Artist’s impression of the deep blue planet HD 189733b
Wide-field view of HD 189733b and surroundings
(DSS2 excerpt, ground-based image)
(DSS2 excerpt, ground-based image)
Wide-field view of the Summer Triangle (ground-based image)
Exotic blue planet HD 189733b (artist’s impression)
Exotic blue planet HD 189733b (labelled artist’s impression)
The colour of HD 189733b compared to our Solar System
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True colour of exoplanet measured for the first time
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 azure blue, reminiscent of Earth’s colour as
seen 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. 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 [1].
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. It has been intensively studied by Hubble and other
telescopes, and its atmosphere has been found to be dramatically
changeable and exotic, with hazes and violent flares (heic0720, heic1209). Now, this planet is the subject of an important first: the first measurement of an exoplanet's visible colour.
"This planet has been studied well in the past, both by ourselves and other teams,"
says Frédéric Pont of the University of Exeter, UK, leader of the
Hubble observing programme and an author of this new paper. "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."
In order to measure what this planet would look like to our eyes, the
astronomers measured how much light was reflected off the surface of HD
189733b — a property known as albedo [2].
HD 189733b is faint and close to its star. To isolate the planet's
light from this starlight, the team used Hubble's Space Telescope
Imaging Spectrograph (STIS) to peer at the system before, during, and
after the planet passed behind its host star as it orbited. As it
slipped behind its star, the light reflected from the planet was
temporarily blocked from view, and the amount of light observed from the
system dropped. But this technique also shows how the light changes in
other ways — for example, its colour [3].
"We saw the brightness of the whole system drop in the blue part of the spectrum when the planet passed behind its star," explains Tom Evans of the University of Oxford, UK, first author of the paper. "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 [4].
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.
HD 189733b presented a favourable case for these kinds of
measurements as it belongs to a class of planets known as "hot
Jupiters". These massive planets are similar in size to the gas giants
in the Solar System, but instead lie very close to their parent star —
this size and proximity to their star make them perfect subjects for
exoplanet hunting. We know that hot Jupiters are numerous throughout the
Universe. As we do not have one close to home in our own Solar System,
studies of planets like HD 189733b are important to help us understand
these dramatic objects.
"It's difficult to know exactly what causes the colour of a planet's atmosphere, even for planets in the Solar System," says Pont [5]. "But
these new observations add another piece to the puzzle over the nature
and atmosphere of HD 189733b. We are slowly painting a more complete
picture of this exotic planet."
Notes
[1] In 2007 NASA's Spitzer Space
Telescope measured the infrared light from the planet, producing one of
the first ever temperature maps for an exoplanet. The map shows that
day- and night-side temperatures differ by about 260 degrees Celsius,
causing fierce winds to roar across the planet. The condensation
temperature of the silicates (over 1300 degrees Celsius) mean these
particles could form very small grains of glass in the atmosphere.
[2] Albedo is a measure of how much incident
radiation is reflected. The greater the albedo, the greater the amount
of light reflected. This value ranges from 0 to 1, with 1 being perfect
reflectivity and 0 being a completely black surface. The Earth has an
albedo of around 0.4.
[3] This technique is possible because the planet's
orbit is tilted edge-on as viewed from Earth, so that it routinely
passes in front of and behind the star. When the planet passes behind
its host star, the light received from the system drops by about one
part in 10 000.
[4] The deep blue colour of HD 189733b is consistent with the "red sunset of HD 189733b" result from the transit spectrum (heic0720).
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.
[5] 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.
Notes for editors
The Hubble Space Telescope is a project of international cooperation between ESA and NASA.
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.
[1] The international team of astronomers in this
study consists of T. Evans (University of Oxford, UK), F. Pont
(University of Exeter, UK), D. K. Sing (University of Exeter, UK), S.
Aigrain (University of Oxford, UK), J. K. Barstow (University of Oxford,
UK), J-M. Désert (California Institute of Technology, USA; Sagan
Postdoctoral Fellow), N. Gibson (European Southern Observatory,
Germany), K. Heng (University of Bern, Switzerland), H. A. Knutson
(California Institute of Technology, USA) and A. Lecavelier des Etangs
(Universite Pierre et Marie Curie, France).
More information
Image credit: NASA, ESA, M. KornmesserLinks
- Science paper
- ESA/Hubble release: Hazy red sunset on extrasolar planet
- ESA/Hubble release: Dramatic change spotted on a faraway planet
- Exoclimes website
- NASA press release
- Images of Hubble
Contacts
Frédéric PontUniversity of Exeter
Exeter, United Kingdom
Tel: +41 77 4185812
Email: f.pont@exeter.ac.uk
Tom Evans
University of Oxford
Oxford, United Kingdom
Tel: +44 1865 273598
Email: tom.evans@astro.ox.ac.uk
Nicky Guttridge
ESA/Hubble
Garching, Germany
Tel: +49-89-3200-6855
Email: nguttrid@partner.eso.org
Source: ESA/HUBBLE - Space Telescope
