The colour of a planet outside our solar system has been observed for the first time – and it is blue like Earth.
The blueness of HD189733b, a gas giant that orbits a star just 63 light years away, was detected by the Hubble Telescope, an international group of scientists reported Wednesday in a news release from the ESA/Hubble Information Centre and in a scientific paper posted online. The peer-reviewed article has been accepted for publication in the Aug. 1 issue of Astrophysical Journal Letters.
The colour is described in the release as "deep azure blue, reminiscent of Earth’s colour as seen from space."
It does not indicate that the planet has oceans. Instead, the researchers report that it is due to a dust — made of specks of glass — in the planet's violently stormy atmosphere that scatter blue light.
"This planet has been studied well in the past, both by ourselves and other teams," said Frederic Pont, a University of Exeter researcher who leads the Hubble observing program, in a statement.
"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."
The colour was not imaged directly by the telescope, but detected with an instrument called the Space Telescope Imaging Spectrograph, which measures the levels of different colours of light in the Hubble's field of view, including the light reflecting off the planet.
As the planet passed behind its star over the course of each orbit, the spectrograph showed that the level of blue light decreased.
"From this, we can gather that the planet is blue, because the signal remained constant at the other colours we measured," said Tom Evans, a University of Oxford researcher who co-authored the paper, in a statement.
Aside from not having oceans, the blue planet has many other features that make it uninhabitable. It belongs to a special class of gas giants called "hot Jupiters." As such, it has a temperature of over 1000 C on the side that faces closest to its star and 7000-kilometre-per-hour winds in its atmosphere that may cause it to "rain" glass sideways. The planet is located so close to its star that the star's gravity keeps the same side of the planet constantly facing it (as the Earth's gravity does to the Moon), while the other side is in perpetual darkness.