Thus far, the existence of extraterrestrial life remains strictly hypothetical due to the total lack of (official) evidence.
The Biochemistry of Extraterrestrial Life
There are two main hypotheses regarding the existence of extraterrestrial life. The first is that extraterrestrial life can arise independently on two separate planets.
The second hypothesis is called panspermia. Panspermia posits that life arises on one planet and spreads to others, either through a natural cataclysm such as an asteroid strike or it becomes sentient and colonizes the neighbouring worlds in its home star system.
All living organisms on Earth require carbon, hydrogen, oxygen and nitrogen in addition to traces of other elements and minerals such as sulphur and phosphorus. Additionally, living organisms on Earth also require water as a solvent in order to facilitate biochemical reactions.
Because most planets are made from so-called “star dust,” the material ejected outward by exploding stars as they die, it is not unreasonable to assume that planets with a similar density and chemical make-up to Earth may exist elsewhere in the Milky Way Galaxy.
Due to the abundance of these materials in the universe, it has been suggested that life forms elsewhere in the universe might also be water-carbon based. However, other possibilities have also been considered.
The commonly suggested alternative to carbon-based life is silicone. It has been theorized that a silicone-based life form would have a crystalline structure. It is also believed that a silicone-based life form could possibly arise on worlds that orbit very closely to their parent star.
The crystalline structure of such a life form would allow it to survive the intense heat. A third possibility is an ammonia-based life form, however, ammonia is considered less ideal as a facilitator of biochemical reactions than water.
The Evolution of Extraterrestrial Life
In addition to the biochemistry of extraterrestrial life, some scientists have also begun to consider extraterrestrial evolution.
The study of evolution on Earth has led to the discovery of a division between universal and restricted or parochial traits.
Universal traits, such as flight or limbs are adaptations that have evolved multiple times over the course of Earth’s natural history. Parochial traits, such as gills, are traits that are only beneficial to certain species.
The Drake Equation
In 1961, astronomer Dr. Frank Drake devised a mathematical equation that attempted to estimate the number of intelligent civilizations in the galaxy. The Drake Equation works in the following way.
– The rate of star formation
– The fraction of stars with planets
– The fraction of stars with Earth-like planets
– The number of planets with life
– The number of planets with intelligent life
– The number of planets with communicative civilizations
– The lifespan of communicative civilizations
Using this formula, Drake believed that there are roughly 10,000 intelligent species in the Milky Way at any given time in its history.
The Search for Extraterrestrial Life
There are currently two approaches to the search for extraterrestrial life. These are the direct approach and the indirect approach. The direct approach has been a guiding force in the design of many of the probes used to explore our solar system.
For the past several decades, scientists have been searching the solar system for signs of microbial life. It is believed that the planets most likely to harbour life in our solar system are Mars and Europa, which is believed to possess a subsurface ocean.
The indirect approach is the method adopted by SETI, and consists of astronomical observation with the goal of picking up radio transmissions from an extraterrestrial civilization.
