At last we have found a truly Earth-like planet orbiting another star. It lies some 500 light years away so having a really close look will not happen any time soon. It is a bit bigger than the Earth, but not enough to make it very different from our world. It orbits its star at the right distance for rivers, lakes and oceans of liquid water to exist on its surface, and for it to have the sorts of weather with which we are familiar. One other plus is that its star is a small one, a little smaller than the Sun, which means it will shine steadily for long enough for life to develop and evolve. Surprisingly, looking at the spectrum of the light from that planet’s star as the planet moves in front of it may be able to tell us whether there could be life on that world. This discovery is just the latest in a surprising succession of developments that have taken place over the last couple of decades.
It was not very long ago that astronomers believed we would never be able to find out whether a star other than the Sun has planets without going there. The reason was glare. Pointing a telescope at a star and looking for planets would be like staring into a searchlight looking for a firefly. However we really did need to know whether other stars had planets. Firstly we want to know whether forming planets usually happens when stars are born, as indicated by our theories. Secondly we want to know if we are alone in the universe. Most scientists believe there must be other worlds out there with living things wandering around on them, but believing is not the same as knowing. So as technologies improved, scientists looked for other methods for finding alien worlds.
The first method used gravity. When two objects orbit each other, they both orbit their common centre of gravity. If you had an apple and a tomato stuck on each end of a piece of dowel about 30cm long, there will be a balance point on the dowel from which you could hang the assembly on a single string, with it remaining horizontal. You can then push either the apple or the tomato to make them orbit each other. You will notice that no matter how much larger the apple is than the tomato, the apple circles the centre of gravity, so that it moves towards you and away as you watch it. It does not sit still in the centre.
The same happens with stars. A star may be hugely larger than any planet orbiting it, but it still moves towards us and away from us by a measurable amount, revealing the presence of planets. It sounds easy, but making actual measurements is difficult, because we’re sitting on a planet that is rotating once a day and orbiting around the Sun once a year. Correcting for those to find a tiny movement of a distant star is hard.
This difficulty probably led to the development of the next method, which was to use sensitive photometers to measure the tiny diminutions in light from a star when a planet moves in front of it. Using this method the Kepler orbiting observatory has found thousands of candidates. This method of planet searching works well from the ground too. Even backyard astronomers have got into the act. A few thousand dollars and a dark site and you can become a planet finder too. How something deemed impossible not long ago is now something we can do successfully in the back yard is astonishing, but maybe not as astonishing as what happened next.
Using latest innovations in optical science and information processing we can now see alien planets directly! We can do something that was once believed impossible. We can blank out the glare of a star and actually see its planets as little, faint dots, and watch their positions change as they orbit their alien “sun”.
At nightfall, Mars lies in the southeast and Jupiter high in the west. Saturn rises just after 9 p.m. and Venus about 5 a.m,. in the predawn twilight. The Moon will reach first quarter on the 6th.
Ken Tapping is an astronomer with the National Research Council’s Dominion Radio Astrophysical Observatory, Penticton.