A night at an Extremely Large Telescope

Spending time at the 4-metre class Anglo-Australian Telescope (AAT) in New South Wales, Australia, gives a hint of the kind of findings that will come from Extremely Large Telescopes, Fred Watson writes. 

Inside the Anglo-Australian Telescope at the Siding Spring Observatory in New South Wales.
Ben Wrigley

Also see: our feature article which examines the new generation of Extremely Large Telescopes and how they will show us, for the first time, what exoplanets are really like.

A hint of the kind of findings that will come from the ELTs is revealed in the work of some of my colleagues as they use Australia’s biggest optical telescope – the 4-metre class Anglo-Australian Telescope (AAT) at the Siding Spring Observatory near Coonabarabran in New South Wales.

The scientists, from the University of NSW, include an old friend, Professor Jeremy Bailey. I am with them in the control room for an hour or so during one of my periodic visits, and find them engaged in work of a very different kind to the routine star survey observing that I do.

They are homing in on a known exoplanet to study its atmosphere using an instrument that detects polarised light – in which the light waves oscillate in specific directions rather than randomly. The instrument rejoices in the acronym HIPPI – high-precision polarimetric instrument. It lives up to its name by measuring polarisation to the astonishing accuracy of a few parts per million.

What does detecting polarised light have to do with studying exoplanets? Think of it this way. If you look at the blue daytime sky wearing a pair of polarising sunglasses, its apparent brightness will change if you tilt your head to left or right. That’s because the light of the sky is polarised – and it’s polarised not just on Earth but on any planet that has an atmosphere. HIPPI does something similar to the sunglasses.

So here we are in the dim, spartan functionality of the AAT control room, measuring polarisation in the atmosphere of a distant planet. My colleagues subtract the unpolarised light of the planet’s parent star to reveal evidence of clouds in the planet’s atmosphere. All done with a telescope as modest as the AAT.

Now imagine a decade or so into the future, when Bailey and his collaborators are using a descendant of HIPPI on an ELT. “Look at this,” someone might say, displaying the polarisation signature of a rainbow – proof positive of liquid in the atmosphere on a exoplanet.

Such direct observation, rather than deductions based on analysing the light of a parent star, will revolutionise our knowledge of distant planets.

Fred Watson is an astronomer at the Australian Astronomical Observatory. His books include 'Stargazer: The Life and Times of the Telescope' and 'Star-Craving Mad: Tales from a Travelling Astronomer'.
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