Over the last decade, there has been an ongoing debate on whether Earth is the only planet that is habitable in our universe. The standard definition for a habitable planet is one that can sustain life for a significant period of time. This discussion has arisen due to the rapid population growth over the last few years. The idea of determining whether a planet could sustain life sounds simple. In reality, there are several factors researchers have been looking into since the 20th century that plays a part in determining the criteria that makes life sustainable.
Researchers have spent the past few decades trying to find traces of the simplest forms of life in other planets which are also known as “Biosignatures”. Although living things are complex, they will always follow the same physical and chemical principles. This same concept applies to the criteria that need to be evaluated to know if it's safe to build a sustainable civilization elsewhere. In 1997, NASA launched a space observatory called “The Keppler”, whose purpose is to search the galaxy for Earth-like planets that fit the standards. NASA was able to find a planet that fit most of these requirements, Kepler-452b. Life has not been found on this rocky planet yet, however, its habitable zone is very similar to Earth’s. In order to determine whether a planet can sustain life, scientists study if the planet contains the following factors:
Water
This compound is essential to life as it works as a solvent. This is important because it is capable of dissolving substances and allows important chemical reactions in animals, plants, and microbial cells. It is also a key to sustaining life as it provides elements for a good habitat such as heat conduction, surface tension, and high boiling and melting points.
Carbon
This element is key as it is a basic building block that allows simple and complex organisms to form things such as proteins, carbohydrates, and fats. Also known for bonding nicely with 3 other key elements which are, oxygen, hydrogen, and nitrogen. This is important as it allows for the forming of fundamental long-chain molecules.
Nitrogen
This element allows for the building of amino acids that are needed for the creation of complex proteins that are used to create life. It is also essential in the making of DNA and RNA which carry the genetic code.
Phosphorus
Although it may come as a surprise, phosphorus is one of the key elements in the building of energy. It is an important component of ATP which transports chemical energy around cells and powers all the cellular processes that require it. Phosphorus is also known for being vital in cell membranes.
Sulfur
This element allows for the creation of an energy source in the case that there is no oxygen or light available. Bacterias called extremophiles have been discovered to gain energy from sulfur and nitrogen. This element can be described as a sign of the simplest forms of life.
Time
Scientists do not disregard that there might be life on other planets; however, it is likely it still has a lot of evolutionary catching up to do. The key building block of life is a cell that makes up all of the complex organisms that follow it. For tissues and organs to form cells, they need to multiply, specialize in a function, and co-operate. All of these processes take time so it's likely we are still early in the development.
Temperature and Location
Temperature is key because it influences liquid water and it can be directly estimated from climate models. A planet cannot be too cold since it will cause water to freeze nor too hot since the water will evaporate. Planets closer to the Sun than Earth are too hot, and those farther away are too cold. Earth falls into what is called the “Goldilocks zone” which means it's the perfect distance from the Sun. Scientists are also on the lookout for planets with a similar distance to their host stars. The size of the star is important as well. Large stars have a shorter life span which gives little time for any kind of life to develop.
There is still a lot of our galaxy and universe that we have yet to discover, but every day scientists have more findings that hopefully will lead us to life on other planets. Our generation may not be able to be here to see the final discoveries, but there is beauty in seeing the process unfold.
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