ONE IN 100 BILLION?
by Sue Nelson
The Earth and its Solar System are in the Milky Way galaxy. Astronomers think in this one galaxy alone there could be hundreds of billions of planets.
Planets that orbit a star outside our Solar System are called exoplanets or extra-solar planets. The first exoplanet was discovered in 1995. Since then about 700 others have been confirmed. Most of these are gas giants, so they are similar to Jupiter.
But the huge number of potential exoplanets in our galaxy suggests our own life-filled planet may not be so rare after all. Recently, the Kepler spacecraft, or space observatory, detected the first Earth-sized rocky planets orbiting a star. NASA (National Aeronautics and Space Administration) launched Kepler in 2009. It is named after the famous German mathematician and astronomer Johannes Kepler (1571 – 1630) and has been specially designed to search for exoplanets.
The rocky worlds Kepler has discovered so far are too close to their star or sun. This means they are too hot for water to exist or for there to be any life on them.
For life to exist, astronomers say a rocky planet needs to be in the habitable or ‘Goldilocks’ zone: not too far from its sun and not too close. In this zone conditions for life could be just right – not too hot and not too cold – or the same as Baby Bear’s porridge in the well-known fairy tale.
Exoplanet discoveries are also changing the way astronomers think solar systems were formed. Last year science fiction became science fact when Kepler found a planet orbiting two stars – so if you were on the planet you would see two suns in the sky. This is just like on Tatooine, a planet in the Star Wars films.
There are different methods of detecting exoplanets. As stars outshine planets, some methods are based on examining the light from a star to find out if it has any orbiting planets.
When a planet speeds around a star in orbit, for example, the star wobbles slightly. This movement can be observed as a change in colour, a phenomenon known as the Doppler shift.
In some cases if a planet transits or passes in front of a star, the planet will block a small amount of the light. So if it’s viewed at the right angle, the light from the star will appear to dim. This indicates it has an orbiting planet.
Finding exoplanets in this way is called the transit method. The Kepler telescope uses the transit method to hunt for planets. It can monitor 150,000 stars in its field of view. These stars are between 3,000 and 25,000 light years away. A light year is the distance light travels in one year, roughly 300,000 kilometres (186,400 miles) per second. Another way of finding exoplanets is called gravitational microlensing. This makes use of the fact that gravity can bend the direction of light. A star will appear brighter if another star passes in front of it. But if the passing star has a planet orbiting around it, the light from the background starmay ‘blip’.
By counting the number of stars and planets, astronomers have recently produced a statistical sample to represent the Milky Way. It suggests many stars in the Milky Way have planets in the Goldilocks zone.
If the Earth is the only planet to contain life in our galaxy, it is one in around 100 billion planets. Yet there are an estimated 100 billion galaxies in the Universe. So even if each has only one life-supporting planet, there are at least 100 billion similar worlds to our own throughout the Universe. We may not be alone after all.