When astronomers use a new telescope or other piece of equipment for the first time they like to make it special. After all enough money and effort has gone into the process, and there's all the excitement of making sure it works. So for the Spectro-Polarimetric High-contrast Exoplanet Research instrument (SPHERE) they picked a star that looks remarkably like the Eye of Sauron.
The purpose of SPHERE is so exciting it didn't really need anything special, but its operators gave it a beginning to remember anyway. Its purpose is to directly view planets around other stars. Although this has been done before, only a handful of planets with just the right combination of characteristics have been able to be imaged this way since 2004.
A new generation of cameras, including SPHERE and the Gemini Planet Imager, hope to change that. The first essential feature is a disk, called a coronagraph, to block the light of the star so that the planet does not get lost in the glare.
However, for any telescope located within the Earth's atmosphere this can only do so much – some of the light will have been scattered before it reaches the telescope. Adaptive optics help here, adjusting the shape of the mirrors to balance out what the air above is doing. Planets can polarise the light they reflect, so the capacity to detect polarization is also important.
The European Southern Observatory (ESO) notes that SPHERE uses another mechanism to pick the tiny planetary signal from the noise of stellar glare. The make it, “Take many pictures of an object, but with a significant rotation of the image in between each. Features in the pictures that rotate are artefacts of the imaging process, and features that stay in the same place are real objects in the sky. “
These features also make SPHERE suitable for other work, and one of those is to provide clear images of planetary systems in formation. The image above is of HR 4796A, part of a wide, eight million year old binary system 237 light years away. After blocking out the HR 4796A we are left with a view of the dust surrounding it, which is eventually expected to form into planets. The bright ring is thought to be the result of a planet, as yet not detected directly, whose gravity is tugging the dust into formation, as Saturn's moons do to its rings.
SPHERE is attached to the Very Large Telescope in Chile, and has been praised by its Principal Investigator Jean-Luc Beuzit, who said, “SPHERE is a very complex instrument. Thanks to the hard work of the many people who were involved in its design, construction and installation it has already exceeded our expectations.”