Dr Beth Biller is an astronomer based at the University of Edinburgh’s (UoE) Institute for Astronomy (IfA). We have caught up with her to find out more about how she will use a new and powerful next-generation instrument in her astronomy research. The instrument is called ERIS, and it’s currently being built for the European Southern Observatory (ESO)’s Very Large Telescope (VLT). The VLT is one of the world’s largest and most advanced telescopes, based at Paranal Observatory in Northern Chile.
ERIS stands for Enhanced Resolution Imager and Spectrograph, and in 2020 it is expected to be operational on the telescope. A component of ERIS, called NIX is being built at STFC’s UK Technology Astronomy Centre (UK ATC), in Edinburgh. NIX, a state-of-the-art camera system, will image in the infra-red. NIX is due to be shipped to Germany, where a team there will integrate it into ERIS. The instrument will then undergo 6 months’ of tests before it is delivered to the VLT.
Beth, what is particularly exciting for you, as an astronomer, about what ERIS will enable?
“First of all, ERIS will be cutting edge from the moment it is operational on the telescope, because it will open up wavelength ranges that are very difficult to access from the ground, specifically from 3-5 µm. It has also been developed with ground-breaking high-contrast imaging technology, with the capability to directly image young giant exoplanets.”
Why is the wavelength range 3-5 µm difficult to see from the ground?
“Visible light is the light that we can see. Typically visible light has a wavelength at about 0.5µm. However, ERIS is designed to work in the near infra-red at wavelengths between 2-5µm. This is a tricky wavelength range to work in, due to what we refer to as ‘thermal radiation’.
“Any object at a given temperature emits light with a specific spectrum and intensity, depending on that temperature, and the peak intensity of light from a given object depends on its temperature. For instance:
“This creates a practical problem for a telescope working at wavelengths longer than 3 µm or so – everything (all of the different mirrors and optical elements in your telescope, the air, your object) is glowing at these wavelengths! Each glowing element in your telescope design adds noise to your image. If this noise level is too high, the light from the object you are trying to be imaged will be completely overwhelmed by it.”
How does ERIS overcome this ’thermal radiation’?
“ERIS will be world-leading particularly in the 3-5µm wavelengths, specifically because it is designed to have fewer optical elements to begin with and many of the elements it does have will be cooled to very low temperatures, thus reducing their glow.
“This means ERIS will have the ability to directly image exoplanets at these wavelengths – while exoplanets have been imaged before at 1-3 µm, not much imaging has been done at 3-5µm due to the above-mentioned issues with the atmosphere and telescope glowing.”
What do you mean by direct imaging?
“Exoplanets are planets that orbit stars that are not our own Sun. They essentially sit outside of our solar system. Thousands of exoplanets have been detected in the last 25 years, but most of those planets have been detected via indirect means, either by their tug on the star that they are orbiting or by slight dips in the light of the star as the planet passes in front of it along our line of sight.
“In contrast, ERIS will be able to actually take pictures of planets, directly collecting light from the planets themselves. This is only possible right now for large and bright planets, otherwise the contrast between the star and planet is just too high to allow imaging.”
Which planets can we image now, with the current tech?
“The planets we can image to date are what I often refer to as “baby Jupiters” – similar in mass (or a bit higher mass) than Jupiter, but much younger. Jupiter is the largest planet in our solar system and is about 318 times as massive as Earth – these planets are even more massive, but more importantly, they have just finished being formed. At this stage, they are much hotter than Jupiter – with temperatures similar to that of a candle flame.”
How will ERIS ‘see’ exoplanets?
“ERIS will ‘see’ in the infrared wavelength, where the contrast difference between the star and the planet is reasonably favourable; and with high-contrast imaging technology such as a coronagraph, it is possible to image them. The NIX part of ERIS (the camera system) has been designed with great state-of-the-art coronagraphs.”
How do coronagraphs work?
“Coronagraphs work by blocking out the direct light from a star so that we can see nearby objects – which otherwise would be hidden in the star's bright glare. I’m hoping to use these coronagraphs extensively in my own research, specifically with a custom filter I designed with help from the science and engineering teams at the UK ATC and have now procured using a Royal Society Research grant.”
How will this custom filter for the NIX part of ERIS be used?
“In the filter wheel within NIX we have three filters (which work to select light at a range of specific wavelengths). One is the custom filter (and the other two are standard filters)
“This custom filter will enable ERIS to sample a part of the spectrum for brown dwarfs as well as giant planets. Brown dwarfs are objects that are more massive than planets but less massive than stars.
“The spectra of both brown dwarfs and planets look quite different from those of stars at these wavelengths. Although we only sample 3 different points of the spectra with our 3 filters, it’s enough to determine if the object we are looking at is a star – or is a lower mass brown dwarf or exoplanet.
So, this means that this filter will be very important for confirming exoplanets orbiting stars?
“Yes. When astronomers suspect they are seeing a candidate exoplanet orbiting a star, they need to check if the candidate exoplanet is moving with the same motion on the sky. Otherwise, the candidate might just be a background star that happens to appear close to the star of interest in the sky.
“To check whether the motion of the candidate exoplanet is similar to that of the star requires several years’ worth of observations. But using the custom filter we have designed for ERIS, we can determine if the candidate exoplanet has a spectrum that looks like that of an exoplanet or that of a star, and thus confirm the candidate exoplanet with a lot less telescope time.”
That is exciting, so as well as being able to discover and image exoplanets we’ve never seen before, the design of the camera system offers astronomers efficiencies too?
“Yes. ERIS provides astronomers with another way to determine if an object is what we think it is. ERIS will help us also to discover free-floating planetary mass objects – essentially planets that are not orbiting stars, because they might have been ejected while forming…”
How are you preparing for using ERIS on the telescope?
“Right now, I am working to design the surveys of the sky that ERIS will do. I’m figuring out which stars are the best targets, with the goal of detecting and confirming extra solar giant planets – and to capture images of them.”
Last updated: 16 September 2019