Seeing Stars: Planets of other stars
Our ancient ancestors were aware of ‘wandering stars’ in the night sky but couldn’t explain what it was that they were seeing. They were actually looking at the movements of planets in our solar system such as Venus, Mars and Jupiter.
It wasn’t until the 17th century and the invention of the telescope that these ‘wandering stars’ were seen as planets.
Since then, we have known of eight planets in our solar system, although evidence is stacking up for a ninth one existing very far out, 20 times the distance of Neptune from the Sun. I haven’t included Pluto here as it was demoted from a planet to a dwarf planet.
For hundreds of years, people have wondered if there were planets around other stars. In 1992, this became real when the first planets around other stars were discovered thanks to huge leaps in technology at that time.
More discoveries have followed and the current total is 5,742 planets orbiting other stars. As technology further improves, this total is likely to soar.
Planets outside our solar system are known as exoplanets or extra solar planets. The ones that have been discovered so far are all nearby, in our Milky
Way galaxy, but it is very likely that every star you look at in the night sky will have planets orbiting them just as we have.
Some 200 billion stars can be found in our galaxy and a fifth of these will possibly have planets around the same size of Earth in what is known as ‘habitable zones’ where the temperature is not too hot or cold and so allows liquid water to exist on the surface.
So it is likely that there are upwards of 10 billion habitable Earth-sized planets in the Milky Way!
Exoplanets are found using three different methods. Space-based telescopes use the ‘transit’ method where they look for changes in the light coming from distant stars caused by planets passing across the face of those stars.
The second method is the ‘wobble’ method where a large planet’s mass causes its star to wobble.
The third method is direct imaging where a telescope takes a photograph of a distant planet.
The best example so far of a solar system other than our own is the Trappist-1 solar system where seven roughly Earth-sized planets orbit a small red star in the constellation of Aquarius. It was discovered in 2016 and is named after the Belgian Trappist robotic telescope located in Chile.
These planets have been studied by telescopes and we now know their masses and diameters. We also know how much energy from their star hits the surface of each planet and this allows scientists to estimate their temperatures.
Scientists can even make a good guess at the colour of the sky, although a lot remains unknown about these seven worlds, including whether they have atmospheres, oceans, or ice sheets.
So next time you look at a star in the night sky, have a think about the planets that might accompany it. You might even be looking towards another Earth!