NASA Finds Super Hot Planet With Unique Comet-Like Tail
WASHINGTON -- Astronomers using NASA's Hubble Space Telescope have confirmed the existence of a baked object that could be called a
"cometary planet." The gas giant planet, named HD 209458b, is
orbiting so close to its star that its heated atmosphere is escaping
into space.
Observations taken with Hubble's Cosmic Origins Spectrograph (COS)
suggest powerful stellar winds are sweeping the cast-off atmospheric
material behind the scorched planet and shaping it into a comet-like tail.
"Since 2003 scientists have theorized the lost mass is being pushed
back into a tail, and they have even calculated what it looks like,"
said astronomer Jeffrey Linsky of the University of Colorado in
Boulder, leader of the COS study. "We think we have the best
observational evidence to support that theory. We have measured gas
coming off the planet at specific speeds, some coming toward Earth.
The most likely interpretation is that we have measured the velocity
of material in a tail."
The planet, located 153 light years from Earth, weighs slightly less
than Jupiter but orbits 100 times closer to its star than the Jovian
giant. The roasted planet zips around its star in a short 3.5 days.
In contrast, our solar system's fastest planet, Mercury, orbits the
sun in 88 days. The extrasolar planet is one of the most intensely
scrutinized, because it is the first of the few known alien worlds
that can be seen passing in front of, or transiting, its star. Linsky
and his team used COS to analyze the planet's atmosphere during
transiting events.
During a transit, astronomers study the structure and chemical makeup
of a planet's atmosphere by sampling the starlight that passes
through it. The dip in starlight because of the planet's passage,
excluding the atmosphere, is very small, only about 1.5 percent. When
the atmosphere is added, the dip jumps to 8 percent, indicating a
bloated atmosphere.
COS detected the heavy elements carbon and silicon in the planet's
super-hot 2,000 degrees Fahrenheit atmosphere. This detection
revealed the parent star is heating the entire atmosphere, dredging
up the heavier elements and allowing them to escape the planet.
The COS data also showed the material leaving the planet was not all
traveling at the same speed. "We found gas escaping at high
velocities, with a large amount of this gas flowing toward us at
22,000 miles per hour," Linsky said. "This large gas flow is likely
gas swept up by the stellar wind to form the comet-like tail trailing
the planet."
Hubble's newest spectrograph has the ability to probe a planet's
chemistry at ultraviolet wavelengths not accessible to ground-based
telescopes. COS is proving to be an important instrument for probing
the atmospheres of "hot Jupiters" like HD 209458b.
Another Hubble instrument, the Space Telescope Imaging Spectrograph
(STIS), observed the planet in 2003. The STIS data showed an active,
evaporating atmosphere, and a comet-tail-like structure was suggested
as a possibility. But STIS wasn't able to obtain the spectroscopic
detail necessary to show a tail, or an Earthward-moving component of
the gas, during transits. The tail was detected for the first time
because of the unique combination of very high ultraviolet
sensitivity and good spectral resolution provided by COS.
Although this extreme planet is being roasted by its star, it won't be
destroyed anytime soon. "It will take about a trillion years for the
planet to evaporate," Linsky said.
The results appeared in the July 10 issue of The Astrophysical Journal.
The Hubble Space Telescope is a project of international cooperation
between NASA and the European Space Agency. NASA's Goddard Space
Flight Center in Greenbelt, Md., manages the telescope. The Space
Telescope Science Institute, operated for NASA by the Association of
Universities for Research in Astronomy, Inc. in Washington, conducts
Hubble science operations.
For illustrations and more information about HD 209458b, visit:
http://www.nasa.gov/hubble
Source: NASA
