A&A 418, L1-L4 (2004)
DOI: 10.1051/0004-6361:20040106
A. Lecavelier des Etangs 1 - A. Vidal-Madjar 1 - J. C. McConnell 2 - G. Hébrard 1
1 - Institut d'Astrophysique de Paris, CNRS, 98bis boulevard Arago,
75014 Paris, France
2 -
Department of Earth and Atmospheric Science,
York University, North York, Ontario, Canada
Received 10 November 2003 / Accepted 4 March 2004
Abstract
The extra-solar planet
HD 209458b has been found to have an
extended atmosphere of escaping atomic hydrogen (Vidal-Madjar et al. 2003),
suggesting that "hot Jupiters'' closer to their parent stars could evaporate.
Here we estimate the atmospheric escape (so called evaporation rate)
from hot Jupiters and their corresponding life time against evaporation.
The calculated evaporation rate of HD 209458b is in excellent agreement with
the H I Lyman-
observations.
We find that the tidal forces and high temperatures in the upper
atmosphere must be taken into account
to obtain reliable estimate of the atmospheric escape.
Because of
the tidal forces, we show that there is a new escape mechanism at
intermediate temperatures at which the exobase reaches the Roche lobe.
From an energy balance, we can estimate plausible values for the planetary
exospheric temperatures, and thus obtain typical life times of
planets as a function of their mass and orbital distance.
Key words: star: individual: HD 209458 - stars: planetary systems
Radiative equilibrium is commonly
used to calculate the temperature of the upper atmosphere
(see Schneider et al. 1998).
But this is not appropriate because it does not apply to the
low density upper atmosphere.
As an example,
in the Solar system,
the temperature of planetary upper atmospheres (thermosphere, exosphere)
is much higher than
,
the effective temperature of the lower atmosphere.
For Earth and Jupiter,
are
250 K and 150 K,
while thermospheric temperatures are
1000 K for both planets
(Chamberlain & Hunten 1987).
Although the observed high temperatures in the giant planets
are not yet explained, extreme and far ultraviolet
fluxes, Solar wind and perhaps gravity waves may contribute
to the heating in uncertain amounts (e.g., Hunten & Dessler 1977).
The atmospheric escape flux strongly depends on the temperature
profile of the atmosphere. As the temperature of the upper atmosphere of
extra-solar planets is
not known, in a first step, we will use it as an input parameter:
.
We consider a simple atmospheric structure similar to that
observed in the giant planets of the Solar system, that is a two
level temperature structure:
in the lower atmosphere
and
above. We assume that the lower atmosphere
(up to the thermobase) is mainly composed of molecular hydrogen
at
.
Above that level,
in the thermosphere and exosphere, the gas is a mixture of atomic
and molecular hydrogen at
(Fig. 1).
![]() |
Figure 1: Vertical structure of the atmosphere of HD 259458b as a function of the temperature of the upper atmosphere (Model A). At the top of the thermosphere, the exobase is the critical level where the mean free path is equal to the distance to the Roche lobe. For high temperatures, the exobase reaches the Roche lobe; this leads to a geometrical blow-off. |
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The quantitative results presented in this letter do not depend on
the estimated
,
but on the density at the
thermobase,
and
for atomic
and molecular hydrogen, respectively. In the following we will use three
different estimates of these densities. In the case of Jupiter and Saturn,
H2 is the main constituent at the thermobase,
where the temperature transition is observed to occur for
cm-3 and
cm-3 (Chamberlain & Hunten 1987);
these values will be used for the Model A. By
evaluating the production of atomic hydrogen with realistic condition for
the lower atmosphere of HD 209458b, Liang et al. (2003) have shown
that the H I mixing ratio could be about 10% at the
thermobase. We thus use the corresponding
cm-3 and
cm-3 for the
intermediate Model B. Finally, at high thermospheric temperatures, it
is likely that most of the molecular hydrogen is dissociated into atomic
hydrogen (Coustenis et al. 1998); we use
cm-3 and
cm-3 for the corresponding Model C.
The atmospheric escape critically depends on atmospheric temperatures.
Below a critical temperature, ,
the escape flux
can be calculated using the Jeans escape estimate which refers to the
escape of particles whose velocity is in the tail of the
Boltzmann distribution and that have enough energy to escape the planets
gravity. For temperatures above
,
the kinetic energy of the atoms or
molecules is sufficient to overcome gravitational forces and they can
stream, or blow-off.
For Jeans escape the flux is calculated at a
critical level corresponding to the exobase (Hunten et al. 1989).
This critical level is the level above which particles
can freely escape without collisions. The exobase is usually defined by
the place above which the mean free path (1/nQ) is larger than H,
the scale height of the atmosphere,
where n is the volume density.
This definition results from the approximate calculation of the integrated
collisional cross-section (Q) from the exobase to infinity:
,
where
is the density at the exobase.
Here this idea must be generalized to take into account the particular
geometry of hot Jupiters; we define the exobase by
the place above which the mean free path is larger than the distance to
the Roche lobe of the planet:
.
The Roche lobe is the last equipotential around the planet beyond which the
equipotentials are open to infinity or to encompass the star.
Thus the exobase is the level above which atoms and molecules can
definitively escape the planet. Note that in the case of an isolated planet,
because the Roche lobe is at infinite distance, the two definitions are
identical.
Then, the total escape flux from the planet is given by
,
where
is the mass of the escaping
elements (H I and H2),
is the radius of the exobase,
and vT is the
thermal velocity at temperature T (Chamberlain & Hunten 1987).
is defined by
,
where
is the
gravitational potential.
We calculated the density profile in the atmosphere
using the barometric law:
.
Recapture of escaped particles is not possible because
of the high radiation pressure pushing away hydrogen atoms at hundred
of kilometer per seconds (Vidal-Madjar et al. 2003).
We can see from Fig. 1 that between
the classical Jeans escape at low temperatures and the dynamical blow-off of the
atmosphere at high temperatures, the tidal forces lead to a new escape
mechanism at intermediate temperatures.
Indeed, for HD 209458b (d=0.047 AU,
,
and
)
and
above
5000 K,
the exobase comes close to the Roche lobe, and
the kinetic energy is similar to the potential energy
needed to reach this limit.
In that case, the escape mechanism is
a geometrical blow-off
which is
due to the filling up of the Roche lobe
by the thermosphere pulled up by the tidal forces.
Indeed, the dynamical blow-off is obtained when the escape velocity
is of the order of (or smaller than) the thermal velocity vT,
that is for small value
of
(
).
With the characteristics of HD 209458b, the dynamical blow-off
takes place for temperatures above 20 000 K.
However, at temperatures between 5000 K and 20 000 K, although
,
we have
,
and most
of the gas at the exobase can freely reach the Roche lobe.
This geometrical blow-off is due to the spatial proximity of the
Roche lobe through which the gas can escape the planet.
In the case of hot Jupiters, the planet's gravity is substantially
modified by stellar tidal forces.
The common assumption has been to neglect tidal forces by considering
isolated planets far from their star. But tidal forces have a significant
influence on the density distribution in the upper atmosphere of hot
Jupiters.
In order to calculate the potential energy ,
we include the difference
between the stellar gravitation and the centrifugal effect in the orbiting
planet reference frame, which results in the tidal forces. The
equipotentials are modified, from quasi-spherical in the lower
atmosphere to asymmetric elongated shapes at the level of the
Roche lobe. As a result,
,
hence the vertical density distribution, strongly depend on the
location in both longitude and latitude on the planet (
,
).
We calculate the escape rate as the sum of
,
the escape flux as a function of longitude and latitude.
Even with a uniform temperature, the
escape flux per unit area is larger toward the star and in the opposite
direction (Fig. 2).
![]() |
Figure 2:
Shape of the exobase and
the corresponding escape rate (Model A,
![]() ![]() |
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![]() |
Figure 3:
Escape flux and life time of HD 209458b
as a function of
![]() |
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Finally, using the above, we estimate the total escape flux of
atomic and molecular hydrogen as a function of the upper atmosphere
temperature for
HD 209458b (Fig. 3).
For temperature above
8000 K (Model A), 7000 K (Model B) and 6000 K (Model C),
the H I escape flux is larger than the minimum flux of
1010 g s-1 needed to explain the occultation depth
of 15% as observed in Lyman
(Vidal-Madjar et al. 2003).
From the escape rate, we can derive
the corresponding life time needed to evaporate the total mass of the
planet
.
However when the planet mass
decreases, the evaporation rate increases. This results in a shorter life
time given by
,
which is typically shorter than
t1 by a factor of 5 to 10 (Fig. 3b).
![]() |
Figure 4:
Upper atmosphere temperature estimated from the energy balance
as a function of the orbital distance of a 0.69
![]() ![]() |
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Although the mechanism responsible for the heating of
the upper atmospheres in the Solar system is not fully
identified, we obtain a plausible estimate of
from a
comparison of heating and cooling mechanisms. A lower limit of the
heating can be estimated from the energy flux of both the stellar
extreme ultraviolet (EUV) and Lyman
photons. Using the EUV and a
monte-carlo simulation of the multiple scattering of Lyman
photons
in the thermosphere, we estimate the minimum energy input to be
erg cm-2 s-1,
where d is the orbital distance to the star.
The cooling is due to a combination of the
heat conduction toward the cooler lower atmosphere, collisional exitation of
the H I electronic levels, collisional ionization (photo-ionization is
negligible), and cooling by
escaping atoms and molecules carrying off their kinetic energy.
We derive the ionization fraction and the collisonal cooling
following Spitzer (1978) and Osterbrock
(1989) for atomic and molecular hydrogen clouds.
With the HD 209458b characteristics,
the ionization fraction is barely above 0.01.
We calculated the heat conduction following the
derivation given by Hunten & Dessler (1977).
If we apply this calculation to Jupiter with UV heating only, we find
a temperature rise at the thermobase
as low as 15 K (Strobel & Smith 1973); clearly other heating mechanisms
are at work in Jupiter (Hunten & Dessler 1977).
Considering the UV heating only, the energy balance provides a lower limit to
the thermospheric temperature
(Fig. 4).
For HD 209458b, we evaluate the lower limit of
to be about 11 100 K, 9800 K and 7900 K for Models A,
B and C, respectively. Cooling is dominated by atmospheric escape
and collisonal excitation of H I.
With these temperatures, the present evaporation rates
of atomic and molecular hydrogen
from HD 209458b are estimated to be
g s-1 and
g s-1 in Model A,
g s-1 and
g s-1 in Model B,
and
g s-1 in Model C,
in agreement with the observational lower limit of
1010 g s-1
(Vidal-Madjar et al. 2003).
We obtain a life time (t2) of 1010 to 1011 years.
During
years,
HD 209458b may have lost 1% to 7% of its initial mass.
![]() |
Figure 5:
Contour plot of the planet life time (
![]() |
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With models of heat conduction within escaping atmospheres,
Watson et al. (1981) showed that the escape rate can be limited
by heat exchange.
Using
their result applied to HD 209458b, we found that the escape
rate is heat-limited to 1012 g s-1,
in agreement
with the results obtained by Trilling (1999) and Lammer et al. (2003).
Although these results neglect
the tidal forces,
our values are below this limit and therefore do not need a full calculation
of the heat exchange within the thermosphere.
At a given temperature, the escape rate is roughly proportional to the thermobase densities and the energy input. However, using the temperature calculated from the energy balance, the escape rate is less sensitive to the assumed input parameters. It is roughly proportional to the thermobase densities to the power of 0.3, and proportional to the energy input to the power of about 0.4.
Finally, we can also estimate the life time of a given planet as a function of its mass and orbital distance (Fig. 5). We concludethat planets with orbital distance lower than 0.03-0.04 AU (corresponding to orbital periods shorter than 2-3 days) have short life time unless they are significantly heavier than Jupiter. This may explain why only few planets have been detected with periods below 3 days. Low-mass hot Jupiters have also short life times, meaning that their nature must evolve with time. These planets must loose a large fraction of their hydrogen. This process can lead to planets with an hydrogen-poor atmosphere ("hot Neptunes''), or even with no more atmosphere at all. The emergence of planets modified by evaporation (and possibly the emergence of the inside core of former and evaporated hot Jupiters) may constitute a new class of planets (see also Trilling et al. 1998). If they exist, these planets could be called the "Chthonian'' planets in reference to the Greek deities who come from hot infernal underground (Hébrard et al. 2003).
Acknowledgements
We warmly thank Drs. G. Ballester, L. BenJaffel, & C. Parkinson for very fruitful discussions. We thank Nhât Võ Trân for discussions on myths and etymology.