Interaction of the Evaporating Hot Neptune GJ 436b & its Host Star's Stellar Wind

An evaporating planet in the wind: stellar wind interactions with the radiatively braked exosphere of GJ436 b

Authors:

Bourrier et al

Abstract

The warm Neptune GJ436b was observed with HST/STIS at three different epochs in the stellar Ly-alpha line, showing deep, repeated transits caused by a giant exosphere of neutral hydrogen. The low radiation pressure from the M-dwarf host star was shown to play a major role in the dynamics of the escaping gas. Yet by itself it cannot explain the time-variable spectral features detected in each transit. Here we investigate the combined role of radiative braking and stellar wind interactions using numerical simulations with the EVaporating Exoplanet code (EVE) and we derive atmospheric and stellar properties through the direct comparison of simulated and observed spectra. Our simulations match the last two epochs well. The observed sharp early ingresses come from the abrasion of the planetary coma by the stellar wind. Spectra observed during the transit can be produced by a dual exosphere of planetary neutrals (escaped from the upper atmosphere of the planet) and neutralized protons (created by charge-exchange with the stellar wind). We find similar properties at both epochs for the planetary escape rate (2.5x108 g/s), the stellar photoionization rate (2x10−5 /s), the stellar wind bulk velocity (85 km/s), and its kinetic dispersion velocity (10 km/s). We find high velocities for the escaping gas (50-60 km/s) that may indicate MHD waves that dissipate in the upper atmosphere and drive the planetary outflow. In the last epoch the high density of the stellar wind (3x103 /cm3) led to the formation of an exospheric tail mainly composed of neutralized protons. The observations of GJ436 b allow for the first time to clearly separate the contributions of radiation pressure and stellar wind and to probe the regions of the exosphere shaped by each mechanism.

Stellar Extreme Ultraviolet Spectral Energy Distribution's Impact on hot Jupiter Atmospheres

The influence of the Extreme Ultraviolet spectral energy distribution on the structure and composition of the upper atmosphere of exoplanets

Authors:

Guo et al

Abstract:

By varying the profiles of stellar extreme ultraviolet (EUV) spectral energy distribution (SED), we tested the influences of stellar EUV SEDs on the physical and chemical properties of the escaping atmosphere. We apply our model to study four exoplanets, HD 189733b, HD 209458b, GJ 436b, and Kepler-11b. We found that the total mass loss rates of an exoplanet, which are determined mainly by the integrated fluxes, are moderately affected by the profiles of the EUV SED, but the composition and species distributions in the atmosphere can be dramatically modified by the different profiles of the EUV SED. For exoplanets with a high hydrodynamic escape parameter (λ), the amount of atomic hydrogen produced by photoionization at different altitudes can vary by one to two orders of magnitude with the variation of stellar EUV SEDs. The effect of photoionization of H is prominent when the EUV SED is dominated by the low-energy spectral region (400-900${\AA}$), which pushes the transition of H/H+ to low altitudes. On the contrary, the transitions of H/H+ move to higher altitudes when most photons concentrate in the high-energy spectral region (50-400${\AA}$). For exoplanets with a low λ, the lower temperatures of the atmosphere make many chemical reactions so important that photoionization alone can no longer determine the composition of the escaping atmosphere. For HD 189733b, it is possible to explain the time variability of \lya\ between 2010 and 2011 by a change in the EUV SED of the host K star, yet invoking only thermal H\,I in the atmosphere.

Simulating the Escaping Atmospheres of hot gas ExoPlanets

Simulating the escaping atmospheres of hot gas planets in the solar neighborhood

Authors:

Salz et al

Abstract:

Absorption of high-energy radiation in planetary thermospheres is believed to lead to the formation of planetary winds. The resulting mass-loss rates can affect the evolution, particularly of small gas planets. We present 1D, spherically symmetric hydrodynamic simulations of the escaping atmospheres of 18 hot gas planets in the solar neighborhood. Our sample only includes strongly irradiated planets, whose expanded atmospheres may be detectable via transit spectroscopy. The simulations were performed with the PLUTO-CLOUDY interface, which couples a detailed photoionization and plasma simulation code with a general MHD code. We study the thermospheric escape and derive improved estimates for the planetary mass-loss rates. Our simulations reproduce the temperature-pressure profile measured via sodium D absorption in HD 189733 b, but show unexplained differences in the case of HD 209458 b. In contrast to general assumptions, we find that the gravitationally more tightly bound thermospheres of massive and compact planets, such as HAT-P-2 b are hydrodynamically stable. Compact planets dispose of the radiative energy input through hydrogen Lyα and free-free emission. Radiative cooling is also important in HD 189733 b, but it decreases toward smaller planets like GJ 436 b. The simulations show that the strong and cool winds of smaller planets mainly cause strong Lyα absorption but little emission. Compact and massive planets with hot, stable thermospheres cause small absorption signals but are strong Lyα emitters, possibly detectable with the current instrumentation. The absorption and emission signals provide a possible distinction between these two classes of thermospheres in hot gas planets. According to our results, WASP-80 and GJ 3470 are currently the most promising targets for observational follow-up aimed at detecting atmospheric Lyα absorption signals.

Hot Neptune Gliese 436b has a Comet-like Tail

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

Authors:

Ehrenreich et al

Abstract:

Exoplanets orbiting close to their parent stars may lose some fraction of their atmospheres because of the extreme irradiation. Atmospheric mass loss primarily affects low-mass exoplanets, leading to the suggestion that hot rocky planets might have begun as Neptune-like, 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. 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 claim (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.

Helium may be Present in Warm Neptune Gliese 436b's Atmosphere

Helium Atmospheres on Warm Neptune- and Sub-Neptune-Sized Exoplanets and Applications to GJ 436 b

Authors:

Hu et al

Abstract:

Warm Neptune- and sub-Neptune-sized exoplanets in orbits smaller than Mercury's are thought to have experienced extensive atmospheric evolution. Here we propose that a potential outcome of this atmospheric evolution is the formation of helium-dominated atmospheres. The hydrodynamic escape rates of Neptune- and sub-Neptune-sized exoplanets are comparable to the diffusion-limited escape rate of hydrogen, and therefore the escape is heavily affected by diffusive separation between hydrogen and helium. A helium atmosphere can thus be formed -- from a primordial hydrogen-helium atmosphere -- via atmospheric hydrodynamic escape from the planet. The helium atmosphere has very different abundances of major carbon and oxygen species from those of a hydrogen atmosphere, leading to distinctive transmission and thermal emission spectral features. In particular, the hypothesis of a helium-dominated atmosphere can explain the thermal emission spectrum of GJ 436 b, a warm Neptune-sized exoplanet, while also consistent with the transmission spectrum. This model atmosphere contains trace amounts of hydrogen, carbon, and oxygen, with the predominance of CO over CH4 as the main form of carbon. With our atmospheric evolution model, we find that if the mass of the initial atmosphere envelope is 1E-3 planetary mass, hydrodynamic escape can reduce the hydrogen abundance in the atmosphere by several orders of magnitude in ~10 billion years. Observations of exoplanet transits may thus detect signatures of helium atmospheres and probe the evolutionary history of small exoplanets.

Doubts Cast on hot Neptune Gliese 436b's Multi Exoplanetary System

On the GJ 436 planetary system

Authors:

Maciejewski et al

Abstract:

The GJ 436 system contains a transiting planet GJ 436 b which is a hot analogue of Neptune on an eccentric orbit. Recently, two additional transiting sub-Earth planets have been postulated in the literature. We observed three transits of GJ 436 b over the course of 3 years using two-meter class telescopes, each with a photometric precision better than one millimagnitude. We studied system dynamics based on the existence of the additional planets. We redetermined system parameters, which were in agreement with those found in the literature. We refined the orbital period of GJ 436 b and found no evidence of transit timing variations. The orbital motion of the GJ 436 c planet candidate was found to be significantly affected by the planet b with variations in transit times at a level of 20 minutes. As the orbital period of the GJ 436 d planet candidate remains unknown, our numerical experiments rule out orbits in low-order resonances with GJ 436 b. The GJ 436 system with the hot Neptune and additional two Earth-like planets, if confirmed, would be an important laboratory for studies of formation and evolution of planetary systems.

Warm Neptune Gliese 436b is Alone and is Depleted in Methane, but Rich in CO/CO2

A global analysis of Spitzer and new HARPS data confirms the loneliness and metal-richness of GJ 436 b

Authors:

Lanotte et al

Abstract:

Context.

GJ 436b is one of the few transiting warm Neptunes for which a detailed characterisation of the atmosphere is possible, whereas its non-negligible orbital eccentricity calls for further investigation. Independent analyses of several individual datasets obtained with Spitzer have led to contradicting results attributed to the different techniques used to treat the instrumental effects. Aims. We aim at investigating these previous controversial results and developing our knowledge of the system based on the full Spitzer photometry dataset combined with new Doppler measurements obtained with the HARPS spectrograph. We also want to search for additional planets.

Methods.

We optimise aperture photometry techniques and the photometric deconvolution algorithm DECPHOT to improve the data reduction of the Spitzer photometry spanning wavelengths from 3-24 {\mu}m. Adding the high precision HARPS radial velocity data, we undertake a Bayesian global analysis of the system considering both instrumental and stellar effects on the flux variation.

Results.

We present a refined radius estimate of RP=4.10 +/- 0.16 R_Earth, mass MP=25.4 +/- 2.1 M_Earth and eccentricity e= 0.162 +/- 0.004 for GJ 436b. Our measured transit depths remain constant in time and wavelength, in disagreement with the results of previous studies. In addition, we find that the post-occultation flare-like structure at 3.6 {\mu}m that led to divergent results on the occultation depth measurement is spurious. We obtain occultation depths at 3.6, 5.8, and 8.0 {\mu}m that are shallower than in previous works, in particular at 3.6 {\mu}m. However, these depths still appear consistent with a metal-rich atmosphere depleted in methane and enhanced in CO/CO2, although perhaps less than previously thought. We find no evidence for a potential planetary companion, stellar activity, nor for a stellar spin-orbit misalignment.

Hot Neptune GJ-436b Comet-like Tail Suggests Exoatmosphere Will Last Billions of Years

Lyman-α Transit Spectroscopy and the Neutral Hydrogen Tail of the Hot Neptune GJ436b

Authors:

Kulow et al

Abstract:

To date, more than 750 planets have been discovered orbiting stars other than the Sun. Two sub-classes of these exoplanets, "hot Jupiters" and their less massive counterparts "hot Neptunes," provide a unique opportunity to study the extended atmospheres of planets outside of our solar system. We describe here the first far-ultraviolet transit study of a hot Neptune, specifically GJ436b, for which we use HST/STIS Lyman-α spectra to measure stellar flux as a function of time, observing variations due to absorption from the planetary atmosphere during transit. This analysis permits us to derive information about atmospheric extent, mass-loss rate from the planet, and interactions between the star and planet. We observe an evolution of the Lyman-α lightcurve with a transit depth of GJ436b from 8.8±4.5% near mid-transit, to 22.9±3.9% ∼2 hours after the nominal geometric egress of the planet. Using data from the time-tag mode and considering astrophysical noise from stellar variability, we calculate a post-egress occultation of 23.7±4.5%, demonstrating that the signature is statistically significant and of greater amplitude than can be attributed to stellar fluctuations alone. The extended egress absorption indicates the probable existence of a comet-like tail trailing the exoplanet. We calculate a mass-loss rate for GJ436b in the range of 3.7×10^6−1.1×10^9 g s−1, corresponding to an atmospheric lifetime of 4×10^11−2×10^14 years.