A mere 60 light-years away, orbiting an orangish star called HD 189733, is a world an Earthling would not want to visit. The planet is a gas giant, like Jupiter or Saturn, but unlike those familiar worlds this one hugs tightly to its host star, orbiting at about one thirtieth the distance at which Earth circles the sun. The exoplanet, labeled HD 189733 b by astronomical convention, stays mighty toasty under its astronomical broiler, with temperatures upward of 900 degrees Celsius.
Thanks to a new study, any hypothetical unfortunates forced to visit HD 189733 b will know which part of the planet is the most infernal. A trio of researchers at the University of Washington in Seattle (U.W.), Columbia University and Northwestern University has produced a thermal map of the planet's atmosphere in both latitude and longitude. Their research appears in the March 10 The Astrophysical Journal Letters.
The map is somewhat crude, but that is no surprise given that HD 189733 b cannot even be seen in the conventional sense. As is the case for most of the 750-plus exoplanets that astronomers have identified to date, its presence and properties are inferred from indirect observations—for instance, by monitoring how much starlight the planet blocks when passing in front of its parent star.
Five years ago, astronomers used the Spitzer Space Telescope to chart the longitudinal difference in thermal emission from HD 189733 b—the dayside versus the nightside—by monitoring the infrared radiation coming from both the star and the planet. By tracking changes as the planet moved through its orbit, the researchers were able to extract not only how much infrared radiation the planet emitted, but also how much came from individual longitudinal bands of the body.
HD 189733 b is tidally locked, meaning that the same hemisphere faces the star at all times, much the same way that the same side of the moon always faces Earth. "After watching it for half an orbit, then we've gotten to see all of its sides," says Eric Agol, a U.W. astrophysicist who contributed both to the 2007 mapping effort and the latest study. "But we don't know where the light is coming from as a function of latitude."
To get a two-dimensional picture, Agol and his colleagues used Spitzer to track HD 189733 b as the planet disappeared behind its star, and then as it reemerged. Because those so-called secondary eclipses occur high on the face of the star, the edge of the star blots out the planet at an angle. That allowed Agol and his colleagues to estimate the thermal brightness of various slanted bands of the planet as HD 189733 b passed behind the star, and as the planet reemerged. "As the planet is passing behind the star, you know how much of the planet should have disappeared," Agol says. "You can compare that to how much flux disappears in that time, and we then can combine that to make a two-dimensional picture of the planet."
In the picture gleaned from seven secondary eclipses, the planet was confirmed to have a significant hot spot, as would be expected on the dayside, under the continuous glare of the star. But that hot spot is shifted along the equator just a bit, perhaps because of the fierce winds that are expected to exist there. Climate models of HD 189733 b have predicted that winds of several kilometers per second may race along the equator. "One hallmark of that band of wind is that the hot spot shouldn't be directly at the substellar point, at high noon," says planetary astronomer Heather Knutson of the California Institute of Technology, who did not contribute to the new study. "That hot gas actually gets carried a little bit downwind."
Knutson, who worked on mapping HD 189733 b in 2007, notes that the two maps pinpoint a similar location for the hot spot. "The really reassuring thing is that by using this different approach they come to a very similar conclusion," she says. Agol's group also revealed that the hot spot is located near the equator, as had been assumed in the one-dimensional longitudinal map. "The nice thing about their approach is that you can start to get some of that north-south information," Knutson says. "It's a really great shortcut to get around the fact that most of these planets are so far away that you can't really see them."
Unfortunately, few other known planets are as favorably positioned for such detailed study. For starters, HD 189733 b is closer to our solar system than all but a few known exoplanets. And it is a giant, about 14 percent larger in diameter than Jupiter, orbiting next to a host star that is about 20 percent smaller than the sun. Those dimensions, coupled with the planet's extreme heat, make for a relatively strong thermal signature that can be isolated from the radiation given off by the star alone. But future infrared observatories, such as NASA's massive James Webb Space Telescope, may be able to map additional exoplanets.
For now, the map that Agol calls a "first stab" at the secondary-eclipse technique is also an impressive résumé booster for Carl Majeau, an undergraduate student and the lead author of the new study. Majeau, who hails from Seattle, approached Agol for a project between school years at Columbia. "This was an idea that I thought would be cool to do with the data, but I just didn't have time to do it," Agol says. "At that point I think he had just finished his freshman year at Columbia, and he wanted a summer job."