Water Worlds
Earlier this month, astronomers announced the first clear detection of water in the atmosphere of an alien world orbiting a distant star in the constellation Vulpecula, "the Fox". The star is formally known as HD 189733, and the planet -- which is slightly larger than the planet Jupiter in our solar system -- is affectionately called HD 189733b. Like the detection earlier this year of an extrasolar planet not much larger than the Earth, finding evidence of water from a distance of 63 light years is a landmark in the search for habitable worlds outside of our solar system.
This particular planet is special -- the alignment of its orbit causes it to pass directly in front of its parent star as seen from the Earth. As a consequence, the planet causes a tiny eclipse during each orbit as it blocks a few percent of the starlight from reaching us. How much starlight it blocks depends on the size of the planet, including its atmosphere. That atmosphere acts as a kind of filter -- allowing some colors of light through easily, while blocking some others. The scientists exploited this fact, along with the known absorption properties of water, to measure the size of the planet in several different colors of infrared light. The pattern of absorption in these colors matched the pattern expected for water, and could not be explained by any other common atmospheric molecule.
After the planet stops blocking any starlight, it swings around to the back side of the star. During this segment of its orbit, the planet exhibits phases just like the moon -- showing a crescent until it reaches the largest separation, and then gradually becoming fully illuminated before it slides out of view behind the star. Although astronomers cannot observe the phases directly, they can monitor the tiny changes in reflected light during this part of the orbit. In this way, it was also recently possible to reconstruct a map of the temperature difference between the day and night sides of HD 189733b. The day side of the atmosphere turned out to be more than 200 C degrees hotter than the night side, and the hottest spot actually occurred slightly before noon local time on the planet.
The ultimate goal of such research is to find water and other possible signs of life on a distant Earth-like planet, motivated by the age old question: "Are we alone in the Universe?". With the demonstration of these powerful techniques to study the atmospheres of alien worlds, we are on our way to finding the answer.
