Thermal escape from extrasolar giant planets

Koskinen, Tommi; Lavvas, P.; Harris, Mary; Yelle, R. V.

doi:10.1098/rsta.2013.0089

Cited by 41 publications

(42 citation statements)

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“…The parameter  in equation (1) is an efficiency factor that is typically taken to be 0.1 Bolmont et al 2017;Koskinen et al 2014;Lammer et al 2013;Owen & Wu 2013 The XUV flux from young FGK stars is largest for the first 100±20 Myr after formation Lammer et al 2012;Ribas et al 2005). Emissions of XUV are saturated during that time and remain approximately constant.…”

Section: Methodsmentioning

confidence: 99%

Rocky Worlds Limited to ∼1.8 Earth Radii by Atmospheric Escape during a Star’s Extreme UV Saturation

Lehmer

Catling

2017

ApJ

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Recent observations and analysis of low mass (<10 M  ), exoplanets have found that rocky planets only have radii up to 1.5-2 R  . Two general hypotheses exist for the cause of the dichotomy between rocky and gas-enveloped planets (or possible water worlds): either low mass planets do not necessarily form thick atmospheres of a few wt. %, or the thick atmospheres on these planets easily escape driven by x-ray and extreme ultraviolet (XUV) emissions from young parent stars. Here we show that a cutoff between rocky and gas-enveloped planets due to hydrodynamic escape is most likely to occur at a mean radius of 1.760.38 (2) R  around Sunlike stars. We examine the limit in rocky planet radii predicted by hydrodynamic escape across a wide range of possible model inputs using 10,000 parameter combinations drawn randomly from plausible parameter ranges. We find a cutoff between rocky and gas-enveloped planets that agrees with the observed cutoff. The large cross-section available for XUV absorption in the extremely distended primitive atmospheres of low mass planets results in complete loss of atmospheres during the ~100 Myr phase of stellar XUV saturation. In contrast, more massive planets have less distended atmospheres and less escape, and so retain thick atmospheres through XUV saturation and then indefinitely as the XUV and escape fluxes drop over time. The agreement between our model and exoplanet data leads us to conclude that hydrodynamic escape plausibly explains the observed upper limit on rocky planet size and few planets (a "valley" or "radius gap") in the 1.5-2 R  range.

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Section: Methodsmentioning

confidence: 99%

Rocky Worlds Limited to ∼1.8 Earth Radii by Atmospheric Escape during a Star’s Extreme UV Saturation

Lehmer

Catling

2017

ApJ

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“…The former choice is representative of the planetary gas found a few planetary radii from the planet (e.g. Koskinen et al 2014). The latter choice is where the gas becomes opaque to Lyman continuum photons (Murray-Clay, Chiang & Murray 2009) and could be caused by escaping gas in a thick column (e.g.…”

Section: Modeling Of Planetary Gas With Cloudymentioning

confidence: 99%

Investigation of the environment around close-in transiting exoplanets using cloudy

Turner

Christie

Arras

et al. 2016

Mon. Not. R. Astron. Soc.

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It has been suggested that hot stellar wind gas in a bow shock around an exoplanet is sufficiently opaque to absorb stellar photons and give rise to an observable transit depth at optical and UV wavelengths. In the first part of this paper, we use the CLOUDY plasma simulation code to model the absorption from X-ray to radio wavelengths by 1-D slabs of gas in coronal equilibrium with varying densities (10 4 − 10 8 cm −3 ) and temperatures (2000 − 10 6 K) illuminated by a solar spectrum. For slabs at coronal temperatures (10 6 K) and densities even orders of magnitude larger than expected for the compressed stellar wind (10 4 − 10 5 cm −3 ), we find optical depths orders of magnitude too small (> 3 × 10 −7 ) to explain the ∼ 3% UV transit depths seen with Hubble. Using this result and our modeling of slabs with lower temperatures (2000 − 10 4 K), the conclusion is that the UV transits of WASP-12b and HD 189733b are likely due to atoms originating in the planet, as the stellar wind is too highly ionized. A corollary of this result is that transport of neutral atoms from the denser planetary atmosphere outward must be a primary consideration when constructing physical models. In the second part of this paper, additional calculations using CLOUDY are carried out to model a slab of planetary gas in radiative and thermal equilibrium with the stellar radiation field. Promising sources of opacity from the X-ray to radio wavelengths are discussed, some of which are not yet observed.

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“…(3.2) in Koskinen et al (2014a) provides a rough estimate of whether escaping heavy species from the stratosphere can be present in the thermosphere. The expression derived in Koskinen et al (2014a) is independent of altitude for an isothermal atmosphere. We applied this expression here using temperatures that are consistent with our model results.…”

Section: Ionospheric Densities: Atmosphere In Hydrodynamic Escape Regimementioning

confidence: 99%

“…Hence there are two different regimes of atmospheric escape depending on orbital distance, stellar heating and dissociation of molecular coolants upon which the composition and structure of the upper atmosphere depends. At large orbital distances (a > 0.2 AU for a Jupiter-like planet orbiting a Sun-like star), the main thermal escape mechanism is Jeans escape and the atmosphere is in a stable state of hydrostatic equilibrium, whereas planets with small orbital distances (a < 0.2 AU around a Sun-like star) undergo hydrodynamic escape (Koskinen et al 2007(Koskinen et al , 2014a. In addition, electrodynamics in the ionosphere can modulate escape rates and influence the structure of the upper atmosphere through ion drag and resistive heating (Koskinen et al 2014b).…”

Section: Introductionmentioning

confidence: 99%

EUV-driven ionospheres and electron transport on extrasolar giant planets orbiting active stars

Chadney

Galand

Koskinen

et al. 2016

A&A

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The composition and structure of the upper atmospheres of extrasolar giant planets (EGPs) are affected by the high-energy spectrum of their host stars from soft X-rays to the extreme ultraviolet (EUV). This emission depends on the activity level of the star, which is primarily determined by its age. In this study, we focus upon EGPs orbiting K-and M-dwarf stars of different ages -Eridani, AD Leonis, AU Microscopii -and the Sun. X-ray and EUV (XUV) spectra for these stars are constructed using a coronal model. These spectra are used to drive both a thermospheric model and an ionospheric model, providing densities of neutral and ion species. Ionisation -as a result of stellar radiation deposition -is included through photo-ionisation and electron-impact processes. The former is calculated by solving the Lambert-Beer law, while the latter is calculated from a supra-thermal electron transport model. We find that EGP ionospheres at all orbital distances considered (0.1−1 AU) and around all stars selected are dominated by the long-lived H + ion. In addition, planets with upper atmospheres where H 2 is not substantially dissociated (at large orbital distances) have a layer in which H + 3 is the major ion at the base of the ionosphere. For fast-rotating planets, densities of short-lived H + 3 undergo significant diurnal variations, with the maximum value being driven by the stellar X-ray flux. In contrast, densities of longer-lived H + show very little day/night variability and the magnitude is driven by the level of stellar EUV flux. The H + 3 peak in EGPs with upper atmospheres where H 2 is dissociated (orbiting close to their star) under strong stellar illumination is pushed to altitudes below the homopause, where this ion is likely to be destroyed through reactions with heavy species (e.g. hydrocarbons, water). The inclusion of secondary ionisation processes produces significantly enhanced ion and electron densities at altitudes below the main EUV ionisation peak, as compared to models that do not include electron-impact ionisation. We estimate infrared emissions from H + 3 , and while, in an H/H 2 /He atmosphere, these are larger from planets orbiting close to more active stars, they still appear too low to be detected with current observatories.

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Thermal escape from extrasolar giant planets

Cited by 41 publications

References 50 publications

Rocky Worlds Limited to ∼1.8 Earth Radii by Atmospheric Escape during a Star’s Extreme UV Saturation

Rocky Worlds Limited to ∼1.8 Earth Radii by Atmospheric Escape during a Star’s Extreme UV Saturation

Investigation of the environment around close-in transiting exoplanets using cloudy

EUV-driven ionospheres and electron transport on extrasolar giant planets orbiting active stars

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