A Neptune-sized transiting planet closely orbiting a 5–10-million-year-old star

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Theories of the formation and early evolution of planetary systems postulate that planets are born in circumstellar disks, and undergo radial migration during and after dissipation of the dust and gas disk from which they formed1,2. The precise ages of meteorites indicate that planetesimals—the building blocks of planets—are produced within the first million years of a star’s life3. Fully formed planets are frequently detected on short orbital periods around mature stars. Some theories suggest that the in situ formation of planets close to their host stars is unlikely and that the existence of such planets is therefore evidence of large-scale migration4,5. Other theories posit that planet assembly at small orbital separations may be common6,7,8. Here we report a newly born, transiting planet orbiting its star with a period of 5.4 days. The planet is 50 per cent larger than Neptune, and its mass is less than 3.6 times that of Jupiter (at 99.7 per cent confidence), with a true mass likely to be similar to that of Neptune. The star is 5–10 million years old and has a tenuous dust disk extending outward from about twice the Earth–Sun separation, in addition to the fully formed planet located at less than one-twentieth of the Earth–Sun separation.

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Figure 1: Light curve of K2-33.
Figure 2: Constraints on astrophysical false-positive scenarios.


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We thank S. Metchev, K. Batygin, B. Benneke, K. Deck, J. Fuller and A. Shporer for discussions, M. Ireland for software used in the aperture masking analysis, and A. Kraus for contributing to the 2011 Keck/NIRC2 data acquisition. T.J.D. is supported by an NSF Graduate Research Fellowship under Grant DGE1144469. E.A.P. is supported through a Hubble Fellowship. I.J.M.C. is supported through a Sagan Fellowship. A.W.H. acknowledges funding from NASA grant NNX16AE75G and NASA Research Support Agreement 1541779. This paper includes data collected by the Kepler mission, funded by the NASA Science Mission directorate. Some 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. We acknowledge the important cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community and we are fortunate to be able to conduct observations from this mountain.

Author information

T.J.D. noted the object as a young star, prepared the light curve, validated the transit, and led the overall analysis and the writing of the paper. L.A.H. analysed the 2015 and 2016 Keck/HIRES spectra, participated in team organization, performed general analysis, and contributed substantially to the writing of the paper. E.A.P. analysed raw K2 photometry and validated the transit, provided general guidance on exoplanets and false positives, and contributed substantially to the writing of the paper. J.M.C. was involved in the K2 proposal that included the object, and was principal investigator on the 2011 Keck/NIRC2, 2015 Keck/HIRES, and ALMA observations of the object. I.J.M.C. led the transit fitting and VESPA analysis. A.M.C. analysed the raw K2 photometry and validated the transit. A.W.H. and H.T.I. obtained, reduced, and analysed the 2016 Keck/HIRES spectra. D.R.C. led the clear aperture adaptive optics contrast curve analysis for the 2016 and 2011 data. C.A.B. wrote the proposal for 2016 Keck/NIRC2 follow-up of K2 sources and participated in the observations of the object. S.H. analysed the Keck/NIRC2 aperture masking data and assessed the temperature of the circumstellar dust. J.E.S. provided a rotational velocity analysis and calculated the kinematic distance. S.A.B. took the 2015 Keck/HIRES spectrum and analysed the ALMA data (referred to, but published separately in ref. 27).

Correspondence to Trevor J. David.

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The authors declare no competing financial interests.

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Nature thanks A. Collier Cameron and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Extended data figures and tables

Extended Data Figure 1 K2 light curve for K2-33 phased on the stellar rotation period of 6.3 days.

Semi-sinusoidal brightness variations due to rotational modulation of starspots. Point colour indicates the relative time of observation, with grey corresponding to earlier in the campaign and dark blue corresponding to later times. Brightness is lowest when the most heavily spotted hemisphere of the stellar surface is along the line of sight. The shape and evolution of the variability pattern depends on the number, geometry, distribution, and lifetime of spots, along with any latitudinal gradient in the rotational speed (differential rotation). The transits of K2-33 b are visible by eye in this figure and are too narrow in rotational phase to be attributed to any feature on or near the stellar surface.

Extended Data Figure 2 Model-dependent age of K2-33.

a, Solid lines show mean stellar density as a function of effective temperature for pre-main-sequence stars having different ages, according to theoretical models19. Grey points represent plausible combinations of density and temperature for K2-33 as determined by light-curve fits and stellar spectroscopy. b, Distribution of implied stellar age based on temperature, density, and pre-main-sequence models. The implied age of 2–7 Myr is consistent with the age we adopted of 5–10 Myr, derived independently. Dark- and light-grey shaded regions indicate 68% and 95% confidence intervals, respectively.

Extended Data Figure 3 Apparent radial velocity variations of K2-33.

Line-of-sight velocities and 1σ uncertainties (standard deviations, indicated by error bars) with respect to the Solar System barycentre from Keck/HIRES are indicated. Radial velocities are mean-subtracted, and the abscissa shows the orbital phase of K2-33 b measured from K2 photometry (mid-transit occurs at zero orbital phase). We rule out radial velocity variations larger than 300 m s−1 at 68.3% confidence, corresponding to a 1.2MJup planet mass. Curves show the expected radial velocity variations for planets having circular orbits and different masses Mp. Radial velocities due to a 1.0MJup planet (blue) are consistent with our observations, while a 4.0MJup planet (red) is ruled out at high confidence.

Extended Data Figure 4 Images of K2-33.

a, K2 target pixel file. b, Sloan Digital Sky Survey (SDSS) optical image. c, Keck/NIRC2 K-band image. Extents of the K2 target pixel file, K2 photometric aperture, and NIRC2 image are shown respectively with black, green, and purple boundaries. In each image, north is up and east is left. Three other sources identified by SDSS reside within the K2 photometric aperture, one of which is a galaxy. All are 7.3–10.1 magnitudes fainter than K2-33 in the SDSS r-filter and below the detection limit of the NIRC2 images, and are thus too faint to produce the observed transits.

Extended Data Figure 5 Sensitivity to non-comoving sources in the vicinity of K2-33.

The blue X marks the star’s position in 2011. Between 2011 and 2016, the star moved by 0.1228″ ± 0.0085″ (red X) owing to proper motion. Contours show the K-band sensitivity to non-comoving stars from adaptive optics imaging from both epochs. The 2011 data set included non-redundant aperture masking, and provided tighter constraints. The combined sensitivity to non-comoving objects is the maximum contrast achieved for either data set. Owing to stellar proper motion, we achieved K-band contrasts of >3.3 mag throughout the ΔRA–Δdec. plane, even at the 2011 and 2016 positions of K2-33.

Extended Data Table 1 Keck/HIRES radial velocities for K2-33

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David, T., Hillenbrand, L., Petigura, E. et al. A Neptune-sized transiting planet closely orbiting a 5–10-million-year-old star. Nature 534, 658–661 (2016) doi:10.1038/nature18293

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