The Kepler space observatory has now found 1,284 confirmed planets orbiting other stars.
Of these, more than 100 of them are roughly Earth-sized, that is, one in thirteen. Of course being the right size is only part of the story; for life as we know it, the surface temperature has to be right for liquid water to be available. Currently, nine of the hundred Earth-sized planets are at the right distances from their stars for this. That is, 0.7 per cent of the 1,284 planets are ìEarth-likeî.
There are between 100 and 300 billion stars in our galaxy ñ the Milky Way. The uncertainty is because much of our view of the galaxy is obscured by clouds of dark clouds of dust and gas. Letís take the middle of the range, and assume there are 200 billion stars in our galaxy. For life to develop and evolve on a planet, its star has to be stable enough not to incinerate or freeze any life forms, and to last long enough for them to evolve into more advanced forms. On our Earth life appeared some 3.5 billion years ago, only a billion years or so after the Solar System formed. If we allow ourselves a few billion years of future, we would like a star to be a stable source of energy for about 10 billion years.
How a star turns out depends upon the mass of hydrogen it grabs when it forms. Really massive stars live life fast and loose, shine extremely brightly and blow themselves up after a few million years. Less massive stars, like the Sun shine fairly steadily for 10 billion years or so. Even less massive stars, such as red dwarfs, are very niggardly with their energy production, and last far longer. Because of their relatively short lives, massive stars form only a tiny percentage of the stars in the galaxy. Almost all of the 200 billion stars are low-mass stars, suitable for a life-bearing planet or two. Since stars typically have one or more planets, there must be at least 200 billion planets scattered around the Milky Way. If 0.7 per cent of them are ìEarth-likeî, there are at least 1.4 billion Earth-like planets.
With such numbers it is very likely that at least one of these worlds lies closer to us than 20 light years. In cosmic terms this is nothing. However a light year is the distance that light travels in a year, almost 10,000,000,000,000 kilometres. If we sent our fastest spacecraft, travelling at, say 250 kilometres a second, the journey would take nearly 26,000 years. At the moment we are not even in sight of the technologies we will need for practical exploration outside our own Solar System.
The interstellar gas and dust clouds from which planets form are loaded with the chemical basis of carbon-based life. All living creatures so far found on Earth are carbon-based. Since these chemical ingredients are so common in the universe, we can expect these to be the basis of life on many other planets. However, there is no reason for carbon-based aliens to look like any Earthly life.
Oxygen is a key element in life on Earth. It is so highly reactive that it does not occur naturally. On Earth it is produced by plants. Moreover it is so reactive that unless it is continually topped up, it will vanish quickly by reacting with organic materials or with the rocks. So oxygen in a planetís atmosphere is a good indicator that life is present. When a planet passes between us and its star, some of the starlight passes through that planetís atmosphere, and picks up the signatures of the gases the atmosphere contains. We can detect those signatures, so we are searching for the signature of oxygen. We have not found any yet, but we have only just started. Maybe when we find a planet or two with an oxygen atmosphere, we will become more determined to develop the technologies we need to get out there and look.
Jupiter is high in the south after sunset. Mars and Saturn rise around 10pm. Mars is the brighter of the two and will be at its closest to Earth on May 30. The Moon will reach Last Quarter on May 29.
Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton.