Spitzer has been observing the exoplanets HD 209458b and HD 18933b as they periodically pass in front of its orbiting star (what is called a transit) and behind its orbiting star (what is called a secondary eclipse). This transit/secondary eclipse is unusual because most exoplanets do not orbit their host star at an angle that is nearly edge-on to the Earth’s line-of-sight. When the exoplanet passes behind its host star, the system's light output slightly dims because of the reduced amount of light that reaches the Earth during this secondary eclipse. Thus, astronomers can measure the difference in the amount of infrared light coming to the Earth with respect to when a planet goes behind its host star and when it is in front of the star.

HD 209458b is a gas-giant extrasolar planet (probably around the size of the planet Jupiter) that orbits its Sun-like star HD 209458 in the constellation Pegasus. It is about 150 light-years (904 trillion miles) from the Earth’s solar system at a distance of about 7 million kilometers (4 million miles) from its host star. The planet is eight times closer to its star than the planet Mercury is about the Sun.

The exoplanet is called a “hot Jupiter” because of its closeness to its star. Spectroscopic studies discovered the exoplanet orbiting HD 209458 on November 5, 1999. Informally called Osiris, the exoplanet was the first transiting extrasolar planet discovered—and now the first extrasolar planet known to possess an atmosphere. Scientists believe it contains an evaporating hydrogen atmosphere that contains oxygen and carbon.

HD 18933b, also known as a “hot Jupiter” planet because of its close orbit about its host star HD 18933, is one of 14 known exoplanets that transits its orbiting star. (About 200 exoplanets are known to exist.) It was discovered in 2005 to be about 61 light-years (370 trillion miles) away in the constellation Vulpecula. High winds on the planet are believed to move heat from the side facing its orbiting star to the cooler dark side of the planet.

So far, independent scientific observations of the two exoplanets have found that they are more dry and cloudy that earlier thought. No water was found, nor was any normally associated molecules such as carbon dioxide or methane found. If water is present, then it might be beneath the thick, dark, waterless clouds surrounding both exoplanets. Some scientists think that these clouds might be composed of dust or tiny sand grains called silicates.

NASA scientist Jeremy Richardson (Goddard Space Flight Center in Greenbelt, Maryland), who has been researching this phenomenon associated with HD 209458b, said, “It is virtually impossible for water, in the form of vapor, to be absent from the planet, so it must be hidden, probably by the dusty cloud layer we detected in our spectrum.”

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Astronomy  NASA  Research  Science  Spitzer  William Atkins