H. Lammer (1), P. Odert (2), M. Leitzinger (2), M. L. Khodachenko (1), M. Panchenko (1),Yu. N. Kulikov (3), T. L. Zhang (1), H. I. M. Lichtenegger (1), J. Weingrill (1) G. Micela (4),

and A. Hanslmeier (2)

(1) Space Research Institute, Austrian Academy of Sciences, Graz, Austria(2) Department of Physics, University of Graz, Graz, Austria

(3) Polar Geophysical Institute, Russian Academy of Sciences, Murmansk, Russian Federation(4) INAF - Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy

[Email: helmut.lammer@oeaw.ac.at]

Abstract. We investigate the orbital distance which selects Jupiter-like exoplanets in hot gas giants which can lose their whole atmosphere by Coronal Mass Ejection's or thermal evaporation if they are not protected by strong magnetic fields or by IR-cooling species in their upper atmospheres, with planets at orbits beyond the evaporation boundary. We show that thermal evaporation becomes relevant for mass losses of hot gas giants at orbital locations ≤ 0.015 AU, while stellar CME plasma interaction could result in efficient non-thermal mass loss from hot Jupiter’s at orbital distances between 0.02 – 0.035 AU. Depending on the host star, “hot Uranus” or sub-Uranus-class exoplanets may lose their hydrogen envelopes at orbital distances ≤ 0.02 AU.

Fig. 3: Minimum (dashed-line) and maximum (solid-line) CME and ICME plasma density as a function of orbital distance [Khodachenko et al., PSS, 2007].

Acknowledgments: The data about the main statistical features of the solar CMEs used in this paper are based on the CME catalogue, generated and maintained by NASA and The Catholic University of America in cooperation with the Naval Research Laboratory (http://cdaw.gsfc.nasa.gov/CME list/). The authors thank also the Austrian Ministry for Science, Education and Culture (bm:bwk) and ASA for funding the CoRoT project. This study was supported by the International Space Science Institute (ISSI) and carried out

in the framework of the ISSI Team “Exoplanet Evolution and Atmosphere Characterisation”. M.L., P.O., A.H. , H.L. and M.L.K. gratefully acknowledge the Austrian Fonds zur Förderung der wissenschaftlichen Forschung (FWF grant P19446) for supporting this study.

Fig. 4: Neutral hydrogen density profile of the upper atmosphere under hydrodynamic conditions of HD209458b as a function of planetocentric distance in units of planetary radii. The dotted and dashed lines mark the magnetopause stand-off distance corresponding to the minimum and maximum CME plasma densities respectively, and the maximum expected magnetic moment Mmax = 0.1 MJup. In the case of the minimum expected magnetic moment of HD209458b Mmin = 0.005 MJup, the magnetopause stand-off distances would lay below the visual planetary radius at about 0.56 – 0.8 rpl. Since at deep atmospheric levels the neutral gas density is very high, one can expect that the CME plasma flow will be deflected around the planet at somewhat higher altitudes which are shown as a shaded area.

CME exposure of HD209458b - like gas giants: CME exposure of HD209458b - like gas giants:

Table 1: Expected CME parameters at HD209458b’s orbital distance of about 0.045 AU [Khodachenko et al., PSS, 2007].

The distribution of planetary energetic atoms is calculated by means of a test particle model which is based on the proton flow in the magnetosheath according to the Spreiter-Stahara model. The total production rate of planetary ions is the sum of the corresponding photoionization, electron impact and charge exchange rates. Once these rates are determined, the trajectories of the newly born H+ ions are followed. The numerical simulation is initialised by dividing the space around HD209458b into a number of volume elements and the CME plasma density nCME in each element is calculated via

where the integration is performed from infinity to the corresponding volume element which is located at the point s1 on the streamline. Here, nCME

(0) is the unperturbed solar wind density, ni the density of the i-th neutral species as a function of altitude, σi is the energy dependent charge exchange cross section between a proton and the i-th neutral species and ds is the line element along the streamline [Khodachenko et al., PSS, 2007].

,

1

)0(

s

iii dsn

CMECME enn

CME induced ion pick up modelling:CME induced ion pick up modelling:

Table 2: H+ ion loss rates on HD209458b due to its interaction with CME’s, for different CME plasma densities and expected planetary magnetic moments.

We studied the possible thermal and non-thermal atmospheric mass loss of 57 known transiting exoplanets around F, G, K, and M-type stars over evolutionary time scales. For stellar wind induced mass loss studies we estimate the position of the pressure balance boundary between Coronal Mass Ejection (CME) and stellar wind ram pressures and the planetary ionosphere pressure for non- or weakly magnetized gas giants at close orbits. We found that the transiting low density gas giant WASP 12b at orbital location of 0.023 AU, lost about 5–12 % of its initial mass. All other transiting exoplanets in our sample experience negligible thermal mass loss (less than about 2 %) during their life time. We show that the ionospheric pressure can balance the colliding dense stellar wind and average CME plasma flows at distances which are above the visual radius of “Hot Jupiters”, resulting in mass losses less than 2 % during evolutionary time scales. However, the ram pressure of fast CMEs cannot be balanced by the ionospheric plasma pressure at orbital distances between 0.02–0.1 AU. Therefore, collisions between hot gas giants and fast CMEs result in large atmospheric mass loss, which may influence the mass evolution of gas giants with masses less than that of Jupiter.

Cores andshrinked Jupiter‘s

could be discovered

Gas giants

can not shrink

Telluricplanets

ConclusionsConclusions: :

[Yelle, Icarus, 2004]

0.1 AU0.045 AU

0.01 AU

0.3 rPl/r r ~ 3.3 rPl

Fig. 3: Stellar wind and CME number density, plasma velocity and n ram pressure as a function of orbital distance. The ✳ correspond to the ionospheric pressure, which is strong enough to ballance the stellar wind but is too weak for CMEs at orbital distances between 0.02 – 0.1 AU [Lammer et al., A.&A., submitted 2009].

s.w.

s.w.

s.w.

CME

CME

CME

at 3 rpl

Information on s.w./CME density & speed change with orbital distance plays crucial role for the study of pressure balance: PSW,CME ~ PIP.

Mass loss

inplanet

mass [%]

K is the non-linear potential energy reduction factor due to the stellar tidal forces [Erkaev et al. A&A, 2007].

Hydrodynamic approach

Energy limitedapproach incl. Roche lobe effect

Fig. 2: The dashed line gives the median X-ray luminosity of G stars, the dark shaded area the 1σ equivalent of the luminosity distributions, as derived by Penz et al. (A&A, 2008). The dash-dotted line and the light shaded area show the same for M dwarfs [Penz and Micela, A&A, 2008]. The solid lines display (from top to bottom) the scaling law from Ribas et al. (ApJ, 2005) for solar analogs in the range 0.1-10 nm, and the scaling from Guinan and Engle (2009) for a sample of M dwarfs, respectively. The dash-dot-dotted line displays the scaling law for G stars from Scalo et al. (Astrobiology, 2007).

X-ray/EUV induced evaporation of hydrogen-rich gas giants: X-ray/EUV induced evaporation of hydrogen-rich gas giants:

Fig. 1: Hydrodynamic and energy limited XUV-evaporation related thermal mass loss rates for the hot Jupiter HD209458bHD209458b, modelled over the planets` lifetime. by considering the XUV evolution of the Sun-like host star [after Penz et al. PSS, 2008].

Fig. 5: Illustration of thermal mass loss integrated over 4 Gyr for low (ρ2=0.4 g cm−3) and high (ρ1 =1.24 g cm−3) density gas giants with initial masses of 0.5MJup located in an area where no exoplanets have been discovered until now. The filled circles show discovered exoplanets in mass range between 0.01–1.0MJup and an orbital distance between 0.01–0.1 AU.

Fig. 7: Modelled hot Jupiter ionsopheric and atmospheric density profiles of a hydrodynamically expanded upper atmosphere as a function of orbital distance [after Yelle, Icarus, 2004]. One can see that the high stellar X-ray and EUV flux ionizes most of the hydrogen at about 3 rPl. This ionization results in high ionospheric pressures which can balance the stellar wind ram pressure (see Table 3).

Table 4: Masses and radii of 57 transiting exoplanets, stellar masses, as well as most of the spectral types are taken from the Extrasolar Planets Enyclopaedia; status February 2008; http://www.exoplanet.eu/. Spectral types which are roughly estimated corresponding to the mass sequenceare indicated with an x. The lost mass is calculated over a period of 4 Gyr for three different values of heating efficiency (10 %, 60%, 100 %). The planetary densities and the Roche lobe related mass loss enhancement factor are also given[Lammer et al., A&A,In press 2009].

Fig. 6: Mass loss of exoplanets (four upper panels) ranging from 12–95 MEarth (initial masses) versus density in the range of 0.2–2 g cm-3. All exoplanets are set to an orbital location of 0.017 AU. The integration time of the mass loss calculations is between 0.01–5 Gyr and the heating eciency is 10 and 25 %, respectively. As host stars we consider two cases, namely a 0.96 MSun K-star and a 0.96 MSun G-star. Two Solar System hydrogen-rich ice giants are over plotted (Uranus and Neptune).

Table 3:

Table 3: The mass loss from “Hot Jupiters” and “Hot Neptunes” with low and high densities at four different orbital distances around a Sun-like G star.

The relevance of the planetary density to the thermal mass lossof close-in exoplanets