Panspermia is a theory that suggests life exists throughout the universe and spreads through space dust, rocks, asteroids, comets, and other space objects. It also includes the idea of spacecraft carrying tiny living things, like bacteria, unintentionally to other places. This theory suggests that life did not begin on Earth but instead developed elsewhere and spread to Earth and other places.

Panspermia has several forms, such as radiopanspermia, lithopanspermia, and directed panspermia. These theories generally suggest that certain tiny living things, like some bacteria or plant spores, can survive in space. These living things might become trapped in debris that is sent into space after collisions between planets and other space objects that have life. This debris can then travel through space on meteors, moving life between planets or even between different planetary systems in a galaxy. Panspermia studies focus on how life might spread through the universe, not on how life began. This idea is sometimes criticized because it does not explain where life first started.

Panspermia is a less popular theory among most scientists. Critics say it does not answer the question of where life began, only where it might have traveled. It is also criticized because it is hard to test with experiments. In the past, debates about this theory focused on whether life is common throughout the universe or only appears in certain places. Some scientists still study panspermia, trying to create math models that explain how life might move naturally through space. Because of its long history, many stories and false claims have also been made about it.

In contrast, pseudo-panspermia is a well-supported idea that small organic molecules needed for life likely formed in space and were carried to planets.

History

Panspermia is a scientific idea that suggests life on Earth may have come from space. This idea has a long history, starting with Anaxagoras, a thinker from the 5th century BCE. He believed the universe was full of life, and that life on Earth began when life from space fell to Earth. However, the version of panspermia known today is different from Anaxagoras’s original idea. The term "panspermia" was first used in 1908 by Svante Arrhenius, a Swedish scientist. Before this, scientists since the 1860s had started to explore the idea. More recently, scientists like Sir Fred Hoyle and Chandra Wickramasinghe supported the theory.

In the 1860s, three major scientific discoveries helped scientists focus on the question of how life began. First, the Kant-Laplace Nebular theory suggested that Earth formed in conditions too harsh to support life, meaning life must have started later. Second, Charles Darwin’s theory of evolution raised questions about where life first began, as evolution requires a starting point. Darwin did not address this in his book Origin of Species. Third, experiments by Louis Pasteur and John Tyndall showed that life cannot arise from non-living matter, disproving the old idea of spontaneous generation.

These discoveries created a problem: if life could not form from non-living material on Earth, how did life begin? Scientists had two main ideas. Some believed life must have formed from non-living materials under unknown conditions on early Earth, a theory called abiogenesis. Others thought life must have come from existing life elsewhere in the universe, leading to the modern study of panspermia.

In 1871, Lord Kelvin proposed that life could travel to Earth like seeds carried by wind, but instead through meteorites. He believed life could only come from life, a principle similar to the idea that matter cannot be created or destroyed. However, Kelvin’s idea was criticized, as scientists argued that life in meteorites would not survive Earth’s atmosphere.

Svante Arrhenius later gave panspermia its modern form. He argued that abiogenesis lacked evidence and believed life has always existed in the universe. He suggested that solar radiation could push tiny organisms, like bacterial spores, through space. This idea provided a possible way for life to travel between planets. However, scientists still questioned how long spores could survive in space and whether the theory could be tested.

Some scientists, like Fred Hoyle and Chandra Wickramasinghe, supported panspermia because they believed Earth’s conditions might not have been ideal for life to begin, and because Earth’s life shows traits that might not explain well if life started here. Hoyle studied space dust and found organic materials, which he thought were building blocks for life. He also suggested that comets might carry viral material to Earth, possibly causing epidemics. However, biologists criticized this idea.

Since the 1970s, space exploration has provided data to test panspermia. While the theory remains unproven, it continues to be studied. Despite its challenges, panspermia has remained a topic of interest for many years.

Overview

Scientists have proposed three main ideas: (1) that organic molecules, which are the building blocks of life, may have formed in space and later reached Earth, (2) that life itself could have started from these molecules outside Earth, and (3) that this life was then brought to Earth. The idea that organic molecules formed in space and spread to Earth is now widely accepted and is called pseudo-panspermia. However, the idea that life itself originated in space and traveled to Earth is still a theory that cannot be tested with current methods.

Bacterial spores and plant seeds are two types of structures that scientists suggest might carry life between planets. According to the theory of panspermia, these spores or seeds could be trapped inside a meteorite, travel through space, and then land on another planet. Once there, they might survive the journey through a planet’s atmosphere and grow into living organisms. For this to happen, the spores or seeds must have formed elsewhere, possibly even in space. Studies of how planets form and the composition of meteorites suggest that some rocky objects in space might have conditions that could support life. For example, heat from radioactive elements inside these objects might melt ice, creating water and energy. Some meteorites found in our solar system show signs of water-related changes, which could mean such processes have occurred. Because there are many such objects in the solar system, some scientists think each might have the potential to develop life. A collision in the asteroid belt could change the path of one of these objects, sending it toward Earth.

Plant seeds are another possible way life might travel. Some seeds can survive extreme conditions, such as cold, vacuum, and ultraviolet light, which are found in space. These seeds are not thought to have formed in space but on another planet. If a plant is damaged during its journey through space, parts of it might still be able to grow in a new environment. However, this depends on whether the environment is sterile, as it is unclear if the new plant could compete with existing life. This idea is based on past research showing that living cells can sometimes regrow from broken parts of algae. Plant cells also contain tiny organisms called endosymbionts, which might survive and reproduce in a new place.

While both plant seeds and bacterial spores are considered possible ways for life to travel, scientists debate whether these structures can survive the long time in space and the intense heat of entering a planet’s atmosphere.

Space probes could also carry life between planets. To reduce the risk of spreading Earth life to other worlds, space agencies follow strict cleaning procedures. However, some microorganisms, like Tersicoccus phoenicis, might be able to survive the cleaning processes used during spacecraft assembly.

Varieties of panspermia theory

Panspermia is divided into two main types: one involves the transfer of life between planets in the same solar system (interplanetary panspermia), and the other involves transfer between different solar systems (interstellar panspermia). These types are further classified based on how life might travel through space.

In 1903, Svante Arrhenius proposed a theory called radiopanspermia. He suggested that tiny, single-celled life forms could travel through space, pushed by the pressure of light from stars. This happens because light can apply a small force on very small particles. Arrhenius believed that particles smaller than 1.5 micrometers (about the size of a single bacterial spore) could be moved quickly by starlight. However, this method only works for extremely small particles, as larger ones are not affected as strongly by light pressure.

Critics, such as Iosif Shklovsky and Carl Sagan, pointed out that space radiation, like ultraviolet (UV) and X-rays, can destroy microorganisms. If enough of these tiny life forms were sent into space, some might reach another planet after traveling through space for about 10 years. However, most would likely die from radiation and the harsh conditions of space. Some scientists still think this theory could be possible.

Experiments like ERA, BIOPAN, EXOSTACK, and EXPOSE showed that bacterial spores, such as those of Bacillus subtilis, can be killed quickly by the full space environment. However, if protected from UV radiation, these spores could survive in space for up to six years when embedded in clay or meteorite-like material. To survive space radiation, spores would need strong protection. Rocks at least 1 meter in size are needed to shield microorganisms from harmful cosmic radiation. Additionally, the vacuum of space alone can damage DNA, making it unlikely for unprotected DNA or RNA to survive long trips through space powered by light pressure.

Other ways for larger, protected spores to travel, such as being captured by comets, are not well understood. There is little evidence to support the radiopanspermia idea.

The idea of panspermia became more studied after discoveries of exoplanets and new data. Lithopanspermia is a theory where life is transferred between planets through rocks, such as those in comets or asteroids. This theory is still unproven. A related idea is that life could travel between star systems on free-floating exoplanets or exomoons.

Although there is no clear proof that lithopanspermia has happened in our solar system, scientists can now test parts of the theory. For example:

• Planetary Ejection: For lithopanspermia to work, microorganisms must survive being ejected from a planet’s surface. This involves extreme forces, high temperatures, and pressures. Studies of Martian meteorites suggest that rocks might experience pressures up to 55 gigapascals, speeds of 3,000 kilometers per second, and temperature increases of up to 1,000 degrees Celsius. Some organisms might survive these conditions.

• Survival in Transit: Once in space, microorganisms must survive the journey. Experiments have shown that some bacteria can form protective layers called biofilms to resist UV radiation. These tests help scientists understand how life might survive in space.

• Atmospheric Entry: When rocks carrying life return to a planet, the organisms must survive re-entry into the atmosphere. Tests using rockets showed that Bacillus subtilis spores could survive re-entry if they were on the sides of rocks, but not on the front-facing parts that became extremely hot. Photosynthetic organisms, like cyanobacteria, might not survive re-entry, as shown in the STONE experiment.

Lithopanspermia can occur between planets or between stars. Scientists have created models to estimate the chances of life traveling between planets. For example, a study of the Trappist-1 system found that lithopanspermia might be much more likely there than between Earth and Mars. This is because the Trappist-1 planets may have a higher chance of life forming naturally. These models suggest that panspermia could help explain how life begins, rather than contradicting it.

Lithopanspermia might also happen between star systems. One study estimated that many rocky or icy objects in the Milky Way could be captured by other planetary systems. However, for this to work, life would need to survive the journey, which depends on how long organisms can live and how fast they travel. No evidence has been found to support this idea.

Lithopanspermia is hard to prove because of the extreme conditions life would need to survive. However, large impacts, like those in the early solar system and still happening today in the asteroid belt, could send rocks carrying life between planets.

In 1972, Nobel Prize winner Francis Crick and Leslie Orgel proposed a theory called directed panspermia. They suggested that life was intentionally brought to Earth by an advanced civilization from another planet. This idea was considered because earlier theories about life traveling via radiation or rocks seemed unlikely. However, Orgel was less serious about this theory.

Hoaxes and speculations

On May 14, 1864, twenty pieces from a meteorite fell in the French city of Orgueil. In 1965, a separate piece of the Orgueil meteorite (kept in a sealed glass jar since its discovery) was found to have a seed inside it. However, the original glassy layer on the outside of the meteorite remained untouched. Scientists were excited at first, but later found that the seed was from a European Juncaceae or rush plant. The seed had been glued into the meteorite and hidden with coal dust. The outer "fusion layer" was actually made of glue. The person who created this trick is unknown, but it is believed they wanted to influence a 19th-century debate about whether life could arise from non-living matter—rather than from space—by showing how inorganic material could become biological.

In 2017, the Pan-STARRS telescope in Hawaii discovered a reddish object that showed large, regular changes in brightness, suggesting it was a thin, spinning object. Analysis of its path showed it was an interstellar object, coming from outside the Solar System. It moved away from the Sun without the usual visible gas release that helps asteroids speed up. Astronomer Avi Loeb says there is no clear natural explanation for this movement and suggests Oumuamua might be a solar sail, which could support the idea of directed panspermia. However, other scientists believe this claim is unlikely.