History of The Search for Life
The question "Are we alone in the Universe?" is a question that has been asked for many thousands of years.
The first scientific attempts to understand our place within the universe started with an Geocentric system (the Ptolemaic System of ancient Greece and Rome) which was increasingly found to be lacking explanation of the movements of the Planets, especially Mars.
In 1543, Nicolaus Copernicus published his new model for the "Universe", which was a Heliocentric (Sun centered) system. By removing the Earth's special place at the centre of the Universe, this new model allowed scientists to think that the Earth might not be the only place to have life.
The first person to publish work discussing the possibility of life on other worlds was a Dominican Friar called Giordano Bruno, who was burned at the stake for Heresy in 1600 due to his claims.
It wasn't until the advent of high quality telescopes in the 1800's that people started to start to explore the possibility of life on other Worlds in greater detail.
In 1877, the Italian Astronomer Giovanni Schiaparelli observed what he thought were canals in the Martian surface.
The video below shows a talk by Sara Seager about her work in the search for life.
Conditions for Life
From the BGE Science course, it has been shown that in order for Life (as we know it) to exist, four fundamental needs must be addressed:-
1. Water (in liquid form).
2. Essential chemicals for living processes.
3. A surface/medium for processes to occur on/in.
4. Energy to fuel these processes.
Liquid Water and The 'Goldilocks' Zone
The main way in which astronomers search for life is to focus on the first fundamental need - water in liquid form.
Liquid water can only exist in locations where the temperature is between the freezing and boiling points of water.
1. Too close to parent star - surface Temperature too high, water in vapour form only or has been lost to space.
2. Too far from parent star - surface Temperature too low, water locked in solid form (ice) .
3. Band 'in between' - Habitable Zone, surface Temperature 'just right' for water in liquid form.
The image above shows a visual representation of our Solar System's Habitable Zone. As can be seen, based upon distance from the Sun, three planets lie within this zone, Venus, Earth and Mars.
Habitable Zone Size
The size of the Habitable Zone varies from star to star. The larger and hotter a star, the further from the star the Habitable Zone is. The below image shows the size of the Habitable Zone around the white-dwarf star Kepler-186 as compared to our Solar System. The planet Kepler-186f is the first Earth-sized planet to be discovered orbiting within its parent star's Habitable Zone:-
Life outside the Habitable Zone
The Habitable Zone is a good place to look for life, but is not the only location life could occur. Even within our own Solar System, there are locations beyond the Habitable Zone which could support life.
Europa - One of Jupiter's moons, possible liquid ocean under a surface layer of ice, kept liquid by Tidal Heating due to Jupiter's immense Gravity.
Titan - One of Saturn's moons, only moon in our Solar System to have a significant atmosphere. liquid hydrocarbon lakes could possibly sustain life in a similar way to the water cycle on Earth.