We live in a golden age for space exploration. Scientists are gathering massive amounts of new information and scientific evidence at a record pace. Yet the age-old question remains unanswered: are we alone?
New telescope technologies, including space-based tools such as the James Webb Telescope, have enabled us to discover thousands of potentially habitable exoplanets that could support life similar to that on Earth.
Gravitational wave detectors have opened a new avenue for space exploration by detecting space-time distortions caused by black holes and supernovae millions of light-years away.
Commercial space ventures have further accelerated these advancements, leading to increasingly sophisticated spacecraft and reusable rockets, signifying a new era in space exploration.
NASA’s OSIRIS-REx mission successfully touched down on asteroid Bennu when it was 207 million miles away from Earth and brought back rock and dust samples.
Several countries have developed the ability to deploy robots on the moon and Mars, with plans to send humans to these celestial bodies in the future.
A central driver of all these ambitious endeavours is still that fundamental question of whether life exists — or ever existed — elsewhere in the universe.
Defining life
Defining life is surprisingly challenging. While we intuitively recognize living organisms as having life, a precise definition remains elusive. Dictionaries offer various descriptions, such as the ability to grow, reproduce and respond to stimuli.
But, even these definitions can be ambiguous.
A more comprehensive definition considers life as a self-sustaining chemical system capable of processing information and maintaining a state of low entropy with little disorder or randomness.
Living things constantly require energy to sustain their molecular organization and maintain their highly organized structures and functions. Without this energy, life would quickly descend into chaos and disrepair. This definition encompasses the dynamic and complex nature of life, emphasizing its ability to adapt and evolve.
Life on Earth, as we currently understand it, is based on the interplay of DNA, RNA and proteins. DNA serves as the blueprint of life, containing the genetic instructions necessary for an organism’s development, survival and reproduction. These instructions are converted into messages that guide the production of proteins, the workhorses of the cell that are responsible for a vast array of functions.
This intricate system of DNA replication, protein synthesis and cellular processes — all based on long strings of molecules linked by carbon atoms — is fundamental to life on Earth. However, the universe may harbour life forms based on entirely different principles and biochemistries.
Something other than carbon
Life elsewhere could use different elements as building blocks. Silicon, with its chemical similarities to carbon, has been proposed as a potential alternative.
If they exist, silicon-based life forms may exhibit unique characteristics and adaptations. For instance, they might use silicon-based structures for support, analogous to bones or shells in carbon-based organisms.
Even though silicon-based organisms have not yet been found on Earth, silicon plays an important role in many existing life forms. It is an important secondary component for many plants and animals, serving structural and functional roles. For example, diatoms, a type of algae found in the ocean, feature glassy cell walls made of transparent silicon dioxide.
This doesn’t make diatoms silicon-based life forms, but it does prove silicon can indeed act as a building block of a living organism. But we still don’t know if silicon-based life forms exist at all, or what they would look like.
The origins of life on Earth
There are competing hypotheses on how life arose on Earth. One is that life’s building blocks were delivered on or in meteorites. The other is that those building blocks came together spontaneously via geochemistry in our planet’s early environment.
Meteorites have indeed been found to carry organic molecules, including amino acids, which are essential for life. It’s possible that organic molecules formed in deep space and were then brought to Earth by meteorites and asteroids.
On the other hand, geochemical processes on early Earth, such as those occurring in warm little ponds or in hydrothermal vents deep in the ocean, could have also provided the necessary conditions and ingredients for life to emerge.
However, no lab has yet been able to present a comprehensive, certain pathway to the formation of RNA, DNA and the first cellular life on Earth.
Many biological molecules are chiral, meaning they exist in two forms that are mirror images of each other, like left and right hands. While both left- and right-handed molecules are typically naturally produced in equal amounts, recent analyses of meteorites have revealed a slight asymmetry, favouring the left-handed form by as much as 60 percent.
This asymmetry in space-derived organic molecules is also observed in all biomolecules on Earth (proteins, sugars, amino acids, RNA and DNA), suggesting it could have arisen from the slight imbalance delivered from space, supporting the theory that life on Earth is extraterrestrial in origin.
Chances of life
The slight imbalance in chirality observed in many organic molecules could be an indicator that life on Earth originated from the delivery of organic molecules by extraterrestrial life. We could well be descendants of life that originated elsewhere.
The Drake equation, developed by astronomer Frank Drake in 1961, provides a framework for estimating the number of detectable civilizations within our galaxy.
This equation incorporates factors such as the rate of star formation, the fraction of stars with planets and calculates the fraction of those planets where intelligent life may emerge. An optimistic estimate using this formula suggests that 12,500 intelligent alien civilizations might exist in the Milky Way alone.
The primary argument for extraterrestrial life remains probabilistic: considering the sheer number of stars and planets, it seems highly improbable that life wouldn’t have arisen elsewhere.
The probability of humanity being the sole technological civilization in the observable universe is considered to be less than one in 10 billion trillion. Additionally, the chance of a civilization developing on any single habitable planet is better than one in 60 billion.
With an estimated 200 billion trillion stars in the observable universe, the existence of other technological species is highly likely, potentially even within our Milky Way galaxy.