We’re learning more and more about HD 189733b, an extrasolar planet some 63 light years from Earth in the direction of the constellation Vulpecula. This transiting ‘hot Jupiter’ orbits once every two days about three million miles out from its primary. David Charbonneau (Harvard-Smithsonian Center for Astrophysics) and team recently measured an unusual spectrum from the planet’s atmosphere using the Spitzer Space Telescope. Looking for water, carbon dioxide and methane, they found instead a flat spectrum that Charbonneau thinks may indicate the presence of dark silicate clouds.
Now we have further work, this time using Hubble Space Telescope data, that points to the presence of haze in the atmosphere of HD 189733b. That’s an interesting finding to which we can add another result: Studying how light varies when the planet makes its transit indicates that this world has neither Earth-sized moons or a discernible ring system. Moreover, we’ve got a fairly good read on the temperature of its atmosphere, a toasty seven hundred degrees Celsius. That’s a lot to learn about a place we can’t image with the best equipment we possess.
Like Charbonneau’s team, the group studying the Hubble data, led by Frédéric Pont (Geneva University Observatory), knew what they were looking for. But there were no signatures characteristic of sodium, water or potassium. Weighing what they found in the entire planetary spectrum, the scientists infer that high level hazes about 1000 kilometers in altitude are present. Likely haze constituents are tiny particles of condensates of iron, silicates and aluminum oxide dust. Another finding: A starspot on HD 189733b’s surface thought to be over 80,000 kilometers across.
Pont points to the significance of the work in terms of future goals:
“One of the long-term goals of studying extrasolar planets is to measure the atmosphere of an Earth-like planet. This present result is a step in this direction. HD 189733b is the first extrasolar planet for which we are piecing together a complete idea of what it really looks like.”
True enough, but let’s be cautious. I can recall when we had a pretty good idea of what the Jovian moons looked like. Then Voyager changed everything, and we realized that the outer planets weren’t simply orbited by small worlds much like our Moon. The stunning variety that exists around Jupiter alone reminds us to hedge our bets when weighing data from so much further out. Still, this work, which studies starlight passing through the atmosphere of a giant planet in transit, is a marker of how we’re learning to use existing equipment to begin filling in planetary details. Bit by bit, HD 189733b is getting to be known. The unanswered question is, how many surprises does it hold in store for the future?
Addendum: The lack of detected sodium in the atmosphere of this world, as one reader has already pointed out (thanks, Luis!), seems to contradict the recent findings from the HET instrument in Texas, discussed in this earlier Centauri Dreams article. Comments?