Since 51 Pegasi b was discovered in 1995 it has been estimated that such planets orbit so close to their parent stars that their upper atmospheres are largely heated to temperatures of ~10,000 K by the immense stellar extreme-UV (EUV) radiation. Since then 209 extrasolar planets have been discovered and over 15% of these are also ``hot Jupiters''.
With the discovery of the transiting hot Jupiter planet HD 209458b new opportunities have come about by measuring the obscuration of the stellar disk produced while the planet is transiting. The planetary size can be determined from the total broadband optical obscuration, found to be 1.45%. The nature of the planet's atmosphere can also be studied from additional signatures in the stellar spectrum due to atmospheric absorption during transit. HD209458b's atmosphere was first detected by the Hubble Space Telescope (HST) by a small 0.02% additional absorption in the yellow D lines produced by atomic sodium at low atmospheric altitudes (Charbonneau et al. 2002). Less sodium was observed than expected.
HST then detected far-UV 15% absorption by an enormous cloud of atomic hydrogen (Vidal-Madjar et al. 2003; see below Publication #1). This was followed by an HST far-UV detection of atomic oxygen and singly ionized carbon also in the huge extended upper atmosphere (Vidal-Madjar et al. 2004; see below Publication #2). The UV observations indicated that the planet has a huge upper atmosphere, extending beyond 3 times the radius of the planet, escaping the planet and forming a comet-like tail.
Even though the upper atmosphere heats up to over 10,000K, the thermal escape is not enough to supply the estimated escape rate of hydrogen atoms of 1-9x1010 gm/s which is 2,400-21,000 tons/s, or on the order of 10,000 tons/s! So in the upper atmosphere of this ``hot Jupiter'', the immense stellar EUV heating should produce a hydrodynamic state, where the gas has a net upward velocity. This is what should produce the sizable expansion of the atmosphere and the escape of neutral hydrogen gas.
In our new work we analyzed some archival HST observations of HD 209458 during the planet's transit, and have now made the first detection of absorption by hot hydrogen in its optical and near-UV signature of the Balmer jump and continuum in any planet. The absorption contrast was 0.03% +/- 0.006%!
Modeling of the observations, based on a model of HD 209458b's upper
atmosphere by Yelle (2004) indicates that the hot hydrogen resides in a
layer that is hot ebough (over 5,000K) and dense enough to produce a
substantial amoung of hot hydrogen atoms. This layer is about 1,000 km
thick and resides in at the bottom of the thermosphere at an altitude
of ~8,500 km. This is in a transition region where the temperature
rises, the atmosphere inflates, and the gas accelerates to escape from
the planet. The results are depicted in the figure below.
The hot hydrogen consists of hydrogen atoms in the first excited state (n=2 energy level). Most of the hydrogen atoms in HD 209458b's extended upper atmosphere are in the ground state (n=1 energy level). As the temperature of the atmosphere increases with altitude, the total density of the atmosphere decreases with altitude. There is a region where the TOTAL number of excited (hot) hydrogen atoms maximizes. This is the layer that produces the maximum Balmer absorption that we have now detected. The Balmer jump, at 364.6 nm, is at the spectral edge where hot hydrogen atoms start absorbing light into the ionization continuum. We detected absorption in the Balmer jump and also across the Balmer continuum at shorter wavelengths spanning from the violet to the near-ultraviolet regions. Solar system planets are too cold to show this signature.
With this new HST detection we have now been able to isolate structure in the atmosphere of an extrasolar planet. We have detected the transition region in between the lower atmosphere (at ~1,200 K in the sub-stellar side) and the extended thermosphere where the temperature reaches 10,000-15,000 K. With this detection, we thus been able to confirm the picture of the hydrodynamic state of HD 209458b's upper atmosphere and of the escape.
Our new work provides a new method to study the atmospheric structure and complex escape processes of extrasolar hot Jupiters. Future HST observations will allow for similar studies of other transiting planets.
The archival HST data used for this work consisted of very high quality
transit observations of the HD 209458 system obtained in 2003 with the
Space Telescope Imaging Spectrograph (STIS) instrument by D.
Charbonneau (Harvard Smithonian Institute) and collaborators.
For press releases see:
Press Release January 31, 2007: Hubble Space Telescope Science Institute
Press Release January 31, 2007: Univ. of Arizona UANews.org
Detection of hot hydrogen in HD 209458b's atmosphere: Simple explanation for the public
For the first time oxygen and carbon have been detected in a planet outside our solar system. This discovery was made with low-resolution far-ultraviolet observations of the transit of HD 209458b using the STIS intrument on HST. What's key here is that we have detected oxygen and carbon in atomic form and in the outermost layers of the planet where we would not normally expect them in the giant planets in our solar system. These species are over 10 times heavier than hydrogen atoms. There must be a strong driving force lifting them up along with the hydrogen gas against the planetary gravity in this close-in giant planet, orbiting at a a distance of only 9 stellar radii from its parent star.
After our first HST detection of a very extended hydrogen upper atmosphere on HD209458b with a large escaping component, described under Publication #1 below, new team member Jack McConnell from York Universtiy, Canada realized that the thermal escape at the top of the atmosphere (at the exobase) would be insufficient even for the large ~10,000 K temperatures estimated for the upper atmosphere. The UV radiation source from the star is so large for this close-in extrasolar giant planet that it produces kinetic energy for macroscopic motion. That is, the upper atmosphere may not be hydrostatically stable. The mean kinetic energy attained by the hydrogen gas becomes larger than the escape energy. In this case the light hydrogen gas starts to flow upwards to escape. Independent detailed modeling of the hydrogen upper atmosphere of HD 209458b has also been made by colleague R. Yelle at the Univ. of Arizona, which also yields hydrodynamic escape (Yelle, "Aeronomy of extra-solar giant planets at small orbital distances", submitted to Icarus, 2004).
What our team members further realized is that hydrodynamic escape can
become a blow-off situation, when the flow speeds of the light gas are
large enough, approaching the local sound speed, that the light gas can
then drags along other heavier species into the flow. The blow-off is
maintained by the constant UV input from the star. In addition, the
planet is so close to the star that the shape of the gravitational
field in the upper atmosphere is progressively altered with altitude,
from a spherical to an elongated shape, allowing even more material to
escape (Lecavelier, Vidal-Madjar, McConnell and Hebrard, "Atmospheric
escape from hot Jupiters", submitted to Astronomy & Astrophysics,
2004). The outer atmosphere should resemble a rugby ball
pointed towards the star. There is still also the outermost escape
component resembling a comet tail.
What species could we then detect with HST? Oxygen and carbon are common solar species with strong resonant absorption lines in the far-UV spectral region accessible with the STIS instrument on HST. Oxygen and carbon have been identified in our Jovian planets bound-up in molecules such as methane (and other hydrocarbons) and water, residing at the bottom of the upper atmosphere. Furthermore, the stellar UV irradiation of close-in extrasolar giant planets is so strong that it should dissociate any such molecular species if at high enough altitudes in the upper atmosphere, and further ionize the carbon atoms in the hot 10,000 K environment (as in irradiated ISM clouds) preferentially over the oxygen atoms.
Since solar-like stars produce many far-UV line emissions including the OI (1304 Ang) and CII (1335 Ang) multiplets, we proposed that absorption by the planet upper atmosphere should be detectable during transit if blow-off is indeed occurring. And this is what we successfully observed with HST. We detected transit absorption depths of 13+/-4.5% and 7.5+/-3.5% respectively for oxygen and carbon. (The new HST low-resolution data also confirmed the first hydrogen detection.)
The HST discovery observations now provide key constraints on the nature of the upper atmosphere of a close-in extrasolar planet, and on the strength of the hydrodynamic escape and blow-off.
Hydrodynamic escape process is what is believed to be at the origin
of the formation of the solar wind. Blow-off has also been proposed to
have possibly removed the early atmospheres of Venus, Earth and Mars
when the solar UV light was much more brighter. Interesting analogies
of the process are when dust is swept up by a whirlwind, air is swept up
in a thunder cloud, and a stick is carried along by a swift moving river.
For press releases see:
Press Release February 2, 2004: Hubble Space Telescope, European Space Agency
Press Release February 2, 2004: Univ. of Arizona UANews.org
The detection of the atmosphere of an extrasolar giant planets has been elusive, yet far from fully pursued. The discovery of the transit of an extrasolar planet HD209458b, a hot-Jupiter orbiting its parent star at a mere distance of 9 stellar radii every 3.5 days, has opened the door for occultation experiments in which the starlight can be studied as it is transmitted through the planetary atmosphere during the 3-hr length of the transit. Most transit observations of HD209458b have focused on the visible spectral region, providing high photometry and accurate measurements of the size of the planet disk (defined by the altitude of the optically-thick visible cloud tops). However, the optical regime is sensitive mostly to minor, heavy or low altitude atmospheric species, such as atomic sodium which was detected with Hubble Space Telescope in the strong yellow lines but only at a 0.02% absorption (Charbonneau et al. 2002).
On the other hand, it is in the ultraviolet where the dominant components of the atmospheres of the Jovian planets, atomic and molecular hydrogen, have their strongest signatures. In fact, many discoveries and studies of the atmospheres of Jupiter, Saturn, Uranus and Neptune have been made in the UV, such as with the UV instruments built by scientists at the Univ of Arizona for the Voyager mission to our own Solar System. Occultation and transit observations can provide information on the composition, temperature and structure of the planet's atmosphere. (UV observations of reflected sunlight, of dayglow emissions, as well as of auroral emissions have also been critical for our understanding of planetary upper atmospheres and their magnetospheric interactions.) Remote sensing UV observations of our Jovian planets are also made with Earth orbiting telescopes (above our own UV absorbing atmosphere). This is now possible only with the Hubble Space Telescope (HST) (and soon by the Cassini UVES at Saturn), as described in other parts of this website.
The upper atmosphere of extrasolar planet HD209458b should be highly inflated given the immense heating from the nearby parent star, and as the lightest element, atomic H will further have the largest scale height. Atomic hydrogen also has a strong ultraviolet absorption signature in the Lyman-alpha line at 121.6 nanometers. Hydrogen is therefore the best suited species to be detected in an extrasolar hot-Jupiter atmosphere. This is the first program to make such a detection. The observations were made with the Space Telescope Imaging Spectrograph (STIS) instrument on HST, and the work has been led by Alfred Vidal-Madjar at the Institute d'Astrophysique de Paris (IAP).
HST measured the stellar chromospheric H Lyman-alpha emission of the parent star HD209458 during three planetary transits, and detected the transit absorption signature of the atomic hydrogen in the upper atmosphere of the planet. We found that the hydrogen atmosphere is indeed highly inflated because it obscures 15% of the star while the visible disk obscures only 1%. This implies that the hydrogen reaches beyond the Roche lobe (ie, beyond 2.7 planetary radii). Hydrogen atoms thus escape the gravitational control of the planet, and are directly exposed to various stellar forces.
Another interesting result is that a good part of the absorption is blue shifted, produced by atoms moving away from the planet and the star, with a significant velocity component towards Earth. Thus, a lot of the escaping hydrogen atoms are forming a cometary tail on this planet.
Right Figure: Planet passing in front of the star, and resulting
transit curve showing the absorption of the stellar Lyman alpha by the
planet hydrogen as detected with HST and from the simulation.
Credit: Jean-Michel Desert, IAP, France
Team member Alain Lecavelier des Etangs at IAP has made a simple model of the hydrogen escape from the outer regions of the planet's atmosphere and the formation of the cometary tail. The gravitational attraction of the star diminishes that of the planet. But more importantly, when the hydrogen atoms absorb the stellar Lyman-alpha photons (the same process that produces the transit signature), the atoms experience a radiation pressure that pushes them away from the star thus forming the tail. The tail is skewed because the planet is orbiting around the parent star fast, at a speed of ~140 km/sec.
The simulation is our first step in the modeling of the system. We still have to understand in more detail the properties of this extended, escaping atmosphere, such as the heating and ionization by the strong stellar UV input. We further have to explore the effects of the stellar wind interaction as an additional source of atmospheric escape and the formation of the tail. We also need to make new UV observations, as already proposed for HST. Soon enough we will need a tailored space-borne satellite to do dedicated UV measurements of extrasolar planets.
The phenomenon that produces the detected absorption is resonant
scattering. The H Lyman alpha transition is so strong that multiple
scatterings will take place and Lyman alpha photons will be emitted
from the planetary environment. If we could get close to the HD209458
stellar system and bring a UV camera, we would image an inflated planet
with an extended tail in the scattered stellar Lyman alpha. The same
is seen in comets in our own solar system, as was imaged for comet
Hyakutake with the WFPC2 camera on HST by M. Combi et al. (1998).
For more information on the observations, and to download movies, etc, please go to our team website at
For press releases see:
March 12, 2003: Hubble Space Telescope Science Institute
March 12, 2003: Hubble Space Telescope, European Space Agency
March 12, 2003: Univ. of Arizona UANews.org