How Does TerraForge's Habitability Score Work?

What the number is, and what it is not

Every planet you build in TerraForge, and every one of the roughly 5,600 real exoplanets in its directory, gets a habitability score from 0 to 100.

The score is a model. It reads the conditions the simulation has worked out for a planet (its temperature, air, water, spin and so on) and asks how well they match what life as we know it needs. It is not a measurement of life, and it is not a probability: 94 does not mean a 94% chance of anything.

The number lands in one of five bands, and the band name is what the builder shows beside it:

• 80 and above: Highly Habitable
• 60 to 79: Habitable
• 40 to 59: Marginal
• 20 to 39: Extremely Harsh
• below 20: Uninhabitable

Most real planets sit at the bottom. Across the catalogue, more than four in five score below 20, about two in five score exactly zero, and only around a hundred reach Habitable or better. That is not the model being stingy. Most planets found so far are giants with no surface, or orbit so close to their stars that they are far too hot, or both. The reasons are covered in why most exoplanets are hot giants. Those worlds fail on the two things that matter most.

The seven factors

The builder’s Habitability panel lists seven “Contributing Factors”, each scored 0 to 100 with a one-line reason. Here is what earns each one its top marks.

Temperatures here are in kelvin (K), the scale scientists use that starts at absolute zero. Water freezes at 273 K and boils at 373 K at Earth’s surface pressure, so the best temperature band is roughly 0°C to 37°C.

Water is scored in two steps. First, abundance: the model takes the smaller of the hydrogen and twice the oxygen (water is H₂O, so you need both) as a share of the whole recipe. Above 15% is “Abundant water” and scores 100; above 10% scores 90; above 5% scores 50; any at all scores 20. Second, phase, meaning whether that water can be liquid. Between freezing and boiling it all counts. Below freezing the credit slides to a floor of 22% at 180 K; above boiling it falls to 15% by 600 K, since steam is water a planet is losing to space. Below about 0.7 Earth masses, weak gravity costs some credit too.

Organic chemistry looks for CHNOPS: carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur, the six elements that make up nearly all of the molecules in living things. A recipe that is at least 5% chlorine loses 25 points, because free chlorine tears apart organic molecules.

Geology matters because an active planet recycles its crust, which on Earth drives the long-term carbon cycle that steadies the climate. Extreme volcanism scores only 40.

Magnetic fieldis a yes or no in this model. A field shields the surface from charged particles streaming off the star. The engine gives a planet a field if it has at least half Earth’s mass and a day shorter than 100 hours (anything over ten Earth masses always has one). Real dynamos are far more complicated, and no rocky exoplanet’s field has been measured, so treat this factor as the roughest of the seven.

How the factors combine

The seven factors are not simply averaged. They split into two groups that do different jobs. Atmosphere, magnetic field, geology, chemistry and rotation are the support factors. They are averaged with equal weight, one fifth each. They describe how good a habitable world is.

Temperature and water are the limiters. They decide whether it is a habitable world at all, so instead of joining the average they multiply it. This is an idea borrowed from agricultural science called the law of the minimum: growth is held back by whatever is scarcest, and a surplus of something else cannot make up for it. A strong magnetic field does not help a planet with no liquid water.

Each limiter is turned into a multiplier between 0 and 1 with a square root, and it stops counting once it reaches 65. In other words, the model asks a planet to clear a bar, not to top a scale. A temperature or water score of 65 or more costs nothing. Below 65 it costs progressively more, and a score of zero on either one zeroes the whole planet.

A worked example

Load the Earth-like preset. Its support factors are atmosphere 100, magnetic field 100, geology 90, chemistry 80 (there is no sulfur in the recipe) and rotation 100. Their average is 94. Temperature scores 100, so it costs nothing. Water scores only 50, “Limited water content”, which is under the bar of 65. Its multiplier is the square root of 50/65, about 0.88. And 94 × 0.88 rounds to 82.

The Water World preset has the same support average but scores 100 on water, so it lands at 94.

Why so many planets tie near the top

Because the limiters stop counting at 65, the top of the scale is set by the support average alone. Geology never scores above 90 on its own, so a planet without a moon tops out at 98. A moon whose tidal flexing keeps the interior active can add 10 points to geology, which is the only route to 100. To see how moons do that, read about moons and tidal heating.

This saturation shows up clearly in the catalogue. At the time of writing, 22 real planets share the top score of 94 and none score higher. One of them is Kepler-442 b, which the engine puts at 264 K. That is below the ideal band, so its temperature factor is 70 rather than 100, but 70 is over the bar, so it ties with planets in the ideal range. With what we know about these worlds, the model has no grounds to rank them against each other. TerraForge infers their chemistry from mass and radius, which puts most of them on recipes with phosphorus but no sulfur. That is why their chemistry factor sits at 80 and they stop at 94.

Caps and penalties that override the arithmetic

After the main calculation, a handful of rules can override it.

• Chemistry caps. With no carbon-hydrogen chemistry at all (a chemistry factor of 20 or less), the total can be no more than 30. With carbon and hydrogen but not the other building blocks (50 or less), no more than 60. Chemistry is a cap rather than a third limiter because, unlike heat and water, organic material can arrive later on comets and meteorites.
• Not a planet. A gas wisp (a cloud that never formed a planet) and a brown dwarf (a failed star) both score 0.
• Hard radiation. Orbiting a black hole, a pulsar or a supernova remnant scales the score down, more steeply the closer you are. A supernova remnant also caps it at 50.
• A radioactive crust. If uranium (plus a tenth of any potassium) makes up more than 5% of the recipe, the score is divided down. At 10% a world keeps about 61% of its score. For comparison, real crustal rock is only a few parts per million uranium.
• Extreme seasons.An axial tilt (how far the spin axis leans) up to 40° costs nothing; Earth’s is 23.4°. From 40° to 55° it costs up to 5% of the total, rising to 25% at 90°, where the poles spend months in constant daylight and then constant dark, as on Uranus. Deep oceans store heat through the year, so a water score of 60 or more halves the penalty.
• Extreme spin. A day under 4 hours cuts the atmosphere and water factors to 30% (to 10% under 2 hours). A day over 200 hours cuts the atmosphere factor to 60% (40% over 500 hours), on top of the low rotation score itself.

The score versus “Life possible”

Separately from the score, the builder decides whether to show a “Life possible” badge and grow vegetation on the surface. That is a different, stricter yes-or-no check, and it does not read the score at all. A surface supports life only if all of these hold:

• the planet can hold on to an atmosphere
• more than 0.5 parts phosphorus, needed for DNA and for ATP, the molecule cells use to carry energy
• more than 2 parts carbon
• more than 2 parts of whichever is scarcer, hydrogen or oxygen
• a temperature above 250 K and below 350 K

When exactly one of these fails, the builder says which, for example by pointing out that there is no phosphorus. A planet that fails most of them gets no message, since there is no single fix.

Because the two are separate, they can disagree, and the disagreement is informative. Proxima Centauri b, the closest known exoplanet, passes every life check, yet scores only 44, Marginal. It orbits so close to its star that the engine assumes it is tidally locked, with one side always facing the star. That makes its day as long as its 11-day year: rotation scores 30, the slow spin loses it its magnetic field, and its atmosphere is partly stripped. The reverse also happens. Move the Earth-like preset out to 1.3 AU and it still scores 50, but at 247 K it falls under the 250 K line and the badge disappears.

Astronomers use a related idea. The Earth Similarity Index, proposed in 2011, rates a planet from 0 to 1 by how close its radius, density, escape velocity and surface temperature are to Earth’s. It answers a narrower question, “how Earth-like is this?”, using measured or estimated physical quantities. TerraForge’s score also leans on things never measured for any exoplanet, such as chemistry, magnetic field and day length, which makes it more descriptive and more dependent on assumptions. Neither number detects life.

See it for yourself

Open the builder, pick Earth-like from Quick Start Presets, and watch the Habitability panel as you change one thing at a time.

• In Element Composition, raise hydrogen from 5 to 10 parts. Water climbs from “Limited” to “Good water content” and the total jumps from 82 to 94.
• Drag Distance from Star out to 1.2 AU. The temperature factor drops to 70, which costs nothing by itself because 70 is still over the bar. The small dip to 76 comes from the water starting to freeze. Go to 1.3 AU and temperature falls to 40, under the bar, and the total drops to 50.
• Set Rotation Speed to 150 hours. The planet loses its magnetic field and the score drops into the Marginal band.
• Tip the Axial Tilt to 90°. No single factor changes, but the total falls to 62.

Then compare real worlds. TOI-700 d scores 82 for the same reason the Earth preset does: a good temperature but limited water. 55 Cancri e, at around 2,000 K, scores 0, because a temperature factor of zero makes everything else irrelevant. Browse the planet type guides, or read how the habitable zone sets the temperature that matters most.