7 things that show us that we are not alone in the Universe

If the emergence of animal life is not a simple matter? What if the origin of multicellular organisms requires a highly improbable combination of geological and astronomical coincidences? These questions gave rise to the so-called rare earth hypothesis. Read extensively in the lines below.

Are we alone in the universe or not?

The Rare Earth hypothesis was proposed in 2000 by geologist Peter D. Ward and astronomer Donald Brownlee and is a major blow to most optimistic astrobiology. This assumes that life is common in the universe, but only in its simplest forms, which have demonstrated the ability to survive in the most extreme environments.

But the same cannot be said for animal life, which is much more sensitive than microbial life to environmental conditions. Furthermore, the processes that allowed complex animal life to emerge are not as common as we are led to believe. Here are the reasons for this:

1. Things near the galactic center are subject to high levels of deadly radiation

Areas with a high density of stars are also off-limits due to the threat of supernovae, which, when they explode, are capable of severely affecting life on planets less than 30 light-years away.

This creates a barren sphere around the galactic center with a radius of 10,000 light-years. The outermost regions are also unsuitable because elements such as iron, carbon, nitrogen, nickel, magnesium, which are essential for life, are rare there. The same is true for elliptical galaxies: life can only appear in spiral galaxies.

2. Not just any star is enough

Evolution takes time, so the star must live long enough. This means that it must not have too much mass because it burns its nuclear fuel too quickly and goes out faster. In addition, they also produce a lot of lethal ultraviolet radiation, sterilizing the planet from the start.

They can’t be stars with too little mass either, because they emit too little energy. If we consider that 95% of the stars in the galaxy are less massive than our Sun, it is a bit of an exaggeration to say that our star is a typical star.

The same is true for all solar-type stars with a low amount of heavy elements: current observations have not discovered any planetary systems in stars with a content of less than 40% of that of our Sun.

3. The planet must be in the right place

Life needs water, so the planet must orbit at a distance from the star where the surface temperature allows liquid water to exist: this is the habitable zone of a star. However, as stars change brightness over their lifetimes, the Habitable Zone changes over time.

This imposes a new constraint: the planet must be in the habitable zone throughout its history. Calculations show that, in our solar system, the distance is between 0.95 and 1.15 times the Earth-Sun distance . What planet is right there? Only ours.

4. The importance of having a Jupiter

The orbital location of a Jupiter-like planet is a delicate matter: if ours had been born a little closer or had a little more mass, the gravitational pull would have prevented Earth from forming. We might think that it would be better if there were no gas giants nearby.

This is a big mistake. The existence of our gas giant has contributed to the “sweeping” of dangerous asteroids and comets from the Solar System : without Jupiter, the number of 10-kilometer objects – apocalyptic asteroids – that would collide with Earth would be 10,000 times higher.

5. Size and geology matter, too

The planet cannot be too small, as is the case with Mars, because it would not exert enough gravitational pull to prevent the atmosphere from escaping into space. On the other hand, plate tectonics plays a key role in the natural greenhouse effect caused by gases such as carbon dioxide, which causes our planet’s average temperature to be 18°C ​​instead of -25°C.

However, CO2 is removed from the atmosphere by reacting with rainwater to form carbonic acid, which ends up on the sea floor. If this CO2 were not returned to the atmosphere, the temperature would drop globally.

Due to plate tectonics, CO2 is returned to the atmosphere by volcanoes, which resulted in an average temperature that allowed animal life to develop. On the other hand, the planet must be geologically active to have a magnetic field strong enough to shield life from cosmic rays.

6. The need for a big moon

Our planet is unique because it has an exceptionally large satellite. If it did not exist, the orientation of the Earth’s axis would not be stable and would undergo chaotic variations over time: thanks to the Moon we enjoy a regular succession of seasons over millions of years.

Without it, the climatological consequences of a chaotic variation in the rotation axis would be catastrophic for any complex life form. Is it rare for a planet to have a large moon? Certainly yes. Ours has one because, when it was in its infancy, it was hit by an object larger than Mars , and the Moon emerged from the debris of this collision.

7. A planet is a dangerous place

On 5 occasions, life on Earth was on the verge of extinction. The most dangerous time was 250 million years ago, when more than 90% of marine species disappeared. No one knows what happened, but the threat to life is always present. Added to this is the snowball phenomenon that our planet has gone through.

The Earth has reportedly frozen completely in four instances, with global temperatures of -50°C. The last occurred about 700 million years ago, before the famous Cambrian explosion, an unprecedented burst of biological diversity that gave rise to all the forms of life we ​​know today. To what extent does the existence of animal life critically depend on this special and unique event?


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