The answer depends on what we are looking for when we look for habitable planets. We could be looking for planets where similar life has developed, planets where any form of life might have developed, or planets where humans could live, perhaps with some terraforming. The classic assumption guiding searches for extraterrestrial life was that a habitable planet must be like Earth, although the possibility of life with ammonia instead of water or silicon instead of carbon has been recognized. Science fiction writers have been free to write about life outside the limits generally accepted by scientists. In the book 2010: Odyssey Two, [1983] Arthur C. Clarke wrote about Europa having developing indigenous life, which was protected by the mysterious monoliths in the story. In our solar system today, the short list where life might have developed includes the plants Earth and Mars, and the moons Europa and Titan.
But the search for life in other solar systems is still narrowed by ruling planets unlike Earth out. Even before planets were found around other stars, it was thought they should be at a distance that would allow liquid water to exist, which was called the “Goldilocks” or habitable zone. The best stars to check were F, G, and K type stars. Bigger stars would have wider habitable zones, but the largest stars had lives that were too short for life to evolve. Smaller stars would have smaller habitable zones, so close to the star the planets were likely to be tidally locked with one side always facing the sun. When large planets were discovered near small stars, it was thought that habitable moons could orbit them, escaping the problem of being tidally locked to the sun.
Wikipedia lists planets judged by seven criteria. One is based on temperatures and humidity which are good for vegetation. Two are based on temperature of the planet. Three bracket planets based on physical properties, making it easy to recognize the good values. The last is also the first of two methods suggested in a paper published in Astrobiology in Dec 2011 by Schulze-Makuch and his co-authors in order to assess exoplanet habitability. It is the most Earth-centric, planets can’t get better than a 1, which is a perfect match for Earth.
Standard Primary Habitability (SPH) is determined by an equation with parameters of temperature and humidity. The values range from 0 to 1, Earth’s habitability may have been rated higher overall in the past. When the equation is applied to areas of Earth, the tropical rain forests rate the highest, other areas quite low. It is hard to see how this can be uniformly applied to exoplanets, where the humidity of the whole planet or different areas is unknown.
Planetary Class (pClass) Classifies planets into 21 groups based on temperature (hot, warm, or cold, where warm is in the habitable zone) and mass (asteroidan, mercurian, subterran, terran, superterran, neptunian, and jovian).
Habitable Class (hClass) — Classifies habitable planets based on the thermal classification for microbial life in the microbiology field. Mesophiles are those microorganisms that growth best at moderate temperatures between 10°C to 45°C. Psychrophiles between -15°C to 10°C, thermophiles between 45°C to 80°C, and hyperthermophiles above 80°C. The names can be further abbreviated as M-planets (mesoplanets), P-planets (psychroplanets), and T-planets (thermoplanets), which ends up resembling the Star Trek classifications. Mesoplanets are Class M, medium-temperature planets, ideal for complex life. whereas Class P or T would only support extremophilic life.
The next three parameters give planets a number based on physical properties where the numbers 1 and -1 represent bracketing limits. The number 0 represents the center between the two parameters, planets that are outside the limits get numbers greater than 1 or less than -1, which can still show if they are close or reveal other interesting properties.
Habitable Zone Distance (HZD) is based on the distance from a star and the temperature and luminosity of the star. That determines the distancefrom a star were a planet could have liquid water on their surface. Values run from -1, at the inner limit of the habitable zone to 1 at the outer edge. In our solar system, Venus is at -1, Earth is about -0.5, and Mars is about 0.5.
Habitable Zone Composition (HZC) is another single parameter equation. It calculates a value based on the density of the planet, between a planet made of iron (-1) and one made of water (1). Most rocky planets are near the middle between iron and water. Gas giants would have values greater than 1, since they are less dense than water. A value below -1 would be very unusual.
Habitable Zone Atmosphere (HZA) Calculates the atmosphere a planet might have based on the escape velocity of atomic nitrogen (-1) and atomic hydrogen (+1). When planets are sorted by this parameter after being sorted by being within the habitable zone, most have a temperature and mass that puts them in the range to have an Earthlike atmosphere. They are massive enough to hold heavier gasses, but not hydrogen or helium. And some are less dense than water on the HZC scale, so they must be gas planets made mostly of nitrogen, water vapor and carbon dioxide.
The last, Earth Similarity Index (ESI) is used to rank planets by the Habitable Exoplanets Catalog maintained by the University of Puerto Rico uses it, along with Wikipedia. It, along with the Planetary Habitability Index (PHI), rate planets on a scale of 0 to 1. Instead of bracketing Earth between values as the HZ series of parameters does, it starts with Earth’s values for Radius, Density, Escape Velocity, and Surface Temperature as normal. A planet with the same radius, density, (and escape velocity), and surface temperature would get a value of 1. The equation calculates how much other planets differ from Earth, each difference pushing the value, which is a product of all the values, closer to 0. The Planetary Habitability Index (PHI) calculates a value using presence of a stable substrate, an energy source, appropriate chemicals, and a liquid solvent. The usual source Earth-centric source for the elements (Sunlight for energy, water for solvent) can be replaced by something different (tidal heating of a moon, other liquid solvents) If any of the 4 elements is missing, the planet or moon will score 0.
