08 09 Alone in the Universe? The Chance of Life Outside our Planet Jago Wall Do you ever find yourself thinking, wondering about things, life, the universe? For hundreds of years, people have speculated whether we, on this tiny insignificant planet, orbiting a relatively small, average star in a fairly average sized galaxy, are the only pieces of life in the entire universe. Or could there be something else out there? The universe is huge; you might think you have an idea about how huge, but you don’t. Even our solar system is a lot bigger than you think. The moon isn’t that far away, right? It’s pretty big in the night sky, you’ve seen lots of images and drawings of the Earth and moon in close proximity. Actually, it’s 384,400 km from Earth. You could fit 30 Earths along that distance, and if you drove there at constant speed of 100 km/h (62mph), it would take over 160 days. But that’s just the Earth and the moon. If we zoom all the way out and view the Milky Way Galaxy as a whole, we can see the location of our solar system, in the centre of the yellow square shown. As you probably know, the Milky Way makes up most of the stars we see in the night sky here on Earth. But did you know that the yellow square also represents something else? Over 99% of the stars we see from Earth are located inside the square. Think of the millions of lights you see when you look up on a clear night, and think about just how much more is out there than that. Our galaxy alone contains over 100 billion stars, and a similar number of planets. It stretches to over 100,000 lights years across, meaning that light given off by a star on one side of the galaxy would take over 100,000 years to reach the other side. Our Sun is currently in the main sequence stage of its life cycle, a stage lasting around 10 billion years before becoming a red giant and engulfing the solar system. It is a relatively small star, with a circumference of around 4.4 million kilometres. The largest known star is UY Scuti, a red hypergiant with a circumference of around 7.5 billion kilometres. To put this into perspective, if our sun was scaled to the size of a regular size 5 football, UY Scuti would be as tall as the Empire State Building. So as you can see, our Sun is nothing special, so what is stopping other stars like the Sun holding planets like ours that can harness life? The main building-block for life is water. As strange as it may sound, there are absolutely no exceptions to this rule; if there is no water there is no life. The second requirement for the development of life is that the water must be liquid, making the temperature range very small, only 0 – 100°C. This means that the planet can’t be too far away from a star, as all the water would be frozen solid, but it also means that it can’t be too close either, as all the water would boil away in the scorching heat. This leaves what is called the ‘Goldilocks Zone’ or ‘habitable zone’ in which the conditions are just right for liquid water to remain on the surface of a planet orbiting a star. Using these criteria, astronomers can narrow down the search for planets that could hold life by looking for those that are within the ‘Goldilocks Zone’ of their stars, and which have similar attributes to Earth, like an atmosphere capable of regulating temperature so there aren’t opposite extremes from day to night. Using this criteria, we have found Earth-like planets with the potential to hold life, although no proof of actual life as of now. The most famous example is Kepler-452b, nicknamed ‘Earth 2.0’ for its Earth-like properties. It orbits the inner edge of the habitable zone of the star Keplar-452, and is located around 1,800 light years away in the constellation of Cygnus. It orbits its star at roughly the same distance as Earth from the Sun; its orbit takes around 384 Earth days (just longer than an Earth year); it has a radius of 1.5 times that of Earth; and a mass of around 5 times that of Earth. However, it is unknown whether the entirety of the planet could be habitable, as it receives slightly more solar energy from its star than the Earth does from the Sun. The other thing we have to consider when looking for life is how long it takes for life to evolve. If Earth’s entire history (4.5 billion years) was 1 day, the time that humanity has been around (300,000 years) would be equivalent to 1 second. That is how long intelligent life has taken to evolve on this planet. This would mean that for intelligent life like us to develop, a planet would have to be in the right location orbiting the right type of star, would have to hold the ability to sustain liquid water and avoid catastrophic events such as an asteroid collision or super eruption, and on top of that would have had to develop life over 4 billion years ago (note that the entire universe is only 13.4 billion years old). The chance of intelligent life therefore on an individual planet is so impossibly tiny.