The Conditions for Life in the Universe and The Habitable ZoneWhat are the conditions for life as we know it, and exactly what are the building blocks of life and this goldilocks zone we hear about?
We talk a lot about exoplanets having the building blocks of life and that they orbit in the Goldilocks zone, but what exactly are the building blocks of life and this Goldilocks zone? There is no absolute definition of life on Earth, so it is really difficult to define life on other planets. By "life" we mean simple bacterial life, not ET as bacterial life is expected to be far more abundant and easier to find once we know where to look and what to look for.
Conditions for Life
Based on what we do know about life on Earth, simple life requires a few basics to appear.
1. Enough Time
Basic life requires that enough time has passed for stars, planets and a suitable environment to form, including heat from within the core - but not too much, geological processes and an atmosphere to form. Also, enough time must have passed for Darwinian evolution to create simple living organisms from organic molecules.
2. Located in the Habitable Zone
The planet needs to be within the habitable zone of the solar system, a location where liquid water can exist. The position within the habitable zone and the planet's mass give a good indication of its surface conditions. An Earth-mass-like planet would not have enough mass to be a gas giant, and it would be too hot to be an icy body so chances are it has a rocky surface. The habitable zone (or Goldilocks zone) is explained later in this article.
3. Enough Raw Materials
There must be enough raw materials such as Carbon, Hydrogen, Iron and Oxygen which have formed inside massive stars and been seeded to plants. These can then combine to form organic molecules or a "primordial soup" where the building blocks of life accumulate.
4. A catalyst for Organic Molecules
Finally, a catalyst is required for the organic molecules to combine and form into organisms.
What is the Habitable Zone?
The habitable zone refers to two areas - one within a solar system and the other within a galaxy. The circumstellar habitable zone is a distance range from a star where a planet can theoretically support life and is defined as "the range of distances from a star for which liquid water can exist on a planetary surface." This habitable zone is situated such that the temperature of the surface is not too hot, not too cold but just right (hence the Goldilocks name).
The range of distance will be affected by the energy output of the star, with hot supergiants having a habitable zone much further out than a red giant.
The Habitable Zone can also refer to the galactic habitable zone, which is the area within a galaxy that has the correct balance of heavy elements and radiation. In general, the outermost parts of a galaxy do not have enough heavy elements, whereas the central core contains too much radiation (x-ray, gamma and so on).
How Did Life on Earth Begin?
There are several theories regarding how life started on Earth. Among the most popular was the theory of spontaneous generation where living organisms developed from inanimate non-living material without any outside influences. The theory has been traced back to Aristotle in the 4th century BC. Spontaneous generation was disproved by Luis Pasteur in 1859 when he experimented with a sealed flask that clearly showed microorganisms could not arise spontaneously within it.
It wasn't until 1024 that a new theory emerged. Russian biochemist Alexander Oparin suggested that life began in a "primordial soup" and that the atmosphere of Earth back then was different than it is today. When life was believed to have been created, the atmosphere of Earth mainly consisted of methane, ammonia, carbon dioxide and nitrogen and the lack of oxygen provided suitable conditions for complex organic molecules to be synthesised from simpler inorganic ones. In 1953 this theory was put to the test by Stanley Miller and has become one of the most famous experiments in history. In this experiment, he placed the constituents of the early Earth's atmosphere into a sealed flask and subjected them to electrical sparks to simulate lightning. This resulted in the synthesis of several amino acids - which are used to form proteins and are generally regarded as the building blocks of life.
Another theory is that life began around hydrothermal vents where chemicals dissolved in sea water become deposited in the vents which causes the distinctive chimney structures. These chimney vents contain small holes and voids that could make ideal conditions for the reactions necessary for life to begin.
Yet another theory is that of panspermia which suggests that life arrived on Earth from outer space on asteroids or comets that impacted the Earth. In the 1970s scientists discovered that interstellar dust does contain complex organic compounds and these have been also found inside asteroids and comets where they could be protected from the high temperatures of entry into the atmosphere.
While we cannot say for certain how life on Earth started, these are some of the most popular scientific theories and they help us to identify locations on other planets and moons where we can look to see if life exists also.
