16 November 2008

Fomalhaut b

What, you may ask, is 'Fomalhaut b'? If you are interested in astronomy you will already know. Fomalhaut bFomalhaut b is a planet circling one of our Sun's nearest neighbour stars.

Fomalhaut b has been imaged twice by the Hubble Space Telescope, once in 2004 and again in 2006. This is important because it's the first time a planet outside our own Solar System has been seen to have moved in its orbit around its central star.

This is extraordinary news indeed. It's the same scale of forward step as Galileo seeing craters on the Moon for the first time, or discovering the rings of Saturn, Halley predicting the return of his eponymous comet, or the Apollo 11 Moon landing in 1969.

Why is it so important? It's a milestone because astronomers have long assumed that other stars have planets. In recent years the presence of such planets has been indirectly detected, but this is the first time we can claim to have seen the light reflected by an exoplanet. You may not have realised it, but you have just lived through a truly historic moment.

The difficulty of making these images is difficult to grasp. Look at the picture again (you can click the image to see the full-size version).

The star - In the middle of the full image (but near the upper left in the article's thumbnail picture) is a small white circle. This is not part of the image, it was added later, but it marks the position of the star (Fomalhaut). In reality the star would be far smaller, just the tiniest speck, it's shown much larger to make it easy to see.

The obscuring disk - if the Hubble telescope had just been pointed at the star, the overpowering brightness would have flooded the image with light so that nothing but glare would be visible.

To see details really close to the star, it's essential to block the direct starlight. This was done by moving an obscuring disk in front of the star, and this is seen in the image as the irregular black area around the central white dot.

The halo of diffracted light - Outside the black zone, some starlight is still diffracted into the surrounding area. This is the circular zone that looks like the iris of an eye, close inspection of the large version of the image reveals that it's made up of lines of light radiating out from the position of the star. This not a real, distant object, it's created by subtle interactions between the starlight and the structure of the telecope.

The debris disk - The oval shape (clearly visible only in the full-size version) is a band of dust, gas, and orbiting rock and ice particles. It's part of a disk of material which is in the process of condensing into planets. Fomalhaut is a young star and is still developing a planetary system.

The planet - Just inside the inner edge of the dusty band is where astronomers thought there might be a planet, and sure enough when they looked they found one! This is a gas giant, probably much like Jupiter though something like twice as large, and it is so bright that many astronomers suspect it must have a ring similar to Saturn's (but larger).

The real clincher is that the planet appears in two Hubble photos of Fomalhaut, taken two years apart. It has moved, as expected, in its orbit around the star.

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