Azure planet a colorful twist

Image Credit: NASA, ESA, M. Kornmesser (Artist impression)

Astronomers using the NASA/ESA Hubble Space Telescope have, for the first time, determined the true colour of a planet orbiting another star. If seen up close this planet, known as HD 189733b, would be a deep cobalt blue, reminiscent of Earth’s colour as seen from space.

Frédéric Pont of the University of Exeter, UK, leader of the Hubble observing programme and an author of a new paper on HD 189733b said:

"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."

We all have heard the expression "pale blue green dot" as a description of earth, HD 189733b's color is azure which is a very bright blue with almost no reflected green light. So whilst the planet is simlar color to earth from space but that's where the similarities end. This "deep blue dot" is a huge gas giant orbiting very close to its host star.

Image Credit: NASA, ESA, and A. Feild (STScI/AURA)

Azure is sometimes used as an evocative adjective not just a color, the azure waters of your favourite beachside destination, or the tiny azure Kingfisher one of Australia's most stunning birds. Dictionaries defines azure as the color of the clear blue sky or a light purple shade of blue. Whilst astronomers love clear blue skies pointing to a night of great observations, the atmosphere of the azure planet HD189733b is anything but plain sailing on a clear day.

Image Credit: JJ Harrison, Julatten, Qld 2011. Wikipedia Commons

Only this week talented amateur astronomer Emmanuel Conseil trapped this planet in front of its parent star during a transit. Emannuel like many other amateur astronomers regularly collaborate on extra-solar planet transits and submit their data to the TRESCA database. HD189733b has over 90 such observations from astronomers around the world.

Image Credit: HD189733b transit - Emmanuel Conseil

At a distance of 63 light-years from us, this turbulent alien world is one of the nearest exoplanets to Earth that can be seen crossing the face of its star. 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.

So the azure blue "seas" here are nothing like the Amalfi coast!

An explanation is provided by Tom Evans of the University of Oxford, UK, first author of the paper:

"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."

The planet's azure blue colour does not come from the reflection of a tropical ocean, but is due to a hazy, turbulent atmosphere thought to be laced with silicate particles, which scatter blue light. Earlier observations using different methods have reported evidence for scattering of blue light on the planet, but these most recent Hubble observations give robust confirming evidence, say the researchers.

Looking out - Looking in!

It is thought that inside the atmosphere the sunsets on HD 189733b could be red. If sodium absorbs red light and dust scatters red light, the atmosphere will redden light shining through it, but will appear blue in reflected light. The colours of Jupiter and Venus are both due to unknown particles within the atmospheres of the planets. Earth looks blue from space because the oceans absorb red and green wavelengths more strongly than blue ones, and reflect the blueish hue of our sky. The shorter blue wavelengths of sunlight are selectively scattered by oxygen and nitrogen molecules in our atmosphere via a process called Rayleigh scattering.

So there we have it an azure blue planet that rains glass - I'll stop right there before I start to think of quirky analogies about what it takes to make glass ceilings come down. ;-)

The new paper, titled "The deep blue colour of HD 189733b: albedo measurements with HST/STIS at visible wavelengths", will appear in the 1 August issue of the journal Astrophysical Journal Letters.

The Hubble Space Telescope is a project of international cooperation between ESA and NASA.