1. Life forms should fill a niche in a balanced ecosystem, based on a food chain of eating and being eaten.
2. Life forms should naturally converge on the safest, most beneficial environments for them.
3. Having said that, any environment that can support life, will eventually support life. Even a harsh environment, like extremely hot and cold places on Earth, provide an advantage to life forms, in that any lichens and microbes that can survive there are safe from other predators that can't.
4. Life forms should naturally optimise their biology to fit their environment. This means forming an optimal size, speed, lifespan, an optimal number of offspring, an optimal daily food intake, an optimal metabolism, and an optimal intellect. This is why so many animals survive for millions of years without improving their intellect.
5. Life forms should mate in such a way as to pair the strongest, healthiest couples to produce the strongest offspring. Offspring should have mutations that over time will make their species stronger and more resilient.
Scientists also consider how an alien planet might differ from Earth, and how this would affect wild life. They ask:
1. What kind of sun is there? How strong is it? A planet is dependent on its sun for the energy required to power life. So any planet would adapt to its sun. Red dwarf suns give off less heat and light, so any plants growing on a planet might appear black, to absorb as much energy as possible.
2. Is the planet in the "goldilocks" distance from the sun? This means it's not too hot, not too cold, but just right for liquid water, or at least liquid something.
3. Another option for a goldilocks environment would be the moon of a very large planet, like Jupiter. Jupiter and Saturn's moons are constantly stretched and squashed in their elliptical orbits, causing a great deal of friction and heat. So, even though there's no atmosphere, and it's quite far from the sun, it can still have warm zones, even on surface. Jupiter's Io has warm spots around its many volcanoes.
4. Is the planet close enough to the sun so that it's "tidally locked", meaning that one side always faces the sun, while the other is always in darkness. Our moon is tidally locked to Earth, which is why we always see the same face on it. This would create fearsome winds on an alien planet.
5. Does the solar system have a large planet like Jupiter to pull away all the asteroids and comets that would otherwise bombard it, killing all life forms?
6. Are there large amounts of carbon, hydrogen, oxygen, and nitrogen? Are they present in an atmosphere? These are all required for life, as we know it.
7. Are there oceans of liquid at a stable temperature (preferably water)? How deep are they? Is the entire planet made of liquid? Scientists regard liquid as necessary, in order to mix various elements in ways that create life - single celled organisms.
8. What's the gravity there? Lower gravity can allow for larger creatures. Higher gravity may require animals with many legs to move around.
9. What's the density of the atmosphere? This determines the possibility of flight, and the size of wingspans necessary for flight. The denser an atmosphere, the easier it is to fly and float.
10. What's the oxygen level in the atmosphere? More oxygen also allows for larger creatures. It also increases the risk of fires.
Based on these concepts, the aliens that scientists most expect to find will be:
1. Micro organisms, Viruses, Bacteria: These are the most likely forms of life. On Earth, they survive in dry deserts, lakes of arsenic, cave walls full of sulphuric acid, and miles underground, some of whom don't even need oxygen to live.
3. simple plants and trees: On Earth, we separate plants and animals, but on alien worlds, there might be life forms that combine aspects of both. On a water world, these plants may have gaseous bladders to keep them buoyant, so they don't sink to the bottom. On a world with a very thick atmosphere, these same plants might use gaseous bladders to float in the air.
4. herbivores, possibly grouping in herds
5. carnivores that hunt them, possibly solitary, almost certainly more intelligent than its prey. Some will hunt in packs in order to corner and ambush prey. Some may even work as a hive.
6. flying creatures similar to birds.
7. If there are oceans of water, scientists expect fish-like creatures, as they are the most efficient shape in water.
8. If a planet commonly experiences high winds, there might be "land whales," an animal that filters micro-organisms from the air, in much the same way that whales do in water. Combine this with a dense atmosphere, and you could have "sky whales," swimming through the air much like how our whales swim in water.