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A giant comet-like cloud of hydrogen escaping the warm Neptune-mass exoplanet GJ 436b



Exoplanets orbiting close to their parent stars may lose some fraction of their atmospheres because of the extreme irradiation1,2,3,4,5,6. Atmospheric mass loss primarily affects low-mass exoplanets, leading to the suggestion that hot rocky planets7,8,9 might have begun as Neptune-like10,11,12,13,14,15,16, but subsequently lost all of their atmospheres; however, no confident measurements have hitherto been available. The signature of this loss could be observed in the ultraviolet spectrum, when the planet and its escaping atmosphere transit the star, giving rise to deeper and longer transit signatures than in the optical spectrum17. Here we report that in the ultraviolet the Neptune-mass exoplanet GJ 436b (also known as Gliese 436b) has transit depths of 56.3 ± 3.5% (1σ), far beyond the 0.69% optical transit depth. The ultraviolet transits repeatedly start about two hours before, and end more than three hours after the approximately one hour optical transit, which is substantially different from one previous claim6 (based on an inaccurate ephemeris). We infer from this that the planet is surrounded and trailed by a large exospheric cloud composed mainly of hydrogen atoms. We estimate a mass-loss rate in the range of about 108–109 grams per second, which is far too small to deplete the atmosphere of a Neptune-like planet in the lifetime of the parent star, but would have been much greater in the past.

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Figure 1: Evolution of the hydrogen Lyman-α emission line of GJ 436.
Figure 2: Lyman-α transit light curves of GJ 436b.
Figure 3: Particle simulation showing the comet-like exospheric cloud transiting the star, as seen from Earth.
Figure 4: Polar view of three-dimensional simulation representing a slice of the comet-like cloud coplanar with the line of sight.


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This work is based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with programmes #11817, #12034 and #12965. The scientific results reported in this article are based on observations made by the Chandra X-ray Observatory. This work was carried out in the framework of the National Centre for Competence in Research ‘PlanetS’ supported by the Swiss National Science Foundation (SNSF). D.E., V.B. and S.U. acknowledge the financial support of the SNSF. V.B., A.L.d.E., X.B. and X.D. acknowledge the support of CNES, the French Agence Nationale de la Recherche (ANR) under program ANR-12-BS05-0012 ‘Exo-Atmos’, the Fondation Simone et Cino Del Duca, and the European Research Council (ERC) under ERC Grant Agreement no. 337591-ExTrA.

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Authors and Affiliations



D.E. proposed and led the HST–Chandra joint observation programme, supervised data reduction and analysis, interpreted the results and wrote the paper. V.B. performed data reduction and analysis, and computer simulations to interpret the results. P.J.W. set up the Chandra X-ray observations, reduced, analysed and interpreted the X-ray data. A.L.d.E. co-designed the simulation programme with V.B. and provided computing resources to run the simulations. A.L.d.E. and G.H. contributed to the observation programme, data analysis and interpretation. S.U., X.B., X.D., J.-M.D., D.K.S. and A.V.-M. contributed to the observation programme and interpretation. All authors discussed the results and commented on the manuscript.

Corresponding author

Correspondence to David Ehrenreich.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Time evolution of GJ 436 Lyman-α line during each HST visit.

There is one spectrum per HST orbit. Colours indicate the phase with respect to the optical transit: out-of-transit (black), pre-transit (blue), in-transit (green) and post-transit (red and magenta for the last HST orbit in visit 1). For visit 1 there is no out-of-transit spectrum, hence we over-plotted the out-of-transit spectrum from visit 2 (dotted line).

Extended Data Figure 2 Evolution of the Lyman-α line of GJ 436 reproduced with the numerical simulation.

ad, The panels show the stellar emission line at different transit phases, here for visit 2 (all error bars are 1σ). a, The out-of-transit reference line (black curve) is the calculated profile best-fitting the observed out-of-transit line profile (blue curve), after taking into account the interstellar medium absorption and convolution by the instrumental line spread function. bd, This theoretical profile is compared to observations (blue curves): the numerical simulation computes absorption in the theoretical profile and adjusts absorbed line profiles (dashed lines) as a function of time to the observations.

Extended Data Figure 3 Light curve from visit 1 data.

a, b, The light curve was calculated from integration of the flux over the red part of the line in the velocity range [+20,+250] km s−1. The temporal (a) and phase-folded (b) light curves are shown. The different colours represent data acquired during different consecutive HST orbits. The vertical dashed lines in a indicate the location of the optical transit contacts. The horizontal dashed lines in b show the best-fit constants to each HST orbit (orbit 1 data—the open circles—are not fitted because of its different phasing). We did not apply any correction to this visit. We also did not find it necessary to trim out the first HST orbit in this visit (or in the subsequent ones) due to possible increased systematics, as described previously18. These time series have a higher sampling (by a factor of 2) than those in Fig. 2, which is made possible by exploiting the time-tag mode of data acquisition. The 1σ error bars have been propagated accordingly.

Extended Data Figure 4 Correction steps for the visit 2 data.

ad, The different colours represent different consecutive HST orbits. The vertical dashed lines in the temporal light curves indicate the optical transit contact points. a, Raw light curve obtained from integration of the flux over the red part of the line in the velocity range [+20,+250] km s−1. b, Breathing correction. This is the same light curve as in a, phase-folded on the HST orbit. The dashed colour lines are linear fits to the different HST orbit data. Data from HST orbit 1 (empty black circles) are not taken into account because of their slightly different phasing. c, Light curve corrected from telescope breathing. A linear trend (dashed line) representing long-term systematics is fitted to the data. d, Light curve corrected for telescope breathing and long-term systematics. All error bars are 1σ.

Extended Data Figure 5 Correction steps for the visit 3 data.

Identical to the description of Extended Data Fig. 2 for visit 2 data. All error bars are 1σ.

Extended Data Figure 6 Chandra X-ray counts of GJ 436.

The different symbols show the different visits. The June 2013 (stars) and June 2014 (squares) observations were contemporaneous with HST visits 2 and 3. The vertical dashed lines represent the contacts of the optical transit. All error bars are 1σ.

Extended Data Figure 7 X-ray spectrum of GJ 436 fitted with a two-temperature model.

a, b, The XMM-Newton EPIC-pn spectrum is shown in black, the combined Chandra ACIS-S spectrum is shown in red. a, Spectra in units of measured counts, with the Chandra spectrum below XMM-Newton because of the lower sensitivity of the instrument. b, Unfolded spectra in flux units; it can be seen that the XMM-Newton and Chandra spectra are consistent with each other. The dotted lines show the contributions of the individual temperature components. All error bars are 1σ.

Extended Data Figure 8 Emission measures for the two temperature components fitted to the individual XMM-Newton and Chandra spectra.

The red points correspond to the low temperature component (0.097 keV) that dominates the spectra below 0.5 keV. The blue points correspond to the higher temperature component (0.72 keV) that dominates above 0.5 keV. The higher temperature component is more variable, probably due to the effect of stellar flares. The less variable lower temperature component is a proxy for the unobserved extreme-ultraviolet emission that dominates the high-energy irradiation of the planet. The last two points are contemporaneous with HST visits 2 and 3.

Extended Data Table 1 Log of the HST and Chandra observations
Extended Data Table 2 Ephemerides of GJ 436b transit

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Ehrenreich, D., Bourrier, V., Wheatley, P. et al. A giant comet-like cloud of hydrogen escaping the warm Neptune-mass exoplanet GJ 436b. Nature 522, 459–461 (2015).

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