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You often hear scientists talk about it: we have found traces of molecules that are necessary for life on Mars. But we all know that Mars is not habitable right now. It might have been, but we cannot live there now.

I am making a space exploration video game, and I'm wondering what properties a planet needs in order for Earth-like life to exist there. I know about the habitable zone around a star, temperature, atmospheric density, gravity, etc.

I would like to get a more complete list of every parameter on planets that makes unaided life possible.

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  • $\begingroup$ Possibly relevant question. $\endgroup$ – ForgeMonkey Jun 11 '15 at 8:04
  • $\begingroup$ We can't, but it seems quite possible that some forms of Earthly life could survive on Mars. Especially underground. Harder for it to originate or get there, but that is being worked on! $\endgroup$ – LocalFluff Jun 11 '15 at 9:38
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    $\begingroup$ You may also be interested in the Worldbuilding Stack Exchange site. $\endgroup$ – a CVn Jun 11 '15 at 11:19
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    $\begingroup$ Take care to distinguish between the following three things: (1) survivability of life in some form; (2) possibility for life to emerge; (3) possibility to have a biosphere like on earth. To get (1) you don't need much beyond some liquid water somewhere, and there might even be "life, but not as we know it" that doesn't even need that. (You also need an energy source, but you can get that from many kinds of rock.) (2) is much harder, since we don't know how life arose on Earth. (There are many theories - don't let anyone tell you they know which is the right one.) ... $\endgroup$ – Nathaniel Jun 14 '15 at 10:18
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    $\begingroup$ ... for (3) you need liquid water on the surface and you probably also need plate tectonics, because without that you have no way to recycle nutrients over sufficiently long time scales. There are several theories that say the existence of life can enhance the probability of plate tectonics and liquid water, so it's a positive feedback loop. The point is that life might exist on a planet (perhaps such as Mars) without forming an Earth-like biosphere. $\endgroup$ – Nathaniel Jun 14 '15 at 10:20
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Planetary Habitability depends on a whole host of parameters, I will attempt to summarise some of them. These represent the absolute basic characterises a planet should have to sustain life. I leave the advanced mechanisms of the actual creation of life to your creative freedom.


The blue planet

Our Earth, the only planet in our infinitely large universe known to harbour life. (Courtesy Wikimedia Commons)


Mass

Geologically, a small planet will have a high surface-to-volume ratio. As a result, most of the energy acquired during formation is lost, and plate tectonics do not tend to occur. Current research suggests that tectonic movement is vital for evolution, and of course, geothermal heat. Do note that exceptions to this rule exist, such as Io, which maintains it's tectonic system due to gravitational stress imparted by Jupiter.

Planets with a very low mass tend not to have the necessary gravitational pull to retain an atmosphere. For example Mars is less massive than the Earth, and has a much thinner atmosphere. A thin atmosphere means less heat insulation, less protection from radiation, and less protection from meteors. Additionally a minimum atmospheric pressure of $0.006 atm$ is required for water to exist in it's liquid form.

On the other hand if a planet is too large, then it's surface gravity starts becoming an issue. Any life on that planet would have to evolve to withstand the high gravity. However presuming such evolution can take place, larger planets are more suited to harbour life. They have a tendency to create magnetic fields from their iron cores, protecting the planet from radiation.


Orbital Characteristics

The Earth has an orbital eccentricity (very roughly, deviation from a circular orbit) of 0.02. This means the the distance between the Earth and the Sun stays more or less the same over the period of a year. Hence, heating is uniform year-round.

If a planet has a high orbital eccentricity, then it will go through massive swings in global climate over the course of it's 'year'. These fluctuations could be too severe for any life to evolve.

Rotation and tilt are two important factors, being responsible for seasons. Rotation should be fast enough to maintain a reasonable day-night cycle. If it is too slow, then the large difference in temperature each day would cause issues. Rotation also influences the magnetic field of the planet by inducing a magnetic dynamo in it's iron core. Finally, there is significant scientific evidence that the presence of the Moon is a crucial factor in stabilising the Earth's tilt.


Geochemistry

Assuming that extra-terrestrial life is similar in composition to life on Earth, four fundamental elements are required; Oxygen, Carbon, Hydrogen, and Nitrogen. However since this is a video game you are developing, I presume you have the artistic freedom to create new, different mechanisms for life. Thus, I shall not elaborate more here.


Others

Once you have your planetary characteristics sorted out, it's up to you to define further parameters for life to exist on that planet. What type of star is it orbiting? Are solar winds frequent, and how do they affect the planet? Does the planet's solar system have an Oort Cloud, hurling massive asteroids through space, causing mass extinctions after a fixed time period?

Life is fascinating because it is the single most complex phenomenon we have observed so far. Only your imagination limits the world you can create. Do some research on your own as well, read up on evolution and biochemistry. I wish you the best for your game!


Sources:

http://en.wikipedia.org/wiki/Planetary_habitability (Structured around this)

http://adsabs.harvard.edu/abs/2007AsBio...7...66R

Further Reading:

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19900013148.pdf

http://web.archive.org/web/20110117011137/http://astrobiology.arc.nasa.gov/roadmap/g1.html

https://www.cfa.harvard.edu/news/2008-02

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  • $\begingroup$ I've actually got several issues with your answer. -Underground life can live without atmosphere. -Seasonal fluctuations might even stimulate evolution, it's been proposed that global ice ages made multicellular life emerge on Earth. -Nothing speaks for high gravity being an issue for life, there are advanced animals in the high pressure deep seas. Lack of tectonics might be a killer in the long term though. The simulated planet should be very geologically active, with stuff like Ionian sulfur volcanoes. Should help the game drama. $\endgroup$ – LocalFluff Jun 11 '15 at 9:55
  • $\begingroup$ @LocalFluff this is meant to be a brain dump of all the parameters affecting life on a planet. There are obviously exceptions to every single thing I have stated here, but I cannot list them all. Conversely, I cannot, for example, remove what I wrote about the atmosphere because it is vital for the majority of life on Earth. OP can use this information to modify his theoretical planet and it's inhabitants accordingly. $\endgroup$ – Vedant Chandra Jun 11 '15 at 10:09
  • $\begingroup$ @Vedant Chandra Thank you for this list. I don't care about getting every parameter right as it is just a video game. Not a program nasa astronauts depend on. So I think the brain dump works best for that. $\endgroup$ – Avanak Jun 12 '15 at 8:32
  • $\begingroup$ @Avanak that was my purpose, glad I could help :) $\endgroup$ – Vedant Chandra Jun 12 '15 at 8:40
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Earth like life ... lots of luck. the proteins and water seem to come from the cold outer areas of a solar system. if in the goldilocks zone and given enough time I would submit that the same thing that happened here would repeat itself.

remember that there was once many zillions of life forms on this planet. with many different mechanisms of biology. then it came to pass that one evolved and created this poisonous gas, killing off most of the life. that gas, oxygen produced by algae, separated the life forms into two basic types aerobic and anaerobic. some of the latter persist to this day. viruses were part of that old pool of life too.

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