Future historians could think back to that time and call it “the era of exoplanets”. We have found over 5,000 exoplanets and will continue to find more. Then we will go beyond their simple discovery and focus our efforts on finding biosignatures, the special chemical fingerprints that living processes imprint on the atmospheres of exoplanets.
But biosignatures are not limited to atmospheric chemistry. On a planet with lots of plant life, light can also be a biosignature.
The search for biosignatures on exoplanets received a boost when the James Webb Space Telescope began its observations. One of the scientific objectives of the telescope is to characterize the atmospheres of exoplanets using its powerful infrared spectrometry. If Webb finds large amounts of oxygen, for example, it indicates that biological processes could be at work and altering a planet’s atmosphere. But the JWST and other telescopes could detect another type of biosignature.
A plant signature
Earth’s abundant plant life changes the “light signature” of our planet. The change is based on photosynthesis and how plant life absorbs certain light frequencies while reflecting others. The resulting phenomenon is called the Vegetation Red Edge (REV.)
Exoplanet scientists have been working on the idea of ERV 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. This helps plants avoid overheating during photosynthesis. This absorption and reflection allows remote sensing to assess plant health, cover and activity, and agronomists use it to monitor crops.
In a new paper, a team of researchers looked at chlorophyll and its sunlight-induced fluorescence (SIF). SIF is the name of the electromagnetic signal emitted by chlorophyll a, the most widely distributed chlorophyll molecule. Some of the energy absorbed by chlorophyll is not used for photosynthesis but is emitted at longer wavelengths as a two-peak spectrum. It covers approximately the spectral range from 650 to 850 nm.
The article is titled “Photosynthetic Fluorescence From Earth-Like Planets Around Sun-Like and Cool Stars”, and it will be published in The Astrophysical Journal. The main author is Yu Komatsu, a researcher at the National Institutes of Natural Sciences Astrobiology Center, National Astronomical Observatory of Japan.
The article focuses on how chlorophyll fluorescence could be detected on Earth-like planets. “This study investigated the biological fluorescence detectability of two types of photosynthetic pigments, chlorophylls (Chls) and bacteriochlorophylls (BChls), on Earth-like planets with oxygen-rich/poor and anoxic atmospheres around the Sun. and M dwarfs,” the authors explain.
Detecting the presence of chlorophyll in another world is complicated. There is a complex interaction between plant life, starlight, land/ocean cover and atmospheric composition. This study is part of an ongoing effort to understand some of the limits of detection and what spectroscopic data can tell scientists about exoplanets. Over time, exoplanet scientists want to determine which detections may be biosignatures under different circumstances.
ERV is a sudden drop in light observed between infrared and visible light. Light in the near infrared (from about 800 nm) is much brighter than light in optics (between about 350 and 750 nm). On Earth, it is the light signature of plant life and its chlorophyll. Chlorophyll absorbs light up to 750 nm, and other plant tissues reflect light above 750 nm.
Satellites like NASA’s Terra can observe different regions of the Earth’s surface over time and observe how the reflectance of light changes. Scientists measure what is called the Normalized Difference Vegetation Index (NVDI). A dense forest location during the peak growing season gives maximum values for NDVI, while regions with poor vegetation give low values.
Scientists can also observe Earthshine, the light reflected from Earth to the Moon. This light is all of the light reflected from Earth, what scientists call a disk-averaged spectrum. “While remote sensing observes local areas on Earth, Earthshine observations provide disc-averaged spectra of the Earth, leading to fruitful insights into exoplanet applications,” the authors write. “The apparent reflectance change in the Earth’s disk-averaged spectrum due to surface vegetation is less than 2 percent.”
The Earthshine we see on the Moon is similar to the light we detect from distant exoplanets. It is the total light relative to the regional surface light. But there is enormous complexity in studying this light, and there are no easy comparisons between Earth and exoplanets. “VRE signals from exoplanets around stars other than a Sun-like star are difficult to predict due to the complexity of photosynthetic mechanisms in different light environments,” the authors explain. But it is always useful to search for an ERV on exoplanets. If scientists observe an exoplanet frequently, they may be able to recognize how the VRE changes seasonally, and they may recognize a step similar to that of the VRE in the spectroscopy of the planet, although it may be at lengths waves different from those of the Earth.
In their paper, the researchers considered an Earth-like planet at different stages of atmospheric evolution. In each case, the planets revolved around the Sun, a well-studied red dwarf named Gliese 667 C, or the even better known red dwarf TRAPPIST-1. (Both red dwarfs have planets in their habitable zones and both represent common types of red dwarfs.) They modeled each case’s reflectance for plant chlorophyll, bacterial chlorophyll-based vegetation, and biological fluorescence without any surface vegetation.
What they came up with is a collection of lightcurves that show what different ERVs might look like on Earth-like exoplanets at different stages of atmospheric evolution around different stars. It is important to look at the different stages of atmospheric evolution because Earth’s atmosphere changed from oxygen-poor to oxygen-rich when life was present.
“We considered fluorescence emissions from Chl and BChl-based vegetation in a clear sky, an Earth-like planet around the Sun, and two M dwarfs,” the authors write.
The study produced a range of reflectance data for Earth-like planets around different stars. The planets have been modeled with different atmospheres which correspond to the different atmospheres of the Earth during its 4 billion years of history. The researchers also varied the amount of land cover versus ocean cover, the amount of coastline, and whether the surface was covered with plants or photosynthetic bacteria.
Tomorrow, we will have ever more powerful space telescopes like LUVOIR (Large UV/Optical/IR Surveyor) and HabEx (Habitable Exoplanet Observatory). Ground-based telescopes like the Thirty Meter Telescope, the Giant Magellan Telescope, and the European Extremely Large Telescope will also come 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.
We are detecting 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 better understand the characteristics of exoplanets. Telescopes like the E-ELT will do this with its 39.3 meter mirror. It will be able to separate exoplanet light from starlight and directly image certain exoplanets. This will trigger a flood of data on exoplanet reflectance and potential biosignatures, and all of this data will need to be evaluated.
If we ever locate an Earth-like planet that is habitable and currently supports life, it will not simply appear in one of our telescopes and announce its presence. Instead, there will be tantalizing clues; there will be indications and contraindications. Scientists will move slowly and carefully, and one day we may be able to say that we have found a planet with life. This research has a role to play in the effort.
“It is important to quantitatively assess the detectability of any potential surface biosignature using the expected specifications of specific future missions,” explain the authors. “This study made the first attempt to investigate the detectability of photosynthetic fluorescence on Earth-like exoplanets.”
This article was originally published on Universe today by EVAN GOUGH. Read the original article here.