BY DR EMILY BALDWIN
Posted: 29 January, 2009
By observing an exoplanet’s rapid rise in temperature as it approached its parent star, astronomers were able to map its atmospheric properties to generate realistic images of what it would look like if you were there.
The University of California Santa Cruz (UCSC) astronomers used NASA's Spitzer Space Telescope to obtain 30 hours worth of infrared measurements of the heat emanating from exoplanet HD 80606b as it orbited its host star. Specifically, the observations were made as the planet passed behind the star, an event known as a secondary eclipse. This well-timed observation allowed the team to make measurements of just the star, thereby enabling accurate temperatures of the planet to be calculated too. The astronomers watched as the planet’s temperature rose from 800 Kelvin to a sizzling 1,500 Kelvin (526 to 1226 degrees Celsius) in just six hours.
These computer-generated images chart the development of severe weather patterns on the highly eccentric exoplanet HD 80606b starting from 4.4 days after the planet's close approach to the star and covering a total of 8.9 days. The glowing blue crescent is starlight that has been scattered and reflected by planet. The night side appears reddish orange as it glows with its own internal heat. Image: NASA/JPL-Caltech/J. Langton/UCSC.
"We watched the development of one of the fiercest storms in the Galaxy," says Gregory Laughlin, professor of astronomy and astrophysics at UCSC. "This is the first time that we've detected weather changes in real time on a planet outside our Solar System."
Feeding this data into computer simulations of the planet's atmosphere enabled the researchers to generate photorealistic images of the planet. "We can't get a direct image of the planet, but we can deduce what it would look like if you were there," says Laughlin. "The ability to go beyond an artist's interpretation and do realistic simulations of what you would actually see is very exciting."
The program was developed by Hubble postdoctoral fellow Daniel Kasen at UCSC to calculate radiative transfer processes in astrophysical applications. "It calculates the colour and intensity of light coming from the glowing planet, and also how starlight would reflect off the surface of the planet," explains Kasen. The resulting images show a thin blue crescent of reflected starlight framing the night side of the planet, which glows red from its own heat, like coals in a fire.
HD 80606b was discovered in 2001 at a distance of 200 light years from Earth. It is four times the mass of Jupiter, and its radius is currently estimated as 1.1 Jupiter radii, but, says, Laughlin, this measurement could be confirmed with future direct observations. HD 80606b also has the most eccentric orbit of any known planet. For the majority of its 111.4 day orbit the planet stays at a distance equivalent to that somewhere between Venus and Earth in our own Solar System, but at closest approach it is brought dramatically close to its parent star, at just 0.03 astronomical units.
As the planet swung through its closest approach to the star, Spitzer observations indicated that it experienced very rapid heating (as shown by the red curve). Just
This close encounter lasts for less than a day, but at the closest point, the sunlight beating down on the planet is 825 times stronger than what it receives at its farthest point from the star. "If you could float above the clouds of this planet, you'd see its sun growing larger and larger at faster and faster rates, increasing in brightness by almost a factor of 1,000," says Laughlin.
By feeding the observational data into a hydrodynamic model of the planet’s atmosphere, Jonathan Langton, a postdoctoral researcher also at UCSC, revealed how the planet handles heat. The conclusion was that intense irradiation from the star is absorbed in a layer of the planet's upper atmosphere that absorbs and loses heat rapidly, an extreme version of seasonal change. Furthermore, he demonstrated that global storms and shockwaves were unleashed in the planet's atmosphere every 111 days as it swings close to its star.
"The initial response could be described as an explosion on the side facing the star," says Langton. "As the atmosphere heats up and expands, it produces very high winds, on the order of 5 kilometers per second, flowing away from the day side toward the night side. The rotation of the planet causes these winds to curl up into large-scale storm systems that gradually die down as the planet cools over the course of its orbit." Laughlin comments that the observations bring to life the “thought experiment” of what would happen if you dragged a planet like Jupiter very close to the Sun.
If the planet's orbit is aligned just right, it will pass in front of the star (an event known as a primary transit) on 14 February, and many professional and amateur astronomers worldwide will be watching to see if this happens. “If the planet is observed to transit on 14 Feb, then we would get a direct measurement of the planet's radius,” Laughlin tells Astronomy Now. “Follow-up observations of the transit from space-based instruments such as the Hubble Space Telescope would allow for measurements of some of the planet's atmospheric constituents, and follow-up spectroscopic measurements from the ground would allow us to determine how the orbit of the planet is tilted with respect to the axis of rotation of the star.” The astronomers report a 15 percent chance of the transit occurring.
The new findings are reported in the latest issue of the journal Nature.