The first factor of the Pfeifer Equation describes whether or not a planet is rocky. The two categories of exoplanets that are considered to be rocky are Terrestrial planets and Super Earths. Most importantly, rocky planets provide a surface. Surfaces provide areas such as oceans where the necessary chemicals can come together and use water as a solvent to undergo reactions that lead to life. Additionally, life would benefit from a surface that allows for easy access to energy of some kind. Exoplanets that fall into the Neptune-Like and Gas Giant categories do not have a surface. They only contain swirling gasses. This prevents the use of water as a solvent, nullifying chances of life. Therefore, the first factor to look at in identifying a habitable planet is whether the planet is Terrestrial or a Super Earth.
The Habitable Zone is defined as the range of distances from a star where liquid water is possible on a planet's surface. Depending on the individual star and its characteristics, the habitable zone varies. It is often referred to as the 'Goldilocks' zone. This is because if a planet is too close to its star, it would be too hot and water would be in a gaseous state. On the other hand, if a planet is too far from its star, it would be too cold and any surface water would freeze. Until proven otherwise, water is considered to be necessary for life as it serves as a solvent for life-forming chemicals. Although being outside the star's habitable zone could be critical for an otherwise habitable planet, it is far from the only criteria required for life. Oftentimes, people will point to an exoplanet in the habitable zone and laud it as a great candidate for life. Not so fast. One of the aims of the Pfeifer Equation is to challenge the relative importance of the habitable zone compared to other (often unmentioned) factors. There definitely should be other factors involved in the discussion when analyzing exoplanets. Potential reasons why they are not brought up might be that other factors are more difficult to measure and we do not yet have the data. Nevertheless, they should not be ignored. Also, the proportion of planets in the habitable zone might surprise you. Check out the estimates page to learn more.
In addition to aligning compasses and producing stunning auroras, the Earth's magnetic field keeps us alive every day. Our Sun is constantly releasing charged particles which travel at high speeds. This phenomenon is known as solar winds. When solar winds reach the Earth, they are dispersed by the magnetic field. Not only does this prevent the particles from bombarding the surface, but the Ozone layer is also protected. The Ozone layer is vital since it shields the Earth from ultraviolet radiation. Life as we know it has no chance of surviving the brunt of solar winds and UV radiation. The Earth is able to generate its magnetic field thanks to its liquid outer core. The outer core rotates with the Earth and undergoes convection. The liquid's ability to conduct electricity then results in our precious magnetic field. Unfortunately, not all planets share these qualities in their cores. Mars and Venus, for example, have deficiencies in their cores which prevent them from generating magnetic fields. Without this protection, the prospects of life are severely hindered.
The atmosphere is yet another aspect of the Earth that allows life to thrive. Firstly, it can have a significant effect on the temperature of a planet. The greenhouse effect occurs when certain gas molecules in the atmosphere re-radiate infrared radiation that would otherwise leave the planetary system. This process warms up the planet, showing that the habitable zone is not the only factor affecting temperature. A planet with a weak greenhouse effect, such as Mars, is too cold. On the other hand, too many greenhouse gasses cause a runaway greenhouse effect. This occurs when greenhouse gasses act as a positive feedback loop, assisting in the production of more greenhouse gasses to the point where a planet's surface temperature becomes way too hot. In such a case, Venus for example, all surface water is evaporated. Fortunately, on Earth, the greenhouse effect traps the right amount of heat to maintain liquid water on the surface and for us to live comfortably. Our atmosphere also provides an air composition which allows cells of all organisms to respire. As mentioned in the discussion of the magnetic field factor, the Ozone layer is a part of the atmosphere which shields life from devastating ultraviolet radiation. Lastly, Earth's weather systems are quite calm compared to other planets. Some planets routinely experience very high wind speeds, which certainly would not be comfortable for any form of life. It may seem easy to gloss over, but Earth's atmosphere has done life many favors, contrary to other atmospheres that leave most other planets in a less fortunate state.
The final factor determining the habitability of a planet is its chemical composition. When it comes to life, an organism can be divided all the way down to the elements that make up its biomolecules. With very few exceptions, all living things on Earth universally contain 6 elements. These elements are carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur (often abbreviated as CHNOPS). Many organisms are not limited to these 6 elements and can have ten-fold the amount of elements navigating through its cells. However, the vast majority of mass in the biosphere (>95%) is attributed to the CHNOPS elements. We have a pretty good idea that the CHNOPS elements are necessary for life. Finding planets that have all of them is another question. Observing the atmospheres of exoplanets and scanning for the presence of essential elements has not really been done yet due to technological constraints. The next best way to get an idea of a planet's composition is to look at its star. Planets form in protoplanetary disks and make use of the same dust that is used to form the star. Therefore, stars and their respective planets generally harbor similar relative abundances of chemicals in their compositions. Astronomers are able to study the chemicals present in stars using a technique called spectroscopy. What this has revealed is that some elements are more scarce than others and our essential CHNOPS elements do not appear in every planetary system.
The formation of life is definitely the most contested factor. This is in large part due to what is unknown to us. No human was around billions of years ago to tell the tale of life's origin. How did life form? Although there are plenty of ideas, nobody truly knows. Our best guess is that all the necessary materials could gather in the same place and react with each other in water, the universal solvent. Somehow, in collaboration with a source of energy, the first cell containing DNA and RNA was born. Seeing how far removed we are from this event, we cannot say with any certainty that we understand how non-living matter becomes living matter. It is not as if we can develop a formula with life as the end result of a sequence of steps. At this moment in time it is an existential question... one that has no right answer. However, despite this lack of knowledge, we can still attempt to reason how likely a life-creating event is. In doing so, we must acknowledge that any estimate is subjective. If we presume that forming life from non-living matter is not guaranteed and instead a matter of chance, the inclusion of this factor implies that not all habitable planets will go on to harbor life, but rather only a fraction of them.
These two factors combine to estimate the total number of planets in the Milky Way galaxy. If we want to estimate how many planets have life, having a value for the size of the population is a good place to start. From there, the other factors narrow down our options. When we multiply the number of stars in the galaxy by the number of planets per star, we are left with a quantity of planets, which is what we want.
Made by Nicholas Pfeifer