shown as shaded regions.
Extended Data Fig. 2 MCMC corner plot for custom combined Spitzer and TESS light-curve analysis for AU Mic.
The full set of model parameters are shown, with the posterior probability distributions along the diagonal, the others are the two-dimensional parameter covariance plots.
Extended Data Fig. 3 One season (July to October 2007) of SuperWASP light curves for AU Mic from the NASA Exoplanet Archive, phase-folded to the rotation period of the star.
Measurements with large photometric uncertainties (>5%) have been excluded from the plot. 1σ measurement uncertainties are suppressed for visual clarity and are typically <1% but occasionally up to 5% at phases where there is more apparent vertical scatter in the measurement values themselves.
Extended Data Fig. 4 Correlation plots of the standard HARPS stellar activity indicators with the RVs.
The bisector values for the cross-correlation function (‘CCF bisector’), but not the activity indicators (Hα, Na D, Ca ii H and K), show a correlation with the RVs, with substantial remaining scatter. Formal uncertainties are smaller than the plotted symbols.
Extended Data Fig. 5 Correlation plots of the HARPS activity indicators with each other.
The activity indicators Ca ii H and K, Hα, and Na D are strongly correlated with one another, but not with the RVs or with the CCF bisector.
Extended Data Fig. 6 The HARPS RVs and standard activity indicators, phase folded to the rotation period of the star.
Blue circles, HARPS RVs; black circles, standard activity indicators. None of the activity indicators show a statistically significant trend with the period of AU Mic b. The Ca and Na activity indicators appear to show (by eye) some cyclic variation with the rotation period of the star. Formal uncertainties are smaller than the plotted symbols.
Extended Data Fig. 7 RV time-series of AU Mic, with fitting residuals, and phased to the orbital period of AU Mic b.
Shown are data from three spectrometers: iSHELL (yellow circles), HIRES (black circles) and HARPS (red squares). Uncertainties shown are 1σ for HARPS and iSHELL. For HIRES, a 5 m s−1 minimum 1σ uncertainty is adopted, although the formal 1σ uncertainties are smaller for all but one epoch at 5.43 m s−1. The maximum-likelihood best fit model is overlaid in blue, with shaded regions indicating the 1σ model confidence interval, with a separate GP for each dataset indicated with different coloured shaded regions. b, Model-subtracted residuals, with the same colours as in a. Because our RVs are undersampled with respect to the stellar rotation period38, the GP best-fit model overfits the AU Mic RV time-series. c, RV measurements are phased to the orbital period of AU Mic b, and binned in phase (red circles). The blue curve is a maximum-likelihood best-fit circular orbit model, after subtracting the best fit GP model of stellar activity and the modelled instrument offsets. The plot is labelled with the best-fit orbital period Pb, velocity semi-amplitude Kb, and the assumed circular orbit (eb = 0).
Extended Data Fig. 8 RADVEL MCMC corner plot for the model parameters for the iSHELL, HARPS and HIRES RV datasets.
Along the diagonal are the one-dimensional posterior probability distributions for a given model parameter; the others are the two-dimensional parameter covariance plots.
Extended Data Fig. 9 Photometric variability amplitudes obtained contemporaneously in four different bandpasses.
The amplitudes (black squares) are from ref. 21. The horizontal error bars correspond to the effective bandpass widths, and the 1σ vertical error bars are set to 1 mmag. A 1/λ trend is shown in red, as would be expected for cool starspots with relatively small temperature contrast35.
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Plavchan, P., Barclay, T., Gagné, J. et al. A planet within the debris disk around the pre-main-sequence star AU Microscopii. Nature 582, 497–500 (2020). https://doi.org/10.1038/s41586-020-2400-z
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