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Since the early 1990's, astronomers have known that extrasolar planets, or "exoplanets,"

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orbit stars light-years beyond our own solar system.

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Because most exoplanets are too far away to be directly imaged, characteristics such as size,

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composition, and atmospheric makeup must be determined through a variety of indirect methods.

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For instance, when an exoplanet passes in front of its star, or transits,

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it blocks a fraction of the star's light and causes a dip in brightness.

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Large planets block more light, so the size of the dip can be used to determine the size of the planet.

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By observing an exoplanet's gravitational pull on its star, astronomers can also determine the planet's mass,

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and thus calculate its density, to see if it is composed of rock like Earth, or gas like Saturn.

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But to fully understand an exoplanet, astronomers must study its atmosphere,

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and the information that they need is encoded during a transit.

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As the planet crosses its star, its atmosphere absorbs certain wavelengths of light, or colors,

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while allowing other wavelengths to pass through.  Because each molecule absorbs distinct wavelengths,

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astronomers spread the star's light into its spectrum of colors to see which wavelengths have been absorbed.

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The dark absorption bands act as molecular fingerprints, revealing the atmosphere's chemical makeup.

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Knowing the depth and density of the atmosphere is also important.

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To figure this out, astronomers observe the transit at many different wavelengths.

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At wavelengths where more absorption occurs, the planet will appear larger,

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with the change in size indicating how deeply the atmosphere extends, and its density at different altitudes.

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Measuring the depth of absorption at each wavelength gives astronomers the planet's transit depth curve,

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which allows them to model the composition, height, and density of the atmosphere, providing a detailed picture of the planet.

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Recent studies suggest that exoplanets and their atmospheres come in a wide variety.

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At one extreme are "hot Jupiters" like WASP 19 b, a boiling gas giant that orbits its star

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far closer than Mercury orbits our Sun.  Visitors who could survive the heat might complain about the air quality: 

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planet WASP 19 b's jagged transit depth curve suggests a deep atmosphere of poisonous hydrocarbons,

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with methane and hydrogen cyanide far more abundant than water.

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By contrast, planet Gliese 1214 b is a comparatively inviting "waterworld."

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Its nearly flat transit depth curve hints at a shallow atmosphere of pure steam,

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enveloping an ocean thousands of kilometers deep, with an interior of hot ice:  water solidified by extreme pressure rather than cold.

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As detection methods improve, astronomers will search the atmospheres of Earth-size planets

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for signs of life such as water vapor, oxygen, and methane,

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taking us one step closer to finding a world like our own, all thanks to some flickering starlight.

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[ music, satellite beeping ]

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[ music, low rumble, wind chimes ]