Astronomers have observed the intense heating of a distant
planet as it swung close to its parent star, providing important
clues to the atmospheric properties of the planet. With that
data, astronomers at the University of California, Santa Cruz
were able to generate 'realistic' images of the planet by
feeding the data into computer simulations of the planet's
atmosphere. The researchers used NASA's Spitzer Space
Telescope to obtain infrared measurements of the heat emanating
from the planet as it whipped behind and close to its star. In
just six hours, the planet's temperature rose from 800 to 1,500
Kelvin (980 to 2,240 degrees Fahrenheit).
Known as HD 80606b, the planet circles a star 200 light years
from Earth, is four times the mass of Jupiter, and has the most
eccentric orbit of any known planet. It spends most of its
111.4-day orbit at distances that would place it between Venus
and Earth in our own solar system, while the closest part of its
orbit brings it within 0.03 astronomical units of its star (one
astronomical unit is the distance between Earth and the Sun).
The planet zips through this dramatic close encounter with its
star in less than a day.
The planet HD 80606b glows orange from its own heat in this
computer-generated image. A massive storm has formed in response
to the pulse of heat delivered during the planet's close swing
past its star. The blue crescent is reflected light from the
star. Image by D. Kasen, J. Langton, and G. Laughlin (UCSC).
"We can't get a direct image of the planet, but we can deduce
what it would look like if you were there. The ability to go
beyond an artist's interpretation and do realistic simulations
of what you would actually see is very exciting," said Gregory
Laughlin, professor of astronomy and astrophysics at UCSC.
Laughlin is lead author of a new report on the findings
published this week in Nature.
At the closest point, the sunlight beating down on the planet is
825 times stronger than the irradiation 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," Laughlin said.
Spitzer observed the planet for 30 hours before, during, and
just after its closest approach to the star. The planet passed
behind the star (an event called a secondary eclipse) just
before the moment of its closest approach. This was a lucky
break for Laughlin and his colleagues, who had not known that
would happen when they planned the observation. The secondary
eclipse allowed them to get accurate measurements from just the
star and thereby determine exact temperatures for the planet.
The extreme temperature swing observed by Spitzer indicates that
the intense irradiation from the star is absorbed in a layer of
the planet's upper atmosphere that absorbs and loses heat
rapidly, Laughlin said.
Coauthor Jonathan Langton, a postdoctoral researcher at UCSC,
fed the Spitzer data into a hydrodynamic model of the planet's
atmosphere to predict its response to the intense heating.
Langton's simulation shows the global storms and shockwaves
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," Langton said. "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."
Daniel Kasen, a Hubble postdoctoral fellow at UCSC, was able to
generate photorealistic images of the planet using a program he
developed to calculate radiative transfer processes in
astrophysics. "It calculates the color and intensity of light
coming from the glowing planet, and also how starlight would
reflect off the surface of the planet," Kasen said.
The resulting images show a thin blue crescent of reflected
starlight framing the night side of the planet, which glows
cherry red from its own heat, like coals in a fire. "These
images are far more realistic than anything that's been done
before for extrasolar planets," Laughlin said.
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
February 14. Both professional and amateur astronomers worldwide
will be watching to see if this happens. The occurrence of
primary transits would enable astronomers to learn more about
this unusual planet by conducting spectroscopic observations.
HD 80606b was originally discovered in 2001 by a Swiss
planet-hunting team led by Dominique Naef of the Geneva
Observatory, Switzerland. Using a method known as the
Doppler-velocity technique, they detected the tell-tale wobble
in the light from the star caused by the gravitational tug of
the planet.
Subsequent observations by Laughlin's colleagues on the
California&Carnegie Planet Search team--Steve Vogt at UCSC and
Paul Butler at the Carnegie Institute of Washington--provided
precise information about the planet's orbit, which was
essential for planning the Spitzer observations. Drake Deming of
NASA's Goddard Space Flight Center contributed his expertise to
the analysis of the Spitzer data. Other coauthors of the Nature
paper include UCSC postdoctoral researcher Eugenio Rivera and
graduate student Stefano Meschiari.
The Spitzer Space Telescope is operated by the Jet Propulsion
Laboratory (JPL), California Institute of Technology (Caltech),
under contract to NASA. Support for this work was provided by
NASA through an award issued by JPL/Caltech.
Article: Laughlin, G., et al. Nature 457, 562-564 (2009)
