Among the numerous problems in actually imaging an exoplanet is the fact that telescopes produce artifacts that can mask the faint planetary signature. Light diffracts as it passes through the aperture of an optical telescope, causing a series of concentric rings to surround the observed star. This effect, known as an Airy pattern, has a bright disk at center whose size determines how small an object the telescope can see.
But there are always ways of making a virtue of necessity. A team led by Niranjan Thatte (Oxford University) and Laird Close (University of Arizona) have developed a technique that effectively uses the artifacts produced by diffraction to determine the position of a dim stellar companion and retrieve its spectrum. The idea is that when the wavelength of light being studied is changed, the telescope artifacts can be seen to shift position, while the actual object around the star will not move.
Here’s how an ESO news release puts the matter:
So if the image has an internal reflection of the star masquerading as a planet, this phantom planet will be in one location in the image when looking in red light, and another when looking in blue; a real planet will stay at the same place no matter what colour of light one examines. Therefore, with the combined detection of spectra and position, one can see what is scaling, subtract it, and be left with what is fixed, that is the target dim object.
The method is called ‘spectral deconvolution.’ Using it with the SINFONI integral field spectrograph on ESO’s Very Large Telescope, the researchers have demonstrated its viability on the stellar system AB Doradus. 48 light years from Earth, the quartet includes AB Doradus A and a faint companion AB Doradus C that orbits a scant three AU away. Imaging AB Doradus C, first accomplished in 2005, is no mean feat, not just because of its proximity to AB Doradus A but also due to the fact that it is almost ten times less massive than the larger star. Getting an undiluted spectrum takes the observations to a new level.
Image (click to enlarge): The left side shows a raw image, while the right side shows the result after the newly developed technique was applied. Thanks to this technique it is possible to study the faint AB Doradus C (about 100 times fainter than its host), once the contamination from the brighter AB Doradus A and the artifacts due to atmospheric turbulence are subtracted. AB Doradus is the closest faint companion ever detected by imaging. Credit: ESO.
With the new results, AB Dor C now reveals itself to be a red dwarf, though one that hangs on the borderline between red and brown dwarf at 93 Jupiter masses. The researchers believe that the star is heating enough that in about a billion years it will begin the process of fusing hydrogen into helium. With a luminosity fully a thousand times less than that of Sol, AB Doradus C gives us a hint what the new observing technique can do. Add future adaptive optics onto existing telescopes and we may find that obtaining the spectrum of a terrestrial-class world is possible not just from space but from right here on Earth.
Is it possible to retrieve a _spectrum_ by these means? The summary sounds as though you need to “integrate” over the full spectrum available to retrieve an image – that is, you get the integrated light curve from the companion and its location, but not a spectrum.
That’s a great question, Alastair. And the answer seems to be that you can indeed get a spectrum, based on this from the ESO release: “Using SINFONI and this new technique, the astronomers could for the first time obtain a spectrum of the object that is free from the light of the brighter companion and that contains all the information necessary for a complete classification.” Which sounds like more than simple location, and provided these researchers with the additional information needed to better understand AB Doradus C.