# An ultrahot Neptune in the Neptune desert

### Subjects

An Author Correction to this article was published on 13 October 2020

## Abstract

About 1 out of 200 Sun-like stars has a planet with an orbital period shorter than one day: an ultrashort-period planet1,2. All of the previously known ultrashort-period planets are either hot Jupiters, with sizes above 10 Earth radii (R), or apparently rocky planets smaller than 2 R. Such lack of planets of intermediate size (the ‘hot Neptune desert’) has been interpreted as the inability of low-mass planets to retain any hydrogen/helium (H/He) envelope in the face of strong stellar irradiation. Here we report the discovery of an ultrashort-period planet with a radius of 4.6 R and a mass of 29 M, firmly in the hot Neptune desert. Data from the Transiting Exoplanet Survey Satellite3 revealed transits of the bright Sun-like star LTT 9779 every 0.79 days. The planet’s mean density is similar to that of Neptune, and according to thermal evolution models, it has a H/He-rich envelope constituting 9.0$${}_{-2.9}^{+2.7}$$% of the total mass. With an equilibrium temperature around 2,000 K, it is unclear how this ‘ultrahot Neptune’ managed to retain such an envelope. Follow-up observations of the planet’s atmosphere to better understand its origin and physical nature will be facilitated by the star’s brightness (Vmag = 9.8).

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## Data availability

The photometric data that support the findings of this study are publically available from the Mikulski Archive for Space Telescopes (http://archive.stsci.edu/) under the TESS mission link. All radial-velocity data are available from the corresponding author upon reasonable request. Raw and processed spectra can be obtained from the European Southern Observatory’s data archive at http://archive.eso.org.

## Code availability

All codes necessary for the reproduction of this work are publically available through the GitHub repository, as follows: EMPEROR, https://github.com/ReddTea/astroEMPEROR; Juliet, https://github.com/nespinoza/juliet; SPECIES, https://github.com/msotov/SPECIES ; ARIADNE, https://www.github.com/jvines/astroARIADNE.

## Change history

• ### 13 October 2020

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

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## Acknowledgements

Funding for the TESS mission is provided by NASA’s Science Mission directorate. We acknowledge the use of public TESS alert data from pipelines at the TESS Science Office and at the TESS Science Processing Operations Center. This research has made use of the Exoplanet Follow-up Observation Program website, which is operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program. Resources supporting this work were provided by the NASA High-End Computing (HEC) Program through the NASA Advanced Supercomputing (NAS) Division at Ames Research Center for the production of the SPOC data products. J.S.J. and N.T. acknowledge support by FONDECYT grants 1161218 and 1201371, and partial support from CONICYT project Basal AFB-170002. M.R.D. is supported by CONICYT-PFCHA/Doctorado Nacional-21140646/Chile and Proyecto Basal AFB-170002. J.I.V. acknowledges support of CONICYT-PFCHA/Doctorado Nacional-21191829. This work was made possible owing to ESO Projects 0102.C-0525 (principal investigator, Díaz) and 0102.C-0451 (principal investigator, Brahm). R.B. acknowledges support from FONDECYT Post-doctoral Fellowship Project 3180246. This work is partly supported by JSPS KAKENHI grant numbers JP18H01265 and JP18H05439, and JST PRESTO grant number JPMJPR1775. The IRSF project is a collaboration between Nagoya University and the South African Astronomical Observatory (SAAO) supported by the Grants-in-Aid for Scientific Research on Priority Areas (A) (numbers 10147207 and 10147214) and Optical and Near-Infrared Astronomy Inter-University Cooperation Program, from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan and the National Research Foundation (NRF) of South Africa. We thank A. Fukui, N. Kusakabe, K. Morihana, T. Nagata, T. Nagayama and the staff of SAAO for their kind support for IRSF SIRIUS observations and analyses. C.P. acknowledges support from the Gruber Foundation Fellowship and Jeffrey L. Bishop Fellowship. This research includes data collected under the NGTS project at the ESO La Silla Paranal Observatory. NGTS is funded by a consortium of institutes consisting of the University of Warwick, the University of Leicester, Queen’s University Belfast, the University of Geneva, the Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR; under the ‘Großinvestition GI-NGTS’), the University of Cambridge, together with the UK Science and Technology Facilities Council (STFC; project reference ST/M001962/1 and ST/S002642/1). P.J.W., D.B., B.T.G., S.G., T.L., D.P. and R.G.W. are supported by STFC consolidated grant ST/P000495/1. D.J.A. gratefully acknowledges support from the STFC via an Ernest Rutherford Fellowship (ST/R00384X/1). E.G. gratefully acknowledges support from the David and Claudia Harding Foundation in the form of a Winton Exoplanet Fellowship. M.J.H. acknowledges funding from the Northern Ireland Department for the Economy. M.T. is supported by JSPS KAKENHI (18H05442, 15H02063). A.J., R.B. and P.T. acknowledge support from FONDECYT project 1171208, and by the Ministry for the Economy, Development, and Tourism’s Programa Iniciativa Científica Milenio through grant IC 120009, awarded to the Millennium Institute of Astrophysics (MAS). P.E., A.C. and H.R. acknowledge the support of the DFG priority programme SPP 1992 ‘Exploring the Diversity of Extrasolar Planets’ (RA 714/13-1). We acknowledge the effort of A. Tokovinin in helping to perform the observations and reduction of the SOAR data.

## Author information

Authors

### Contributions

J.S.J. led the TESS precision radial-velocity follow-up programme, selection of the targets, analysis and project coordination, and wrote the bulk of the paper. M.D., N.T. and R.B. performed the HARPS radial-velocity observations, P.T. observed the star with Coralie and M.D. analysed the activity data from these sources. N.E. performed the global modelling, with P.C.-Z. performing the TTV analysis, and R.B., M.G.S. and A.B. performing the stellar characterization using the spectra and evolutionary models. P.A.P.R. worked on the EMPEROR code and assisted in fitting the HARPS radial velocities. E.D.L. created a structure model for the planet, and in addition to G.W.K. and P.J.W., performed photoevaporation modelling. J.N.W. performed analysis of the system parameters. D.R.C. led the Keck NIRC2 observations and analysis. G.R., R.V., D.W.L., S.S. and J.M.J. have been leading the TESS project, observations, organization of the mission, processing of the data, organization of the working groups, selection of the targets and dissemination of the data products. C.E.H., S.M. and T.K. worked on the SPOC data pipeline. C.J.B. was a member of the TOI discovery team. S.N.Q. contributed to TOI vetting, TFOP organization and TRES spectral analysis. J.L. and C.P. helped with the interpretation of the system formation and evolution. K.A.C. contributed to TOI vetting, TFOP organization, and TFOP SG1 ground-based time-series photometry analysis. G.I., F.M., A.E., K.I.C., M.M., N.N., T.N. and J.P.L. contributed TFOP SG1 observations. J.S.A., D.J.A., D.B., F.B., C.B., E.M.B., M.R.B., J.C., S.L.C., A.C., B.F.C., P.E., A.E., E.F., B.T.G., S.G., E.G., M.N.G., M.R.G., M.J.H., J.A.G.J., T.L., J.M., M.M., L.D.N., D.P., D.Q., H.R., L.R., A.M.S.S., R.H.T., R.T.-W., O.T., S.U., J.I.V., S.R.W., C.A.W., R.G.W., P.J.W. and G.W.K. are part of the NGTS consortium who provided follow-up observations to confirm the planet. E.P. and J.J.L. helped with the interpretation of the result. C.B. performed the observations at SOAR and reduced the data, C.Z. performed the data analysis, and N.L. and A.W.M. assisted in the survey proposal, analysis and telescope time acquisition. All authors contributed to the paper.

### Corresponding author

Correspondence to James S. Jenkins.

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### Competing interests

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## Extended data

### Extended Data Fig. 1 Normalised TESS pre-search data conditioning timeseries photometry for LTT 9779.

with the optimal model (black curve) overplotted on the data (top). The model residuals are shown in the lower panel.

### Extended Data Fig. 2 Independently constrained system parameters from the EMPEROR MCMC runs of the 31 HARPS radial-velocities.

From top to bottom we show the posteriors of the velocity amplitude, the orbital period, and the eccentricity of the orbit. Overplotted on each histogram is a gaussian distribution with the same input parameters as those calculated from the posterior distributions. We also show the values obtained from the distributions. The histograms reveal that the signal is well constrained with the current data in hand, and the period in particular is in excellent agreement with that from the TESS lightcurve.

### Extended Data Fig. 3 Spectral line bisector inverse slope measurements as a function of the radial-velocities.

The orange diamonds and blue circles relate to measurements made using HARPS and Coralie, respectively. The best fit linear trend is shown by the dashed line, and a key in the upper left indicates the origin of the data points.

### Extended Data Fig. 4 Companion sensitivity for the Keck NIRC2 adaptive optics imaging and the SOAR Adaptive Optics Module (SAM).

For NIRC2 (left), the black points represent the 5σ limits and are separated in steps of 1 FWHM (~ 0.05”); the purple represents the azimuthal dispersion (1σ) of the contrast determinations (see text). The inset image is of the primary target showing no additional companions within 3” of the target. For SAM (right) the black curve also represents the 5σ limit, and the black data points mark the sampling. The inset also shows the speckle image of the star, constructed from the Auto-Correlation Function.

### Extended Data Fig. 5 Stellar density as a function of R_p/R*.

when modelling the TESS, NGTS, and LCOGT lightcurves with a log-uniform prior on the stellar density and the planetary eccentricity constrained to be zero.

### Extended Data Fig. 6 Observed minus computed mid-transit times of LTT 9779 b.

The residuals (TTV) of the transit times are shown considering the proposed linear ephemeris. The dashed line corresponds to zero variation and the grey area is the propagation of 1σ uncertainties, considering the optimal transit time from EXOFASTv2 and the period from juliet. The epoch 0 is the first lightcurve obtained by TESS and is also the corresponding epoch of the optimal transit time. The TTV values shown in this plot fit accordingly with the proposed linear ephemeris ($${\chi }_{red}^{2}=1.23$$= 1.23).

## Supplementary information

### Supplementary Information

Supplementary Tables 1 and 2.

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Jenkins, J.S., Díaz, M.R., Kurtovic, N.T. et al. An ultrahot Neptune in the Neptune desert. Nat Astron (2020). https://doi.org/10.1038/s41550-020-1142-z