The Drake Equation: Conditions Why should other lifeforms require similar conditions to those here on Earth? Drake's Equation starts with a premise that we expect that other lifeforms will require similar conditions to exist as we have here on Earth. This needs to be justified. This notion is based on two observations about life here. Firstly, light elements such as Hydrogen, Carbon, and Oxygen which make up a significant fraction of the mass of all lifeforms here on Earth are amongst the most common elements found in the Universe. They are made by nuclear fusion in stars and ejected into space in the cataclysmic explosions at the end of their lives. Secondly, the chemistry of Carbon, the basis of our life, is by far the most diverse of any chemical element, so allowing a vast range of organic compounds to exist. The same chemistry These two facts would imply that most, if not all, of other lifeforms would be based on a similar chemistry. This would then require the presence of liquid water as a medium in which life could evolve so we would need a location where the temperature lies between zero and 100 or so °C. (Note that at high pressure, such as at the bottom of oceans, the temperature of liquid water can exceed 100°C!). A further likely requirement is that the temperature should remain within "reasonable" bounds for long periods of time. This implies that we need a location on or below the surface of a planet in a stable, roughly circular, orbit around its star. Extending the boundaries We should not, however, forget that liquid water, warmed by tidal heating, could also exist under a surface crust of ice of a moon orbiting a giant planet, as this may possibly be the case on the Jovian moon Europa. In this connection, it is also of interest that the known boundary conditions for life on the Earth are being extended in different directions, especially through the discovery of extremophiles, i.e. life forms that have been able to adapt themselves to extreme conditions.