When I was in secondary school, the first few pages of my geography book proclaimed our place in the universe. Earth was the third planet from the sun. One of nine planets in orbit around it. That was it, nine planets. Sure, there was a lot of talk about planets around other stars and Star Trek’s starship Enterprise would pay a visit to some of them every week. But in the real world we knew of nine.
That was until 1992 when the first planet outside our solar system was discovered. Once the first one was found, more followed and pretty soon the numbers started to become quiet significant. Today, that number stands at over 3750 extra solar planets (exoplanets) with the number growing all the time. Our own solar system wasn’t immune from discovery with large asteroids/ planets discovered outside the orbit of Pluto. This meant that either we had more than nine planets or Pluto was just one of many small rocky worlds orbiting the sun at great distances. Unfortunately, Pluto got demoted and from 2006, our solar system was pronounced to have eight planets with Pluto and other bodies like it becoming “dwarf planets”.
Today the variety of exoplanets discovered is immense and ranges from large hot balls of gas that travel around their parent star at breakneck speed to smaller rocky worlds orbiting at a more leisurely rate and every possible combination in between.
The holy grail when it comes to finding life on other worlds are rocky earthlike planets residing in the habitable or Goldilocks zone, a region where the temperature is not too hot and not too cold, but is just right to support liquid water on its surface. Unfortunately rocky worlds tend to be much harder to find than the larger gas planets and so the number of confirmed terrestrial exoplanets is much smaller than the total number of exoplanets discovered.
In 2007, NASA launched the Kepler space telescope to discover more exoplanets and establish their frequency. Data from Kepler has already confirmed over 1000 exoplanets, with a further 3000 exoplanets to be confirmed. The good news is that using data from Kepler, scientists estimate that there are over 40 billion earth size planets in our Milky Way galaxy alone, 11 billion of which orbit sun like stars. In spite of this knowledge and the proliferation of alien worlds, we are still no closer to answering the question “Are we alone?”