Look at the Exoplanet timeline: http://www.nasa.gov/externalflash/PQTimeline/
To astronomers, a "potentially habitable" planet is one that could sustain life. Habitability depends on many factors, but having liquid water and an atmosphere are among the most important. Thus determining the characteristics of the atmospheres of exoplanets and the possible implications of these characteristics on habitability are key areas of research in astrobiology.
A history of the detection of exoplanet atmospheres is brief because the technology that it takes to do this work has been recently invented and applied. Visible-light telescopes can detect exoplanets and determine certain characteristics, such as their sizes and orbits, but not much can be inferred about their atmospheres or what they look like. Therefore, exoplanet atmospheric studies was largely pioneered in 2005 by Spitzer, when it became the first telescope to directly detect photons from an exoplanet by examining their infrared light and using spectrometry to detect the molecules present in the exoplanet atmospheres by their unique ‘spectral fingerprint.’ Since then, Spitzer, along with NASA's Hubble Space Telescope, has studied the atmospheres of several hot Jupiters (very hot, giant gas planets that orbit very closely to their star), finding water, methane, carbon dioxide and carbon monoxide. The first near-infrared emission spectrum obtained for an exoplanet was that of HD 189733b.
The Hubble telescope was conceived primarily for observations of the distant universe, yet it is also important for exoplanet atmospheric studies because we can use Hubble's near infrared camera and multi-object spectrometer to study infrared light emitted from planets. Gases in the planet's atmosphere absorb certain wavelengths of light from the planet's hot glowing interior and the molecules leave a unique spectral fingerprint on the radiation from the planet that reaches Earth. This observation is best done on planets with orbits tilted edge-on to Earth because of the usefulness of eclipses. The eclipses allow an opportunity to subtract the light of the star alone, when the planet is blocked, from that of the star and planet together prior to eclipse. That isolates the emission of the planet and makes possible a chemical analysis of its atmosphere. Searching for molecules in exoplanet atmospheres is important because molecules present are expected to: (1) influence strongly the balance of atmospheric radiation, (2) trace dynamical and chemical processes and (3) indicate the presence of disequilibrium effects. Thus searching for molecules is a high priority because they have to potential to
reveal atmospheric conditions and chemistry which will help us infer the habitability of the planet.
05.09.07, “NASA Finds Extremely Hot Planet, Makes First Exoplanet Weather Map.”
On this date, researchers using NASA's Spitzer Space Telescope learned what the weather is like on two distant, exotic worlds—giant gas planets: HD 189733b and HD 149026b. They mapped the weather patterns on HD 189733b by using the Spitzer telescope to measure the infrared light coming from the planet as it circled around its star and from this, they created a map of the temperature of the entire surface of the planet based from a quarter of a million data points. They found that the planet is likely whipped by roaring winds and that temperatures are fairly even considering the planet is tidally locked: 650 degrees C (1,200 F) on the dark side to 930 degrees C (1,700 F) on the sunlit side. It is assumed that many hot Jupiters are tidally locked so it is interesting that the planet's overall temperature variation is mild and at the time, scientists believed winds must be spreading the heat from its permanently sunlit side around to its dark side. Such winds occur at up to 9600 kph (6,000 mph); in comparison, jet streams on Earth travel at 322 kph(200 mph). This is interesting because now we are beginning to see how these hot Jupiters deal with the incredible amount of energy blasted at them—20,000 times more energy per second than Jupiter. Also of interest, HD 189733b has a warm spot 30 degrees east of ‘high noon.’ Assuming the planet is tidally locked to its parent star, the researchers at the time suggested that this implies that fierce winds are blowing eastward.
The main interesting finding regarding HD 149026b was that it was the hottest planet found to date at a scorching 2,038 degrees C (3,700 F), which is even hotter than some low-mass stars. This suggests that the heat is not being spread around in contrast to HD 189733b. HD 149026b is the smallest and densest known transiting planet, with a size similar to Saturn's and a core suspected to be 70 to 90 times the mass of Earth. It also probably reflects almost no starlight, instead absorbing all of the heat which means HD 149026b might be the blackest planet known, in addition to the hottest.
The near-infrared transmission spectrum of the planet HD 189733b, described above, shows the presence of methane and water vapor. This is the first time that water was found to exist in the atmosphere of an exoplanet. The discovery of methane was interesting because, on thermochemical grounds, carbon monoxide is expected to be abundant in the upper atmosphere of hot-Jupiter planets like HD 189733b. The detection of methane rather than carbon monoxide in such a hot planet could signal the presence of a horizontal chemical gradient away from the permanent dayside, or it may imply an ill-understood photochemical mechanism that leads to an enhancement of methane. The Hubble Space Telescope also discovered carbon dioxide in the atmosphere. This marked an important step toward finding chemical biotracers of extraterrestrial life even though in this instance, HD 189733b is way too hot for life to be feasible. But because organic compounds can be a by-product of life processes, their detection on an Earth-like planet someday may provide the first evidence of life beyond our planet.
In contrast to the larger than expected amount of methane found on HD 189733b, GJ 436b has very little methane. This is peculiar because models of planetary atmospheres indicate that any world with the common mix of hydrogen, carbon and oxygen, and a temperature up to 1,000 K (1,340 degrees F)—cooler than HD 189733b—should have a large amount of methane and a small amount of carbon monoxide. However, at about 800 Kelvin (or 980 F), GJ 436b does not. This again demonstrates the lack of scientific understanding of atmospheres of exoplanets and also the diversity of exoplanets that exist.
There have been various models created to better understand the possible atmospheric characteristics of different exoplanets. One type of planet that has been modeled is that of planets occurring in the habitable zone of red dwarfs. Planets within the habitable zones of M dwarfs are likely to be synchronous rotators (tidally locked with a fixed light and dark side). Scientists have made three-dimensional simulations of the atmospheres of such planets and determine that near the ground, a thermally direct longitudinal cell exists, transporting heat from the dayside to the nightside. The circulation is three-dimensional, with low-level winds returning mass to the dayside across the polar regions and higher up, the zonally averaged winds display a pattern of strong super-rotation due to these planets' finite (albeit small) rotation rate. The main claim is that planets orbiting M stars can support atmospheres over a large range of conditions and, despite constraints such as stellar activity, are very likely to be habitable.
Simulations of atmospheres of Earth-like aquaplanets that are tidally locked to their star have also been created and described. These models illustrate how planetary rotation and insolation distribution shape climate. In these models, the winds are ‘approximately isotropic and divergent at leading order in the slowly rotating atmosphere but are predominantly zonal and rotational in the rapidly rotating atmosphere.’ Furthermore, ‘free-atmospheric horizontal temperature variations in the slowly rotating atmosphere are generally weaker than in the rapidly rotating atmosphere.’ A curious result is that the surface temperature on the night side of the planets does not fall below ∼240 K in either the rapidly or slowly rotating atmosphere which means that heat transport from the day side to the night side of the planets efﬁciently reduces temperature contrasts in either case. This creates a relatively mild temperature difference across even a tidally locked aquaplanet. Also described is the distribution of winds, temperature, and precipitation, with rotational waves and eddies of primary influence in the rapidly rotating atmosphere and simpler divergent circulations influential in the slowly rotating atmosphere. Both the slowly and rapidly rotating atmospheres exhibit equatorial superrotation which varies non-monotonically with rotation rate, however, the surface temperature contrast between the day side and the night side does not vary strongly with changes in rotation rate.
In the near future, astronomers look forward to using the James Webb Space Telescope (to be hopefully completed around 2014) to look spectroscopically for biomarkers on a terrestrial planet near the size of Earth. The Webb telescope should be able to make much more sensitive measurements of these primary and secondary eclipse events than the technology currently available which will hopefully bring us a much greater ability to detect and characterize exoplanet atmospheres.