Aeronomy of extrasolar giant planets Tommi Koskinen (APL) APEX Meeting, Thursday 26th October 2006 HD b (an artist’s impression), © ESA 2004 (A.Vidal-Madjar)

aeronomy (noun) The science of the upper atmosphere, esp. those regions where there is significant ionization of gases.

Exoplanet statistics 210 exoplanets found so far 170 planetary systems 20 multi-planet systems Extrasolar Planet Encyclopaedia (

Most of the known exoplanets have been detected by using the Doppler technique (Marcy and Butler, 1996) This technique favors the detection of massive planets orbiting relatively close to their parent stars. Most of the new planets are extrasolar giant planets (EGPs) Marcy and Butler (

Transiting planets 14 exoplanets have been found transiting their host star Transit light curves allow us to deduce the radius and the mass of the transiting planet accurately and even to detect its atmosphere Photometric time series for HD b binned into 5 m averages, plotted as a function of time from the centre of the transit (Charbonneau et al. 2000) © Lynette R. Cook

Secondary eclipses Observe the decrement in IR flux from the system when the planet passes behind the star (Spitzer space telescope) Deduce the effective temperature and Bond albedo of the planet Charbonneau et al. (2005)

HD b (Osiris) The first exoplanet observed to transit its star (Henry et al. 2000, Charbonneau et al. 2000) P ~ 3.52 days, a = AU R P = 1.32 (± 0.25) R jup, M P = 0.69 (± 0.05) M jup Mean density  ~ 372 kg/m 3 confirms that HD b is a gas giant Circular orbit due to tidal forces, rotationally synchronised

HD b - Atmosphere Charbonneau et al. (2002): detection of Na I resonance doublet at nm Depleted sodium abundance or high cloud deck predicted Charbonneau et al. (2002)

HD b - Atmosphere Vidal-Madjar et al. (2003): 15 ± 4 % absorption in the stellar H Lyman  ( nm) line observed Absorption strength corresponds to an occultation by an atmosphere extended to 4.3 R jup. This is beyond the Roche limit and thus some H is escaping Minimum mass loss rate g/s

HD b - Atmosphere Vidal-Madjar et al. (2004): detection of O I (130.2 nm) and C II (133.5 nm) in the upper atmosphere of HD Hydrodynamic flow of H I drags heavier components with it - hydrodynamic “blow off” Vidal-Madjar et al. (2004)

HD b (an artist’s impression), © ESA 2004 (A.Vidal-Madjar) HD b - Atmosphere

Deming et al. (2005): detection of infrared radiation (24  m) with Spitzer space telescope Brightness temperature of T 24 = 1130 ± 150 K derived. This confirms heating by stellar irradiation.

IR observations IR fluxes have been measured for HD189733b (16  m), HD b (8  m) and TrES-1 (4.5  m, 8  m, T eff = 1060 ± 50 K, A = 0.31 ± 0.14)

Evaporation The rate of evaporation depends on the exospheric temperature The upper atmosphere is tenuous and absorbs the energetic stellar X-rays and UV photons => the exospheric temperature is much higher than the effective temperature Lammer et al. (2003) calculated exospheric temperatures for EGPs within 1 AU from their star. They assume heating by XUV radiation and cooling by downwards heat conduction.

Lammer et al. (2003)

H 3 + Cooling It has been proposed that infrared emissions from H 3 + ions cool the upper atmospheres of EGPs (Miller et al. 2000) Lammer et al. (2003) do not take into account any radiative cooling or atmospheric circulation. Their exospheric temperatures may be too high.

EXOTIM Based on a thermospheric GCM for Saturn (Mulller-Wodarg et al. 2005) 3D coupled exoplanet thermosphere-ionosphere circulation model Solves the full Navier-Stokes equations for momentum, energy and continuity by explicit time integration Includes detailed ion density calculations and cooling by H 3 + ions

Assumptions The thermosphere is in hydrostatic equilibrium between 2  bar and 0.04 nbar Close-in EGPs are rotationally synchronised H 3 + emissions based on line lists of Neale et al. (1995) and modified for non-LTE Photochemical equilibrium (i.e. ion transport neglected) Ion densities are negligible compared to neutral densities

Table 1: Chemical reactions included in the model. Two-body rates are given in cm 3 s -1 and three-body rates are given in cm 6 s -1. Photoionisation rates are calculated explicitly by using the same photoionisation cross sections as Muller-Wodarg et al. (2005). The electron temperatures are taken to be the same as neutral temperatures.

Formation of H 3 + H H 2 —> H H H 2 + h —> H e H + + H 2 ( ≥ 4) —> H H

Recombination of H 3 + H e —> H 2 + H H e —> H + H + H

Dissociation of H 2 H 2 + M —> H + H + M (thermal dissociation) H 2 + h —> H + + H + e (dissociative photoionisation)

Upper boundary temperatures x - models that do not include H cooling ∆ - models that include H cooling ◊ - effective temperatures of the planets

Temperature and winds at 0.06 nbar of a slowly rotating EGP at 0.24 AU from a solar type host star. Maximum wind speed is roughly 1 km/s.

Composition At 0.24 AU, H 2 remains dominant throughout the thermosphere Ion number densities are much smaller than the overall neutral number density solid - H 3 +, dots - H +, dashes - H 2 + solid - H 2, dots - He, dashes - H

H 3 + emissions

Conclusions Transit observations are the only way to detect EGP atmospheres directly at present. Infrared emissions from H 3 + ions appear to play a significant role in cooling the thermospheres of EGPs within 1 AU from their host stars but the resulting fluxes are not detectable with current technology. On close-in EGPs H 3 + may not survive. Further modeling needed…