Astrophile is our weekly column on curious cosmic objects, from the solar system to the far reaches of the multiverse
Object: the exoplanet 55 Cancri b
Distance: about 40 light years
Wrapped up snug in a warm, puffy atmosphere, the gas giant planet 55 Cancri b may not realise it is living on the edge.
But it turns out the planet is at just the right distance from its star for its gassy covering to start slipping away. The discovery of this slowly evaporating exoplanet could help us better understand atmospheres in our solar system, including why some worlds have them while others don't.
David Ehrenreich of the University of Geneva, Switzerland, and colleagues had been using the Hubble Space Telescope to examine 55 Cancri e, a super-Earth in the same star system, and one that might be made of diamond.
That planet transits, or crosses, in front of its star, and the team was hoping to find out if it has an atmosphere by looking for changes in the light emitted during a transit.
They did detect hints of an atmosphere, but not the one they expected. Their results showed that nearby 55 Cancri b has an unusually large atmosphere, and that its uppermost layers skim the star.
Puffed up
Ehrenreich's team calculates that 55 Cancri b has a puffed-up atmosphere rich in hydrogen. Such puffiness has been seen before among so-called hot Jupiters, which are Jupiter-like gas giants that orbit very close to their stars. The intense heat inflates their atmospheres, which make up most of their mass, giving them low densities as a result. It also causes their atmospheres to evaporate rapidly.
"We do not observe such extended atmospheres for the giant planets in our solar system," says Ehrenreich. "Since we do observe them in the case of extrasolar planets, it means there is probably a critical distance at which the upper atmosphere becomes unstable and starts to evaporate."
And as it happens, 55 Cancri b orbits around 17 million kilometres from its star, far enough away that it is more of a "warm" Jupiter ? the first known to have an extended atmosphere. The planet is probably losing its atmosphere 10 to 100 times slower than previously studied hot Jupiters, suggesting it is orbiting at very close to the critical distance.
That makes 55 Cancri b a perfect tool for studying how atmospheres evolve based on a planet's mass and distance from its star, says Ehrenreich.
Link between worlds
"It's kind of a link between the very hot Jupiters and the solar system giant planets, which are much colder," he says. "It could [also] give us a clue about the chance of Earth-mass planets having an atmosphere, depending on their distance to the star."
It's possible that the inner planets in our solar system ? Mercury, Venus, Earth and Mars ? had thicker hydrogen atmospheres in their early years that eventually boiled away, says Helmut Lammer of the Space Research Institute of the Austrian Academy of Sciences in Graz. "The small planets may have looked like small gas giants."
Further studies of the 55 Cancri system may offer clues to why we see such a variety of atmospheres around our solar system's inner worlds.
Journal reference: arxiv.org/abs/1210.0531
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