As far as planets go, HD 189733b, a giant, sizzling Jupiter-like world that swoops around its parent star every 2.2 days, couldn’t be more different from Earth.
But the planet, located 63 light-years away in the constellation Vulpecula (the Fox), has one feature that is familiar: it’s blue.
'[It's] about as alien a planet as you could possibly imagine.'
- Astronomer Heather Knutson with Caltech
Astronomers weren’t specifically thinking about HD 189733b’s color per se when they requested observation time on the Hubble Space Telescope. They were following up previous studies showing the planet had clouds with an attempt to learn more about what is in its atmosphere.
As HD 189733b paraded around its star, astronomers used Hubble’s light-splitting spectrograph to home in on specific wavelengths of light reflecting off the planet’s surface.
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HD 189733b is a so-called “transiting planet” meaning it passes in front of and then behind its host star, relative to the telescope’s line of sight. Taking data before, during and after eclipses often yields scientific treasures -- and in this case an aesthetic one as well.
When HD 189733b slipped behind the star, the light seen by Hubble dropped deeply into the blue part of the electromagnetic spectrum, while all other colors remained the same, a telltale sign of the planet's color.
HD 189733b is far too hot for liquid water, but there are other molecules that could scatter blue light, mirroring what happens in Earth’s atmosphere. Scientists believe HD 189733b has clouds made of liquid glass.
“Our best guess is that the color is due to a combination of reflection by silicate clouds and absorption by sodium atoms,” astronomer Frederic Pont, with the University of Exeter in the United Kingdom, wrote in an email to Discovery News.
“Other factors may be photochemical aerosols -- i.e. smog -- and absorption by other atoms or molecules than sodium,” though presently are no specific candidates,” he added.
Driving the planet’s extreme environment is its unenviable position 30 times closer to its parent star than Earth orbits the sun. At that distance, surface temperatures reach more than 1,800 degrees Fahrenheit.
To boot, the planet is likely gravitationally locked with one side permanently facing its star and the other in darkness. That dichotomy can generate wild winds that surpass 4,350 mph.
“I think of this planet in some ways as being about as alien a planet as you could possibly imagine,” astronomer Heather Knutson, with the California Institute of Technology (Caltech) in Pasadena, told Discovery News.
“If we could actually see it in person, I think we would find that there’s no good comparison we could make with anything that we’re familiar with. That’s what makes it interesting,” she added.
The research will be published in the Aug. 1 issue of Astrophysical Journal Letters.