It’s a Planet… No, It’s a Comet…

No… Maybe It’s A Cometary Planet

An artist’s concept of an exoplanet being baked by its star, with the planetary atmosphere streaming out to space like a comet tail. Courtesy NASA, ESA and Greg Bacon (STScI).

Astronomers used the Hubble Space Telescope’s Cosmic Origins Spectrograph (COS) to study the ultraviolet-light fingerprints of elements in light streaming away from a star located 153 light-years from Earth. What’s so important in that starlight?  According to astronomer Jeffrey Linsky of the University of Colorado, Boulder, the evidence from the spectrograph shows that there’s a “baked” object orbiting the star.

The chemical elements in the starlight are actually evidence that the planetary atmosphere is being heated by the nearby star and escaping into space. The fleeing atmospheric gases are being swept into a tail, giving the planet — named HD 209458b — a comet-like appearance.

“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 Linsky, who is the 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 is not Earth-like, but instead is more Jupiter-like. It orbits 100 times closer to its star than Jupiter does to the Sun. That gives this roasted planet a very short year — only 3.5 days long.  For this reason, plus its proximity to Earth and the fact that it is one of the few known exoplanets that can be seen passing in front of (transiting) its star, HD 209458b is one of the most intensely scrutinized alien worlds in our part of the galaxy.  The fact that this star transits its star so frequently allowed Linsky and his team to use COS to analyze the planet’s atmosphere as it passed in front of the star.

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 detection isn’t the first time a Hubble Space Telescope instrument has detected this cometary planet’s atmosphere. The Space Telescope Imaging Spectrograph (STIS) first observed the planet in 2003 and found the first evidence of this world’s evaporating atmosphere.  COS’s followup study gave astronomers much  more detail and is allowing them to study the action in ultraviolet wavelengths.