Worlds bustling with plant life should glow in a detectable wavelength of infrared light

Future historians might look back on this time and call it the “age of the exoplanets.” We’ve found more than 5,000 exoplanets, and we’ll continue to find more. Next, we’ll move beyond just finding them, and direct our efforts to finding biosignatures, the special chemical fingerprints that living processes imprint on exoplanets’ atmospheres.

But there is more to biosignatures than atmospheric chemistry. On a planet with so much plant life, light can be a vital signature, too.

The search for biosignatures on exoplanets got a boost when the James Webb Space Telescope began observations. One of the scientific goals of the telescope is Characterize the atmospheres of the outer planets With a powerful infrared spectrometer. If Webb finds large amounts of oxygen, for example, that’s an indication that biological processes may be at work and alter the planet’s atmosphere. But JWST and other telescopes can detect another kind of biosignature.

Earth’s abundant plant life is changing the “light signature” of our planet. The change depends on photosynthesis and how plant life absorbs some frequencies of light while reflecting others. is called the resulting phenomenon Red edge vegetation (VRE.)

Exoplanet scientists have been working on the idea of ​​the VRE as a biosignature for a few years. It is based on the fact that chlorophyll absorbs light in the visible part of the spectrum and is almost transparent in the infrared. Other cellular structures in vegetation reflect infrared radiation. This helps plants avoid overheating during photosynthesis. This absorption and reflection makes it possible for remote sensing to measure plant health, coverage and vigor, and is used by agricultural scientists to monitor crops.

In a new paper, a team of researchers looked at chlorophyll and its components Solar induced fluorescence (SIF.) SIF is the name for the electromagnetic signal emitted by chlorophyll a, the most common chlorophyll molecule. Part of the energy absorbed by chlorophyll a It is not used in photosynthesis but is emitted at longer wavelengths as a two-peaked spectrum. They cover the spectral range approximately 650-850 nm.

These two pictures help explain Red edge vegetation and solar luminosity. (L) Shows the wavelength of the VRE. (Credit: Terrence et al. 2010.) (R) Shows the absorption and fluorescence of two types of chlorophyll: Chl is plant chlorophyll, and BChl is bacterial chlorophyll. (Credit: Komatsu et al. 2023.)

The paper is “Photoluminescence from Earth-like Planets around Sun-like and Cool Stars,” and will be published in The Astrophysical Journal. The lead author is Yu Komatsu, researcher at the Center for Astrobiology of the National Institutes of Natural Sciences, National Astronomical Observatory of Japan.

The paper focuses on how fluorescence from chlorophyll can be detected on Earth-like planets. “This study examined the possibility of detecting bioluminescence from two types of photosynthetic pigments, chlorophylls (Chls) and bacterial chlorophylls (BChls), on Earth-like planets with oxygen-rich/poor, oxygen-deficient atmospheres around the Sun and M dwarfs,” he explains.

Detecting the presence of chlorophyll in another world is complicated. There is a complex interaction between plant life, starlight, land/ocean coverage, and atmospheric composition. This study is part of an ongoing effort to understand some of the limitations of the discovery and what the spectroscopic data can tell scientists about exoplanets. Over time, exoplanet scientists want to identify finds that could be biosignatures in different circumstances.

Machinery within the chloroplasts of plant cells convert sunlight into energy, emitting fluorescence in the process. Scientists can detect a fluorescence signature in the satellite data. Credit: NASA Goddard Conceptual Image Lab/T. Chasing

VRE is a sharp dip in the observed light between infrared and visible light. Light in the near infrared (starting at about 800 nanometers) is much brighter than light in the optical (between 350 to 750 nanometers) on Earth. This is the photosynthetic signature of plant life and chlorophyll. Chlorophyll absorbs light up to 750 nanometers, and other plant tissues reflect light above 750 nanometers.

like satellites NASA Earth It can observe different regions of the Earth’s surface over time and see how the reflection of light changes. Scientists measure what is called the Natural Variance Vegetation Index (NVDI). The location of dense forest during peak growth season gives peak NDVI values, while areas poor in vegetation give low values.

Scientists can also take note Earth’s radianceLight reflected from the Earth on the Moon. This light is all light reflected from the Earth, what scientists call a Disc-average spectrum. “While remote sensing monitors local regions on Earth, geoobservations provide mid-disc spectra for Earth, leading to fruitful insights for exoplanet applications,” the authors write. “The apparent reflectance change in the average spectrum over the Earth’s disk due to surface vegetation is less than 2%.”

The sunlit crescent contrasts with the darker illumination of twice reflected light provided by sunlight reflecting off our planet.  Credit: Bob King
The sunlit crescent contrasts with the darker illumination of twice reflected light provided by sunlight reflecting off our planet. Credit: Bob King

The brightness of the Earth we see on the Moon is similar to the light we detect from distant exoplanets. It is the sum of the light versus the regional surface light. But there is an enormous amount of complexity involved in studying this light, and there are no easy comparisons between Earth and exoplanets. “It is difficult to predict VRE signals from exoplanets around stars other than sun-like stars due to the complexity of photosynthesis mechanisms in different light environments,” the authors explain. But there is still value in searching for VRE on exoplanets. If scientists observe an exoplanet more frequently, they may be able to get a sense of how the VRE changes seasonally, and they may recognize a step similar to the VRE in the planet’s spectroscopy, although it can be at different wavelengths than Earth’s.

In their paper, the researchers considered an Earth-like planet in various stages of atmospheric development. In each case, the planets revolved around the Sun, a well-studied red dwarf Gliss 667cor even the more famous red dwarf TRAPPIST-1. (Both red dwarfs have planets in their habitable zones, and both are common types of red dwarfs.) They modeled the reflectance from each case of vegetative chlorophyll, chlorophyll-based bacterial flora, and biological fluorescence without any surface vegetation.

What they found is a set of light curves that show what different VREs might look like on Earth-like exoplanets at different stages of atmospheric development around different stars. It is important to look at the different stages of atmospheric evolution because the Earth’s atmosphere changed from oxygen-poor to oxygen-rich during the existence of life.

“We studied fluorescence emissions from Chl- and BChl-based vegetation in the clear-sky condition
An Earth-like planet around the Sun has two M dwarfs.”

This figure from the study shows just one set of the team's findings.  This is a set of typical light curves for a young, Earth-like planet with oxygen atmospheres around three stars: the Sun, red dwarf GJ667C, and red dwarf TRAPPIST-1.  The column on the left is a planet whose entire surface is covered with vegetation;  The middle pillar of a planet with 70% oceans, 2% coastline, and 28% land covered in vegetation;  Right column of modern Earth.  When scientists study the light of exoplanets with powerful telescopes in the future, they can compare their observations with this study as part of their interpretation of the data.  Image credit: Komatsu et al.  2023.
This figure from the study shows just one set of the team’s findings. This is a set of typical light curves for a young, Earth-like planet with oxygen atmospheres around three stars: the Sun, red dwarf GJ667C, and red dwarf TRAPPIST-1. The column on the left is a planet whose entire surface is covered with vegetation; The middle pillar of a planet with 70% oceans, 2% coastline, and 28% land covered in vegetation; Right column of modern Earth. When scientists study the light of exoplanets with powerful telescopes in the future, they can compare their observations with this study as part of their interpretation of the data. Image credit: Komatsu et al. 2023.

The study produced a set of reflectivity data for Earth-like planets around different stars. Planets have been modeled with different atmospheres corresponding to Earth’s different atmospheres over its four billion year history. The researchers also varied the amount of land cover versus ocean cover, the amount of shoreline, and whether the surface was covered by plants or photosynthetic bacteria.

In the future, we will use more powerful space telescopes such as LUVOIR (Large Ultraviolet/Optical/Infrared Surveyor) and HabEx (Habitable Exoplanet Observatory). The European Very Large Telescope will also be launched online in the near future. These telescopes will generate an unprecedented amount of data on exoplanets, and this study is part of the preparation for that.

This artist's impression shows the European Extremely Large Telescope (E-ELT) in its cover.  E-ELT will be a 39-meter aperture optical and infrared telescope.  ESO/L. Calzada
This artist’s impression shows the European Extremely Large Telescope (E-ELT) in its cover. E-ELT will be a 39-meter aperture optical and infrared telescope. Image credit: ESO/L. Calzada

We are discovering more and more exoplanets and building a statistical understanding of other solar systems and the distributions, masses and orbits of exoplanets. The next step is to gain a deeper understanding of the properties of exoplanets. Telescopes like the E-ELT will do so with its 39.3-meter mirror. It will be able to separate exoplanet light from starlight and directly image some exoplanets. It will unleash a flood of data on exoplanet reflectivity and potential biosignatures, and all of that data will have to be evaluated.

If we ever identify an Earth-like planet that is habitable and currently supports life, it won’t just show up in one of our telescopes and announce its existence. Instead, there will be tantalizing hints, pointers and counter-indications. Scientists will slowly and carefully work their way forward, and one day we may be able to say we’ve found a planet with life. This research has a role to play in this endeavour.

“It is important to quantitatively assess the detection potential of any potential surface biosignature using the specifications expected for specific future missions,” the authors explain. “This study provided the first attempt to investigate the possibility of detecting photoluminescence on Earth-like exoplanets.”

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