---
title: "Panspermia"
chunk: 3/5
source: "https://en.wikipedia.org/wiki/Panspermia"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T03:09:26.597459+00:00"
instance: "kb-cron"
---

== Overview ==

=== Core requirements ===
Panspermia requires:

that organic molecules originated in space (perhaps to be distributed to Earth),
that life originated from these molecules, extraterrestrially,
that this extraterrestrial life was transported to Earth.
The creation and distribution of organic molecules from space is now uncontroversial; it is known as pseudo-panspermia. The jump from organic materials to life originating from space, however, is hypothetical and currently untestable.

=== Transport vessels ===
Bacterial spores and plant seeds are two common proposed vessels for panspermia. According to the theory, they could be encased in a meteorite and transported to another planet from their origin, subsequently descend through the atmosphere and populate the surface with life (see lithopanspermia below). This naturally requires that these spores and seeds have formed somewhere else, maybe even in space in the case of how panspermia deals with bacteria. Understanding of planetary formation theory and meteorites has led to the idea that some rocky bodies originating from undifferentiated parent bodies could be able to generate local conditions conducive to life. Hypothetically, internal heating from radiogenic isotopes could melt ice to provide water as well as energy. In fact, some meteorites have been found to show signs of aqueous alteration which may indicate that this process has taken place. Given that there are such large numbers of these bodies found within the Solar System, an argument can be made that they each provide a potential site for life to develop. A collision occurring in the asteroid belt could alter the orbit of one such site, and eventually deliver it to Earth.
Plant seeds can be an alternative transport vessel. Some plants produce seeds that are resistant to the conditions of space, which have been shown to lie dormant in extreme cold, vacuum, and resist short wavelength UV radiation. They are not typically proposed to have originated in space, but on another planet. Theoretically, even if a plant is partially damaged during its travel in space, the pieces could still seed life in a sterile environment. Sterility of the environment is relevant because it is unclear if the novel plant could out-compete existing life forms. This idea is based on previous evidence showing that cellular reconstruction can occur from cytoplasms released from damaged algae. Furthermore, plant cells contain obligate endosymbionts, which could be released into a new environment.
Though both plant seeds and bacterial spores have been proposed as potentially viable vehicles, their ability to not only survive in space for the required time, but also survive atmospheric entry is debated.
Space probes may be a viable transport mechanism for interplanetary cross-pollination within the Solar System. Space agencies have implemented planetary protection procedures to reduce the risk of planetary contamination, but microorganisms such as Tersicoccus phoenicis may be resistant to spacecraft assembly cleaning.

== Varieties of panspermia theory ==

Panspermia is generally subdivided into two classes: either transfer occurs between planets of the same system (interplanetary) or between stellar systems (interstellar). Further classifications are based on different proposed transport mechanisms, as follows.

=== Radiopanspermia ===
In 1903, Svante Arrhenius proposed radiopanspermia, the theory that singular microscopic forms of life can be propagated in space, driven by the radiation pressure from stars. This is the mechanism by which light can exert a force on matter. Arrhenius argued that particles at a critical size below 1.5 μm would be propelled at high speed by radiation pressure of a star. However, because its effectiveness decreases with increasing size of the particle, this mechanism holds for very tiny particles only, such as single bacterial spores.

==== Counterarguments ====
The main criticism of radiopanspermia came from Iosif Shklovsky and Carl Sagan, who cited evidence for the lethal action of space radiation (UV and X-rays) in the cosmos. If enough of these microorganisms are ejected into space, some may rain down on a planet in a new star system after 106 years wandering interstellar space. There would be enormous death rates of the organisms due to radiation and the generally hostile conditions of space, but nonetheless this theory is considered potentially viable by some.
Data gathered by the orbital experiments ERA, BIOPAN, EXOSTACK and EXPOSE showed that isolated spores, including those of B. subtilis, were rapidly killed if exposed to the full space environment for merely a few seconds, but if shielded against solar UV, the spores were capable of surviving in space for up to six years while embedded in clay or meteorite powder (artificial meteorites). Spores would therefore need to be heavily protected against UV radiation: exposure of unprotected DNA to solar UV and cosmic ionizing radiation would break it up into its constituent bases. Rocks at least 1 meter in diameter are required to effectively shield resistant microorganisms, such as bacterial spores against galactic cosmic radiation. Additionally, exposing DNA to the ultrahigh vacuum of space alone is sufficient to cause DNA damage, so the transport of unprotected DNA or RNA during interplanetary flights powered solely by light pressure is extremely unlikely.
The feasibility of other means of transport for the more massive shielded spores into the outer Solar System—for example, through gravitational capture by comets—is unknown. There is little evidence in full support of the radiopanspermia hypothesis.

=== Lithopanspermia ===
This transport mechanism generally arose following the growth of planetary science with the discovery of exoplanets and the sudden availability of data. Lithopanspermia is the proposed transfer of organisms in rocks from one planet to another through planetary objects such as in comets or asteroids; it remains speculative.  A variant would be for organisms to travel between star systems on nomadic exoplanets or exomoons.
Although there is no concrete evidence that lithopanspermia has occurred in the Solar System, the various stages have become amenable to experimental testing.