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# A giant planet candidate transiting a white dwarf

## Abstract

Astronomers have discovered thousands of planets outside the Solar System1, most of which orbit stars that will eventually evolve into red giants and then into white dwarfs. During the red giant phase, any close-orbiting planets will be engulfed by the star2, but more distant planets can survive this phase and remain in orbit around the white dwarf3,4. Some white dwarfs show evidence for rocky material floating in their atmospheres5, in warm debris disks6,7,8,9 or orbiting very closely10,11,12, which has been interpreted as the debris of rocky planets that were scattered inwards and tidally disrupted13. Recently, the discovery of a gaseous debris disk with a composition similar to that of ice giant planets14 demonstrated that massive planets might also find their way into tight orbits around white dwarfs, but it is unclear whether these planets can survive the journey. So far, no intact planets have been detected in close orbits around white dwarfs. Here we report the observation of a giant planet candidate transiting the white dwarf WD 1856+534 (TIC 267574918) every 1.4 days. We observed and modelled the periodic dimming of the white dwarf caused by the planet candidate passing in front of the star in its orbit. The planet candidate is roughly the same size as Jupiter and is no more than 14 times as massive (with 95 per cent confidence). Other cases of white dwarfs with close brown dwarf or stellar companions are explained as the consequence of common-envelope evolution, wherein the original orbit is enveloped during the red giant phase and shrinks owing to friction. In this case, however, the long orbital period (compared with other white dwarfs with close brown dwarf or stellar companions) and low mass of the planet candidate make common-envelope evolution less likely. Instead, our findings for the WD 1856+534 system indicate that giant planets can be scattered into tight orbits without being tidally disrupted, motivating the search for smaller transiting planets around white dwarfs.

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## Data availability

We provide all reduced light curves and spectra with the manuscript. The Spitzer images are available for download at the Spitzer Heritage Archive (http://irsa.ipac.caltech.edu/applications/Spitzer/SHA/), and the TESS images and light curves are available from the Mikulski Archive for Space Telescopes (https://archive.stsci.edu/tess/). Source data are provided with this paper.

## Code availability

Much of the code used to produce these results is publicly available and linked throughout the paper. We wrote custom software to analyse the data collected in this project. Though this code was not written with distribution in mind, it is available online at https://github.com/avanderburg/.

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## Author information

Authors

### Contributions

A.V. led the TESS proposals, identified the planet candidate, organized observations, performed the transit and flux limit analysis, and wrote the majority of the manuscript. S.A.R. helped to organize observations, performed independent data analysis, and wrote portions of the manuscript. S.X. helped to organize observations, obtained and analysed the Gemini data, measured fluxes from the Spitzer data, and helped to guide the strategy of the manuscript. I.J.M.C., L. Kreidberg, V.G., B.B., D.B., J.L.C., D.D., C.D., X.G., S.R.K., F. Morales and L.Y. acquired and produced a light curve from the Spitzer data. S.A.R., J.C.B., L.N., B.Z., F.C.A. and J.J.L. investigated the formation of the WD 1856 system. B.G., F. Murgas, T.G.K., E.P., H.P., A.F. and N.N. acquired follow-up photometry. S.B., P.D. and K.G.S. determined the parameters of the white dwarf, and A.W.M. and E.R.N. studied the M-dwarf companions. C.M., G.Z., W.R.B., R.T., B.K., L.A.B., A.E.D. and A.I.H. acquired spectra of the white dwarf and/or M-dwarf companions. B.M.M., K.H. and T.D. performed an independent analysis of the TESS data, and J.A.L. performed an independent analysis of the white dwarf SED. C.V.M. provided expertise on brown dwarf models, and L. Kaltenegger investigated the system’s implications. L.A.P. determined parameters for the binary M-dwarf orbits and white dwarf/M-dwarf orbits, A.C. investigated the system’s galactic kinematics. G.R.R., R.K.V., D.W.L., S.S., J.N.W., J.M.J., D.A.C., K.A.C., K.D.C., J.D., A.G., N.M.G., C.X.H., J.P., M.E.R. and J.C.S. are members of the TESS mission team.

### Corresponding author

Correspondence to Andrew Vanderburg.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Peer review information Nature thanks Artie Hatzes, Steven Parsons and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

## Extended data figures and tables

### Extended Data Fig. 1 Archival imaging of WD 1856.

a, From the Palomar Observatory Sky Survey on a photographic plate with a blue-sensitive emulsion. b, From the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) survey in the i band. c, From the Pan-STARRS survey in the i band, zoomed out to show the co-moving M-dwarf pair (labelled G 229-20). d, Coadded TESS image from sector 14. The photometric apertures for the three sectors of TESS observations (14, 15 and 19) are shown as red-, purple- and blue-coloured outlines, respectively. The present-day location of WD 1856 is shown with a red cross in all images.

### Extended Data Fig. 2 All transit observations of WD 1856.

From top to bottom, we show the light curves (arbitrarily offset for visual clarity) from TESS; data from several private telescopes in Arizona (operated by B.G. and T.G.K.) with odd and even-numbered transits shown separately; simultaneous light curves in four colours from MuSCAT2; a light curve from the GTC, and a light curve from Spitzer. The individual two-minute-cadence TESS flux measurements are shown as grey points, and the rose-coloured points are averages of the brightness in roughly 30 s in orbital phase. The TESS data have been corrected for dilution from nearby stars so that the transit depth matches that of the GTC data. Source data

### Extended Data Fig. 3 Spectral energy distribution of WD 1856. Photometric measurements from Pan-STARRS148, 2MASS149, WISE150 and Spitzer are shown as blue, orange, dark red and pink points, respectively.

The formal 1σ (standard deviation) photometric uncertainties on the Pan-STARRS, WISE, and Spitzer points are smaller than the symbol size. Four different SED models are shown as solid curves: a pure hydrogen atmosphere model (red), a 50% hydrogen, 50% helium model (blue), a pure helium model (gold), and a blackbody curve (black). None of the SED models capture all of the SED’s features, but all four yield mostly consistent effective temperatures and stellar parameters.

### Extended Data Fig. 4 Spectrum of WD 1856 near the Hα line.

Our summed Hobby–Eberly/LRS2 spectrum (black connected points) is shown in comparison with three atmosphere models: a pure hydrogen model (red), a 50% hydrogen, 50% helium model (blue), and a pure helium model (gold). We disfavour a pure hydrogen atmosphere on the basis of our non-detection of an Hα feature in our LRS2 spectra, but otherwise remain uncertain about the precise composition of the envelope of WD 1856.

### Extended Data Fig. 5 Posterior probability distributions of transit parameters.

This ‘corner-plot’ shows correlations between pairs of parameters in our MCMC transit fit (with circular orbits enforced) and histograms of the marginalized posterior probability distributions for each parameter. For clarity, we have plotted correlations with the inclination angle i instead of the fit parameter cosi and subtract the median time of transit (tt). The orbital inclination i, scaled semimajor axis a/R⁎, and the planet–star radius ratio Rp/R⁎ are strongly correlated, owing to the grazing transit geometry, but constrained by the prior on the stellar density. We do not include rows for the GTC and Spitzer photometric jitter terms because these are nuisance parameters that showed no correlation with the other physical parameters.

### Extended Data Fig. 6 Posterior probability distributions of transit parameters when eccentric orbits are allowed.

This ‘corner-plot’ shows correlations between pairs of parameters in our MCMC transit fit (allowing eccentric orbits) and histograms of the marginalized posterior probability distributions for each parameter. This plot shows a subset of the parameters that correlate with the orbital eccentricity. For clarity, we have plotted correlations with the eccentricity e, argument of periastron w and orbital inclination i instead of the fit parameters $$\sqrt{e}\cos \,\omega$$, $$\sqrt{e}\sin \,\omega$$ and δ.

### Extended Data Fig. 7 Hα equivalent width for G 229-20 A/B compared to other nearby M dwarfs.

The histogram shows the Hα equivalent widths for a large sample of M dwarfs with similar spectral types from the Sloan Digital Sky Survey103. G 229-20 A/B (shown as a blue arrow) has a lower than average Hα equivalent width, but falls well within the distribution of field M dwarfs.

### Extended Data Fig. 8 Theoretical relationships between the star’s radius and the mass of its core.

We show MIST120 evolution tracks in the radius–core mass plane for solar composition models with masses ranging from 1M–2.8M. The RGB phase is clearly identifiable for core masses between 0.2M and 0.47M, whereas the thermal pulses on the AGB are readily recognized at higher core masses of 0.5M. The lime-green curve is the analytic expression given by equation (8). The vertical lines for each star mark the point where the envelope has been exhausted by the AGB wind.

### Extended Data Fig. 9 The minimum value of the efficiency parameter αλCE required for WD 1856 b to form via common-envelope evolution as a function of the progenitor stellar mass.

The two dashed curves show the minimum αλCE values from our analytic calculation (equation (11)) required for a 15MJ object to eject the primary star’s envelope. The purple dashed curve is taken directly from equation (11), and the brown dashed curve results if the progenitor star has lost 0.1M in a stellar wind by the time of the common envelope. The three solid curves show the minimum αλCE computed directly from MIST tracks in three different situations: before the star reaches the AGB (red), before more than 30% of the star’s envelope mass has been lost (black), and at any point in the star’s evolution, regardless of the mass lost (blue). Stars in the grey region at low masses evolve too slowly for the system to have left the main sequence more than 5.85 Gyr ago and are not viable solutions. For values of αλCE > 1 (horizontal grey line), one must invoke the internal energy of the star to help to unbind the envelope during the common-envelope phase. Before mass is lost during the AGB phase, it is difficult for WD 1856 b to eject the common envelope, but it is possible that WD 1856 b could have ejected its progenitor’s envelope if the common-envelope phase began after the progenitor reached the AGB. We have smoothed the lower two curves to remove some unphysical scatter that is probably due to numerical artefacts in the model grids.

## Supplementary information

### Supplementary Data

This file contains a comma separated value file with spectroscopic data on the M-dwarf companions.

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Vanderburg, A., Rappaport, S.A., Xu, S. et al. A giant planet candidate transiting a white dwarf. Nature 585, 363–367 (2020). https://doi.org/10.1038/s41586-020-2713-y

• Accepted:

• Published:

• Issue Date:

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