Extremely hot exoplanet

Using Spitzer, NASA's infrared space telescope, Harrington and his team observed the tiny planet disappear behind its star and reappear. Although the planet cannot be seen separately from the star, the dimming of the light that reached Spitzer told the scientists how much light the hot planet emits. From this they deduced the temperature on the side of the planet facing its star.
Discovered in 2005, HD 149026b is a bit smaller than Saturn, making it the smallest extrasolar planet with a measured size. However, it is more massive than Saturn, and is suspected of having a core 70-90 times the mass of the entire Earth. It has more heavy elements (material other than hydrogen and helium) than exist in our whole solar system, outside the Sun.

There are more than 230 extrasolar planets, but this is only the fourth of these to have its temperature measured directly. It is simple to explain the temperatures of the other three planets. However, for HD 149026b to reach 2,040 degrees Celsius (3,700 degrees Fahrenheit), it must absorb essentially all the starlight that reaches it. This means the surface must be blacker than charcoal, which is unprecedented for planets. The planet would also have to re-radiate all that energy in the infrared.

"The high heat would make the planet glow slightly, so it would look like an ember in space, absorbing all incoming light but glowing a dull red," said Harrington.

Drake Deming, of NASA's Goddard Space Flight Center in Greenbelt, MD, and a co-author of the Nature paper, thinks theorists are going to be scratching their heads over this one.

"This planet is off the temperature scale that we expect for planets, so we don't really understand what's going on," Deming said. "There may be more big surprises in the future."

Image Credit: NASA/JPL-Caltech/T. Pyle (SSC)

This artist's concept illustrates the hottest planet yet observed in the universe. The scorching ball of gas, a "hot Jupiter" called HD 149026b, is a sweltering 3,700 degrees Fahrenheit (2,040 degrees Celsius) -- about 3 times hotter than the rocky surface of Venus, the hottest planet in our solar system. The planet is so hot that astronomers believe it is absorbing almost all of the heat from its star, and reflecting very little to no light. Objects that reflect no sunlight are black. Consequently, HD 149026b might be the blackest known planet in the universe, in addition to the hottest.

The temperature of this dark and balmy planet was taken with NASA's Spitzer Space Telescope. While the planet reflects no visible light, its heat causes it to radiate a little visible and a lot of infrared light. Spitzer, an infrared observatory, was able to measure this infrared light through a technique called secondary eclipse. HD 149026b is what is known as a transiting planet, which means that it crosses in front of and passes behind its star -- the secondary eclipse -- when viewed from Earth. By determining the drop in total infrared light that occurs when the planet disappears, astronomers can figure out how much infrared light is coming from the planet alone.

The Spitzer observations of HD 149026b also suggest a hot spot in the middle of the side of the planet that always faces its star. Even though the planet is black, the spot would glow like a black lump of charcoal. HD 149026b is thought to be tidally locked, just as our moon is to Earth, such that one side of the planet is perpetually baked under the heat of its sun.

Astronomers think that HD 149026b is probably blazing hot on its sunlit side, and much cooler on its dark side. A similar phenomenon was observed previously by Spitzer for the planet Upsilon Andromedae b. In the case of both planets, heat is not being evenly distributed across their surfaces. This is the opposite of what happens on Jupiter, where temperature differences are minimal all around.

HD 149026b is located 256 light-years away in the constellation Hercules. It is the smallest known transiting planet, with a size similar to Saturn's and a suspected dense core 70 to 90 times the mass of Earth. It speeds around its star every 2.9 days.

Harrington's team on this project also included Statia Luszcz from the Center for Radiophysics and Space Research at Cornell University, who is now a graduate student at the University of California, Berkeley. Sara Seager, a theorist in the Departments of Earth, Atmospheric, and Planetary Sciences and of Physics at the Massachusetts Institute of Technology, and Jeremy Richardson, an observer from the Exoplanet and Stellar Astrophysics Laboratory at NASA Goddard, round out the team.

Harrington is no stranger to significant findings. His research was published in Science magazine in October 2006 and in Nature in February 2007. In the first of those papers, Harrington's team used Spitzer to make the first measurement of day and night temperature variation on a different extrasolar planet. That research found that a Jupiter-like gas-giant planet circling very close to its sun is as hot as fire on one side, and potentially as cold as ice on the other, a condition that may also hold for HD 149026b.

February's publication documented a landmark achievement. In a project led by Richardson, the group captured enough light from an exoplanet to spread it apart into a spectrum and find signatures of molecules in the planet's atmosphere - a key step toward being able to detect life on alien worlds.

Harrington's team fared well in this year's stiff competition for observing time on NASA's orbiting infrared facility. They will observe HD 149026b using all of Spitzer's instruments in the coming year, to gain a better understanding of the planet's atmosphere. Harrington is a professor in UCF's growing program in planetary sciences.

University of Central Florida News Release