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The benefit of hindsight February 26, 2009

Posted by Sarah in science.
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Left: The image of the HR8799 planetary system from data taken with the Keck telescope. Right: the 1998 Hubble data (credit: NRC) (a) original Hubble image, (b) with "traditional" speckle subtraction method, (c-d) 2 images reprocessed showing the planet above the noise (credit: Lafrenière et al., 2009).

An interesting paper turned up on astro-ph last week. Remember HR8799, the star with a whole family of exoplanets imaged directly last year? A Canadian-American team of scientists went back through the archive and re-analysed data taken with the Hubble Space Telescope in 1998. And lo and behold, using new analysis techniques they managed to tease the outermost of HR8799’s planets out of the noise. Very cool. After all, 1998 was only three years after the first ever detection of an exoplanet! Obtaining a direct image of one really was just a glint in our starry eyes back then.

It shows that we don’t necessarily need bigger telescopes or more sophisticated instruments to do better science. Particularly in the field of exoplanets, many recent advances have been made by new calibration and data analysis techniques. The difficulty with imaging exoplanets is that the planets are invariably tiny compared with the host star, and because they appear relatively close by, the faint light from the planets themselves is washed out by many orders of magnitude by the starlight and noise. Even when the light from the central star is suppressed using coronagraphy, e.g. by placing a mask at the position of the central star to block out its light, the image is inundated by photons from it that are scattered and diffracted by the optics. From the ground, the situation is worse yet, as turbulent gases cause the light to spread out even more over the field of view. A carefully designed optical layout and higher quality mirrors help enormously with reducing these effects, but to get rid of it we have to apply clever calibration and post-processing.

One technique for example uses the fact that the position of the diffraction speckles depend on the wavelength of observation, while the exoplanet, given the same instrumental configuration, should be in the same position independent of the wavelength observed. By observing the star at different wavelengths and calibrating out the moving speckles, the image of the exoplanet may emerge.

This new detection from 1998 data essentially uses a new algorithm to combine many images of stars from the telescope taken in a particular position to get a kind of “master image” that can be subtracted from the data to remove far more noise from the image than before. Where before it was swamped by the starlight, the planet was now clearly visible on top of the residual noise. This begs an obvious question: how many more could we detect by going through all this archive data? Ground-based data too could be revisited using this new post-processing algorithm, but the authors suggest, and I would agree,  that atmospheric turbulence may make the noise and speckles too variable in time to get a reliable “master image” as was done for HST. But the success of their method on space-based data bodes well for the exoplanet-spotting capabilities of Hubble’s successor, the James Webb Space Telescope.

Reference: Lafrenière, Marois, Doyon & Barman, HST/NICMOS detection of HR8799 in 1998, ApJL in press, 2009.


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