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Capturing polarised light in the search for alien plants

A new way to decipher the light from distant worlds could give us unmistakable evidence of extraterrestrial photosynthesis, and maybe alien plants, finds astronomy author Colin Stuart in the New Scientist. In his article, he describes the work of the group led by Leiden astronomer Rob van Holstein. They were the first team to detect polarised light from an exoplanet.

Astronomers are in little doubt that a plant-filled planet exists somewhere in the universe. The challenge, however, is to find one and prove that it exists. The team of Rob van Holstein has devoted itself to this task. They do so by searching for polarised light coming from exoplanets (see box).

Polarised light? - New Scientist explains

Think about light as an electromagnetic wave. Usually the light we encounter from the sun or a light bulb is unpolarised – the wave vibrates in various directions. Polarised light, on the other hand, is restricted to vibrating only in certain directions. It just so happens that starlight is polarised when it reflects off a planet’s surface, and the way it is polarised should contain clues about exactly what is doing the polarising – including life.

Light with the signature of life

Polarised light might make it possible to nail down the presence of liquid water on other planets, for example. But it might get even better: What if polarised light could dazzle us with unmistakeable evidence of biology: life!

Big breakthrough for Leiden astronomers

Unfortunately, polarised light reflecting of distant planets is extremely difficult to observe: the light is very dim and only one per cent of the light is potentially polarised.

Yet, in January this year, the team of Rob van Holstein at Leiden Observatory managed to capture polarised infrared light from a disc of dust and gas surrounding the exoplanet DHTaub. They did so using the Very Large Telescope in Chile.

Three images of the exoplanet DH Tau b. The left image shows all light, both unpolarised and polarised. The middle image shows only polarised light. The right image additionally shows the direction of the polarised light. In polarised light, the planet DH Tau b is visible, which points to a disk of dust and gas around this planet. The disk around the star is also visible. (c) ESO/VLT/SPHERE/Van Holstein et al.

Using the Earth as a test planet

The next challenge is to repeat this success for smaller, Earth-like planets. And furthermore, to figure out what signatures plant life would leave in any polarised light we can detect.

To test how life exactly polarises the light, how this light behaves in space and how the resulting polarisation signal looks like, astronomers use the Earth as a test planet. They for example study the earthshine: the light of our own planet reflected back to us by the moon.

Leiden astronomer wants to take selfie standing on the moon

But Dora Klindžić, also from the Leiden Observatory, had a better idea. 'We can't study the light from Earth properly until we step away from the Earth and take a selfie from the moon,' she says. 

Klindžić is the driving force behind the Lunar Observatory for Unresolved Polarimetry of Earth mission. Her idea is to piggyback a polarimeter onto a future trip to the moon and drink in the polarised light from Earth as it hits the lunar surface. Rather than capturing a beautifully focused image of Earth, this picture would cram all of Earth’s light into a single, unresolved pixel. The idea is to mimic the way we see the light from distant exoplanets.

New generation of telescopes to answer age-old question

Within a year, Klindžić hopes to have the first prototype to test. The other big project in the pipeline is the Extremely Large Telescope (ELT), with a mirror almost 5 times larger than the Very Large Telescope, and a state-of-the-art spectropolarimeter. Van Holstein defenitely believes the ELT will enable detecting and characterising rocky exoplanets in the near future.

'It could well herald the dawn of a new era in our search for life elsewhere,' concludes Stuart. 'One that gives us the best chance yet of answering that age-old question: are we alone in the universe?'

Further reading

Read the whole article in New Scientist: Colin Stuart, Far-flung flora, New Scientist, Volume 250, Issue 3333, 2021, Pages 46-49.

Read the paper by Holstein et al: A survey of the linear polarization of directly imaged exoplanets and brown dwarf companions with SPHERE-IRDIS – First polarimetric detections revealing disks around DH Tau B and GSC 6214-210 B. By: R.G. van Holstein et al. Accepted for publication in Astronomy & Astrophysics.

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