Spinning starlight into little whirlpools could be the key to seeing weird and wonderful new planets circling distant stars.
Imaging extrasolar planets directly is a huge challenge because they hide in the glare of their parent stars. But by turning starlight into an 鈥渙ptical vortex鈥� with a dark core, astronomers could remove the nuisance starlight from the picture.
More than 160 extrasolar planets have been detected over the past decade. Most have been detected indirectly, because their gravity makes their parent stars appear to wobble. Even if a telescope has enough magnification to separate the star and the planet in the sky, it is almost impossible to measure the planet鈥檚 reflected light because the star is typically more than 10 million times brighter.
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One possible solution would be to use a tiny opaque disc in a telescope, positioned so that the starlight is blocked while the planet鈥檚 light sneaks around the side. The trouble is that the starlight will bend around the obstacle and interfere, creating bright diffraction bands in which the planet can get lost.
Spiral staircase
But now Grover Swartzlander, Gregory Foo and David Palacios from the University of Arizona in Tucson, US, have come up with another solution. They think the starlight could be removed by using a helical, transparent mask with a shape that resembles a single revolution of a spiral staircase (see diagram).
When laser light enters the mask, it travels more slowly than in air. When the phase difference between light that traverses the thin parts of the mask and light that traverses the thick parts is equivalent to two full waves, something unexpected happens.
Light in the central core destructively interferes to create an 鈥渙ptical vortex鈥� with a dark core, the light spinning out into a bright ring around it. 鈥淪urprisingly, it creates a large disc of zero intensity,鈥� says Swartzlander.
Calculations by Swartzlander鈥檚 team suggest the dark core could act as a window through which a planet鈥檚 light can freely pass with the starlight almost entirely removed. In lab trials using laser light to mimic a star and planet, their vortex mask cut the intensity of starlight by 100 to 1000 times without blocking the planet鈥檚 light at all.
Improvements needed
For the technique to be practical, Swartzlander says the optical quality of the mask would have to be dramatically improved and it would have to work over a much wider range of wavelengths. 鈥淏ut these are the first experiments to demonstrate the basic idea,鈥� says Swartzlander. 鈥淭heoretically, our technique would make a dim planet visible.鈥�
His team hopes it might be useful for projects like NASA鈥檚 Terrestrial Planet Finder (TPF), two space observatories being planned that will try to image Earth-like planets and scan them for signs of life.
鈥淪eeing the light of an Earth-like planet next to a star is extremely hard,鈥� says Steve Kilston of Ball Aerospace in Boulder, Colorado, a company contracted by NASA to work on the TPF design.
He says the current inability of the optical vortex mask to work with multicoloured light is a big drawback: 鈥淭here have been various solutions proposed, many of which have great promise and all of which have great challenges. We need to follow several of them until we find the one that鈥檚 going to work the best.鈥�
Journal reference: Optics Letters (vol 30, p 3308)
