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

Abstract

Exoplanets orbiting close to their parent stars could lose some fraction of their atmospheres because of the extreme irradiation 1-6 . Atmospheric mass loss primarily affects low-mass exoplanets, leading to suggest that hot rocky planets 7-9 might have begun as Neptune-like 10-16 , 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 spectrum 17 . 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 ~2 h before, and end >3 h after the ~1 h optical transit, which is substantially different from one previous claim 6 (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 ~10 8 -10 9 g s −1 , which today 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.Three transits of GJ 436b, which occur every 2.64 days, were observed on 7 A stellar spectrum acquired using similar settings in January 2010 (ref. 17) (visit 0) was retrieved from the archive for comparison purposes. HST data in visits 2 and 3 were complemented with simultaneous Chandra X-ray observations. The HST data consist of timetagged, far-ultraviolet spectra obtained with a grating dispersing light over the 1,195-1,248 Å domain, with a spectral resolution of ~20 km s −1 at 1,215.6 Å (the Lyman-α transition of 2 atomic hydrogen, H I). Exposure times of 1,500 s to 2,900 s were used to observe the star for four successive HST orbits during each visit. Each HST orbit lasts for 96 min, during which GJ 436 is visible for 56 min before being occulted by the Earth, yielding 40 min gaps in the data.The most prominent spectral feature is the H I Lyman-α emission (Fig. 1). Absorption in the blue wing of this line has been reported in other systems, during transits of hot Jupiters. This is interpreted by the presence of escaping hydrogen exospheres surrounding giant planets 1,5,[18][19][20] . Tentative evidence that the Neptune-mass planet GJ 436b possesses such an extended atmosphere was drawn from visit 1 data despite the signal being observed after one optical transit 6 , raising questions on its planetary origin. Visits 2 and 3 were carried out to determine the signal nature.We performed a careful analysis to check for the existence of instrumental systematics in the data and correct for them (see Methods). Large variations are detected over a part of the Lyman-α line at times corresponding to the optical transit, which cannot be explained by any known instrumental effects. The most notab...

Habitability of planets on eccentric orbits: Limits of the mean flux approximation

“…It is therefore possible that these planets experience little water escape, as on Earth (Lammer et al 2003;Kulikov et al 2007;Selsis et al 2007). We note that an extreme case for water escape has been observed for a hot Neptune (Ehrenreich et al 2015). However, we expect a much lower escape rate here as the planet is located much farther away.…”

An evaporating planet in the wind: stellar wind interactions with the radiatively braked exosphere of GJ 436 b

“…While radiative braking explains the size of the coma and the blueshifted velocity range of the observed absorption signatures up to about −120 km s −1 well, it does not account for the variations in the depth and duration of the absorption signal at the different phases of the transits. Furthermore, even though the overall signature of the GJ 436 b exosphere is very similar in the three different epochs, specific features to each visit cannot be explained by the radiation pressure, which is due to the Lyman-α line that was shown to be extremely stable over time (Ehrenreich et al 2015;Bourrier et al 2015a). Our goal in this paper is to investigate the coupled effects of stellar wind interactions and radiation pressure on the exosphere, using numerical simulations of the GJ 436 system with the EVaporating Exoplanet code (EVE).…”

“…The brightness of the host star (V = 10.7) and its close proximity to Earth (d = 10.14 pc) makes GJ 436 a good target for transit observations in the Lyman-α line. Using HST/STIS, Kulow et al (2014) identified a deep absorption signature from neutral hydrogen after the end of the optical transit, but their interpretation was misled by an inaccurate transit ephemeris and by the lack of an out-of-transit reference for the flux in the stellar Lyman-α line (Ehrenreich et al 2015). Using two additional HST observations, Ehrenreich et al (2015) revealed a deeper signature repeated over the three epochs of observations, which shows that GJ 436 b is surrounded by a giant coma of neutral hydrogen that is large enough to occult the stellar disk several hours before the optical transit, and that is trailed by a long cometary tail that could remain detectable for many hours after the optical transit (Fig.…”

“…Transit observations in the Lyman-α line of neutral hydrogen led to the detection of extended exospheres around the hot Jupiters HD 209458b (Vidal-Madjar et al 2003 and HD 189733b (Lecavelier des Etangs et al 2010Etangs et al , 2012, the warm Neptune GJ 436 b (Kulow et al 2014;Ehrenreich et al 2015), and the warm Jupiter 55 Cnc b (Ehrenreich et al 2012). The intense stellar X-ray and extreme ultraviolet energy input at the base of a hydrogen-rich thermosphere has been shown to be responsible for the expansion of the upper atmospheric layers (e.g., Lammer et al 2003;Lecavelier des Etangs et al 2004;Koskinen et al 2013a,b).…”

Ground-based near-UV observations of 15 transiting exoplanets: constraints on their atmospheres and no evidence for asymmetrical transits

“…These observations can be subdivided into two groups: asymmetric and symmetric light curves. There are five exoplanets (55 Cnc b, GJ 436b, HD 189733b, HD 209458b, WASP12b) where asymmetries in their light curves are observed (VidalMadjar et al 2003(VidalMadjar et al , 2004(VidalMadjar et al , 2008(VidalMadjar et al , 2013Ben-Jaffel 2007Fossati et al 2010;Ehrenreich et al 2012Ehrenreich et al , 2015Haswell et al 2012;BenJaffel & Ballester 2013;Kulow et al 2014;Nichols et al 2015). For the symmetric transits, nine hot Jupiters (HAT-P-1b, HAT-P-12b, WASP43b, are observed to have a constant planetary radii from near-UV to optical wavelengths Turner et al 2013;Bento et al 2014;Nikolov et al 2014;Pearson, Turner & Sagan 2014;Mallonn et al 2015;Ricci et al 2015;Sing et al 2016).…”