Astronomers have also mapped the surface temperature of one of those next-to-hottest planets, the larger and less dense HD 189733 b. They conclude that winds are evening out its day and night temperatures by stirring together hot and cold gas.
For HD 149026 b to reach such blistering heat, researchers say, it must suck up nearly all the energy it receives from its big bluish star. If so, the gaseous planet could be nearly pitch-black in color. Experts cannot fully explain the planet's intense heat, but they speculate that it may have something to do with its unusually high concentration of heavy elements.
"This is a weird planet, and this is yet another weird thing about it," says planetary scientist Joseph Harrington of the University of Central Florida in Orlando.
The two planets are among 17 or so known to transit in front of their stars as viewed from Earth. Dubbed hot Jupiters for their typical size and closeness to their stars, they always present those stars with the same face. By comparing the intensity of infrared starlight as one of the gas giants goes behind and front of its star, researchers can deduce the temperatures of its day and night sides.
Harrington and colleagues used the Spitzer Space Telescope (SST) to gauge the daytime temperature of HD 149026 b's upper atmosphere in this way. Infrared data had pegged three other exoplanets, including HD 189733 b, in the 1,000-to-1,200-kelvin range, which implied that the planets reflected about 30 percent of incoming starlight.
To reach 2,300 kelvins, HD 149026 b must have zero reflectivity, or albedo, say Harrington and colleagues in a report published online today by Nature. Moreover, it must radiate energy back into space as quickly as it receives it. The atmosphere would heat up by absorbing blue-white starlight, radiating out lower energy infrared light, and pocketing the energy difference.
Harrington says the team has debated what exactly the planet would look like. Zero albedo is "blacker than coal," he says, but the infrared light could spill into red where the heat is strongest. He says he pictures a deep-black planet glowing like an ember at the region closest to the star [see image above].
HD 149026 b was already an oddball among hot Jupiters for its Saturn-like size and mass and its high density. More than two thirds of the planet must consist of elements heavier than helium, which are uncommon in gas giant planets and may introduce unexpected compounds into the atmosphere that contribute to the still mysterious total absorption, Harrington says.
A separate team trained the SST on HD 189733 b for 33 hours of its 2.2-day orbital period, giving them a map of its surface heat. In a second Nature paper they report that its day- and night-side temperatures were relatively similar—1,200 and 970 kelvins (about 1,700 and 1,285 degrees F), respectively—and that the day side's hottest spot did not face the star dead-on but was offset by 30 degrees longitude [see image above].
Both features are signs of extreme wind speeds of perhaps 5,000 to 6,000 miles per hour, says astronomer Heather Knutson, a graduate student at the Harvard-Smithsonian Center for Astrophysics. "It tells us that winds are kind of shifting things around in the atmosphere," she says.
The size and mass of HD 189733 b is average among hot Jupiters, Knutson says, but time will tell if such strong winds are also typical.
The researchers say their next goal is to study the planets at other infrared frequencies in order to get more accurate readings and possibly learn about their atmospheres' constituents. As Harrington notes, "we're at the beginning of understanding how planets handle their heat."