That’s the kind of headline that can leave us scratching our heads. How can you see tree shadows on other worlds, when those planets are tens or hundreds of light years—or even further—away. As it turns out, there might be a way to do it.
One team of researchers thinks that the idea could potentially be used to answer one of humanity’s long-standing questions: Are we alone?
At some point in their lives, almost every thinking person comes up against this question: “Are we alone in the Universe?” There’s no answer, yet, but we keep looking. For many people, finding life means more than finding some struggling extremophile, a single-celled organism in a squalid, hot, hole in some planet surface somewhere. Single-celled organisms would be a revelation to scientists, and to science-minded people. But it’s hard for many people to relate to a layer of biological scum on a cave wall.
It’s complex life that we want. Complex, multicellular life that tells us that yes, life can develop and grow into more complexity—and even become intelligent—somewhere other than on Earth. And while we now know of thousands of exoplanets, with more being discovered all the time, most of them aren’t suitable for life.
Some of those planets might be, and we’re getting close to the day when we can reliably examine their atmospheres for biosignatures. (We’re looking at you, James Webb Space Telescope). But in our rapidly-approaching future, astrobiologists will need more tools they can apply to the task.
One group of researchers has their eye on the future; and on trees. Soon, we’ll be able to reliably directly image some of our closest exoplanet neighbours, and to hunt for signs of life. What should we be looking for? Tree shadows, according to a new study.
“Earth has more than three trillion trees, and each casts shadows differently than inanimate objects.”
Christopher Doughty, Lead Author, Northern Arizona University
The new study is titled “Distinguishing multicellular life on exoplanets by testing Earth as an exoplanet.” The lead author is Christopher Doughty, an assistant professor at the School of Informatics, Computing and Cyber systems, Northern Arizona University. The paper is published in the International Journal of Astrobiology.
This study looks specifically at complex, multicellular life, but not at technology-using life. The authors wanted to know if there’s a way to detect that outside of our Solar System. On Earth, the largest, most ubiquitous form of multicellular life is trees. And trees cast shadows. Was there a way to detect tree shadows on other planets?
Forest on Mount Garibaldi, Garibaldi Provincial Park, British Columbia, Canada. Image Credit: By TheSimkin (talk · contribs) – Own work by the original uploaderTransferred from en.wikipedia to Commons by Siebrand., Public Domain, https://commons.wikimedia.org/w/index.php?curid=3957472
“Earth has more than three trillion trees, and each casts shadows differently than inanimate objects,” said lead author Doughty in a press release. “If you go outside at noon, almost all shadows will be from human objects or plants and there would be very few shadows at this time of day if there wasn’t multicellular life.”
So how can we detect a trillion shadows from a trillion trees? The effort to do so falls under the umbrella of what’s called bidirectional reflectance distribution function (BRDF.) Basically, BRDF is the change in observed reflectance with changing view angle or illumination direction, and part of that is shadows.
There’s a crucial difference, the authors write, between complex multicellullar life and simple life. Abundant, upright multicellular life like trees cast shadows at high Sun angles. That’s a critical distinction, argue the authors, and one that can be used to detect life from a great distance. Future powerful telescopes can be used to look for those shadows.
But there are limitations.
This figure from the study shows A: the crater field used for Apollo astronaut training as seen in 1967. B: An example of UAV flyover measuring NDVI at 5 am in 2018 with a current Google Earth image as a background image. C: Closeups of both a tree and a crater in NIR at different times of the day. The key finding is that while the crater produces a shadow at 5 am only, the tree produces a shadow throughout the day. Image Credit: Doughty et al, 2020.
“The difficult part is that any future space telescope will likely only have a single pixel to determine if life exists on that exoplanet,” said co-author Andrew Abraham. “So, the question becomes: Can we detect these shadows indicating multicellular life with a single pixel?”
A single pixel is not much to work with. And other natural structures like craters can cast shadows, too. The question for the researchers became, ‘How can we test this?’
“It was suggested that craters might cast shadows similar to trees, and our idea would not work,” said David Trilling, associate professor of astronomy. “So, we decided to look at the replica moon landing site in northern Arizona where the Apollo astronauts trained for their mission to the moon.”
An aerial image of the Cinder Lake Crater Field shortly after its completion in 1967. Image Credit: NASA/USGS
The Cinder Lake Crater Field is a replica Moon landing site near Flagstaff Arizona. Its volcanic gravel is a good analogue for Moon rock. NASA used tons of dynamite to create craters in the field for astronauts to train on. Over a four day period of controlled explosions, they recreated a portion of the Moon’s surface.
One of the controlled explosions used to create the replica Moon landing site in Arizona. Image Credit: NASA/USGS
To test whether or not crater shadows were too similar to tree shadows, the team employed drones. They flew over the crater field during different times of day to image the shadows. The team found that crater shadows are in fact different enough from tree shadows.
But that was just a small scale test. How would this all play out on planetary scales?
Next, the team of researchers turned to the POLDER (Polarization and Directionality of Earth’s Reflectance) satellite. They were able to observe shadows on the Earth’s surface at different times of the day, with the resolution reduced enough so that Earth was only a single pixel. That was to simulate what distant observers might see.
This figure from the study shows the team’s conceptual design of a distant observer monitoring Earth and the change in backscattering as it revolves around the sun. ? is the azimuth angle, ?i is the solar zenith angle, ?v is the view angle and ? is the phase angle. Image Credit: Doughty et al, 2020.
With that data in hand, they compared it to similar data from Venus, Mars, the Moon, and even Uranus. They wanted to see if Earth’s multicellular life and the shadows it casts appeared different from the other types of lifeless worlds.
The results were mixed, but strong enough to warrant further thought.
This figure from the study shows histograms in NIR for a crater at 5:30 am and 9 am (top), and a tree at 9 am and 11 am (middle). The bottom is the NDVI (Normalized Difference Vegetation Index) for trees (green), craters (black) and bare ground (blue) at 11 am. Image Credit: Doughty et al, 2020.
The team had hypothesized that when compared to Mars, Venus, the Moon, and Uranus, Earth’s directional reflectance would lie somewhere between rocky bodies with no weathering (Mars and the Moon) and cloudy bodies like Venus and Uranus. In their studies they made use of models and empirical results. The modelling results put Earth closer to Mars, while empirical results showed Earth is more in line with Venus.
“If each exoplanet was only a single pixel, we might be able to use this technique to detect multicellular life in the next few decades.”
Christopher Doughty, Lead Author, Northern Arizona University
Next, the team looked at large areas like the Amazon, where trees are abundant. There they could distinguish multicellular life. But when they “zoomed out” to the planet as a whole, it was much more difficult.
For lead author Doughty, the results hold promise for the future. How far in the future is yet to be determined. But the future has a way of arriving when we least expect it. How will this technique fit into our search for life?
“If each exoplanet was only a single pixel, we might be able to use this technique to detect multicellular life in the next few decades,” he said. “If more pixels are required, we may have to wait longer for technological improvements to answer whether multicellular life on exoplanets exists.”
This approach to finding life could dovetail nicely with the work that the James Webb Space Telescope does. The JWST will be able to probe exoplanet atmospheres for biosignatures. But it can’t reveal anything about the source: single-celled or multicellular life.
In an email exchange with lead author Christopher Doughty, he told us “Certain trace gases estimated by the JWST may be indicative of life on that planet. However, it would be difficult to distinguish whether life on that planet could have advanced to the “multicellular” stage as our planet had ~500 million years ago just based on atmospheric trace gases. The technique we have developed would potentially allow us to distinguish whether multicellular life has evolved on that exoplanet.”
“…we are not aware of other techniques to distinguish an exoplanet with multicellular life…”
From “Distinguishing multicellular life on exoplanets by testing Earth as an exoplanet”
In the conclusion of the paper the authors acknowledge some shortcomings in their work. This type of work is in its initial phases, and there are bound to be ways to improve it.
“Overall, in theory, BRDF could distinguish between multicellular
and single cellular life on exoplanets,” the authors write, “but we have recognized issues with both our models and our empirical observations
that must be improved before this technique could be used with confidence.”
In an email, Doughty explained that “The model we used was especially good for vegetation and terrestrial surfaces, but not as good for clouds and oceans.” To improve the method, the team needs a better exoplanet model.
“We could model those planets assuming they had no life, only microbial life, and a planet full of upright vegetation. We would then carefully monitor that exoplanet as it revolved around its star and classify it according to which scenario best fit the modelled results – no life, microbial life, or multicellular life.”
At the end of their paper, the authors ask the most pertinent question: “Should this line of research therefore be abandoned? Theoretically, it could still work and since we are not aware of other techniques to distinguish an exoplanet with multicellular life, we believe further research should still continue.”