Alien

**Alien** is a hypernym that refers to the vast number of extraterrestrial organisms that humanity has encountered during its colonisation of the Milky Way. They are also known as **extraterrestrials**. More than 200 planets are host to complex multicellular life, and hundreds more are home to single-celled organisms. In accordance with Interstellar Taxonomy Society guidelines, the taxonomic classification and naming sytems for alien life are the same as those for Earth life.

Despite the vast number of ecosystems discovered in the Settled Systems, no extant intelligent life has ever been found; fossil evidence, however, points to the existence of an advanced spacefaring race, the travellers, which disappeared before the evolution of *Homo sapiens*.

## Biochemistries

 Arsenic biochemistry
 Carbon-Water biochemistry
 Hydrogen fluoride biochemistry
 Methane biochemistry (Azotosome)
 Non-green photosynthesizers
 Silicon biochemistry (Organosilicon)
 Xeno Nucleic Acids (XNA)

### Arsenic Biochemistry

Arsenic, which is chemically similar to phosphorus, is poisonous to humans and most other multicellular lifeforms. However, it is incorporated into the biochemistry of some marine algae, which incorporate arsenic into complex organic molecules such as arsenosugars and arsenobetaines. Fungi and bacteria can produce volatile methylated arsenic compounds. Additionally, some prokaryotes can use arsenate as a terminal electron acceptor during anaerobic growth and some can utilize arsenite as an electron donor to generate energy. Due to the hazardous properties of arsenic, humans typically avoid colonising arsenic-rich worlds.

### Non-Water Solvents

While most organisms - including humans - require water as a solvent, some alien lifeforms have been found to use alternatives such as hydrocarbons, hydrofluoric acid, and ammonia. Water is the most common as it is complex enough to undergo a large number of reaction paths; thermodynamic stable; does not combust with oxygen; has a large temperature range in which it remains liquid; a high heat capacity; and the ability to dissolve a wide variety of compounds.

### Silicon Biochemistry

After carbon, silicon is the most common biochemical system as it has many chemical properties similar to those of carbon and is in the same group of the periodic table, the carbon group. Like carbon, silicon can create molecules that are sufficiently large to carry biological information. Silicon-based life has been surveyed on at least twenty-five worlds, though only two are home to multicellular silicon-based lifeforms.

However, silicon has several drawbacks as an alternative to carbon. Silicon, unlike carbon, lacks the ability to form chemical bonds with diverse types of atoms as is necessary for the chemical versatility required for metabolism, and yet this precise inability is what makes silicon less susceptible to bond with all sorts of impurities from which carbon, in comparison, is not shielded. Compared to carbon, silicon interacts with very few other types of atoms; moreover, when it does interact with other atoms, silicon creates molecules that have been described as "monotonous". This is because silicon atoms are much bigger, having a larger mass and atomic radius, and so have difficulty forming double bonds.

Silanes, which are chemical compounds of hydrogen and silicon that are analogous to the alkane hydrocarbons, are highly reactive with water, and long-chain silanes spontaneously decompose. Molecules incorporating polymers of alternating silicon and oxygen atoms - instead of direct bonds between silicon, known collectively as silicones - are much more stable. Silicone-based chemicals are thus more stable than equivalent hydrocarbons in a sulfuric-acid-rich environment; this is why almost all silicon-based lifeforms have been found on such worlds.

## Recurrent Forms

### Flora

Most forms of extraterrestrial plant life - or "xenoflora" - fall into one of four categories: terran-type chlorophyllic; non-terran-type chlorophyllic; terran-type non-chlorophyllic; and non-terran-type non-chlorophyllic.

Terran-type chlorophyllic xenoflora are Earth-like plants that use the pigment chlorophyll to absorb red and blue wavelengths during photosynthesis, producing green leaves. They occur primarily on Earth-like worlds orbitting Sol-like stars. Non-terran-type chlorophyllics use chlorophyll as a pigment but different photosystem units.

Terran-type non-chlorophyllics use Earth-like photosystems but do not use chlorophyll for the process of photosynthesis; among the most common alternative pigments is phycocyanin, a protein that absorbs orange and red light to produce a light green-blue colour. Non-terran-type non-chlorophyllics are the rarest xenoflora and utilize enzymes, protein units and pigments not seen outside of their homeworld.

### Fauna

While hundreds of planets have been found to host organic lifeforms, less than 200 are home to complex multicellular fauna. The diversity of biochemistries and environments across these life-supporting worlds has given rise to a myriad of animal forms; some are very similiar to Earth life both past and present, while others are completely unique and found nowhere else. Like xenoplants, alien fauna are named according to the naming conventions for Earth life, including the taxonomic ranking system.

#### Xenoarchosaurs

   A common motif among a few of these complex lifeforms is that they are similar, morphologically speaking, to the archosaurs of Earth: the group containing birds, crocodillians and dinosaurs. These creatures, dubbed "xenoarchosaurs", do not form a consistent taxonomic group; instead, like the word "alien" itself, the term is a hypernym referring to a range of creatures that fill a similar ecological niche to the archosaurs of Earth.

## Notable Life-Sustaining Worlds

* Asgard
* Callisto
* Cassini
* Columbia
* Ditsö̀
* Earth
* Europa
* Gagarin
* Ganymede
* Garneau
* Harmony
* Hesperus
* Magellan
* Monarch
* Nibiru
* Schwarzschild
* Teegarden
* Tethys
* Titan
* Vanaheim
* Vanderburg