Our research team based at the University of Arizona's Lunar and Planetary Laboratory and the Institut d'Astrophysique de Paris is excited to announce on January 31, 2007 the discovery of hot hydrogen in the atmosphere of the extrasolar planet HD 209458b. Our paper describing this work is a letter in the scientific journal Nature, published in the February 1, 2007 issue.

Below is some background information and explanation of this discovery.

Extrasolar Planets

In just the last thirty years astronomers have learned that about 200 stars in the Sun's neighborhood have planets. Planets orbiting stars other than our Sun are called extrasolar planets; most are gas giants similar in size to Jupiter, the largest planet in our Solar System. Surprisingly, about ten percent of these extrasolar planets are "hot Jupiters" which orbit very close to their parent stars, usually closer to the star than the planet Mercury is to our Sun! The atmospheres of these planets are heated by the immense irradiation by the parent star, unlike much cooler Jupiter, which is almost 484 million miles from the Sun, or five times the distance from the Earth to the Sun.

A Hot Jupiter in Pegasus

A very interesting and well studied extrasolar planet is called HD 209458b, a "hot Jupiter" which orbits around an Sun-like star in the constellation of Pegasus, the winged horse. This planet was first discovered in 1999 by G.W. Henry of Tennessee State University and G. Marcy of the University of California at Berkeley and marked the first time in history that astronomers witnessed an extrasolar planet passing in front of its parent star. When they measured the decrease in brightness of the star as the planet moved across its bright disk, they proved that extrasolar planets do exist, a fact which had been strongly suspected but not proven up to that point. Scientists had measured "wobbles" in the paths of some stars before this confirmation, which they postulated was due to planets' gravitational tugs on their parent stars. Note that when a planet passes across the disk of a star, it is said to be in transit. In our own Solar System, the planets Venus and Mercury sometimes transit across the Sun, which may be observed by amateur astronomers. However, HD 209458b is so distant that astronomers cannot take an image of the planet's shadow directly, but instead can measure how much the star dims as the planet is in transit and passes over the star's bright disk.

This hot-Jupiter planet is in orbit around a star listed as HD 209458 in the Henry Draper star catalog, and is located in the Pegasus constellation. The stellar coordinates of the sky are:

Right Ascension: 22h 03m 10.8s
Declination: +18° 53' 04"

HD 209458 is also known as V376 Pegasi. This is an F9-G0 type of star that is very similar to our Sun (a G2 star), and is about 150 light years from Earth. You cannot see the star with the naked eye since its apparent magnitude from Earth is +7.65, but it is easily seen with binoculars or a modest telescope. The planet orbiting the star, however, cannot be resolved (seen) with an amateur astronomer's equipment. Even the Hubble telescope cannot resolve the planet visually!

About one-third larger than Jupiter (its radius is 1.32 times Jupiter's), HD 209458b contains about two-thirds of the mass of Jupiter. It orbits very close to its parent star, only at a distance of only 9 times the radius of the star! It is about ten times closer to its parent star than tiny Mercury is to our Sun.

Like Jupiter and other gas giants, HD 209458b is made up mostly of hydrogen gas. Though it is similar in size and composition to Jupiter, the immense heat from the nearby star causes a hydrodynamic state in the planet's atmosphere, with the layers of the upper atmosphere subject to outward, fluid motion. This heat energy causes sizeable expansion of the planet's upper atmosphere and causes hydrogen gas to escape. The expanded gas forms a large sphere around the planet, similar to the coma on a comet. Some hydrogen atoms around the planet are then pushed away from the star by radiation pressure, which forms a comet-like tail, trailing behind the planet as it moves in its orbit.

Hot Hydrogen In A Gas Giant Planet

You may ask, "If we can't directly observe this hot-Jupiter planet, how is it studied?" We learn about HD 209458b the same way that many discoveries about distant stars are made: studying the light from the star in detail by analyzing its spectrum. If you've ever used a prism to split sunlight, the rainbow of colors created is a spectrum of light from the Sun in the visible wavelengths. Different elements, such as hydrogen and helium, cause specific patterns in a spectrum and allow the different elements to be detected and analyzed.

For this discovery of hot hydrogen in HD 209458b, spectra of the star in the ultraviolet and visible wavelength regions were studied using archival data obtained by the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope, both when the planet was in transit and also when it was away from the star. By comparing the two spectra, the planet's transit spectrum was extracted. This planetary spectrum in the violet and near-ultraviolet showed a feature discovered by our team called the Balmer Jump and continuum, which is usually produced in stars by the presence of hot hydrogen.

Hot-Jupiters have different layers in their atmospheres, like the Earth and other planets. Previous theoretical studies and observations by the Spitzer space telescope have indicated that the hot-Jupiter's "cold layer" in the lower atmosphere is about 1,200 Kelvins in temperature, at least on the dayside where it faces its star. Previous HST observations require a hot extended upper atmosphere. Predictions obtained from hot-Jupiter computer models by Dr. Roger Yelle (University of Arizona) and others describe the layers of hot hydrogen.

The Balmer absorption we have discovered is from hot (excited, n=2) hydrogen in a layer that is about 1,000 km thick, about 5,000 K in temperature, and at an altitude of about 8,500 km from the planet. Above that layer is where the atmosphere is very extended and reaches temperatures estimated from computer models to be 10,000 K (17,400° Fahrenheit) or hotter and hydrodynamically escaping (A. Vidal-Madjar, et. al, 2003 and 2004; A. Lecavelier, et. al, 2004; R. Yelle 2004 and 2006).

In the hot hydrogen layers of the planet's upper atmosphere, the temperature rises with altitude due to the irradiation of the star. However, the higher the altitude, as the atmospheric layers expand, the hot hydrogen becomes distributed over more space and become fewer and fewer, or less dense in space. The layer with maximum Balmer absorption occurs where the increasing temperatures and population density create the largest amount of hot, excited hydrogen atoms.

Temperatures this hot had only been detected in the atmospheres of stars previously. The outer layer of our Sun, called the photosphere, is about 11,000° Fahrenheit, for example, similar to the temperature of the planet's hot hydrogen layer detected. (In this example it is important to remember that stars are complex, and the layers of stars vary in temperature depending on where and how you are measuring it.)

Our team's discovery of hot hydrogen is very interesting and unexpected, and will also help to probe the atmospheres of these hot-Jupiter planets, by placing empirical constraints on the lower part of the thermosphere, the hot and thin outer layers of the planets. In other words, since we cannot directly probe all of the layers of this planet's atmosphere, we can now detect the Balmer absorption from hot hydrogen in a hot-Jupiter's transit spectrum, which tells us about one layer of the atmosphere and helps indicate how much energy the planet absorbs from its star. Using this measured information, computer models of the atmosphere can help fill in the unknown details of the other layers of the atmosphere.