# What are the minimum set of physical characteristics to define an Earth like planet?

## Introduction

I have a vague notion of building some sort of strategic space game. Similar to other strategic games, locations are assigned value based upon some abstract evaluation. Following in that vein, I'd like to assign value(s) to planets based upon a number of factors including colonization.

If you feel that this particular question can be answered in two (or more) ways depending upon what I mean by "Earth like", then for this question assume I mean for purposes of human habitation. If you can easily slip in other measures of value, feel free to do so but be sure to distinguish how this value is different than that for colonization.

## Setting

Humans have figured out some mechanism for traveling to other star systems. When we get to these new destinations we want to evaluate the "value" of the planets. One basis for this valuation will be colonization ease (how easy to transplant humans & human life to the planet) and potential (what is the total population this planet is likely to be able to support).

## Likely Measures

Measures I think are likely to be crucial:

• Surface Gravity (higher than 1 g is worse than lower than 1 g)
• Surface Pressure (higher than STP is worse than lower than STP)
• Surface Temperature (higher than STP is worse than lower than STP)

Of course matching atmosphere to what we need would be good but as a starter planet that we can terraform, we might be able to begin with a planet with Earth normal atmospheric pressure. Humans would just require oxygen masks as long as the atmosphere wasn't corrosive or toxic.

Can you think of any other valuation criteria?

Has some other scientific body considered this question already? If so, what did they think?

Also, I like the original Traveller RPG. It had a hex scoring of a planet based upon a variety of factors (gravity, temperature, pressure, radiation, etc.) but that valuation works better on a scalar value like pressure than it does for something as diverse as atmosphere.

I was thinking of doing something like a "snow line" scale for my linear scale and then some other marker if the atmospheric composition deviates from the norm.

Snow line - planet composition chart

• I don't have enough info for a complete answer, but regarding your last question, yes, it has been considered already. For an example of an earth similarity index primarily based upon surface gravity and surface temperature see: phl.upr.edu/projects/earth-similarity-index-esi – fantasia Jul 4 '15 at 14:26
• You should submit that as an answer. I'll at least up vote it! – Jim2B Jul 4 '15 at 14:55
• Great job on the formula. I'd weight it differently though. I don't care about size, density, etc. as long as surface gravity is sufficiently close. And even surface gravity isn't that important if you're within a relatively broad range. It gets very important if you go too far outside the range. – Jim2B Jul 4 '15 at 15:00
• Π is actually the Greek letter pi, albeit the upper-case version, which is why it looks a bit different - although similar enough for you to get it right when you were guessing. It is quite different from π in mathematics though, and is used as a product operator. The way it is expressed in this particular equation means it is an operator for Cartesian products - which I had never even heard of up until 2 minutes ago. You learn as long as you live, right? :) – fantasia Jul 4 '15 at 18:42
• In this case (earth-similarity-index-esi/ESI%20Equation.png) the absolute value of the difference over the sum is a common way to scale a value to account for the zero point. The identical thing appears at the bottom of this page in manner that's easy to see: the contrast of dark vs. light stripes. The 1-minus reverses it, so 1 is match and approaching 0 is no match. Then raise to a power to weight that factor. The inverse power n goes with the Pi, meaning this is just a geometric mean of the weighted similarity values. – JDługosz Jul 5 '15 at 3:22

Angular Momentum, Orbital Ellipse, Mass, Diameter, Number of Moons, Core Temperature, Atmospheric Pressure, Magnetic Fields,

. . . . The list goes on and on!

I've been working on a RT4X space colony simulator and have/am faced/facing similar issues. One way I suggest tackling this is by identifying an optimal parameterization for your world's class constructor. remember that many of the above factors are related, such as Mass, Atmospheric pressure, orbital ellipse, etc. And many (the ellipse, atmospheric composition) are themselves a set of parameters. Surface temperature is based on atmospheric pressure and distance from the sun directly. However, both of these have to do with the total mass of your planet and shape of the orbital ellipse. The Core temperature would effect the strength of your magnetic field and tectonic activity, and you might be able to determine both my addressing how old your world is, or how long since it aggregated. The proportions and distribution of elements is also affect by how far from the sun the planet formed.

Bare with me here - it might be worth considering how your entire solar system evolved. I'm not saying you actually create some sort of nebula simulator. Although that would be pretty cool too. You can fake that by maybe randomly sampling statistic ranges. ANYWAY for example let's say you'd like a planet of mass m, age a, orbiting along ellipse e. That's only three values, and yet already you can design a class constructor which might be able to generate all the factors which would determine the habitability of a planet.

Your 'value' therefore could be derived from just a few basic parameters - how far the planet deviates from the home-world, for example.

so if your species' homeworld is H and the prospective colony is on C, then the square root of {(H(a)-C(a))^2 + (H(m)-C(m))^2 + (H(e)-C(e))^2} could give you very rough idea of how your planets differ, and thus a value.(assuming H would have a MAXIMUM value) YOu probably don't have to square or root anything and you might have more to consider but that's just a simple example of a simple method, just to give you an idea.

There is something else to consider. A low gravity ball of hydrocarbons and Tholins would serve much better as a 'trading post' or a 'refueling station' than a food producing colony world. Dwarf and protoplanet are ideal for robotic mining operations. A hefty 2g world might confer a +1 strength bonus any Espatier trained thereupon.

As you can no doubt intuit, my own project is mired in complexity! Such a topic is hard to KISS. Good luck to you.

• I agree with your last comments. We'd put different emphasis on different aspects of the body depending upon its use (space craft way point, research outpost, colony, etc.). But yeah we'd need to consider a propellant station based availability of propellant and ease of getting there and proximity to routes taken by space craft. Pluto does us no good for propellants if you're only going to Mars! – Jim2B Jul 4 '15 at 20:08
• While I agree that all of the factors you mentioned to impact the habitability, I think the more direct measures (temperature, gravity, composition, etc.) already account for formational things like orbital shape, number of moons, & distance from star (e.g. planets with high temperature standard deviations might not be habitable). – Jim2B Jul 14 '15 at 13:23

Two overlooked evaluation criteria:

A magnetosphere is beneficial to human life in at least two ways:

1) It reduces the amount of cosmic radiation that reaches surface dwellers

2) It prevents solar winds and cosmic rays from stripping the planet from its atmosphere

A planet's sidereal day is of importance primarily for the temperature variances on the surface. A pleasant average temperature, on paper, can in fact be a mean value calculated from the scorching day's and freezing night's respective temperature. Vast oceans and an atmosphere will of course mitigate these effects, but on a planet that rotates very slowly around its own axis (where one night might last several Earth months, for example) the variations would be quite noticeable and therefore sidereal day should probably be included when a habitability index is calculated.

Of note, the Drake Equation. You are attempting to quantify: ne

Habitable planets require a moon to reign in their tilt, providing nominal seasons. Our solar system has a shepherd (Jupiter) which helps keep comets and asteroids away. The planet can't just be physically akin to Earth, the entire system must be of similar ratios and content.

E.g., if the sun was twice as large and the orbits unchanged, we might be having this conversion on Mars. However, with the facts that it lacks sufficient gravity to hold an atmosphere and it's geologically inactive (probable lack of water and no magnetosphere), perhaps not.

Circumstellar Habitable Zone: (Goldilocks Zone)

An example of a system based on stellar luminosity for predicting the location of the habitable zone around various types of stars. Planet sizes, star sizes, orbit lengths, and habitable zone sizes are not to scale. Cite: Habitable_zone-en.svg

Conjecture has been proposed that the safest solar systems would have two shepherds, ours being Jupiter and Saturn. Long ago, these two planets shared a harmonic orbital resonance and are thought to have finally 'cleared' our system of debris with these compounded perturbations.

• Do habitable planets require a moon to keep the axial tilt stabilized, or is that merely a theory that hasn't been proven yet? And does Jupiter keep asteroids and comets away from Earth, or is that another unproven theory? I think that you should have listed those as factors that merely might be necessary. – M. A. Golding Jun 23 '18 at 15:42
• @M.A.Golding - According to Patrick Stewart (narrator) If we had no moon a lot bad things would happen. And as explained here, there's an episode of The Universe that "surmises" J&S's resonant orbit. This stuff is hard to prove, and why I say "thought to have". – Mazura Jun 23 '18 at 18:26
• >Periodic motions of the Moon and of Earth in its orbit cause much smaller (9.2 arcseconds) short-period (about 18.6 years) oscillations of the rotation axis of Earth, known as nutation, which add a periodic component to Earth's obliquity. – Axial tilt – Mazura Jun 23 '18 at 18:34
• >The presence of a near resonance may reflect that a perfect resonance existed in the past, or that the system is evolving towards one in the future. ... A past resonance between Jupiter and Saturn may have played a dramatic role in early Solar System history. A 2004 computer model by Alessandro Morbidelli of the Observatoire de la Côte d'Azur in Nice suggested that the formation of a 1:2 resonance between Jupiter and Saturn (due to interactions with planetesimals that caused them to migrate inward and outward, respectively) ... – Mazura Jun 23 '18 at 18:42
• ... created a gravitational push that propelled both Uranus and Neptune into higher orbits, and in some scenarios caused them to switch places, which would have doubled Neptune's distance from the Sun. The resultant expulsion of objects from the proto-Kuiper belt as Neptune moved outwards could explain the Late Heavy Bombardment 600 million years after the Solar System's formation and the origin of Jupiter's Trojan asteroids. – Orbital resonance – Mazura Jun 23 '18 at 18:43

For most of its history, Earth was not "Earthlike". The oxygen atmosphere accumulated over the last 850 million years though it got started 2.4 billion years ago.

One of my points is that just adding oxygen to an atmosphere that is already the right pressure (or would that be 80% of the current pressure?) is not going to work because it is soaked up as fast as it is produced. Think about the total amount of mass that went into the banded iron strata! How could you do that rapidly, on a human time scale? Even if you did introduce that much oxygen, the reaction would generate more heat than easily handled.

Given the idea of changing the atmosphere, what difference does the original pressure make?

If you want people to work outside, the hostility matters. Pressure high enough but unbreathible? You just need masks, you point out. But if its too hot or cold, you would need special gear anyway.

Another thing is the composition. Is it harmless just lacking oxygen? CO2 can't be tolerated actually, so a full mask to provide a breathing mixture is needed. Living planets (once life spreads like ours) will give an atmosphere that's out of chemical equlibrium. But perhaps there are more choices than oxygen! We can handle corrosive oxygen because we're built for it, but what about chlorine or hydrogen flouride?

So, I ponder that you can rank conditions based on whether it is benign or outright hostile, based on the equipment needed to work in it and the strictness necessary to remain isolated from it.

• You could use oxygen content for the atmosphere as well as pressure, like 20% content is green and edges are 60% and 0% (both red levels)
• Water levels would be important in that they would determine the required amount of water transported to the planet as well as (maybe) the type of habitat you need to have created to live there
• As mentioned by fantasia, magnetosphere is important for blocking radiation & solar 'weather', protecting the planet. The time between inversions in the magnetosphere would also be important because it could affect electronics
• Again, fantasia mentioned temperature ranges, as opposed to average temperatures. The temperature range could be put on a single bar, with human-acceptable levels in the green area
• Existence of sentient creatures could be useful, because if your 'Linguistics' and your 'Diplomacy' or 'Propaganda' skills are high enough, you can either make alliances with them or make them into slaves (assuming you want this in the game)

My habitability index so far includes the following:

### Primary (measures of habitability)

Temperature
Pressure
Primary atmosphere component
Secondary atmosphere component
Tertiary atmosphere component
Atmosphere notes
Hydrogology
Surface Gravity

### Secondary (measures of desirability)

Mineralogy
Sidereal Day
Expected half-life of atmosphere
Star's spectral class
Add other interesting planet characteristics here

I'm not going to write up the details on all of these because there's just too much material (maybe I'll add that later). But I did want to discuss two things: temperature and pressure.

Temperature
From the Habitable Planets for Man

To make this scale reasonable for the range of possible temperatures, I recommend a $\text{log}_2$ scale of the temperature in K. On this scale, the coldest spot in the Solar System (the Moon's South Pole) rates a value of 4.9.

• -405 F / 30 K (coldest spot in the Solar System/Moon's South Pole) equates to 4.9 on my scale
• 0 F / 255 K (probably the minimum average temperature) equates to 8 on my scale.
• 110 F / 316 K (probably the maximum average temperature for human life) equates to 8.3 on my scale.
• 501 F / 534 K (surface of Venus) equates to 9 on my scale

Rather than a single value, we probably need to measure the temperature in 3 locations: equator, pole, and 1/2 way in-between.

We'd need to account for a range of possible temperatures too, so we'll want values for

• Equatorial Average
• Equatorial Standard Deviation
• Temperate Average
• Temperate Standard Deviation
• Polar Average
• Polar Standard Deviation

## Pressure

Atmospheric pressure matters a lot.

It affects many aspects of how liveable that planet might be. When it's too high, even normally inert gases like Nitrogen become troublesome. When it's too low, you need to increase the percent of oxygen to keep it liveable and 100% oxygen makes just about everything - including you - flammable.

I'd use a $\text{log}_5$ scale for atmospheric pressure. On this scale,
0 - vacuum
4.0 - Mars pressure
6.5 - min pressure for human survival
7.2 - Earth pressure
7.4 - Titan's pressure
9.6 - max pressure for human survival
10. - Venus' pressure

### Composition

I'd probably just use a letter code to denote the primary constituent gases and percentages. For ease of use, assume any atmosphere with Hydrogen will have a proportional amount of Helium - so you can omit the Helium content.

Then you could also include a note about the qualitative nature of the atmosphere (reducing, oxidizing, corrosive, toxic, etc.). Humans could survive some atmospheres without spacesuit like protection, but we can assume that corrosive and toxic atmosphere require some sort of special protection. Although our skin might handle reducing atmospheres (like hydrogen and methane), we'd need to be very careful with our breathing oxygen lest we turn our breathers into flame throwers.

It's trickier with gases like $CO_2 + CO$ our bodies can handle certain quantities of both but too much is toxic. In the case of $CO_2$ too little of the gas is bad too.

For any other world builders out there, I highly recommend reading what Winchell Chung put together on Atomic Rockets