July 15th, 2013 by James Ayre
HD 189733b — an exoplanet located about 63 light-years away — has now had its actual visible-light color deduced, thanks to new research done with NASA’s Hubble Space Telescope. If seen directly the exoplanet would appear as a ‘deep blue dot’ — looking a lot like the way that the Earth appears from space. That’s where the similarities end though — HD 189733b is an enormous gas-giant with day time temperatures that soar as high as 2000 degrees Fahrenheit, and winds that howl around the planet at speeds of up to 4500-miles-per-hour. It also seems likely — according to the researchers — that the planet rains molten glass.
The deep cobalt blue color of the planet isn’t the result of a world covered in water like it is on the Earth — HD 189733b is blue as the result of its hazy blow-torched atmosphere and also possibly as a result of high clouds laced with silicate particles. “The condensation temperature of silicates could form very small drops of glass that would scatter blue light more than red light.”
The relatively turbulent exoplanet is one of the closest exoplanets yet discovered — as a result it has been intensively studied, and some light has been shed on its complex atmosphere.
Clouds usually play important roles in the atmospheric dynamics of planets, as a result, detecting the presence and make-up of clouds on exoplanets provides valuable information. “We obviously don’t know much on the physics and climatology of silicate clouds, so we are exploring a new domain of atmospheric physics,” stated researcher Frederic Pont of the University of Exeter, South West England, the United Kingdom.
The Space Telescope Science Institute continues:
The team used Hubble’s Space Telescope Imaging Spectrograph to measure changes in the color of light from the planet before, during, and after the passage of the planet behind the parent star. This technique is possible because the planet’s orbit is tilted edge-on as viewed from Earth; therefore, it routinely passes in front of and then behind the star.
Hubble measured a small drop in light — about one part in 10,000 — when the planet went behind the star, and a slight change in the color of the light, too. “We saw the light becoming less bright in the blue, but not in the green or the red. This means that the object that disappeared is blue because light was missing in the blue, but not in the red when it was hidden,” said Pont.
Earlier observations have reported evidence for the scattering of blue light on the planet. But this most recent Hubble observation gives confirming evidence, said the researchers.
The planet HD 189733b was discovered in 2005. At a distance of only 2.9 million miles from its parent star, the planet is so close that it is gravitationally ‘tidally locked’ so that one side always faces the star and the other side is always dark.
In 2007 NASA’s Spitzer Space Telescope measured the infrared light, or heat, from the planet. This observation produced one of the first-ever temperature maps of an exoplanet. The map shows that the dayside and night-side temperatures differ by about 500 degrees Fahrenheit. This temperature difference should cause fierce winds to roar from the daytime to nighttime side. The complementary visible-light Hubble observations reduce contamination from the planet’s own hot glow, and focus on atmospheric composition.
The researchers note that it’s rather difficult — even with regard to planets in our solar system — to ascertain the exact causes of the color of a planet’s atmosphere. As an example, Jupiter’s reddish color is thought to be the result of ‘unknown’ color-carrying molecules. Venus is a similar situation — the planet doesn’t reflect ultraviolet (UV) light as the result of an ‘unknown’ UV absorber in its atmosphere.
“Earth looks blue from space because the oceans absorb red and green wavelengths more strongly than blue light. In addition, the oceans reflect Earth’s blue sky where the shorter blue wavelengths of sunlight are selectively scattered by atmospheric oxygen and nitrogen molecules in a process called Rayleigh scattering.”
The new research was just published in the August 1 issue of the Astrophysical Journal Letters.
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