Is there life elsewhere in the Milky Way
early as 4.4 billion years ago, which means that the surface temperature must have remained within the freezing and boiling points of water, 0°C and 100°C.
Earth has kept this stable climate through three main processes:
• Firstly, through greenhouse gases in the atmosphere, these are gases with three or more atoms in, such as carbon dioxide, water vapour, methane and o zone. These increase the Earth’s ability to trap heat, ensuring the temperature is not too cold. • Secondly, through the ‘ tectonic thermostat f eedback’. This feedback mechanism enhances weathering when carbon dioxide in the atmosphere or temperature is too high, releasing calcium ions into the oceans, leading to removal of carbon dioxide as calcium carbonate, lowering the greenhouse gas concentration. On the other hand, when carbon dioxide in the atmosphere or temperature is too low, less weathering occurs, allowing for a greater build-up of carbon dioxide from volcanoes, increasing the greenhouse gas concentration. • Finally, the planet’s surface reflectivity, represented on a scale called Albedo, affects howmuch radiation is absorbed from the sun. Earth’s A lbedo is 0.3, compared to Venus’s Albedo which is 0.7. H igh Albedo lowers the temperature of the planet’s surface. 6 The volumes of the oceans have remained constant over Earth’s history, through a balance of removal and addition of water. Water is removed from high in the atmosphere due to ultra-violet radiation which causes the H 2 O molecule to split up into hydrogen and oxygen molecules, the hydrogen can escape to space whilst the oxygen is too heavy and slow so reacts with iron, sulphur, or carbon. However, to counter this, water is added to the surface from Earth’s interior, via the movement of the plates, and the cracks in the crust, hence the importance of plate tectonics on a planet’s habitability. Furthermore, the Earth has a ‘water trap’, where , as altitude increases, temperature decreases rapidly, down to – 60°C where water vapour cannot exist and hence forms clouds preventing the water from being split up by the UV radiation just above the atmosphere. 7 From investigating all these constraints and the data from the Kepler Space Mission, which is attempting to observe and record Earth-like planets in our galaxy, it is proposed that there are approximately 30 billion Earth-like planets orbiting stars, out of the 100 billion stars in the galaxy hence our value for n e is 0.3. 8 From here on, all the variables we shall discuss are very uncertain and subject to disagreement. Arguably, the hardest variable to calculate is f l as we only have one sample of data to investigate, Earth. Life on Earth seems to have arisen as soon as the conditions were favourable, hence implying the number could be close to 1, however this could be an anomaly, we will never know, unless life is found to have arisen on Mars, Europa or anywhere else.
Life seems to have originated from one Universal Common Ancestor (UCA), from which all life since has evolved from. 9 If life forming was common, there should have been multiple lineages from which all life can be linked back to, but there is not, implying that life forming is a rare and uncommon process
6 Langmuir 1985. 7 Ibid. 8 Howell 2018b. 9 Yonezawa 2010.
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