Planetary evaporation by UV and X‐ray radiation: basic hydrodynamics

By J. Owen, Alan P. Jackson. 2012

312 citations recorded by the source corpus.

We consider the evaporation of close-in planets by the star's intrinsic extreme-ultraviolet (EUV) and X-ray radiation. We calculate evaporation rates by solving the hydrodynamical problem for planetary evaporation including heating from both X-ray and EUV radiation. We show that most close-in planets (a < 0.1 au) are evaporating hydrodynamically, with the evaporation occurring in two distinct regimes: X-ray driven, in which the X-ray heated flow contains a sonic point, and EUV driven, in which the X-ray region is entirely sub-sonic. The mass-loss rates scale as LX/a2 for X-ray driven evaporation, and as for EUV driven evaporation at early times, with mass-loss rates of the order of 1010–1014 g s−1. No exact scaling exists for the mass-loss rate with planet mass and planet radius; however, in general evaporation proceeds more rapidly for planets with lower densities and higher masses. Furthermore, we find that in general the transition from X-ray driven to EUV driven evaporation occurs at lower X-ray luminosities for planets closer to their parent stars and for planets with lower densities. Coupling our evaporation models to the evolution of the high-energy radiation – which falls with time – we are able to follow the evolution of evaporating planets. We find that most planets start off evaporating in the X-ray driven regime, but switch to EUV driven once the X-ray luminosity falls below a critical value. The evolution models suggest that while ‘hot Jupiters’ are evaporating, they are not evaporating at a rate sufficient to remove the entire gaseous envelope on Gyr time-scales. However, we do find that close in Neptune mass planets are more susceptible to complete evaporation of their envelopes. Thus we conclude that planetary evaporation is more important for lower mass planets, particularly those in the ‘hot Neptune’/‘super Earth’ regime.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/8089b40f914b5bb4c198f10f32b860824c71723d

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In Astronomy and Astrophysics

1. Jon Fortney
2. J. Owen
3. N. Madhusudhan
4. C. Mordasini
5. Yanqin Wu
6. D. Charbonneau
7. G. Marcy
8. Y. Lithwick

Citations in the corpus

• A New Window into Escaping Exoplanet Atmospheres: 10830 Å Line of Helium (2017, background): “In highly irradiated planets, atmospheric escape can be very efficient and act collectively on the atmosphere as a fluid (e.g. Owen & Jackson 2012), instead of on a particle-by-particle basis.”
• Outgassing History and Escape of the Martian Atmosphere and Water Inventory (2015, methodology): “Because X-rays dominate at young stellar ages, Owen and Jackson (2012) studied the”
• The Hubble Space Telescope PanCET Program: Exospheric Mg ii and Fe ii in the Near-ultraviolet Transmission Spectrum of WASP-121b Using Jitter Decorrelation (2019, background): “These photons ionize atmospheric species and deposit enormous heat through electron collisions, that can greatly expand the upper atmosphere and drive hydrodynamic outflow and mass escape (Yelle 2004; Garćıa Muñoz 2007; Murray-Clay et al. 2009; Owen & Jackson 2012).”
• LYα TRANSIT SPECTROSCOPY AND THE NEUTRAL HYDROGEN TAIL OF THE HOT NEPTUNE GJ 436b (2014, background): “According to Owen & Jackson (2012) for a Neptune mass planet with a density of 1 g cm−3 at a separation of 0.025 AU (for GJ436b: mass = 1.35 MNep, density = 1.69 g cm−3, separation = 0.029 AU) the critical X-ray luminosity at which the planetary wind will transition from X-ray driven to EUV driven…”
• THE MUSCLES TREASURY SURVEY. I. MOTIVATION AND OVERVIEW (2016, background): “Soft Xray heating of planetary atmospheres enhances evaporation and atmospheric escape (Scalo et al. 2007; Owen & Jackson 2012) which may impact the long-term stability of an exoplanetary atmosphere.”
• DENSITY AND ECCENTRICITY OF KEPLER PLANETS (2012, background): “For more detailed treatments see Owen & Jackson (2012); Lammer et al. (2013).”
• UNDERSTANDING THE MASS–RADIUS RELATION FOR SUB-NEPTUNES: RADIUS AS A PROXY FOR COMPOSITION (2013, background): “Although photo-evaporation ca have a large impact on the composition of a planet (e.g. Baraffe et al. 2006; Hubbard et al. 2007; Lopez et al. 2012; Owen & Jackson 2012), the effect on the thermal state of the interior is relatively minor (Lopez & Fortney 2013).”
• SUPER-EARTH ATMOSPHERES: SELF-CONSISTENT GAS ACCRETION AND RETENTION (2015, background): “In addition to this spontaneous mass shedding, gas envelopes are also vulnerable to evaporation by high-energy stellar photons (see, e.g. Lopez et al. 2012; Owen & Wu 2013).”
• HOW ROCKY ARE THEY? THE COMPOSITION DISTRIBUTION OF KEPLER’S SUB-NEPTUNE PLANET CANDIDATES WITHIN 0.15 AU (2014, background): “That said, the photoevaporation of subNeptunes is an active area of theoretical research (e.g. Owen & Jackson 2012; Lopez et al. 2012; Lammer et al. 2013), and so we err on the side of a simple yet theoretically motivated prescription to arrive at a realistic result.”
• KEPLER PLANETS: A TALE OF EVAPORATION (2013, methodology): “Owen & Jackson (2012) develop realistic models of hydrodynamic evaporation for low-mass planets, including both X-ray and EUV irradiation.”
• THE IMPRINT OF EXOPLANET FORMATION HISTORY ON OBSERVABLE PRESENT-DAY SPECTRA OF HOT JUPITERS (2016, methodology): “This can drive atmospheric escape (e.g. Lammer et al. 2003; Baraffe et al. 2004; Erkaev et al. 2007; Murray-Clay et al. 2009; Owen & Jackson 2012; Lopez & Fortney 2013).”
• Water loss from terrestrial planets orbiting ultracool dwarfs: implications for the planets of TRAPPIST-1 (2016, background): “We include tidal gravity, but neglect the effects of the Coriolis force as it is a small while the outflow remains sub-sonic (Murray-Clay, Chiang, & Murray 2009; Owen & Jackson 2012).”
• Compositional Imprints in Density–Distance–Time: A Rocky Composition for Close-in Low-mass Exoplanets from the Location of the Valley of Evaporation (2017, background): “Typically, atmospheric escape is in the X-ray-driven regime during the early evolution stage (Owen & Jackson 2012; Jin et al. 2014).”
• Atmospheric Escape and the Evolution of Close-In Exoplanets (2018, methodology): “Owen & Jackson (2012) showed that the “efficiency parameter” was far from constant, generally decreasing with increasing depth of the gravitational potential, due to more energy loss from radiative cooling with longer flow timescales.”
• ALL SIX PLANETS KNOWN TO ORBIT KEPLER-11 HAVE LOW DENSITIES (2013, background): “The efficiency of photoevaporative mass loss changes as a function of irradiation and stellar age (Owen & Jackson 2012).”
• PLANETARY POPULATION SYNTHESIS COUPLED WITH ATMOSPHERIC ESCAPE: A STATISTICAL VIEW OF EVAPORATION (2014, methodology): “Nevertheless, atmospheric evaporation may play a vital role in the thermal evolution of low-mass planets; this can alter the size distribution of close-in planets with radii < 2 R⊕ (Owen & Jackson 2012; Lopez et al. 2012; Lammer et al. 2014).”