Given the vastness of the universe, it’s possible that 1 billion intelligent civilizations exist.
So why is there no sign of these civilizations?
It is likely that all advanced civilizations are eventually destroyed by a “Great Filter”. And it’s probable that humanity is currently going through its Great Filter right now.
First, Let’s Examine Why Alien Life Should Exist
The Milky Way, our galaxy, is home to approximately 100 to 400 billion stars. Assuming that a portion of these stars are central to solar systems, similar to our own, it’s conceivable that the Milky Way could contain tens to hundreds of billions of such systems.
Adding to this, the discovery of thousands of exoplanets, many found in systems with multiple planets through missions like Kepler and TESS, reinforces the idea that planetary systems are a common feature around stars. This widespread occurrence of exoplanets suggests that a significant majority of stars throughout the universe might host their own solar systems.
Expanding this view to the larger cosmos, with an estimated 2 trillion galaxies in the observable universe, each potentially holding as many stars as the Milky Way on average, the numbers become even more astronomical. This implies a universe where hundreds of trillions to quadrillions of solar systems could exist, each with its unique characteristics and potentially life-supporting conditions.
To crudely estimate the number of solar systems with planets that have conditions favorable to life, let’s use the higher end of the estimate: one quadrillion solar systems in the universe. If 1% of these solar systems have planets with conditions favorable to life, there could potentially be about 10 trillion solar systems with planets that have conditions conducive to life.
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