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This exoplanet weather map shows temperatures on a hot Jupiter known as "HAT-P-2b".
Among the hundreds of new planets discovered by NASA's Kepler
spacecraft are a class of exotic worlds known as "hot Jupiters." Unlike
the giant planets of our own solar system, which remain at a safe
distance from the sun, these worlds are reckless visitors to their
parent stars. They speed around in orbits a fraction the size of
Mercury’s, blasted on just one-side by starlight hundreds of times more
intense than the gentle heating experienced by Jupiter here at home."
Meteorologists watching this video are probably wondering what
kind of weather a world like that might have. The short answer is "big."
Heather Knutson of Caltech made the first weather map of a hot Jupiter in 2007.
"It's not as simple as taking a picture and--voila!—we see the
weather," says Knutson. These planets are hundreds of light years from
Earth and they are nearly overwhelmed by the glare of their parent
stars. "Even to see the planet as a single pixel next to the star would
be a huge accomplishment."
Instead, Knutson and colleagues use a trick dreamed up by Nick
Cowan of Northwestern University. The key, she explains, is that "most
hot Jupiters are tidally locked to their stars. This means they have a
permanent dayside and a permanent night side. As we watch them orbit
from our vantage point on Earth, the planets exhibit phases--e.g.,
crescent, gibbous and full. By measuring the infrared brightness of the
planet as a function of its phase, we can make a rudimentary map of
temperature vs. longitude."
NASA’s Spitzer Space Telescope is the only infrared observatory
with the sensitivity to do this work. Since Knutson kick-started the
research in 2007, nearly a dozen hot Jupiters have been mapped by
astronomers using Spitzer.
The most recent study, led by Nikole Lewis, a NASA Sagan Exoplanet
Fellow working at MIT, shows a gas giant named HAT-P-2b. "We can see
daytime temperatures as high as 2400 K," says Lewis, "while the
nightside drops below 1200K. Even at night," she marvels, "this planet
is ten times hotter than Jupiter."
These exoplanet maps may seem crude compared to what we’re
accustomed to on Earth, but they are a fantastic accomplishment
considering that the planets are trillions of miles away.
The maps show huge day-night temperature differences typically
exceeding 1000 degrees. Researchers believe these thermal gradients
drive ferocious winds blowing thousands of miles per hour.
Without regular pictures, researchers can’t say what this kind of
windy weather looks like. Nevertheless, Knutson is willing to speculate
using climate models of Jupiter as a guide.
"Weather on hot Jupiters," she predicts, "is really big."
Over the years, planetary scientists have developed computer
models to reproduce the storms and cloud belts in Jupiter’s atmosphere.
If you take those models and turn up the heat, and slow down the
rotation to match the tidally-locked spin of a hot Jupiter, weather
patterns become super-sized. For instance, on a hot Jupiter the Great
Red Spot might grow as large as a quarter the size of the planet and
manifest itself in both the northern and southern hemispheres.
"Just imagine what that would look like--a pair of giant eyes staring out into space!" says Lewis.
Meanwhile, Jupiter’s famous belts would widen so much that only two or three would fit across the planet’s girth.
Ordinary clouds of water and methane couldn’t form in such a hot
environment. Instead, Knutson speculates that hot Jupiters might have
clouds made of silicate—that is, "rock clouds."
"Silicates are predicted to condense in such an environment," she
says. "We're already getting some hints that clouds might be common on
these planets, but we don’t yet know if they’re made of rock."
For now just one thing is certain: The meteorology of hot Jupiters is out of this world.
Nikole Lewis of MIT is a NASA Sagan Exoplanet Fellow. The Sagan
Fellowship Program is administered by the NASA Exoplanet Science
Institute as part of NASA's Exoplanet Exploration Program at JPL.
Caltech manages JPL for NASA