Expanding the Goldilocks Zone: Redefining Environments Habitable for Life in the Universe
Astrobiology, the study of life in the universe, is a quickly expanding field. Using high powered telescopes, scientists can infer information about planetary systems light years away from Earth. The "Goldilocks Zone" is the distance at which a planet can orbit its star while maintaining liquid water on the planetary surface. Some planets are too close to their sun to retain liquid water- it just boils away. Other planets are too cold and any surface water that may be present is in the form of solid ice. Water is vital to life as we know it, but by limiting the definition of the Goldilocks zone to require liquid surface water may be a mistake. Life is very complex, and it is possible that there are organisms in the universe that do not exhibit the characteristics of what humans currently consider to be life, but are still living beings. Liquid water may not be vital to all life forms.
Whether or not a planet could potentially support life as we know it is affected by many factors including (but not limited to) the planet's mass, axial tilt, orbital speed, and rotational speed. The mass of a planet determines its capacity to maintain an atmosphere- weak gravity allows an atmosphere to simply drift away into space. It is thought that an atmosphere is vital to life. Planets without atmospheres are vulnerable to extreme temperature changes and solar radiation, conditions that are not compatible with carbon-based life. There are, however, microbes on Earth that can reproduce below 0 degrees centigrade and some that can reproduce in environments with temperatures exceeding 100 degrees centigrade. There are no known organisms that can reproduce in both extremely hot and cold conditions, but that does not necessarily mean it's not possible. Some tubeworms at deep sea hydrothermal vents have bodies that span water of extremely different temperatures, pressures, and chemical consistencies. For example, one part of their body may be in water up to 50 degrees cooler than another part. The ability of these organisms to survive such extreme conditions testifies to the persistence of life and evolution.
The Kepler telescope has led to the discovery of 54 planets in what is currently considered the Goldilocks zone. Planets are currently classified as habitable or hostile even though there are some planets that exhibit fractional habitability, meaning a certain percentage of a planet has the conditions necessary to maintain liquid surface water. A planet's distance from it's sun, however, should not be the only determinant as to whether or not it is capable of supporting life. Some planets have internal heat sources- not all surface heat must come from a star. Certain planets exhibit tidal heating. As the planets orbit their star they are stretched and squeezed by variations in gravitational pull from that star. This stretching and squeezing creates enough internal friction to generate heat. It could also stimulate volcanic activity, which could result in the formation of an atmosphere.
Studying planets in the Goldilocks zone is a good start to the search for proof of life in the universe, but we shouldn't limit our definition of habitability to include liquid water, as we may be overlooking alternative lifeforms. Also, the distance of a planet from its star is not the only thing that determines the surface temperatures and conditions of said planet. As the field of astrobiology expands so must our definition of what environments are capable of sustaining life.
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