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Planetary News: Extrasolar Planets (2007)Water Found on ExoplanetBy Amir AlexanderJuly 11, 2007
“Follow the water” goes the mantra of the search for life beyond Earth, and so far it has led planetary scientists to some very unusual places. To Mars, for example, where the weight of the evidence suggests that primordial oceans lapped ancient shorelines billions of years ago, and to Jupiter’s moon Europa, which harbors a subterranean sea. And now, in an article published in today’s edition of Nature magazine, comes news that traces of water have been detected much further away, on a planet orbiting a star 64 light-years from the Sun. It is the first time that water vapor had been detected with a high degree of certainty on a planet outside our solar system. The planet in question, designated HD 189733b, is not a likely candidate to harbor life. It is an enormous gas giant, 15% more massive than Jupiter, and orbiting a Sun-like star at a distance of only 5 million kilometers (3 million miles) – 30 times closer than Earth is to the Sun. As a result the planet’s average temperature is a scorching 1000 degrees Kelvin (1300 degrees Fahrenheit), making the presence of water-based life as we know it unthinkable. Nevertheless, said Giovanna Tinetti of University College in London (UCL), who is lead author of the Nature article, the detection of water on an exoplanet is big news for the search for extraterrestrial life. “Although HD 189733b is far from being habitable, “she said, “our discovery shows that water might be more common out there than previously thought, and our method could be used in the future to study more ‘life-friendly’ environments. In making the discovery, Tinetti and her international group of colleagues took advantage of the fact that HD 189733b is one of a select group of exoplanets that “transit,” meaning that they pass directly in front of their star as seen from Earth. Whereas most exoplanets are known solely through the wobble they cause their star by their gravitational pull, the fact that HD 189733b also transits provides scientists with additional valuable information about it. In particular, the degree to which the star dims as the planet passes between it and Earth is a strong indication of the planet’s diameter. Significantly, however, a star’s dimming during a planetary transit is dependent on another crucial factor: how much of the starlight is absorbed by the planet’s atmosphere? This is especially the case when the planet in question is a gas giant, which means that its atmosphere encompasses a large portion of its total volume. If the atmosphere blocks out much of the light from the star then it will appear dimmer and, by inference, the planet will appear larger; but if the atmosphere blocks out only a small portion of the light, then the star will appear brighter and the planet’s diameter will be thought smaller. The crucial point for Tinetti and her colleagues was that the atmosphere’s degree of light-absorption is not uniform across the spectrum. Depending on the composition of gasses in the atmosphere, the planet will block more of the star’s light at certain wavelengths and less of its light at others. Each gas, furthermore, has its own signature wavelengths (or spectrum) in which its light absorption peaks. In observing HD 189733b, Tinetti and the other authors, were looking to see if they could detect changes in the planet’s light absorption at different wavelength in the infrared range. If the planet appeared “bigger” at a certain frequency, this meant that it was absorbing more light, whereas if it appeared “smaller” it was absorbing less. If the overall pattern matched the known absorption wavelengths of water vapor, then Tinetti and her colleagues could conclude with a high degree of certainty that the planet’s atmosphere does indeed contain water. To make their measurements, the researchers turned to the only instrument accurate enough to measure the slight discrepancies in the star’s infrared emissions – the Spitzer Space Telescope. They measured the HD 189733b’s apparent size at three different wavelengths – 3.6 microns, 5.8 microns, and 8 microns and found that in the first instance the diameter of the planet was 2.356% the diameter of the star, in the second instance 2.436%, and in the third 2.39%. They then compared their results to a new list of 500 million(!) water absorption lines calculated by two of the papers authors, Jonathan Tennyson and Bob Barber of UCL. As they had hoped, the pattern matched: unmistakably, the atmosphere of HD 189733b contained water vapor. Finding water vapor in the atmosphere of a fiery gas giant is not the same as discovering water on a potentially life-bearing planet. Nevertheless, said Tinetti, it is a crucial step in the right direction. “The ‘holy grail’ for today’s planet hunters is to find an Earth-like planet that also has water in its atmosphere” she said. “Finding water on an extrasolar gas giant is a vital milestone along that road of discovery.” Did you like this article? Send
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