Extraterrestrial life

Extraterrestrial life refers to any life that originates outside of Earth. While we use the term "aliens" in movies, scientists use the term astrobiology to study this possibility. So far, no extraterrestrial life has been conclusively detected by science. This life could be very simple, such as prokaryotes, which are single-celled organisms. It could also be intelligent beings with advanced civilizations. Some might be much more advanced than humans, while others might be less so.

The building blocks of life are actually created inside stars. This happens through a process called stellar fusion. During this process, stars combine hydrogen nuclei to create helium nuclei. This releases a large amount of energy. As a star evolves, it begins combining helium nuclei to form carbon nuclei. Larger stars can then create oxygen, silicon, neon, sulfur, and eventually iron. When these stars reach the end of their lives, they blow these materials back into space. These elements become part of new stars and planets. This makes the raw components of life ubiquitous, or found everywhere, in the cosmos.

Humans have wondered about inhabited worlds since antiquity. Early thinkers like Democritus and Epicurus discussed the idea of countless worlds. In the 15th century, William Vorilong suggested that Jesus might have visited other worlds. Around 1440, Nicholas of Cusa theorized that all celestial bodies, including the Sun, could host life. Later, Rene Descartes noted that we could not prove stars were not inhabited by intelligent creatures.

Since the mid-20th century, the search has become more formal. Scientists now use telescopes and radio signals to look for life. As of October 2025, we have confirmed 6,022 exoplanets in 4,490 different systems.

To find life, scientists look for the circumstellar habitable zone. This is often called the "Goldilocks zone." It is the area around a star where temperatures allow water to exist as a liquid. If a planet is too close, water becomes steam. If it is too far, water freezes into ice. However, being in this zone does not guarantee a planet is habitable. For example, Venus is in the habitable zone, but its atmosphere prevents liquid water. Gas giants, or Jovian planets, are also unsuitable due to crushing atmospheric pressures.

Space is incredibly vast, which makes studying other worlds very difficult. Humans have only walked on the Moon and sent robotic probes to other moons. The New Horizons probe required nine years of travel to reach Pluto. Even the Voyager 2 probe, traveling at 50,000 kilometers per hour, would take 100,000 years to reach Alpha Centauri. This is the closest star system to Earth, located 4.4 light years away. Because of these distances, we mostly rely on telescopes to study extrasolar systems.

There are two main scientific arguments regarding the likelihood of life. The first is the idea that life is common. This relies on the fact that the universe is massive and the physical laws are universal. Proponents like Carl Sagan and Stephen Hawking used the Copernican principle to argue that Earth is not in a unique position. They suggest that since the ingredients for life are everywhere, life should be too. The second idea is the Rare Earth hypothesis. This suggests that life requires a very specific, unlikely combination of factors to survive.

In 1961, astronomer Frank Drake created the Drake equation to help scientists discuss the search for intelligence. This equation is a probabilistic argument used to estimate the number of communicative civilizations in the Milky Way. It uses several variables, such as the rate of star formation and the fraction of stars with planets. It also considers how many of those planets could support life and how long a civilization might broadcast signals. While the equation is a useful tool for dialogue, many of its values remain speculative. It helps us organize our search, even if we do not have all the answers yet.