Categories: Astrobiology

Searching For The Red Edge: How The Earth’s Forests Are Telling The Aliens Where We Live

People are always worried that alien civilizations will detect the transmissions from our old radio shows and television broadcasts, and send in the invasion fleet. But the reality is that life itself has been broadcasting the existence of life on Earth for 500 million years. 

Blame it on the plants.

In addition to filling the atmosphere with oxygen, plants give off a very specific wavelength visible in infrared radiation. It’s the kind of signal that other civilizations could search for as they’re scanning the galaxy.

It’s what we’ll be looking for too. 

But don’t just blame the plants. Other forms of life have been giving off signals too, signals we can search for as we discover new exoplanets and wonder if they have life there.

NASA’s Galileo spacecraft was launched in October 18, 1989. Its mission, of course, was to fly out to Jupiter and go into orbit, studying the planet and its moons for years. 

Unfortunately, NASA didn’t have the heavy lift upper stage rocket they were hoping to use to send the spacecraft directly to Jupiter. Instead, they planned out a series of clever flyby maneuvers that would give the spacecraft the speed it needed to get out to Jupiter.

First it flew past Venus on February 10, 1990, then Earth on December 8th, and then Earth again exactly two years later.

Galileo’s image of the Earth and Moon as it performed a flyby. Image credit: NASA/JPL/USGS

As Galileo passed Earth, it captured photographs of the Earth and Moon, showing our planet from a unique vantage point.

Carl Sagan looked at the pictures and data coming back from Galileo and declared that the spacecraft had found “evidence of abundant gaseous oxygen, a widely distributed surface pigment with a sharp absorption edge in the red part of the visible spectrum and atmospheric methane in extreme thermodynamic disequilibrium”

In other words, Galileo had discovered life on Earth.

A color composite image of Earth taken on Sept. 22 by the MapCam camera on NASA’s OSIRIS-REx spacecraft. Credit: NASA/Goddard/University of Arizona

In fact, when NASA’s OSIRIS-REx mission took a similar flyby, researchers with the mission performed the experiment again, this time noting that the atmosphere of Earth contained levels of methane, oxygen, and ozone that were much higher than what you would expect from a dead world.

Once again, astronomers discovered that there’s life on Earth.

They also found 2017 levels of carbon dioxide were 14% higher, as well as 12% more methane from when Galileo made the same observations 30 years earlier.

Examples of Earth at various eras: Illustration by Wendy Kenigsberg/Cornell Brand Communications

Can we use this technique to find life on other worlds?

In a recent journal article entitled “Expanding the Timeline for Earth’s Photosynthetic Red Edge Biosignature”, researchers Jack T. O’Malley-James and Lisa Kaltenegger explore what the Earth would have looked like in different eras in its history over the past billions of years. And what kinds of signals they would give off, detectable by our telescopes.

Visit almost any spot on Earth and you’ll see plants everywhere. Trees, jungles, grasses, even the oceans are filled with plants.

And for the last 500 million years or so, chlorophyll has been everywhere, giving plants their green color, which is because they’re reflecting a lot of light at 500 nanometers. 

There are many things that can look green in visible wavelengths. But plants are highly reflective in the infrared spectrum, between approximately 700 and 750 nm wavelength. Like, an order of magnitude more reflective than any other part of the spectrum.

Image of Earth captured by NASA’s MESSENGER spacecraft, highlighting the forests of South American in infrared. Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Look at the Earth in this very specific wavelength, and see it blazing away. That’s the red edge.

But according to this new paper, not only plants will give off an obvious signal. The researchers modeled life on Earth backwards in time at various eras to simulated what our planet would look like to distant observers.

Before plants took hold, the most successful life forms were lichen, a symbiotic partnership between photosynthetic bacteria and fungi. A landscape of lichen looks sage color to mint green. This lichen coverage would have also created a photosynthetic red-edge signature, that was distinctly different from a planet covered by plants.

Between 500 million years and 1.2 billion years ago, the Earth would have been broadcasting in the signal of lichen.

A phytoplankton bloom in Lake Erie off the coast of Toledo, OH. Image Credit: NASA

Before that, cyanobacteria, like the algae that covers ponds, would have been dominant, covering part parts of the planet. And once again, this would have generated its own red edge signal as well.

From 1.2 billion to 2 billion years ago, the Earth was broadcasting cyanobacteria.

What if alien worlds don’t have plants on them? Other forms of life generate a red edge too. According to the researchers, some types of corals are even more reflective in the infrared. They’re not widespread here on Earth, but maybe they could dominate an alien world.

Even some animals, like sea slugs, have a red edge increase of 35%. Imagine a planet of sea slugs. 

We do need to be careful, though, there are some minerals that could give off a false positive. For example, a completely dead planet with exposed rocks containing mercury sulfide could mimic the red edge.

So now we know that chlorophyll or a similar chemical could be a clear indication of life on an extrasolar planet, what telescopes are in the works to actually observe them? When will we actually be able to observe a planet and know if there are alien plants growing there.

Radial velocity measurement detects doppler shift from gravitational interaction of a planet and its star. Credit: NASA/JPL

Our methods of detecting planets right now use the radial velocity method, where the wavelength of light from a star is red and blue-shifted as its planets yank it around with their gravity.

This tells us the mass of the planets, but doesn’t show us what they’re made of.

Transit method measures light that’s blocked as a planet passes in front of a star. Image credit: NASA/JPL

The transit method measures the amount of light blocked as a planet passes directly between us and a star. By measuring the amount of starlight that’s dimmed, astronomers can estimate the size of the planet.

In just the last few years, astronomers have developed a technique to analyze the light coming from the planet itself. They measure the chemical spectrum of light coming from the star and the planet together, and then separate what’s just coming from the planet.

The three planets discovered in the L98-59 system by NASA’s Transiting Exoplanet Survey Satellite are compared to Mars and Earth in order of increasing size in this illustration. Credit: NASA’s Goddard Space Flight Center

Using this technique, astronomers have found brutally hot planets with clouds containing iron and rock. As usual, astronomers start out discovering extreme worlds, and then refine their techniques as they get better tools.

But the most productive method will be the direct imaging method. With this, an Earth or space-based telescope uses a coronograph to block the light from the star, allowing only the light from the planet to be observed. 

Using this technique, a powerful telescope could analyze the light from just the atmosphere of a planet. We’ve done a whole episode about this technique, but ESA’s ARIEL mission, due for launch in 2028 will be one of the first instruments dedicated to scanning the atmospheres of other worlds.

Ground-based super observatories like the Magellan Telescope and the European Extremely Large Telescope will be able to directly observe exoplanet atmospheres from the ground as well. They’ll come online over the next half decade, so it won’t be too long to wait.

One last idea, is really cool, using a kind of reflected light called planetshine. When the Moon is at a very thin crescent, only a tiny slice of the Moon is illuminated by the Sun. The rest is being illuminated by reflected light from the Earth. We call this Earthshine.

3D Rendering of the Moon showing Earthshine in the shadowed portion of the Moon. Credit: NASA/GSFC

By observing only the reflected light on the Moon, astronomers could actually learn a tremendous amount about the Earth. Changes in brightness could allow astronomers to map out the continents on Earth and work out the size of our planet’s oceans. They could see weather patterns, and as the seasons change, snow cover near the poles would change the amount of light reflected off the Moon.

And the reflected infrared radiation could show the presence of plant life on Earth, thanks to the reflected red edge.

Whenever scientists propose sending a signal out into space, to inform extraterrestrial civilizations that we’re here, don’t worry about an alien invasion. Any aliens close enough to receive those signals already know we’re here. Our plants, lichen and bacteria gave us up millions and even billions of years ago.

But take solace, as our new telescopes come online, their plants will betray them too.

Fraser Cain

Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

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