Back in December 1990, during its flyby of Earth, NASA’s Galileo spacecraft, which studied the planet Jupiter and its moons, as well as several other Solar System bodies, pointed its instruments towards Earth, at the urging of Carl Sagan. And, it has found evidence of life on our planet. This can be a key to detecting vegetation on exoplanets – which is key to a possible intelligent extraterrestrial life.

In a paper published in Nature, researchers wrote “The Galileo spacecraft found evidence of abundant gaseous oxygen, a widely distributed surface pigment with a sharp absorption edge in the red part of the visible spectrum, and atmospheric methane in extreme thermodynamic disequilibrium. Together, these are strongly suggestive of life on Earth.”

It was the first time in history that scientists detected the signs of life on a planet. The planet in question here was, though, the one we already know that life exists on it: the planet Earth. But, the discovery is still important. Because, since we can detect the signs of vegetation, which means the signs of life on Earth, with more advanced technology, we can also detect life (or vegetation on exoplanets).

Dr. Shawn Domagal-Goldman, a research space scientist and astrobiologist at NASA Goddard Space Flight Center says: “Based on our understanding of how life operates on Earth, we are starting to derive principles of the signals that life creates that we could then look for on these planets around other stars.”

Earth can be a model for detecting vegetation on exoplanets
Earth can be a model for detecting vegetation on exoplanets: this image of the Amazon rainforest is from a 2010 global map of the height of the world’s forests based on multiple satellite datasets. Image: NASA

Earth’s natural history also can be a guide to spot vegetation on exoplanets

Over the last 500 million years, Earth’s surface has changed dramatically, from being ice-covered to having huge forests spread out over the land. For most of our home planet’s early history, land plants did not exist, but around 465 million years ago, the plants colonized the land.

Compared to modern trees, the first plants, mosses (see notes 1), show only a weak vegetation signature that is difficult for astronomers to find remotely.

Lisa Kaltenegger, associate professor of astronomy at Cornell University and director of the Carl Sagan Institute, explains “… as plants evolved on Earth, the vegetation signal that reveals their presence became stronger”.

So, in the future, if we can detect an exoplanet that has a strong signature of vegetation, that could mean the life on this particular exoplanet is most probably old, and it can be a good place to look for extraterrestrial intelligence.

Earth's natural history also can be a guide to spot vegetation on exoplanets
As plants evolved on Earth, the vegetation signal that reveals their presence became stronger.


Artist conception of Galileo spacecraft
Artist’s concept of Galileo at Io with Jupiter in the background; the high-gain antenna is fully deployed. Image: NASA

Named after the Italian astronomer, physicist, engineer, philosopher, and mathematician Galileo Galilei (15 February 1564 – 8 January 1642), Galileo was an American uncrewed spacecraft that studied the planet Jupiter and its moons, as well as several other Solar System bodies. Consisting of an orbiter and an entry probe, it was delivered into Earth orbit on October 18, 1989, by Space Shuttle Atlantis.

The spacecraft was named after Galileo Galilei, because the four largest moons of Jupiter (Io, Europa, Ganymede, and Callisto) were first seen by the Italian astronomer in January 1610, and recognized by him as satellites of Jupiter in March 1610. These four moons are now named “Galilean moons“.

After gravitational assist flybys of Venus and Earth, Galileo arrived at Jupiter on December 7, 1995, and became the first spacecraft to orbit Jupiter.

To eliminate the possibility of contaminating local moons with terrestrial bacteria, on September 21, 2003, after 14 years in space and 8 years in the Jovian system, and after a lot of discoveries, Galileo’s mission was terminated by sending it into Jupiter’s atmosphere at a speed of over 48 kilometers per second (30 miles per second).

Galileo also observed the first extraterrestrial collision of Solar System objects that humanity has witnessed: Comet Shoemaker-Levy 9’s collision with Jupiter in July 1994.


  1. Mosses are small flowerless plants that typically grow in dense green clumps or mats, often in damp or shady locations. They are typically 0.2-10 cm (0.1-3.9 in) tall, though some species are much larger. Dawsonia, the tallest moss in the world, can grow to 50 cm (20 in) in height.


  • “A New Clue in the Search for Forests on Distant Planets” on The Atlantic
  • “Astronomers use Earth’s natural history as a guide to spotting vegetation on new worlds” on
  • Galileo (spacecraft) on Wikipedia
  • Study: “A search for life on Earth from the Galileo spacecraft” on Nature
  • Moss on Wikipedia
M. Özgür Nevres
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