Dry, hot, dusty alien worlds

Bad Astronomy
By Phil Plait
Feb 21, 2007 10:26 PMNov 5, 2019 12:27 AM


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Update (Feb. 21, 2007): Two papers are online describing these results. They're technical, but if you're interested: one on HD 189733b and one on HD 209458b.

Astronomers using the orbiting Spitzer Space Telescope have, for the first time, seen the emission spectrum from planets orbiting another star! This is extremely cool, as you'll see.

First off, a spectrum is what you get when you break up light into its individual colors. Think of it like a rainbow, but with a lot more colors. By carefully examining the brightness of each color, you can tell a lot about the object giving off that light -- for example, its temperature, its chemical composition, and even how it's moving. In this case, astronomers used Spitzer to examine the spectra of two planets in the infrared, light invisible to the naked eye. Many interesting molecules emit infrared light, so it's a good wavelength to investigate. Spitzer was designed to observe in the IR, so a team of astronomers

pointed Spitzer at two stars: HD 209458 (about 150 light years away in the constellation of Pegasus), and HD 189733 (about 60 light years away in the constellation of in Vulpecula). These two stars are known to have giant planets orbiting them. The planets are very close to their stars: they orbit at 4 million and 3 million miles out, respectively-- for comparison, the Earth is 93 million miles from the Sun, so these planets are really close to the stars! The orientation of the planet' orbits is such that every orbit they pass behind their parent stars, creating a planetary eclipse. This allowed the astronomers to use a technique that reveals the planets' spectra. The astronomers observed the stars when the planets were hidden from view, so they got spectra of just the stars. Then they repeated the observations when the planets were next to their stars, yielding spectra of the two together (from Earth, the planet and star appear as a single point of light because they are so far away from us). Once they have both spectra, they can carefully subtract the light from the star form the combined star and planet, leaving just the planet: star + planet - star = planet. Bang! Done.

Well, it's not that easy. I've worked on this kind of observation myself, and it's incredibly painstaking and detailed. It takes a huge amount of work, and my hat is off to these guys who did this observation.

So what did they see? Perhaps the biggest thing is what they didn't see: water! Theoretically, water (actually, steam; these planets are hot) should be present in the planets' atmospheres, and should have been really obvious. Water is excellent at absorbing infrared light (it's a greenhouse gas, after all) and so the presence of water would have been revealed by dips in the spectra at specific wavelengths. They saw nothing like that! Does this mean there is no water in the planets' air? Not necessarily; more likely the water is lower down in the atmosphere, and is covered by high altitude dry clouds of some other chemical, and the light is blocked. This is supported by the detection of silicate dust in the atmospheres. This means the planets have a high layer of dust surrounding them, unlike any planet in our own solar system. So that's pretty interesting as well. Not only that, they found a feature in the spectrum at 7.8 microns they can't identify! That's unusual, and they need to take more observations to try to nail that down. One more thing, and to me it's the most fascinating result they found. The shape of the spectrum can tell you a lot as well. These planets are so close to their parent stars that they are tidally locked

, meaning they always show one face to their star, like the Moon does to the Earth. You might expect that one side of the planet gets incredibly hot while the dark side gets cold. Well, that can be determined by looking carefully at the shape of the planet's spectrum. What they found was that the heat from the day side is somehow being transported to the dark side. The easiest way to do this is through winds. So what we have here are planets more massive than Jupiter yet only a few million miles from their stars. They have one face locked toward their star, and the other forever facing away. They have deeply buried clouds of steam, high altitude bands of dust, and a lot of wind which whips around the planets, warming up the night sides and keeping the atmosphere from literally freezing out. You know, we have names for these objects like "extrasolar planets", and "exoplanets". But maybe we should call them as they are:

alien worlds

. But now, because we're smart, and because we want to learn, they won't stay alien for long. This is the first time in history we've been able to observe alien planets in this way, but it's not the last. We'll find more, and we'll learn more. We always do.

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