The atmospheric properties of distant worlds are becoming increasingly clear. The latest observations reveal fluorescent emission from methane in the upper atmosphere of a Jupiter-like extrasolar planet.

The first extrasolar planet found to be orbiting a Sun-like star was detected less than 15 years ago. As astronomers detect more and more planets orbiting stars other than the Sun in our Galactic neighbourhood, increasing attention is being paid to probing their atmospheres, and for good reason. Planetary atmospheres can be easily altered by geophysical1, photochemical2 and biological3 processes. Owing to the relatively small amount of mass in an atmosphere, its properties can bear the signatures of processes driven from the planetary interior or surface that would not otherwise be observable. A notable example is the presence of the powerful greenhouse gas methane (CH4) in planetary atmospheres.

In Earth's atmosphere, the dominant sources of non-anthropogenic methane are anaerobic bacteria and methanogens, which inhabit wetland and oceanic sediments, and the digestive tracts of some organisms (for example, ruminants and termites). Methanogens, like most archaeal microorganisms, can thrive in a wide range of conditions, including many that would be harmful to the bulk of complex life forms on Earth. Jupiter's methane, meanwhile, is photochemical in origin, and so is dependent on the radiation field incident on the atmosphere and the specific abundances of carbon, oxygen and hydrogen4.

To disentangle the various processes that affect an atmosphere, it is crucial to obtain as much information as possible on a diverse sample of planetary atmospheres. On page 637 of this issue, Swain et al.5 report the discovery of fluorescent emission from methane in the upper atmosphere of a nearby, Jupiter-mass extrasolar planet6, HD 189733b
 

Fluorescence occurs when an atom or molecule absorbs a photon, is excited into a higher energy state and subsequently de-excites, emitting light at lower energies. It requires the relatively low particle densities that occur high in planetary atmospheres, where the time between collisions is longer than the time required for radiative relaxation. Methane, like all molecules, can only make such transitions among specific permitted electronic, rotational and vibrational energy pathways that are unique to its molecular structure. The result is a characteristic spectrum of emission lines, and it is one of these emission lines, distinctive to methane, that has been detected by Swain and colleagues5.

Fluorescent emission is relatively common in astrophysical environments. It has been detected, for example, in the accretion disks surrounding supermassive black holes7, in the interstellar medium8 and in comets9. Specifically, the fluorescent methane emission detected by Swain et al.5 on HD 189733b has also been observed in the atmospheres of Jupiter, Saturn and Titan10. Fluorescence of other organic compounds has been detected on Venus and Mars11. These detections provide a probe of the physical structure of the upper atmosphere of these planets from the perspective of a minor atmospheric constituent. Methane is particularly important because it may help us to find and evaluate possible biological influences on extrasolar planetary atmospheres.

The upper atmosphere is a fascinating and important region where minor molecular constituents, such as methane, can play an indispensable part in establishing the overall heat budget of a planet, thereby altering the thermal profile of a considerable portion of the atmosphere. Powerful winds and vertical mixing of high-altitude atmospheric layers present the possibility for temporal and spatial variability of the fluorescent emission from such molecules. In addition, ionized particles in the upper atmosphere are affected by any global magnetic field — as is dramatically exemplified on Earth by another form of emission-line radiation, the aurorae. Because HD 189733b is extremely close to its host star (less than one-tenth the distance between Mercury and the Sun), energetic particles from its star will interact with any magnetic field the planet may have, possibly resulting in stronger emission-line displays than we see on Earth or Jupiter. Many other observed extrasolar planets are similarly close to their host stars, so variability in emission components because of magnetic effects may be common in these systems.

Swain and colleagues' detection5 of a fluorescent emission line of methane on HD 189733b paves the way for future observations of alternative fluorescent emission lines of methane and other molecules in extrasolar planetary atmospheres. These observations will require sensitive, high-spectral-resolution instruments that can resolve the emission-line profile of fluorescent signals. In addition, this discovery highlights a crucial theme in modern astronomy: the necessity of making complementary observations from the ground and from space. Depending on the wavelengths of the relevant spectral features, Earth's own atmosphere may or may not interfere with detection. The authors' observations5 were made with the 3.0-metre NASA Infrared Telescope Facility located at the summit of Mauna Kea in Hawaii. Our increasing understanding of this one extrasolar planet — from its discovery6 to the growing list of known constituents of its atmosphere, which includes sodium12, carbon monoxide13, carbon dioxide13, water vapour13 and methane14 — has been possible only with observations from both the ground and space.

During the past few years, we have made a transition that deserves some rumination. Rather than speculating about the possibility of other worlds in the cosmos, we can now identify them specifically and enumerate their various characteristics. Many stars that are readily visible to the naked eye, at least from relatively dark sites that are not heavily polluted by artificial light, have planets orbiting them, the masses and orbital characteristics of which we know. A number of other worlds, soon to be discovered, will be small and rocky like Earth, and will have atmospheres that we can detect, inventory and monitor. It is quite possible that, within our lifetimes, atmospheric studies of these extrasolar planets will provide the first evidence of biological life beyond Earth. Swain and colleagues' detection of the fluorescence of a hydrocarbon in the upper atmosphere of an extrasolar planet not only provides insight into the structure of the atmospheres of other worlds, but is also an important step in the far-reaching journey to uncover what may be below them