It’s an Alien Blue World—But Nothing Like Earth

Artist's conception of exoplanet HD 189733b—a "deep blue dot" reminiscent of Earth from a distance. Image Credit: NASA / ESA / M. Kornmesser
Artist’s conception of exoplanet HD 189733b—a “deep blue dot” reminiscent of Earth from a distance. Image Credit: NASA / ESA / M. Kornmesser

The planets and moons in our Solar System come in a wide variety of colors, and the natural beauty can be breathtaking. Now, the true color of a much more distant planet has been seen for the first time—and it’s blue!

The discovery, announced last Thursday, was made by astronomers using the Hubble space telescope; observations of the planet HD 189733b, about 63 light-years away, revealed it to be a deep azure blue color, similar to Earth seen from space. This planet, however, is nothing like ours. It is a gas giant, like Jupiter and Saturn, and orbits very close to its star. It’s atmosphere is scorching hot, about 1,800 ˚F (1000 ˚C) or more, and is very turbulent—wind speeds are estimated to be a whopping 4,350 miles (7,000 kilometres) per hour! It is also thought that silicate glass particles rain sideways in those winds. It’s worth noting also that Neptune is a lovely blue color, too, and Uranus is blue-green, but they are ice giants much larger and different from our own planet. Sometimes, initial appearances can be deceiving.

According to Frédéric Pont of the University of Exeter, UK, leader of the Hubble observing programme and an author of the new paper: “This planet has been studied well in the past, both by ourselves and other teams. But measuring its colour is a real first—we can actually imagine what this planet would look like if we were able to look at it directly.”

Comparison of the colors of some of the planets in our solar system with HD 189733b. Image Credit: NASA / ESA / A. Feild (STScI/AURA)
Comparison of the colors of some of the planets in our Solar System with HD 189733b.
Image Credit: NASA / ESA / A. Feild (STScI/AURA)

The fact that astronomers were able to see the planet’s actual color is impressive, as it is very faint and close to its star. They used Hubble’s Space Telescope Imaging Spectrograph (STIS) to watch the planet before, during, and after it passed behind the star as it orbited. As it moved behind the star, the light reflected from the planet was temporarily blocked from view, and the amount of light observed dropped. This technique allowed astronomers to see how the color of the light changed as well. As Tom Evans, of the University of Oxford, UK, and first author of the paper, explains: “We saw the brightness of the whole system drop in the blue part of the spectrum when the planet passed behind its star. From this, we can gather that the planet is blue, because the signal remained constant at the other colours we measured.”

So if this planet isn’t very Earth-like, then where does the blue color come from? It is thought to result from the silicate particles, which reflect blue light, suspended in the hazy atmosphere. Cool.

At first glance, HD 189733b may look like a “deep blue dot” similar to the “pale blue dot” as Earth is sometimes affectionately called, but it is seemingly a very different and exotic world. With thousands of other exoplanets now being discovered, how many other blue planets are out there? And are any of them really blue waterworlds like ours?

The paper is available here.

This article was first published on The Meridiani Journal.

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