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Taking extrasolar planet imaging to a new level with vector vortex coronagraphy

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Vortex coronagraph helps spot planet formation in the act

A new device at the Keck Observatory in Hawaii has delivered its first image, showing a planet forming in the swirling gas and dust around a young star. Based on the latest vortex coronagraph technology, the device permits the direct image of distant planets closer to their host stars than was previously possible.

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Young stars are often surrounded by a rotating ring of dust and gas known as a protoplanetary disk. The material in this disk can clump together, indicative of a planet in the process of forming. A combination of dedicated high-contrast imaging equipment installed in the Keck II telescope and sophisticated image processing techniques led a team of researchers from Belgium and Sweden to take direct images of the accumulated matter around the star MWC 758. This feat has long been considered vital in the search for planets that orbit relatively close to their star just like our own.

Exoplanets galore

The study of exoplanets has seen remarkable discoveries over the past two decades. There are more than 3 000 confirmed exoplanets, but the overwhelming majority of these discoveries are a result of indirect imaging techniques. These can be either a measurement of the changing wavelength of the light the star emits, which is caused by the gravity of the orbiting planet, or the minute dimming of the star when the planet passes in front. Direct imaging has been technically challenging: being more than a million times fainter, exoplanets are lost in the glare of their host star as seen from Earth. Although large telescopes worldwide have recently started to employ high-contrast imaging techniques, current coronagraph technology can only detect ‘lonely’ planets that orbit their star at a great distance. The orbital period exceeds 20 years. In comparison, all rocky planets in our solar system orbit the Sun in less than two years.

Taking a closer look at Earth-like planets

The EU-funded vortex project pioneered new vortex coronagraph instruments, which enable astronomers to get closer to the realm of short-period planets. The refined instruments enable high-contrast imaging of planetary systems at minute angular separations between the planet and the star. “Young stars are not close to our solar system, so we have to look farther away in the galaxy where stars and planets appear closer together. The ability of our vortex coronagraph technology to survey distant stars for planets is important for catching planets still forming,” notes project coordinator Olivier Absil. The secret behind the project success was the improvements in the existing manufacturing process for the vortex phase mask. This part of the vortex coronagraph redirects light away from the detectors by combining light waves and cancelling them out. Vortex phase masks consist of concentric microstructures that force the star light waves to swirl about the mask centre, creating the vortex singularity. Researchers etched the concentric microstructures into a diamond. “Our vortex phase masks can cancel stellar light by a factor larger than 1 000, which is the highest rejection ratio ever measured,” says Absil. Researchers built on their successful work by training a machine learning system on what to look for in the data. The aim was to discern real exoplanets from residual stellar light. This is the first time ever that researchers have applied machine learning to hunt exoplanets. Results so far show that it outperforms other data processing techniques. Until the 1990s, planet formation theories were based on our solar system. “We now understand there is a huge variety of planetary systems out there, and so our solar system should not be regarded as an archetype,” concludes Absil.


vortex, exoplanet, vortex coronagraph, vortex phase mask, high-contrast imaging, machine learning, MWC 758, Keck Observatory

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