Life on Exoplanets
The Ingredients of Life
Astrobiologists believe that there are three ingredients which are vital for life. They are water, energy and organic molecules such as carbon (C). Energy and organic molecules are abundant in the Universe. Yet water in its liquid form is rare.
Water on Earth is in its liquid form at sea-level, where temperatures are 0 °C - 100 °C. But, the Earth has geographical, seasonal, and daily variations. These variations can drive temperatures down to well below freezing. Then, water is found as ice. Temperatures in Antarctica can reach as low as -89 °C, yet life can still survive there.
Most terrestrial (land-based) organisms can't survive temperatures above 45 °C for long. Yet some marine life can. Thermophiles have been found thriving around hydrothermal vents (black smokers). These are found in the deepest parts of the ocean. Here, pressures are extreme and temperatures exceed 110 °C.
It is important to understand the extreme conditions in which life can survive on Earth. But, we should not let this limit where we expect to find life in the rest of the universe. Life may have evolved in unique ways. Like the ability to 'breathe' other atmospheres.
The Habitable Zone
The 'Habitable Zone' (HZ) is a term to describe an area around a star. It is the area where a planet could have water in liquid form. The term 'Goldilocks Zone' is perhaps better in describing a region that is neither too hot, nor too cold.
The size and location of the HZ depends on the type of the star in question. Typically, hot, bright stars have wide HZs. Let's use the star Sirius as an example. Sirius is 26 times brighter than the Sun. This means if we want to enjoy temperatures the same as those on Earth, we would need to be further away. We would need to orbit Sirius at about the distance of Jupiter from the Sun. In contrast, lets imagine we replaced the Sun with 40 Eridani (see image). 40 Eridani has about half the Sun's brightness. The Earth would need to be in a Venus orbit to receive its present level of warmth.
So, could life exist elsewhere in our galaxy? There are 200 million stars in our galaxy. Imagine one in a thousand had a small rocky planet. They have a stable orbit within the star's HZ. That would mean 200,000 planets where life might exist.
Life in the Solar System
We have mentioned life around other stars. But, there is a chance that life may exist out in our own Solar System. The discovery of primitive life, or fossils, on Mars would extend the HZ of the Sun. In fact, evidence shows that Mars was once warmer and wetter. These conditions may have allowed primitive organisms to evolve. Ongoing and future missions to Mars will search for these signs of life.
Much further from the Sun are the gas giants. Planets like Jupiter and their large moons. At first sight these moons may seem inhospitable to life. However, the moons Europa and Callisto are subject to tidal heating. This may lead to the existence of large amounts of sub-surface liquid water. This would increase the likelihood of aquatic life.
Click here to find out how astronomers are looking for extra-terrestrial life.