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15 May, 2007

First Map Ever From Another Solar System

Will 2007 be a year that everyone remembers? Will my great-great-grandchildren turn to me and say, 'Gosh, Great-Great-Granddad, were you alive when that happened? Were there street parties and did the World President declare a public holiday?' (The cute little mites can't imagine a time before the World Government and the Abolition of Poverty Act.)

'Well, actually, no, kids,' I'll tell them. 'It just happened, very quietly, and nobody seemed to notice. Check it out.' And I'll pass them the space-time co-ordinates for the event so they can check the media for that momentous day – 9th May 2007 – when the very first map of an extrasolar planet was published.

Based on a 33 hour exposure on 28th and 29th October 2006, using the Spitzer space telescope, the map, pictured below, shows the surface temperature of planet HD 189733b (I remember the day when we used to call planets things like 'Mars' and 'Venus' - how silly we were back then.) HD 189733b is what planet-hunters call a 'hot Jupiter'. That is, it is a very large planet, a little larger than Jupiter itself (1.15 times the mass, 1.26 times the diameter) but, unlike our own, icy neighbour, HD 189733b is orbiting extremely close to its star (just a third of one percent of Earth's orbit) and tears around its sun at a rate of one orbit every 2.2. days! Being so close to a star, makes HD 189733b hot. The surface temperature, as revealed by the new map, varies between 650 and 930 degrees Celsius.

See the big white blob to the right of centre on the map? That's the highest temperature region. One of the side effects of orbiting so close to its star is that a planet's orbit becomes 'tidally locked' so that the same face always points towards the sun (just as the same face of our Moon always points towards the Earth). This means the sunward-facing side of the planet gets a lot hotter than the outward-facing side and hence the bright patch near the middle. Yet two things are odd about HD 189733b's temperature. The first is that we might expect a much bigger difference between the sunward and the outward side, yet the sunward side is just 50% hotter than the outward. The other is that we would expect the hottest area to be exactly in the middle of the map (which is arranged so that the middle is the point where the star is directly overhead) not shifted 30 degrees to the right. An explanation that fits both these facts is that the planet's atmosphere is whirling around the planet at tremendous speeds – 6,000 km per hour winds – carrying heat from the sunward side to the outward side and at the same time pushing the hottest spot in the direction in which it is blowing.

It's not much of a map, I know. All we get to see is how hot the air is – no beautiful swirling colours (as on our own Jupiter), no surface features, nothing much at all. Yet HD 189733b is 63 light years away. That's about sixty thousand billion kilometres away (in case you don't have your calculator handy). Given that the fastest spaceship ever built by us went at 253,000 km/hour (although we can certainly do better than that) to get to HD 189733b would be a journey lasting hundreds of billions of years using current technology.

Yet we have a map of the place.

I suppose there wasn't much of a fuss when the Babylonians drew their first maps of our own world. Probably no-one ran through the streets announcing the fact. It might not even have had a mention on the evening news for 9th May 3007 BC (or whenever it was). Nevertheless, it was an incredible achievement, just as our own map of HD 189733b is.

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