After the discovery of exoplanets, astronomers have a chance to be the first to detect signs of life outside the Earth. Some people think that first you need to look for planets covered with vegetation. And to find clues, astronomers looked at Earth through the eyes of a hypothetical exoplanet inhabitant.
Several dozen of the discovered exoplanets are as far from their stars as is best for life to emerge. But even with the most powerful telescopes, it is impossible to see any details on them. Astronomers can only estimate the spectrum of light reflected from their surface. The famous astrophysicist Carl Sagan suggested in the early 1990s that the "plant" worlds have a special "label". Plants absorb visible light well and reflect rays in the near infrared range. Jack O'Malley-James and his colleagues at Cornell University decided that the standard for comparison should be the reflection of light from our planet.
A hypothetical exoplanet with vegetation is unlikely to be the same age as Earth. Therefore, astronomers, using computer simulations, recreated the "glow" of the Earth in the past and tried to look into the future.
The model showed that a special "mark" appeared near the Earth about 500 million years ago, although the mosses that inhabited the planet then reflected infrared rays worse than modern forests and jungles. Moreover, the best time to notice vegetation on Earth from another planet was the Cretaceous period, which ended about 67 million years ago. At the time, the difference between absorbed light and reflected infrared rays was twice as high as it is today. However, in the future, the plant "mark" will become more visible. The researchers put two scenarios into the model: the emergence of new desert areas, or, conversely, wet forests. In the "dry" scenario, the Earth is inhabited mainly by cacti, which perfectly reflect infrared rays.
Exoplanets may not have as much water as we do. Therefore, the first open world with vegetation could be a planet with cacti. Such alien cactus-like plants can live in anticipation of rare rainfall and reflect rays only for a few months a year, the authors suggest in a publication in Astrobiology.
About the jungle, researchers say that they are most likely a rarity on exoplanets, but if they are, they will strengthen the "mark". And although infrared reflection is not enough to be completely sure that the planet is covered with vegetation, astronomers will know which of the planets to pay attention to first.
Author: Sergey Surzhenko