No Space Aliens? Are We Alone in the Universe?

Why Are There No Visitors From Outer Space?

Part I – The strangely wet Earth; Simple cell biology

Originally, scientists thought that intelligent life should be somewhat common in the universe. All one had to do was to estimate the number of planets in the habitable zones of their stars in our galaxy, and there were an unlimited number of candidates. The prebiotic molecules that help kick-start life can be found throughout the universe. So life made up of single cells is probably common on many planets.

However, a closer look at how life formed on Earth suggests that a number of very unlikely events had to occur for the planet to be habitable and for life to develop past basic single-celled organisms. First, the sun and its planet have to be favorable.

Our Sun, a very stable and bright star called a G-type main-sequence star, formed approximately 4.6 billion years ago. It will remain fairly stable for billions of years more. It is estimated to be brighter than about 85% of the stars in our Milky Way galaxy, most of which are red dwarfs. This stability, along with its brightness, has given life on Earth a generous amount of time to evolve, using abundant solar radiation as an energy source.

Bottleneck 1 – No Early Source of Abundant Water

The planets in our solar system can be divided into two groups. The ones closest to the sun, inside the orbit of the asteroid belt, are rocky planets that formed without some volatile elements, including hydrogen and carbon. For those closest to it, the sun's heat kept volatile elements from condensing out of the dust and gas disk cloud which formed the planets. The planets further from the sun, from the center of the asteroid belt on, were able to accumulate volatile elements, including hydrogen, and form water.

Proto-Earth is thought to have formed as a rocky planet near its current orbit about 4.5 billion years ago. The hypothetical planet Theia developed as an ice world beyond the asteroid belt, and contained carbon as well as abundant water ice.

In the very early solar system, the proto-planets' gravitational forces influenced each other as they grew larger, causing orbital instability. Theia, thought to be Mars-sized, was moved out of its orbit until it eventually collided with Earth, about 9 million years after Earth's formation. The collision, very fortunate in its speed and angle, resulted in a merger giving Earth a metallic core larger than normal, a tilted axis, carbon, abundant water, and a moon much larger in relation to its planet's size than the moon of any other planet in our solar system.

The larger core gave Earth a stronger, longer-lasting magnetic field that shields it from solar wind particles that harm living organisms. The slightly tilted axis gave it seasons. The carbon not only made life possible, but allowed the Earth to have a solid inner core by combining with molten iron. As an ice world, Theia added abundant water early in Earth's history, allowing tectonic plates to form and giving life extra time to evolve. Ocean water today is 0.023% of Earth's mass and covers 71% of its surface. The large moon, tidally locked, provided rotational stability.

The Earth's orbit lies within the habitable zone of the sun, where it is neither too hot for liquid water to exist, nor so cold that water freezes. This habitable zone will continue for one billion years more.

Earth's manner of collision with Theia was an unlikely event that made it even more suitable for the evolution of life. The odds of it happening in the habitable zone of other solar systems to produce an Earth-like planet with a stable sun are exceedingly low.

Bottleneck 2 – No Tectonic Plate Activity

Earth is the only planet in our solar system to have tectonic plate activity. It is thought that these plates are caused by the weight of the large oceans which bear down on the Earth's crust and cause cracks, leading to upwelling at certain plate boundaries and subduction at others. The introduction of a huge amount of water all at once, very early in Earth's history, made tectonic plates possible.

The oxygen and carbon dioxide levels in the atmosphere are primarily the creation of biological activity (photosynthesis) over millions of years. However, it is also thought that tectonic plate activity along with rain assists in the long term management of carbon dioxide levels.

The erosion of silicate rocks in mountain ranges by rain leads to carbon and calcium being carried to the oceans where sea creatures use it to make their shells. When the creatures die, their shells fall to the ocean bottom. Subduction then buries these carbon deposits where they will later be recycled as carbon dioxide by volcanic activity.

A second part of this mechanism is that, as carbon dioxide levels rise and the Earth gets warmer, more phosphorous is carried off by rain into the oceans. The phosphorous fuels the growth of algae, which use photosynthesis to capture carbon. When the algae die, they sink to the ocean bottom, building up their stored carbon and phosphorous on the seafloor at an increasing rate.

As oceans get warmer, the amount of dissolved oxygen in the water decreases, resulting in phosphorous being released from the seabed and recycled, leading to a more rapid growth of algae. A positive feedback loop is created as oceans warm, causing more carbon sequestration at a higher rate by algae, that in millions of years can lead to global cooling when large amounts of carbon dioxide have been pulled out of the air.

Earth's atmosphere today consists of about 78 percent nitrogen, 21 percent oxygen, 0.9 percent argon, and only a trace of carbon dioxide (0.042 percent). That tiny amount of carbon dioxide is all it takes to trap heat radiating from Earth and warm the planet. For advanced life to emerge, planets need at least 18 percent oxygen. If oxygen drops below this level, there will not be enough free oxygen for open-air combustion. Without fire, technologies such as metalworking would be impossible, preventing the rise of any civilization past stone age.

It is important that photosynthesis, subduction, and volcanic activity keep carbon dioxide levels in bounds. If carbon dioxide levels become too low, photosynthesis is no longer possible and atmospheric oxygen will no longer be produced. If carbon dioxide levels become too high, the Earth develops a runaway greenhouse effect. It is believed that without tectonic plates to reduce carbon availability, a greenhouse gas effect could cause air temperatures to become hot enough to break up water molecules and destroy life. The planet Venus is a good example. Venus probably never had large seas appear quickly enough to create tectonic plates. It acquired its carbon and water little by little through asteroid bombardments.

One final note. As of October, 2025, astronomers have discovered 6000 exoplanets in other solar systems. None of them are like Earth.

Bottleneck 3 – No Eukaryotes

The first primitive single-celled organisms to appear were prokaryotes, classified into bacteria and archaea. Prokaryotes have a problem. As they grow larger, their volume and energy needs increase much faster than their surface area. Since they obtain energy by interacting with their environment through their outer surface, they very quickly reach a size limit and must stay tiny.

A chance solution to the energy problem resulted when a new cell type, the eukaryote, emerged. Unlike prokaryotes, a eukaryote has an internal membrane-bound nucleus and other internal organelles, one being a mitochondria, which provides far more energy than could be produced by the cell itself.

Only single-celled organisms called eukaryotes have the energy and size to evolve into complex multicellular organisms. The manner in which eukaryotes came about on Earth, and the limited time frame they had to develop, suggests that their presence was an unlikely chance event. Without eukaryotes, intelligent life cannot evolve.

Eukaryotes apparently came about when [1] the production of oxygen by photosynthetic bacteria led to a world-wide catastrophe which forced other prokaryotes to mutate to be compatible with oxygen, or die. [2] Eukaryotes developed as a merger, called a symbiogenesis, between two cell types, an anaerobic (oxygen incompatible) archaeon and an aerobic (oxygen compatible) bacterium. The aerobic bacterium became the mitochondria, and the merger became an alternative way for at least one species of archaea to survive oxygenation.

There was a relatively short time frame when oxygen stress was beginning to kill many prokaryotes, but before all anaerobic cells died. The merger came about in part because of a unique characteristics of the two cells.

The archaeon had developed a DNA mutation that provided greater flexibility of its cell membrane, allowing it to surround and pull inside objects touching it for ingestion. The aerobic bacteria could perform a chemical exchange with the archaeon that reduced its oxygen load, and supplied it with energy, in the form of ATP molecules as a waste product. When the aerobic bacteria slipped into a fold of the archaeon, possibly as a parasite, it was engulfed by the archaeon.

This led to a symbiotic arrangement where the archaeon did not consume the aerobic as it normally would, but kept it as an organelle. The short time frame for this arrangement to occur, plus the rare DNA capability and compatibility of both cells, make the formation of eukaryotes a chance event less likely to happen on other planets.

Eukaryotes, with their better energy source, are able to grow much larger than prokaryotes, create more elaborate DNA, and evolve into complex multicellular organisms that lead to intelligent life.

So on many planets without large oceans, eukaryotes probably do not form, and intelligent life never emerges. This is yet another reason why we have no visitors from other planets. While the probability of eukaryotes forming on most planets seems unlikely, it happened on Earth, resulting in the evolution of modern humans.

In Part II we look at other roadblocks to the development of intelligent life on a planet, assuming that the planet is suitable to life and eukaryotes develop.