So far, Earth is the only place in the Universe where life is known to exist. This includes not just intelligent life, but anything that meets the definitions of a living entity. Although there is no standard scientific definition, life is often considered to mean a biological organism (or even, possibly, an artificially created organism) that possesses the following traits:

• Homeostasis – internal regulation to maintain a constant state such as temperature
• Organisation – structural composition of one or more cells
• Metabolism – transformation of energy and chemicals, for example to create cellular components and maintain homeostasis
• Growth – increases in size in all of its parts, rather than simply accumulating matter
  Adaptation – the ability to change over time in response to the environment
• Response to stimuli – this can range from simple responses, such as contraction or motion, to complex reactions using multiple senses
• Reproduction – the ability of an organism to create copies of itself

Examples of the search for extraterrestrial life include the first attempts to study the moon and planets for signs of life, following the development of the telescope, to modern attempts to use space probes sent to other planets, such as Mars, to analysis the composition of soil for traces of organic chemicals, or to use spectroscopic analysis of planetary atmospheres for traces of biogenic gases such as oxygen.

Attempts to confirm or rule out the existence of life in our solar system have so far been inconclusive. It has been speculated that microbial life might exist in the martian soil or even in the hostile atmosphere of Venus. It is even possible that multicellular life forms might have developed in hidden oceans that could exist beneath the ice on some of the moons of the planets in the outer solar system, such as Jupiter‘s moon Europa.

Considering the vast number of stars and planets in our Universe, it would seem to be incredibly unlikely that Earth is the only place where life has developed. However, until life is found elsewhere in the Universe, it is not really possible to derive a meaningful estimate for the probability that it has developed independently anywhere other than on Earth, although attempts at this have been made (see the Drake Equation below, for example).

Even if life is found on planets or moons in our solar system, it could still be possible that it was seeded from microorganisms that originated on Earth and were transferred to other parts of the solar system through a meteorite impact, for example. This means that the discovery of life elsewhere in our solar system might not provide us, straightaway, with an indication of how likely it is for life to evolve elsewhere in the Universe.

Conversely, it is even possible that life didn’t originate on Earth, but was seeded from space by dust, meteoroids, asteroids or comets. This is known as the panspermia hypothesis, although this is not generally considered to be a mainstream scientific theory. It is even possible that life was brought to Earth by advanced extraterrestrial civilisations from elsewhere in the Universe, although this is considered to be even more of a speculative hypothesis.

SETI – Intelligent Life

The search for extraterrestrial intelligence, often abbreviated to SETI, refers to attempts by astronomers to find evidence of intelligent life elsewhere in the Universe.

The term ‘intelligent life’ covers living organisms that have the ability to learn and to understand. This could include low-level intelligence, such as that possessed by animals, or high-level intelligence such as that possessed by humans.

This search is often considered to rely on the detection of artificially-produced signals from space, which would require quite an advance level of civilisation.

Since radio astronomy was first developed, in the early part of the twentieth century, searching for radio signals from deep space has been the primary method by which astronomers have attempted to identify the presence of extraterrestrial civilisations.

Originally, attempts focused on finding intelligent life on other planets and moons within our solar system. Although it is still unknown whether life exists elsewhere in our solar system, the existence of intelligent life elsewhere in our solar system has been ruled out.

The Drake Equation

The Drake Equation is an attempt to estimate the number of active, communicative extraterrestrial civilisations in our Milky Way Galaxy.

The equation was formulated in 1961 by Frank Drake, as an attempt to summarise and stimulate scientific debate around the main concepts that scientists must consider when attempting to search for radio-communicative life, rather than as a serious attempt to determine a precise number of communicative extraterrestrial civilisations.

The Drake Equation takes the following form:

N = R_∗ x f_p x n_e x f_l x f_i x f_c x L

where N = the number of civilisations in our galaxy with which communication might be possible and:
R_∗ = the average rate of star formation in our Galaxy
f_p = the fraction of those stars that have planets
n_e = the average number of planets that can potentially support life per star that has planets
f_l = the fraction of planets that could support life that actually develop life at some point
f_i = the fraction of planets with life that actually go on to develop intelligent life (civilizations)
f_c = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
L = the length of time for which such civilizations release detectable signals into space

Depending on the assumptions made for the above parameters, the Drake Equation can give a wide range of outcomes for N, from values well below 1 – meaning that we are likely to be alone in the galaxy – to values as high as 15 million potentially communicative civilisations.