Exoplanets – Planets that orbit stars outside our solar system are possibly the most exciting discovery in Astronomy since the advent of the telescope back in the time of Galileo. The first exoplanet was only confirmed in 1995 and today we know of at least 3200 of them with more being found on a daily basis. A statistical estimate suggests that on average there is at least one planet for every star, which means around a trillion planets in our galaxy alone.
So could any of these support carbon based lifeforms such as ourselves?
All lifeforms on Earth require water in liquid form so any exoplanet will need this for life (as we know it ) to be sustained. The nearest potential exoplanet that could be similar to the Earth, that could have surface water, according to the University of Puerto Rico Potentially Habitable Exoplanets is 13 Light years away (about 130 trillion km) so its a long way away!
How do we know about something so far away that we cant even see?
Exoplanets can be spotted using the transit method amongst other ways. When a planet moves across a star between the star and the observer, it cuts out some of the light. Our own solar eclipses are an extreme case of this, the moon temporarily blocks the sunlight from reaching parts of the Earth – The next total solar eclipse is on 21st August 2017 and will pass across the USA. If we measure the intensity of the light we will get a small dip as the exoplanet transits the star. For more detail on this Smithsonian has an in depth report . A nice video that shows the transit method in action as Venus passed between the Earth and the Sun is below
The Institute of Physics has a fantastic free resource on Exoplanets that has classroom activities for 11-14 year olds (or grown ups!) that explains all of these concepts.
So we can spot exoplanets as they move across the star how can this tell us if they might have surface water?
By watching the star we can work out how long the exoplanet takes to orbit. The dip in light we saw in the transit method will happen regularly and so we can work out the period of the orbit – how long a year would be on that planet. From this period we can work out how far the exoplanet is from the star. If you want to know the maths look here and you can also calculate the mass (and the density) of the exoplanet. From these you can calculate the radius of orbit and this will allow us to work out if life could be sustained.
The Goldilocks Zone
On a cold day if you go too close to a fire you’ll get too hot. Too far away you’ll be too cold, at some point your temperature will be just right! Similarly, if the exoplanet is too close to its star then its temperature will be too hot for surface liquid water, if it is too far away then it is likely to be too cold and any water would be locked away as ice. Only exoplanets in this Goldilocks Zone where the temperature is within the region for surface water are considered to be in the habitable zone. We have just found lots of them !
So how many exoplanets are there ?
Taking how many we have found and assuming the rest of the universe is similar there may be in the order of 2 sextillion potentially habitable exoplanets , that’s 2 000 000 000 000 000 000 000 exoplanets ! That’s around 1000 times as many grains of sand on the Earth!
If we take this as an overestimate and only 0.0000000000000001% of them are habitable that still leaves us with a million possibilities
So are we alone?
Become an exoplanet hunter !
You could spot an exoplanet no one else knows about then have a look at planethunters where people collaborate to classify and help the scientists.
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