Define criteria for a planet viability scale

Question

It has been a really long trip since we left our home planet. We have seen so many stellar systems but no planet was over 5 on our Viabilometer©... I was desperate until today ! My Viabilometer© is blinking ! We found one ! A planet with a score of 19.583 !

I am trying to establish a "planet viability scale" which can give a score (say from 0 to 20) reflecting the chances of a planet to be suitable for life. More precisely the potential of evolution of a life-form if I drop a microorganism on the planet.

For example :

• A planet with a score of 1/20 allows only days of survival for the microorganisms, thus they would not evolve a lot.
• A planet with a score of 10/20 allows some microorganisms to survive long enough to allows sub-aquatic vegetable and animal life-forms, but they would never go on earth and thus their evolution is limited.
• Finally, a planet with a score of 18/20 allows microorganisms to develop well and "quickly" enough to be able to escape their home planet and travel through stars.

In order to build this scale, I need some criteria that help evolution or curb it. What are the planet criteria that influence evolution of life ?

I know that distance from star, the type of star, presence and composition of the atmosphere, temperature are capital for example. Are there any others ? In what extent are they important ?

Answer 1

H. Lammer in What makes a planet habitable? (2009) proposed the following classification system for water dependent life bearing planets.

• Class I habitats are planetary bodies on which stellar and geophysical conditions allow liquid water to be available at the surface, along with sunlight, so that complex multicellular organisms may originate.
• Class II habitats include bodies which initially enjoy Earth-like conditions, but do not keep their ability to sustain liquid water on their surface due to stellar or geophysical conditions. Mars, and possibly Venus are examples of this class where complex life forms may not develop.
• Class III habitats are planetary bodies where liquid water oceans exist below the surface, where they can interact directly with a silicate-rich core. Such a situation can be expected on water-rich planets located too far from their star to allow surface liquid water, but on which subsurface water is in liquid form because of the geothermal heat. Two examples of such an environment are Europa and Enceladus. In such worlds, not only is light not available as an energy source, but the organic material brought by meteorites (thought to have been necessary to start life in some scenarios) may not easily reach the liquid water. If a planet can only harbor life below its surface, the biosphere would not likely modify the whole planetary environment in an observable way, thus, detecting its presence on an exoplanet would be extremely difficult.
• Class IV habitats have liquid water layers between two ice layers, or liquids above ice. If the water layer is thick enough, water at its base will be in solid phase (ice polymorphs) because of the high pressure. Ganymede and Callisto are likely examples of this class. Their oceans are thought to be enclosed between thick ice layers. In such conditions, the emergence of even simple life forms may be very difficult because the necessary ingredients for life will likely be completely diluted.

According to him any of those could bring forth life. Just each higher class makes it more likely. You could make those in 1/20, 5/20, 10/20 and 15/20. Gives you a baseline to work with.

What you haven't listed explicitely but could be covered by type of star is radiation. The type and amount of radiation that hits the surface. Now you say wouldn't the atmosphere cover that? Not neccesarily. Jovian moons can be protected by the magnetic field of their host. Kilometers of ice too can block enough radiation.

Beyond liquid water not really anything seems required. Radiation would probably be the second thing to get that greatly increases the odds of life. Gamma bursts can whipe a planet clean. Impacts the size of what created the Moon really screw with life too.

So a third requirement could be a quiet galacted neighborhood. Beyond that anything is icing on the cake. They'll help but aren't required.

Magnetic field that protects the atmosphere against solar flares stripping it away? That's nice for your higher life forms. But under the surface smaller life might still develop regardless.

So in summary

1. Liquid water, either by being in the habitable zone or energy by the pressure of gravitational pull of nearby bodies. For example moons of Jupiter.
2. Radiation protection, magnetic fields, layers of ice. Something to shield our organisms from deadly radiation.
3. Quiet neighborhood, speaking of radiation. Even if regular radiation won't kill you a galactic disaster could strip your planet clean.
4. Magnetic field, to retain an atmosphere long term.

Answer 2

There are five dominant factors that determine a planet's habitability. These are discussed in greater detail in this made-for-worldbuilding table.

Temperature influences how quickly atoms and molecules move. Too high of temperature and they won't stick together. Too low, and they won't come together in the first place.

Water is a universal solvent and an agent of transportation for chemicals. It is a necessity for life as we know it.

Atmosphere gives life shelter against harmful radiation and helps regulate surface temperature, as well as supplying necessary chemicals to air-breathing lifeforms.

Energy is the basis of life. Every organism needs energy to perform life functions, whether that comes from light or chemical sources.

Nutrients are what build and enable the growth of organisms. Without nutrients, life cannot develop.

Answer 3

Since other replies are addressing the planet criteria, I'll address the scale itself. A number from 1 to 20 can only measure one parameter, or rather I can only convey one concept with this scale: Viability.

Sequential numbers imply there are steady increments of viability along this scale. If terraforming were possible, you'd have to go through each sequential state.

If the conditions are not sequential but each state is more or less unique, you'd use letters of the alphabet. They would probably correspond to the order in which they were discovered and codified.

But I suggest you consider a compound number system that can convey more than one parameter. For example J7 would not be the seventh J, rather it conveys two separate concepts represented by a letter and a number.

For inspiration: the web uses hexadecimal numbers to describe colors. The first 2 digits are the color red, the next 2 are the color green, etc. After you learn the system you can estimate how bright and the general hue by comparing the 1st, 3rd, and 5th digits. Also interestingly the numbers count from 0 to 9, then from A to E. Throwing in Greek letters helped Star Trek sound esoteric (or possibly math/academic). So consider a compound number system where the digits are represented by various counting systems.