So far, our planet is the only body in our solar system that has certainly produced life, but could there also have been or could there still be life on other planets or moons?
Life as we know it is linked to very specific conditions. The most important criterion is the presence of water. In addition, it is crucial that a planet is the right distance from its sun, thereby ensuring moderate temperatures. Another crucial factor is the presence of a magnetic field, as only Earth's magnetic field protects us from solar wind and other cosmic radiation, such as gamma rays, which would otherwise be harmful to our cells and could ultimately lead to the extinction of all life.
Other factors that have enabled carbon-based life include Earth's nitrogen-oxygen atmosphere, the moon—which slows down Earth’s rotation through its gravitational pull, making a 24-hour rhythm possible—and Earth's volcanic activity.
For millions of years, life on Earth was dominated by dinosaurs. It was only when they became extinct 65 million years ago due to an asteroid impact on the Mexican Yucatan Peninsula that smaller mammals got their chance. Millions of years later, the first humans evolved from these mammals.
But could the miracle of life have been repeated on other bodies in our solar system? The answer is yes. After our solar system formed from the remnants of the first generation of stars and the planets were exposed to a constant cosmic bombardment of comets and asteroids, not only metals but also water were added to these bodies. Metals are formed in the interior of giant stars, the only places where the pressure is high enough to fuse atoms. When these stars exhaust their hydrogen reserves, they are propelled into space by supernova explosions.
Which planets come into question?
In addition to Earth, Mars and Venus developed in similar ways. Numerous images of ancient riverbeds on Mars indicate its wet past. For a long time, it was unclear where the water had gone; it was assumed to have either escaped into space due to the planet's low gravity or remained trapped underground. In 2002, the Mars Odyssey probe discovered large deposits of water ice beneath the surface at the South Pole, sufficient to fill Lake Michigan twice over.
How did Mars and Venus acquire their current appearances?
Today, Mars is a cold, dusty place that bears little resemblance to its original form. Apart from its low gravity, another reason for the loss of water is the absence of a global magnetic field, which allows solar winds to continually strip away the upper layers of the atmosphere.
Another factor making early life on Mars probable is the existence of the Martian meteorite ALH 84001. Found in Antarctica in 1984, this meteorite contained possible fossil traces of Martian bacterial life. Although NASA's findings are still debated, the meteorite's biosignature included a type of bacteria that produces magnetite, suggesting past life.
However, Venus presents even more extreme conditions today. With temperatures around 460°C, pressures a hundred times greater than on Earth, and acidic sulfur clouds, Venus today is the result of a self-sustaining greenhouse effect that continues to intensify.
Yet, life is not definitively ruled out. Scientists speculate that microbes could exist in Venus's atmosphere, where unexpectedly large amounts of carbon monoxide are rare but sulfur dioxide and hydrogen sulfide are abundant. Since these gases typically react with each other and do not coexist without a production source, and because carbonyl sulfide—a compound difficult to produce inorganically and often considered a strong indicator of biological activity—was also found, researchers speculate about microbial life acting as catalysts for these reactions in Venus’s atmosphere.
Are the other planets also a possibility?
The answer here is clearly no. The closest planet to the sun, Mercury, has no atmosphere and, like Earth's moon, is covered in craters due to the absence of erosion. The other planets are all gas giants with no solid surface.
Could there be life on some moons?
From today's perspective, the moons of Jupiter—Europa and Callisto—and Saturn's moon Titan are the most likely candidates for life. These moons all have an ocean beneath their ice shells. We also know from images taken by the Galileo probe that these ice shells have been fragmented in some places by asteroid impacts. Additionally, data from the Galileo probe indicate that Europa is still volcanically active. Similar to the black smokers in Earth’s Mariana Trench, which thrive on hydrothermal vent chemistry rather than photosynthesis, there could also be life forms in these extraterrestrial environments.
Saturn's moon Titan is a special case. It has an atmosphere comparable to that of early Earth, lacking only direct solar warmth necessary for traditional life forms. Life might become more plausible when the sun expands into a red giant, potentially providing more heat to Titan. However, by then, the sun will have depleted most of its hydrogen and will be transitioning into a white dwarf, marking the end of its life cycle.
Note: You can find out a lot more about this topic in my book 'Exoplaneten - Die Suche nach einer zweiten Erde,' which is in German.
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