By looking at Earth’s natural history, Cornell University’s Carl Sagan Institute astrobiologists Jack O’Malley-James and Lisa Kaltenegger have found a template for vegetation fingerprints to determine the age of habitable exoplanets.
If the development of life on other inhabited worlds follows a similar timeline to life on Earth, vegetation red edge (VRE) biosignatures are only present for a fraction of a planet’s habitable lifetime once vegetation covers the surface; a VRE biosignature similar to that on the present-day Earth would be difficult to detect without very high-precision instruments; however, the team’s models show that the VRE signature increases with geological time, as well as with increasing vegetation surface fraction and decreasing cloud coverage. Image credit: Jack O’Malley-James / Wendy Kenigsberg / Brand Communications.
The geological record of the last 500 million years shows that Earth’s surface has changed dramatically, from being ice-covered to having huge forests spread out over land.
For most of our planet’s early history, land plants did not exist, but plants eventually became widespread on its surface.
The first plants, mosses, show only a weak vegetation signature that is difficult for astronomers to find remotely, compared to modern trees.
“Our models show that Earth’s vegetation reflectance signature increases with coverage of our planet’s surface, but also with the age of our planet,” Dr. Jack O’Malley-James said.
“We use Earth’s history as a key for finding life in the Universe. Our work shows that as plants evolved on Earth, the vegetation signal that reveals their presence became stronger, making older exoplanets really interesting places to look for vegetation,” Dr. Kaltenegger said.
Exoplanets may be parched, arid with clear skies and endless cacti forests, or hot jungle worlds covered in tropical forests.
“Over interstellar distances, these places might be the best targets to spot vegetation,” Dr. Kaltenegger noted.
When NASA’s Galileo mission left Earth for Jupiter in 1989, the late Cornell astronomer Carl Sagan requested the spacecraft’s instruments look at Earth to see how light reflected from an inhabited, life-rich planet.
Observations in December 1990 revealed a distinctive boost in reflectance between the red and infrared spectrum, just beyond the limits of human vision, due to vegetation — the so-called vegetation red edge.
“The signal Galileo detected for Earth was similar to what observations of an exoplanet in another star system might look like, but, of course, Galileo was much closer to us,” Dr. O’Malley-James said.
“Observing an exoplanet is more challenging, but telescope technology is getting better at spotting tiny signals.”
“And factoring Earth’s changing landscapes into our models will make it easier to detect vegetation in the future on other worlds.”
“Looking at how life altered Earth’s biosignatures over time helps us to figure out which planets are most likely to show the strongest signs of life, ultimately giving us the best chances of successfully pinpointing life, if it is there,” Dr. Kaltenegger said.
The team’s work is published in the Astrobiology Journal.
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Jack T. O’Malley-James & Lisa Kaltenegger. 2018. The Vegetation Red Edge Biosignature Through Time on Earth and Exoplanets. Astrobiology 18 (9); doi: 10.1089/ast.2017.1798
