a different refractive index than freshwater rain, so the angle of the rainbow tells you what it is you are looking at. This is similar to how colleagues of mine in the 70s, worked out that on Venus the clouds were sulphuric acid, as it creates a rainbow under a different angle. To find liquid water we want to see water clouds.”
Searching for the Light of Life Frans Snik of the FALCONER project describes how his research findings will help the world’s biggest telescopes, including the Extremely Large Telescope (ELT), find signs of life on far off exoplanets, with the bonus of creating spin-off innovations that will have profound implications for research. In 1991 we only knew the planets in our own solar system. Today, in contrast, we have evidence of over 4,200 worlds orbiting stars light years away. With advances in telescopic technology and with the largest telescope in history, the aptly named Extremely Large Telescope (ELT), now only five years from completion, we can expect vastly improved imaging of exoplanets. Within 20 years, aided with the next generation of space telescopes launched after 2035, it’s expected we’ll be able to detect life on worlds that appear Earth-like. The FALCONER project is pioneering direct imaging of exoplanets, with an aim to suppress the bright glare from far off stars that make it impossible to see the smaller planets that would orbit the habitable zones around them. In addition, the researchers are looking into novel ways to detect definitive signs of life from observing planet-light. “We now know that there are more planets than stars, so every star you see and every star you don’t see, statistically has at least one planet. Take five stars and one will have a planet that looks like Earth. It will be rocky and if it’s the right distance from the star, if there is water, it will be liquid, and life as we know it could emerge. We have not yet seen any of them! We know they are there from indirect observations. A few that pass in front of their star we can start to characterise, but the vast majority of them will not be accessible to this transit method. The effects from the light of the parent star to these planets blocks our view of them. The challenge is to remove most of this starlight and see planets that are orbiting the star, so we can analyse their light, and thus characterise their atmospheres and potential surfaces, and, ultimately, find signs of life ,” said Frans Snik. The art of planet hunting and planet viewing is fraught with difficulty. Snik adds: “To see older planets and to see planets that may have life, you need to get a lot closer to the star than current techniques allow. Most of all, you require a much higher contrast than currently possible. We can now see young gas giants in the infrared at a contrast of 1:100,000. For Proxima b we need 1 in 52
Moonlighting FALCONER PhD students David Doelman and Steven Bos (standing) with collaborators Zane Warriner and Julien Lozi (sitting) at the Subaru telescope during first light for the vAPP coronagraph installed inside the SCExAO instrument.
Image of the star eta Crux, as observed with one of our liquid-crystal vAPP coronagraphs at the MagAO instrument. The light is mostly split into the two images on the left and right, where dark holes are created very close to the image of the star, on either side. Otten et al. ApJ 834(2) id.175 (2017)
ten million, down to 1 in ten billion for an Earth around a solar-type star. To put this in perspective, if the halo of the star was Mount Everest, if you want to detect a Jupiter sized planet then that would be a red blood cell in comparison – and the Earth would be a bacteria. So, you could say that bulldozing down Mount Everest is our challenge and that’s what we call coronagraphy.”
ET is in the Dark The simple version of a coronagraph is trying to block starlight but this does not always work well, for example if your telescope vibrates, starlight leaks around it and blurs the view. “Instead of blocking it, we redistribute starlight,” continued Snik. “In the FALCONER
project we accomplish this with liquid crystals. This is the same stuff you are looking at in most smartphone screens but we are using it in a different way. We are literally painting liquid crystals on pieces of glass. When you put this glass between polarisers, just like polarised sunglasses, you see this pattern that corresponds to the phase pattern that we want to impose. With a flat piece of this glass stuck into a telescope at the right location, it manipulates the light from the star that comes into the telescope, such that the halo of starlight is suppressed, while both the core of the image of the planet and also the star make it through. The cool thing is that with these liquid crystals we can apply phase patterns that do not depend on wavelength, unlike classical techniques. This
Photos of vAPP coronagraphs with different liquid-crystal patterns between crossed polarizers.
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allows us to achieve contrast performance over a broad range of wavelengths, enabling spectroscopy of our targets.” Snik and his team carried out this technique with several telescopes and it removes starlight right next to the star down to 1:100,000, close to the fundamental limit for ground-based telescopes. They are integrating additional optical methods to distinguish exoplanets from residual starlight, and thus enhance the contrast further by orders of magnitude. With the ELT and other proposed ground-based telescopes, exciting opportunities for imaging known exoplanets open up. “With the ELT, we’ll focus on nearby stars and known targets and use the observing time to characterise them. We know that small planets in the habitable zone are ubiquitous – we just have to learn how to see them and that is what we are doing here. With the ELT, we will have a thirty nine metre telescope. Just in terms of mirror surface, this is larger than every telescope that’s ever been built. For the first time we will make sharp enough images to enable us to look next to nearby red dwarf stars, to see if there are rocky planets that can support life. If there is one and if there is life, we will pick it up. If they are not there, the next generation of space telescopes, that we aim to have equipped with our technology, will fly twenty years from now and they will do it for us, specifically for Earth analogs orbiting Sun-like stars. So at least within our lifetimes, generally speaking – we will see the first signs of extraterrestrial life if it is present anywhere in our cosmic neighbourhood.”
our sun from all the different angles, the same way it will be possible to observe exoplanets. Snik elaborates: “We are building this instrument that will hitch a ride to the moon and we are working with companies and countries that are landing on the moon in the next couple of years. Our instrument is called LOUPE; Lunar Observatory for Unresolved Polarimetry of Earth. The instrument will be only slightly larger than a Euro coin. It will measure Earth as a whole lot of dots, as a function of wavelength, as a function of polarisation. This will be a training run on our own Earth while we wait for these big telescopes to come online.”
Another indicator to chase, to confirm life on a planet, is molecular oxygen in the atmosphere. For a long time, there was just algae on Earth and all it produced was oxygen. All the oxygen we breathe should not chemically be here on planet Earth by rights. It is the algae, forests and plants that produce the oxygen that we breathe – they are solely responsible for it. “If you see a planet with oxygen, and, or, other particular molecules in its atmosphere, then you know stuff that chemically should not be there is, and that means we may have a strong indicator for life. We’re collaborating with scientists who are building models to show how oxygen shows up in spectra and in polarisation of reflected light. We need to validate these models, so colleagues of mine have already done one bit of validation by looking at the dark side of the moon. If you look carefully at the dark side of the moon it is not totally dark, there is a little bit of light and that light comes from us, like looking into the mirror – it’s the Earthshine. By taking a large telescope and pointing it at the moon they did detect life on Earth by detecting oxygen and the green and infrared light originating from vegetation.” The moon is an ideal test platform, for continuously observing a world, namely Earth, for signs of life and we plan to send an instrument to the moon and point it toward Earth. As Earth will always be visible in the sky, it would only take a month to observe Earth illuminated by
Whilst detecting signs of water and oxygen are encouraging as signs for life, they are not 100% definitive, so what is? There is one remarkable signature of life that might provide the conclusive proof astronomers and biologists are looking for. Life’s molecular building blocks are amino acids and sugars. They each have a ‘twin’ but not identical, like your right and left hand will mirror each other but are not the same, for example you could not put your right hand in a left-handed glove. This is known as ‘chirality’. Amino acids linked with life on Earth are all ‘left-handed’ in their formation and all sugars characterising life on Earth are ‘right-handed’. Frans Snik is working with biologists to see if this could be used as the best way to detect life signs in the cosmos, as the molecular handedness leaves an imprint on the handedness of light in the form of circular polarisation. After large-scale testing with beetles and vegetation, a machine was devised to detect life, on the basis of this signal. “We built this machine which worked in the lab and then we put it on the roof of a building in Amsterdam, so we started looking around for
Artificial Turf
Distant Trees
Single-handed Detection
Finding Signs of Life So how do you find signs of planetary life, simply by observing light? There are basic ingredients that you need for life to exist on a planet. You need a stable environment, six elements from the periodic table, carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur to build biomolecules, heat from the parent star, or from the planet itself, and liquid water. Starting with how to find water. Any liquid has a special relationship with how it interacts with light. “For example, sea water splash has
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Circular spectropolarimetric observations of artificial grass (left) and a living forest (right). Patty et al. Astrobiology 19(10) 2019
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Falconer Forging Advanced Liquid-Crystal Coronagraphs Optimized for Novel Exoplanet Research
Project Objectives
The FALCONER ERC StG team at Leiden University develops advanced liquid-crystal optics for telescopes around the world and in space to manipulate light from stars and planets. The main goal is to suppress starlight, and analyse light from exoplanets to characterise them. They also use the Earth for “target practice” to learn how to detect signs of life.
Project Funding
FALCONER ERC StG 678194 MONOCLE H2020 776480 EU-citizen.science H2020 824580
Project Partners
All telescope/instrument teams are indicated on the world map to the right. • NCSU Geometric Phase Photonics Laboratory • ImagineOptix for liquid-crystal optics production • TU Delft + Cosine for LOUPE
Contact Details
Project Coordinator, Frans Snik Assistant Professor Leiden University 2333 CA Leiden Room number 1116c E: snik@strw.leidenuniv.nl General introduction to coronagraph technologies: Snik et al. Proc. SPIE id.107062L (2018) The vAPP coronagraph: Snik et al. Proc. SPIE id.84500M (2012) Otten et al. ApJ 834(2) id.175 (2017) Doelman et al. Proc. SPIE id.104000U (2017) Bos et al. A&A 632, id.A48 (2019) Other liquid-crystal optics: Doelman et al. Proc SPIE id.107010T (2018) Doelman et al. Opt. Lett. 44(1) (2019) Snik et al. Opt. Mat. Expr. 9(4) (2019) Doelman et al. PASP 132(1010) (2020) LOUPE: Klindžić et al. Philosophical Transactions A (submitted) Circular polarization and homochirality: Patty et al. Astrobiology 19(10) 2019 LSDpol instrument for the ISS: Snik et al. Proc. SPIE id.111320A (2019) SPEX: Snik et al. Proc. SPIE id.77311B (2010) Van Harten et al. Atm. Meas. Tech. 7(12) (2014) iSPEX: Snik et al. Geophys. Res. Lett. 41(20) (2014)
Dr Frans Snik
Dr Frans Snik is assistant professor at Leiden Observatory, Leiden University, the Netherlands, and member of the Dutch Young Academy. His group develops advanced optical technology for astronomy (specifically exoplanet observations) and Earth observation (climate science, pollution and vegetation monitoring). In addition, he pursues citizen science and collaborations with artists.
Telescopes and instruments where vAPP coronagraphs and other advanced liquid-crystal optics have been, or will be installed.
life and we pointed it at sports fields and didn’t get any signal at all. We thought our instrument wasn’t working or our hypothesis was flawed but then it turns out this is artificial grass! We pointed it at a forest 3 km away and we got this very nice, positive detection of life. The preferred handedness of molecules leaves an imprint in the form of preferred handedness of light: circular polarisation. We are now building a version to plug into the International Space Station to map our entire planet in circular polarisation from orbit.”
Spin-offs and Innovations Fundamental sciences like astronomy can have spin-offs on Earth that are of immediate relevance. With astronomical techniques it’s possible to work out what’s inside atmospheres of planets far away and of course this is also useful to figure out what is in Earth’s own atmosphere. This holds real value in the era where climate change is a serious threat for us all. One application is, for example, measuring dust pollution and other anthropogenic aerosols. Astronomers know how to measure dust and there is a lot of it in our atmosphere that we are ingesting and it can reduce life expectancy, as well as have significant implications for affecting climate. There is a lot of information we don’t really have that would be useful. Some dust particles scatter sunlight back into space, while others retain heat. The size of aerosol particles can have implications for how deeply it impacts on our health or say, the jet engines of a plane in case of volcanic ash clouds. Whilst we can measure how much dust pollution is in the atmosphere by weight, these measurements are also quite limited and very local. “We need to measure the properties of all aerosols on a global scale to measure the effects on climate change and on our health,” said
Snik. “With measurement techniques derived from astronomy, we can infer the amounts of dust in the atmosphere and the sizes and compositions of the particles. To achieve this, we invented a new technique to accurately measure polarisation spectra of sunlight scattered by aerosols. We now have an instrument suite called SPEX to measure dust and aerosols in our own atmosphere. One of them is now flying on what used to be a U2 spy plane and this technology has been selected to fly on the next NASA climate mission PACE, where we will measure the effects on climate change from a satellite.” This technology, a spin-off itself, spawned a further spin-off innovation. A device with this functionality was adapted to attach to smartphones (iSPEX), so anyone could measure air pollution and have it recorded on a map in a large-scale ‘citizen science’ project. “We produced twenty thousand of these add-ons to smartphones so people across the Netherlands were performing measurements on air pollution. This added valuable information. The air pollution pilot project gained momentum covering cities like Barcelona, Rome and London during 2015. We are now developing iSPEX2.0 for most modern smartphones to measure both air pollution and water quality. This is for the MONOCLE project, which is a current EU project. In fact, we are very active in the EU, promoting citizen science as a method of research.” As with any experimenting, innovation and scientific curiosity, what started with looking at ways to understand exoplanets, has spawned technologies that are now assessing our own world. The FALCONER project will await the largest telescopes to come online to detect life elsewhere in the universe, whilst simultaneously making life on Earth better for us all. © ESO
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