Future historians might look back on this time and call it the ‘exoplanet age.’ We’ve found over 5,000 exoplanets, and we’ll keep finding more. Next, we’ll move beyond just finding them, and we’ll turn our efforts to finding biosignatures, the special chemical fingerprints that living processes imprint on exoplanet atmospheres.
But there’s more to biosignatures than atmospheric chemistry. On a planet with lots of plant life, light can be a biosignature, too.
The search for biosignatures on exoplanets got a boost of energy when the James Webb Space Telescope began observations. One of the telescope’s science objectives is to characterize exoplanet atmospheres with its powerful infrared spectrometry. If Webb finds large amounts of oxygen, for example, it’s an indication that biological processes might be at work and are changing a planet’s atmosphere. But the JWST and other telescopes could detect another type of biosignature.
Earth’s abundant plant life changes our planet’s ‘light signature.’ The change is based on photosynthesis and how plant life absorbs some light frequencies while reflecting others. The resulting phenomenon is called the vegetation red edge (VRE.)
Exoplanet scientists have worked on the idea of the VRE as a biosignature for a few years. It’s 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 the vegetation reflect the infrared. This helps plants avoid overheating during photosynthesis. This absorption and reflection make it possible for remote sensing to gauge plant health, coverage, and activity, and agricultural scientists use it to monitor crops.
In a new paper, a team of researchers looked at chlorophyll and its solar-induced fluorescence (SIF.) SIF is the name of the electromagnetic signal emitted by chlorophyll a, the most widely-distributed chlorophyll molecule. Part of the energy absorbed by chlorophyll a is not used for photosynthesis but is emitted at longer wavelengths as a two-peak spectrum. It covers roughly the 650–850 nm spectral range.
These two images help illustrate the vegetation red edge and the solar-induced fluorescence. (L) shows the wavelength of the VRE. (Credit: Terrence et al. 2010.) (R) shows the absorption and the fluorescence for two types of chlorophyll: Chl is vegetative chlorophyll, and BChl is bacterial chlorophyll. (Credit: Komatsu et al. 2023.)
The paper is “Photosynthetic Fluorescence from Earth-Like Planets around Sun-Like and Cool Stars,” and it will be published in The Astrophysical Journal. The lead author is Yu Komatsu, a researcher at the National Institutes of Natural Sciences Astrobiology Center, National Astronomical Observatory of Japan.
The paper focuses on how the fluorescence from chlorophyll could be detected on planets similar to Earth. “This study examined the detectability of biological fluorescence from 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 on another world is complicated. There’s a complex interplay between plant life, starlight, land/ocean coverage, and atmospheric composition. This study is part of an ongoing effort to understand some of the limitations to detection and what spectroscopic data can tell scientists about exoplanets. Over time, exoplanet scientists want to determine which detections can be biosignatures in different circumstances.
Machinery inside the chloroplasts of plant cells converts sunlight to energy, emitting fluorescence in the process. Scientists can detect the fluorescence fingerprint in satellite data. Credit:
NASA Goddard’s Conceptual Image Lab/T. Chase
The VRE is a sharp drop in observed light between infrared and visible light. Light in the near-infrared (starting at about 800 nm) is much brighter than the light in the optical (between about 350 to 750 nm.) On Earth, this is the light signature of plant life and its chlorophyll. The chlorophyll absorbs the light up to 750 nm, and other plant tissues reflect light above 750 nm.
Satellites like NASA’s Terra can observe different regions on Earth’s surface over time and watch how the light reflectance changes. Scientists measure what’s called the Normalized Difference Vegetation Index (NVDI.) A dense forest location during peak growing season gives peak values for the NDVI, while vegetation-poor regions give low values.
Scientists can also observe Earthshine, the light reflected from Earth onto the Moon. That light is the entirety of the light reflected by Earth, what scientists call a disk-averaged spectrum. “Whereas remote sensing observes local areas on Earth, Earthshine observations provide disk-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%.”
The bright sunlit crescent contrasts with the darker lighting of twice-reflected light supplied by sunlight reflecting off our own planet. Credit: Bob King
The Earthshine we see on the Moon is similar to the light we detect from distant exoplanets. It’s the totality of the light vs regional surface light. But there’s an enormous amount of complexity involved in studying that light, and there are no easy comparisons between Earth and exoplanets. “The VRE signals from exoplanets around stars other than a Sun-like star are challenging to predict due to the complexity of photosynthetic mechanisms in different light environments,” the authors explain. But there’s still value in looking for a VRE on exoplanets. If scientists observe an exoplanet frequently, they may be able to recognize how the VRE changes seasonally, and they may recognize a similar VRE-like step in the planet’s spectroscopy, though it could be at different wavelengths than on Earth.
In their paper, the researchers considered an Earth-like planet in different stages of atmospheric evolution. In each case, the planets orbited the Sun, a well-studied red dwarf named Gliese 667 C, or the even more well-known red dwarf TRAPPIST-1. (Both red dwarfs have planets in their habitable zones, and both represent common types of red dwarfs.) They modelled the reflectance from each case for vegetation chlorophyll, bacterial chlorophyll-based vegetation, and biological fluorescence without any surface vegetation.
What they came up with is a collection of light curves that shows what different VREs might look like on Earth-like exoplanets in different stages of atmospheric evolution around different stars. It’s important to look at different stages of atmospheric evolution because Earth’s atmosphere changed from oxygen-poor to oxygen-rich while life was present.
“We considered fluorescence emissions from Chl- and BChl-based vegetation in a clear-sky condition
on an Earth-like planet around the Sun and two M dwarfs,” the authors write.
This figure from the study shows just one set of results the team produced. This is a set of modelled light curves for a modern Earth-like planet with an oxygen atmosphere around three stars: the Sun, the red dwarf GJ667C, and the red dwarf TRAPPIST-1. The column on the left is for a planet with vegetation covering the entire surface; the middle column is for a planet with 70 % ocean, 2% coast, and 28% land covered with vegetation; the right column is for the modern Earth. When scientists study exoplanet light 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 range of reflectance data for Earth-like planets around different stars. The planets were modelled with different atmospheres that correspond to Earth’s different atmospheres over its four billion-year history. The researchers also varied the amount of land cover vs ocean cover, the amount of coastline, and whether the surface was covered in plants or in photosynthetic bacteria.
In the future, we’ll be wielding 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 be coming online in the near future. These telescopes are going to generate an unprecedented amount of data on exoplanets, and this study is part of preparing for that.
This artist’s impression shows the European Extremely Large Telescope (E-ELT) in its enclosure. The E-ELT will be a 39-metre aperture optical and infrared telescope. Image Credit: ESO/L. Calçada
We’re detecting more and more exoplanets and are building a statistical understanding of other solar systems and the distributions, masses, and orbits of exoplanets. The next is to gain a deeper understanding of the characteristics of exoplanets. Telescopes like the E-ELT will do that with its 39.3-meter mirror. It’ll be able to separate an exoplanet’s light from the star’s light and directly image some exoplanets. It’ll unleash a flood of data on exoplanet reflectance and potential biosignatures, and all of that data will have to be evaluated.
If we ever locate an Earth-like planet, one that’s habitable and currently supporting life, it won’t just appear in one of our telescopes and announce its presence. Instead, there’ll be tantalizing hints, there’ll be indications and contra-indications. Scientists will slowly and carefully work their way forward, and one day we might be able to say we’ve found a planet with life. This research has a role to play in the endeavour.
“It is important to quantitatively evaluate the detectability of any potential surface biosignature using expected specifications of specific future missions,” the authors explain. “This study made the first attempt to investigate the detectability of photosynthetic fluorescence on Earth-like exoplanets.”
NAIROBI, Kenya (AP) — The body of Ugandan Olympic athlete Rebecca Cheptegei — who died after being set on fire by her partner in Kenya — was received Friday by family and anti-femicide crusaders, ahead of her burial a day later.
Cheptegei’s family met with dozens of activists Friday who had marched to the Moi Teaching and Referral Hospital’s morgue in the western city of Eldoret while chanting anti-femicide slogans.
She is the fourth female athlete to have been killed by her partner in Kenya in yet another case of gender-based violence in recent years.
Viola Cheptoo, the founder of Tirop Angels – an organization that was formed in honor of athlete Agnes Tirop, who was stabbed to death in 2021, said stakeholders need to ensure this is the last death of an athlete due to gender-based violence.
“We are here to say that enough is enough, we are tired of burying our sisters due to GBV,” she said.
It was a somber mood at the morgue as athletes and family members viewed Cheptegei’s body which sustained 80% of burns after she was doused with gasoline by her partner Dickson Ndiema. Ndiema sustained 30% burns on his body and later succumbed.
Ndiema and Cheptegei were said to have quarreled over a piece of land that the athlete bought in Kenya, according to a report filed by the local chief.
Cheptegei competed in the women’s marathon at the Paris Olympics less than a month before the attack. She finished in 44th place.
Cheptegei’s father, Joseph, said that the body will make a brief stop at their home in the Endebess area before proceeding to Bukwo in eastern Uganda for a night vigil and burial on Saturday.
“We are in the final part of giving my daughter the last respect,” a visibly distraught Joseph said.
He told reporters last week that Ndiema was stalking and threatening Cheptegei and the family had informed police.
Four in 10 women or an estimated 41% of dating or married Kenyan women have experienced physical or sexual violence perpetrated by their current or most recent partner, according to the Kenya Demographic and Health Survey 2022.
TEL AVIV, Israel (AP) — A rare Bronze-Era jar accidentally smashed by a 4-year-old visiting a museum was back on display Wednesday after restoration experts were able to carefully piece the artifact back together.
Last month, a family from northern Israel was visiting the museum when their youngest son tipped over the jar, which smashed into pieces.
Alex Geller, the boy’s father, said his son — the youngest of three — is exceptionally curious, and that the moment he heard the crash, “please let that not be my child” was the first thought that raced through his head.
The jar has been on display at the Hecht Museum in Haifa for 35 years. It was one of the only containers of its size and from that period still complete when it was discovered.
The Bronze Age jar is one of many artifacts exhibited out in the open, part of the Hecht Museum’s vision of letting visitors explore history without glass barriers, said Inbal Rivlin, the director of the museum, which is associated with Haifa University in northern Israel.
It was likely used to hold wine or oil, and dates back to between 2200 and 1500 B.C.
Rivlin and the museum decided to turn the moment, which captured international attention, into a teaching moment, inviting the Geller family back for a special visit and hands-on activity to illustrate the restoration process.
Rivlin added that the incident provided a welcome distraction from the ongoing war in Gaza. “Well, he’s just a kid. So I think that somehow it touches the heart of the people in Israel and around the world,“ said Rivlin.
Roee Shafir, a restoration expert at the museum, said the repairs would be fairly simple, as the pieces were from a single, complete jar. Archaeologists often face the more daunting task of sifting through piles of shards from multiple objects and trying to piece them together.
Experts used 3D technology, hi-resolution videos, and special glue to painstakingly reconstruct the large jar.
Less than two weeks after it broke, the jar went back on display at the museum. The gluing process left small hairline cracks, and a few pieces are missing, but the jar’s impressive size remains.
The only noticeable difference in the exhibit was a new sign reading “please don’t touch.”
VICTORIA – The British Columbia government is partnering with a bear welfare group to reduce the number of bears being euthanized in the province.
Nicholas Scapillati, executive director of Grizzly Bear Foundation, said Monday that it comes after months-long discussions with the province on how to protect bears, with the goal to give the animals a “better and second chance at life in the wild.”
Scapillati said what’s exciting about the project is that the government is open to working with outside experts and the public.
“So, they’ll be working through Indigenous knowledge and scientific understanding, bringing in the latest techniques and training expertise from leading experts,” he said in an interview.
B.C. government data show conservation officers destroyed 603 black bears and 23 grizzly bears in 2023, while 154 black bears were killed by officers in the first six months of this year.
Scapillati said the group will publish a report with recommendations by next spring, while an independent oversight committee will be set up to review all bear encounters with conservation officers to provide advice to the government.
Environment Minister George Heyman said in a statement that they are looking for new ways to ensure conservation officers “have the trust of the communities they serve,” and the panel will make recommendations to enhance officer training and improve policies.
Lesley Fox, with the wildlife protection group The Fur-Bearers, said they’ve been calling for such a committee for decades.
“This move demonstrates the government is listening,” said Fox. “I suspect, because of the impending election, their listening skills are potentially a little sharper than they normally are.”
Fox said the partnership came from “a place of long frustration” as provincial conservation officers kill more than 500 black bears every year on average, and the public is “no longer tolerating this kind of approach.”
“I think that the conservation officer service and the B.C. government are aware they need to change, and certainly the public has been asking for it,” said Fox.
Fox said there’s a lot of optimism about the new partnership, but, as with any government, there will likely be a lot of red tape to get through.
“I think speed is going to be important, whether or not the committee has the ability to make change and make change relatively quickly without having to study an issue to death, ” said Fox.
This report by The Canadian Press was first published Sept. 9, 2024.
