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Planetary News: Extrasolar Planets (2007)Spitzer and Hubble Join Forces to Decipher Distant World’s AtmosphereBy Amir AlexanderDecember 14, 2007
As the list of known planets orbiting distant suns has expanded, our knowledge of what these distant worlds are actually like has not kept pace. For most of the 200 odd planets detected so far, the radial velocity measurements tells scientists only the approximate mass of the planet and its distance from its star. In a few cases in which a planet “transits” in front of its star, as viewed from Earth, researchers have also been able to deduce its diameter, and hence density. But that’s all. The planets are too small, too dark, and too distant to be observed directly, and so scientists are left with this rudimentary information. In the case of one faraway world, however, this situation is beginning to change. Two recent papers relying on the highly imaginative use of space telescopes, have begun to lift the veil from the tightly held secrets of a planet known as HD 189733b. Together, the two articles offer scientists something truly rare in the field of exoplanet studies: real verifiable data on the composition of a distant world. HD 189733b is not a world any of us would wish to visit. A gas giant slightly larger than Jupiter, it orbits a star about three quarters the mass of our Sun, 64 light years away. Unlike our stately Jupiter, which takes twelve years to make its stately journey around the Sun, HD 189733b moves at a dizzying speed, completing each orbit in a frantic 2.2 days. Since the planet is so close to its star, it is also extremely hot, with temperatures in its gassy atmosphere averaging 1000 degrees Kelvin, or more than 700 degrees Celsius. Fortunately for astronomers HD 189733b is a “transiting” planet, its orbit taking it through the line of site between Earth and its star. Since the planet is large, and the star relatively small, this causes a dip of 2.4% in the star’s measured luminosity every 2.2 days. Such a substantial dip occurring frequently and regularly has made HD 189733b into a favorite among scientists trying to squeeze as much information as possible about the characteristics of distant planets from a very limited data set. Already last July a group of scientists led by Giovanna Tinetti of University College, London, published a paper in the journal Nature announcing the detection of water vapor and other gasses in HD 189733b’s atmosphere. Making use of the Spitzer Space Telescope, which observes in the infrared portion of the spectrum, Tinetti and her colleagues made repeated observations of the star HD 189733, noting the extent to which it dimmed during the planet’s transit. Since this temporary darkening is caused by the passage of the planet across the face of the star, the degree of dimming provides scientists with a strong indication of the planet’s size. The darker the star during transit, the larger the planet must be. Tinetti and her colleagues noted however, that the measured size of HD 189733b differed, depending on the precise wavelength at which the observations were made. This is because the planet’s atmosphere absorbs some of the light that emanates from the star. If it absorbs a lot, the star appears darker, and the planet, by implication, larger; if it absorbs less, the star appears more luminous, and the planet is, by implication, smaller. Now the various gasses in the atmosphere absorb more light at certain wavelengths than at others, causing the planet to appear larger or smaller accordingly. By analyzing the precise wavelengths at which absorption took place, Tinneti’s group was able to conclude that HD 189733b’s atmosphere contained significant amounts of water vapor, potassium, and sodium. If Tinetti’s model for the atmosphere was correct, observing the planet in the visible range of the spectrum would clearly show the presence of these three ingredients. To follow up on this prediction, Frederic Pont of the Geneva University Observatory and an international group of astronomers set out to observe the planet’s transits in the visible range of the spectrum. Using the Hubble Space Telescope’s Advanced Camera for Surveys (ACS) they recorded the luminosity of HD 189733 in minute detail at wavelengths ranging from 550 to 1050 nanometers. They then subjected the results to a rigorous analysis, eliminating such factors as the camera’s own systematics and giant dark starspots on the surface of HD 189733, which might have influenced the data. In the end they were left with a light curve representing the star’s true luminosity as the giant planet passed before it.
It was not what they had expected. Pont and his colleagues found not a trace of the water vapor, sodium, and potassium predicted in Tinetti’s study. In fact, Pont’s measurements with the ACS indicated that HD 189733’s spectrum during the transit was flat, meaning that it did not show strong absorption at any wavelength. Furthermore, the star appeared measurably darker than it did to Tinetti’s infrared measurements. This means that when viewed in the visible range, the planet’s diameter measured up to 1000 kilometers more than it did in the infrared range. In other words, when viewed by the Hubble Space Telescope in the visible range of the spectrum , the gas giant HD 189733b appeared both more opaque and larger than it did to the Spitzer’s infrared observations. What could explain such inconsistent findings? The answer, according to Pont, can be found in our very own solar system. The planet Venus and Saturn’s moon Titan both have thick atmospheres that are nearly impenetrable when viewed with visible light. The light from the Sun is reflected right off the top of these worlds’ atmosphere, its spectrum unchanged, and to all appearances – flat. But these worlds are observed in the infrared range, the spectrum tells a very different story. Penetrating much deeper than the visible light, certain wavelengths of the infrared light are absorbed by the various gasses in the atmosphere. When the light is reflected back, the gaps in its spectrum tell the tale of the atmosphere’s composition. Planetary scientists today have a good understanding of why Venus and Titan reflect visible light but allow infrared to penetrate deep into their atmosphere. The cause, simply put, is that the two worlds are coated with a thick haze, composed of minute particles floating in the upper atmosphere. These tend to block out light at wavelengths that are shorter than the particles’ own diameter, but are quite transparent to the longer wavelength in the infrared range. Since the atmosphere of HD 189733b, like that of Venus and Titan, is also permeable to infrared light but opaque at the visible range, it stands to reason that the cause in three all cases is the same. Like Venus and Titan, this hot Jupiter 64 light-years away, is hazy. Although this result was quite unexpected in the case of HD 189733b, it is, in fact, consistent with current theories of composition of extrasolar planets. Models of the atmospheres of hot Jupiters predict that silicates and iron compounds, and possibly other compounds as well, can form into solid particles that might circulate in the upper atmosphere. This is evidently the case for HD 189733b. The emerging picture of the atmosphere of HD 189733b is quite complex. Tinetti’s studies with the Spitzer Space Telescope clearly pointed to the presence of water vapor, potassium, and sodium, as well as methane, ammonia, and carbon monoxide. Surrounding this rich mix is a haze of tiny particles, consisting most likely of condensates of iron, silicates, and aluminum oxide. Nothing remotely like this rich detail is available for any other world outside the solar system. “HD 189733b is the first extrasolar planet for which we are piecing together what it really looks like,” said Pont. Did you like this article? Send
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