Investigation: How many planets are there in the universe?

When we are presented with a problem, we need to figure out what information will help us find an answer. For example, let’s think about how we could answer the question: How many planets are in the whole universe?

First, let’s think about what planets are and where we might find planets in the Universe. Our own planet, Earth, goes around a star which we call the Sun. In the same way, there are planets orbiting every star that you see in the night sky.

Every star you see in the night sky has its own planets. Image Credit: ESO. 

In our own solar system, we have 8 planets (sorry Pluto!). But we don’t know how many planets go around the other stars. It could be 8, or it could be more or less. Therefore, the first piece of information we need is the average number of planets orbiting each star.

Once we know how many planets there are per star, we need to know how many stars there are in the universe. No one has ever tried to count how many stars there are in the whole universe, so let’s break it down a bit. The universe is made up of a huge number of galaxies. Each galaxy is made up of a huge number of stars. Therefore, if we know how many stars there are in an average galaxy, and how many galaxies there are in the universe, we will be able to figure out how many stars there are in the universe.

Every tiny dot in this photo is a star. Image Credit: NASA

Every tiny dot in this photo is a galaxy. Image credit: NASA

So there we have it! In order to find out how many planets there are in the universe, we need to know:

1. The average number of planets per star
2. The average number of stars per galaxy
3. The total number of galaxies in the universe

Scientists are working hard to work out each of these numbers, but they can already make some good guesses. Question number 1 is actually the hardest one to answer. At the moment, scientists can only be sure that there is on average at least one planet per star, but don’t know much more than that. The other information, you can find by doing a little research. Click the links below to see!

1. Average number of planets per star = 1
2. Average number of stars per galaxy = ?

3. Total number of galaxies in the universe = ?

Once you have all the numbers, multiply them together to see how many stars there are in the universe!

Practice questions

Now that you know how to go about answering complex problems, try your skills on the next few problems. First, try to break it down into a list of pieces of information that you need, then use the internet to try to find those pieces of information.

1. How much do all the people in the world weigh?             
2. How much do all the ants in the world weigh? Is it more or less than the weight of all the people?
3. How many helium balloons would it take to lift a person?
4. How many different species of animals are there which have still not been discovered?