An HST/STIS Optical Transmission Spectrum of Warm Neptune GJ 436b

Lothringer, Joshua D.; Benneke, Björn; Crossfield, Ian J. M.; Henry, Gregory W.; Morley, Caroline; Dragomir, Diana; Barman, Travis; Knutson, Heather A.; Kempton, Eliza M.-R.; Fortney, Jonathan J.; McCullough, P. R.; Howard, Andrew W.

doi:10.3847/1538-3881/aaa008

Cited by 42 publications

(51 citation statements)

References 94 publications

(195 reference statements)

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“…Our fiducial stellar parameters for GJ 3470 are in T = 3652±50 K, log(g) = 4.843±0.035, and Z = 0.2±0.1 dex 34,54,55 , and we perform the same wide sensitivity tests as detailed Ref. 48 . We account for the cadence of the observations by numerically integrating in time from the start to end of the individual exposures.…”

Section: Transit Model and Mcmc Analysismentioning

confidence: 99%

A sub-Neptune exoplanet with a low-metallicity methane-depleted atmosphere and Mie-scattering clouds

et al. 2019

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18] NASA Hubble FellowWith no analogues in the Solar System, the discovery of thousands of exoplanets with masses and radii intermediate between Earth and Neptune was one of the big surprises of exoplanet science. These super-Earths and sub-Neptunes likely represent the most common outcome of planet formation 1,2 . Mass and radius measurements indicate a diversity in bulk composition much wider than for gas giants 3 ; however, direct spectroscopic detections of molecular absorption and constraints on the gas mixing ratios have largely remained limited to planets more massive than Neptune 4-6 . Here, we analyze a combined Hubble/Spitzer Space Telescope dataset of 12 transits and 20 eclipses of the sub-Neptune GJ 3470 b, whose mass of 12.6 M⊕ places it near the half-way point between previously studied exo-Neptunes (22-23 M⊕) 5-7 and exoplanets known to have rocky densities (7 M⊕) 8 . Obtained over many years, our data set provides a robust detection of water absorption (>5σ) and a thermal emission detection from the lowest irradiated planet to date. We reveal a low-metallicity, hydrogendominated atmosphere similar to a gas giant, but strongly depleted in methane gas. The low, near-solar metallicity (O/H=0.2-18) sets important constraints on the potential planet formation processes at low masses as well as the subsequent accretion of solids. The low methane abundance indicates that methane is destroyed much more efficiently than previously predicted, suggesting that the CH4/CO transition curve has to be revisited for close-in planets. Finally, we also find a sharp drop in the cloud opacity at 2-3 µm characteristic of Mie scattering, which enables narrow constraints on the cloud particle size and makes GJ 3470b a keystone target for mid-IR characterization with JWST.

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Section: Transit Model and Mcmc Analysismentioning

confidence: 99%

A sub-Neptune exoplanet with a low-metallicity methane-depleted atmosphere and Mie-scattering clouds

et al. 2019

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“…Clouds and/or hazes in planetary atmospheres can obscure and mute the larger spectral features indicative of clear hydrogen-rich atmospheres. There is ample evidence for the presence of clouds and hazes in the atmospheres of planets across all masses, from hot Jupiters (e.g., Sing et al 2016) to exo-Neptunes (e.g., Knutson et al 2014a;Lothringer et al 2018) to super-Earths (e.g., Kreidberg et al 2014a). Hazes in these atmospheres are of particular astrobiological interest.…”

Section: Introductionmentioning

confidence: 99%

Limits on Clouds and Hazes for the TRAPPIST-1 Planets

Moran

Hörst

Batalha

et al. 2018

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The TRAPPIST-1 planetary system is an excellent candidate for study of the evolution and habitability of M-dwarf hosted planets. Transmission spectroscopy observations performed on the system with the Hubble Space Telescope (HST) suggest that the innermost five planets do not possess clear hydrogen atmospheres. Here we reassess these conclusions with recently updated mass constraints. Additionally, we expand the analysis to include limits on metallicity, cloud top pressure, and the strength of haze scattering. We connect recent laboratory results of particle size and production rate for exoplanet hazes to a one-dimensional atmospheric model for TRAPPIST-1 transmission spectra. In this way, we obtain a physically-based estimate of haze scattering cross sections. We find haze scattering cross sections on the order of 10 −26 to 10 −19 cm 2 are needed in modeled hydrogen-rich atmospheres for TRAPPIST-1 d, e, and f to match the HST data. For TRAPPIST-1 g, we cannot rule out a clear hydrogen-rich atmosphere. We modeled the effects an opaque cloud deck and substantial heavy element content have on the transmission spectra using the updated mass estimates. We determine that hydrogen-rich atmospheres with high altitude clouds, at pressures of 12 mbar and lower, are consistent with the HST observations for TRAPPIST-1 d and e. For TRAPPIST-1 f and g, we cannot rule out clear hydrogen-rich cases to high confidence. We demonstrate that metallicities of at least 60× solar with tropospheric (0.1 bar) clouds are in agreement with observations. Additionally, we provide estimates of the precision necessary for future observations to disentangle degeneracies in cloud top pressure and metallicity. For TRAPPIST-1 e and f, for example, 20 ppm precision is needed to distinguish between a clear atmosphere and an atmosphere with a thick cloud layer at 0.1 bar across a wide range (1× to 1000× solar) of metallicity. Our results suggest secondary, volatile-rich atmospheres for the outer TRAPPIST-1 planets d, e, and f.

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“…However, we were able to take advantage of a large number of measurements to average out stochastic variations from the baseline flux. Bourrier et al (2018c) and Lothringer et al (2018) used photometric observations acquired from 2003 to 2017 with the T12 0.80 m automatic photoelectric telescope (APT) at the Fairborn Observatory (Henry 1999). They found evidence for rotational modulation with a period of 44.09 days and a peak-to-peak amplitude of 0.0032 mag.…”

Section: Discussionmentioning

confidence: 99%

“…A consistent behavior is also seen in optical photometric monitoring of GJ 436; based on fig. 1 of Lothringer et al (2018), the epoch when the star becomes the brightest in optical, which represents the minimum spot coverage, roughly corresponds to the minimum of S -index around 2012 (see Fig. 4).…”

Section: Assessing the Presence Of A Long-lived Active Region Or An Amentioning

confidence: 96%

The Hubble PanCET program: an extensive search for metallic ions in the exosphere of GJ 436 b

Santos

Ehrenreich

Bourrier

et al. 2019

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Context. The quiet M2.5 star GJ 436 hosts a warm Neptune that displays an extended atmosphere that dwarfs its own host star. Predictions of atmospheric escape in such planets state that H atoms escape from the upper atmosphere in a collisional regime and that the flow can drag heavier atoms to the upper atmosphere. It is unclear, however, what astrophysical mechanisms drive the process. Aims. Our objective is to leverage the extensive coverage of observations of the far-ultraviolet (FUV) spectrum of GJ 436 obtained with the Cosmic Origins Spectrograph (COS) to search for signals of metallic ions in the upper atmosphere of GJ 436 b, as well as study the activity-induced variability of the star. Methods. We analyzed flux time-series of species present in the FUV spectrum of GJ 436 and successfully performed geocoronal contamination removal in the COS Lyman-α profiles obtained near the Earth's night-side. Results. GJ 436 displays flaring events with a rate of ∼10 d −1 . There is evidence for a possibly long-lived active region or longitude that modulates the FUV metallic lines of the star with amplitudes up to 20%. Despite the strong geocoronal contamination in the COS spectra, we detected in-transit excess absorption signals of ∼50% and ∼30% in the blue and red wings, respectively, of the Lyman-α line. We rule out a wide range of excess absorption levels in the metallic lines of the star during transit. Conclusions. The large atmospheric loss of GJ 436 b observed in Lyman-α transmission spectra is stable over the timescale of a few years, and the red wing signal supports the presence of a variable hydrogen absorption source besides the stable exosphere. The previously claimed in-transit absorption in the Si iii line is likely an artifact resulting from the stellar magnetic cycle. The non-detection of metallic ions in absorption could indicate that the escape is not hydrodynamic or that the atmospheric mixing is not efficient in dragging metals high enough for sublimation to produce a detectable escape rate of ions to the exosphere.

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An HST/STIS Optical Transmission Spectrum of Warm Neptune GJ 436b

Cited by 42 publications

References 94 publications

A sub-Neptune exoplanet with a low-metallicity methane-depleted atmosphere and Mie-scattering clouds

A sub-Neptune exoplanet with a low-metallicity methane-depleted atmosphere and Mie-scattering clouds

Limits on Clouds and Hazes for the TRAPPIST-1 Planets

The Hubble PanCET program: an extensive search for metallic ions in the exosphere of GJ 436 b

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