This illustration shows HD 189733b, a huge gas giant that orbits very close to its host star HD 189733. The planet’s atmosphere is scorching with a temperature of over 1000 degrees Celsius, and it rains glass, sideways, in howling 7000 kilometre-per-hour winds. Credit: NASA, ESA, M. Kornmesser
Move over, Earth. There’s another blue planet in town — or at least in our corner of the Milky Way.
Astronomers using the Hubble Space Telescope deduced for the first time the atmospheric hue of a planet outside our own solar system — and it turns out to be a “deep cobalt blue.”
But the similarities between HD 189733b, as the alien world is unpoetically known, and Earth, pretty much end there: While oceans of liquid water give our world its azure tint, that’s unlikely the case with HD 189733b, which orbits a star just 63 light years away from us.
The planet in question is what’s known as a “hot Jupiter” — a term that describes both its large mass and nearness to its parent star. Nature elaborates, describing the weather on HD 189733b as extremely hot and windy, with occasional glass rain:
“Although the planet seems to be the shade of a deep ocean, it is unlikely to host liquid water. The exoplanet is a giant ball of gas, similar to Jupiter, and was previously often painted brown and red in artists’ impressions.
“The blue color may come from clouds laden with reflective particles that contain silicon — essentially raindrops of molten glass. Evidence for this idea dates to 2007, when Hubble observed the planet passing in front of its star. Light from the star seemed to be passing through a haze of particles.”
But Hubble’s optical resolution isn’t good enough to actually “see” the planet. Instead, astronomers analyzed spectroscopically the light from the parent star and the planet together (during an eclipse from our vantage point), then measured it again when the planet was behind the star. The observation from the star minus the planet was less blue, indicating that is the color of the planet itself. According to Nature:
“During the eclipse, the amount of observed blue light decreased, whereas other colours remained unaffected. This indicated that the light reflected by the planet’s atmosphere, blocked by the star in the eclipse, is blue. …
” ‘This is the first time this has been done for optical wavelengths,’ said Alan Boss, an astrophysicist at the Carnegie Institution for Science in Washington DC. ‘It’s a technical tour de force.’ The amount of visible light bouncing off a planet is typically small compared to light fluctuations in a star, making planets difficult to distinguish. Fortunately, HD 189733 b is large relative to other exoplanets — and well illuminated.”