Click on image for QuickTime Movie
Scientists using NASA's Spitzer Space Telescope were able to create the
first-ever map of the surface of a planet beyond our solar system. The
planet, a hot and cloudy gas giant called HD 189733b, is located 60
light-years from Earth in the constellation Vulpecula. It is so far away
that even the best telescopes can't distinguish the light of the planet
from that of its star. So how did astronomers see this planet's cloudy
surface?
They used Spitzer's infrared vision to observe the HD 189733 system as the
planet, HD 189733b, first crossed in front of its parent star, then passed
behind, as illustrated in this movie. HD 189733b is what is known as a
transiting planet, which means that it is inclined in such a way that it
eclipses its star from Earth's point of view. This planet is also thought
to be tidally locked to its star, meaning that one face, termed the day
side, always "looks at" its fiery hot sun.
Spitzer began observing the planet when it was between us and the star,
with its cooler, dimmer night side in view. As the planet swept around,
the hotter and brighter day side rotated into view, and the total infrared
light measured by Spitzer went up. The size of this increase in infrared
brightness over time told astronomers how the temperature across the
entire surface of the planet varies.
Why did the scientists use infrared light? This astronomical trick works
best with infrared light, because a planet's own light stands out more in
comparison to its glaring sun at this wavelength. For example, if this
star/planet system were viewed in visible light, the light from the planet
itself would be washed out. Since infrared light tells astronomers how
much heat is coming from a planet, it can also be used to determine
temperatures.
The map reveals that HD 189733b is about 1,200 degrees Fahrenheit (650
degrees Celsius) on its dark side, and about 1,700 degrees Fahrenheit (930
degrees Celsius) on its sunlit side. Such a mild temperature variation
indicates that strong winds are sweeping hot gas from the sunlit side over
to the dark side.
