The TV show The Cosmos, outlined the “Cosmic address” of where the Earth resides in the universe – this is:
- The Solar System
- The Milky Way
- The Local Group
- The Virgo supercluster
- The observable universe
But how big are each of these and how many planets are there? Below we outline the size of each of these and review the latest assessment of the number of stars and planets in the universe.
1. How many planets are in the Solar System?
There are 8 planets in our Solar System. These are (in order from the Sun):
The Sun is, of course, a star and one of a large number of stars in the galaxy that we are in – the Milky Way.
Wait, I thought there were nine planets in the solar system?
There used to be nine planets in the Solar System, with Pluto being the additional one. However, in 2006 it was downgraded and taken off the list. This was because the definition of what makes a “planet” changed and Pluto was no longer considered one, essentially because it is too small:
- When Pluto was first discovered in 1930 nobody knew how big it was. Later in the 20th century it was discovered that it was in fact tiny in comparison to the other planets of the Solar System. It is just one-sixth the size of Earth and smaller than our moon.
- The dwarf planet Eris was discovered by the astronomer Michael E. Brown in the same area of space (the Kuiper Belt). It is larger than Pluto, but not considered a planet itself.
Pluto was therefore downgraded to a dwarf planet when an official definition of a planet was agreed by the International Astronomical Union in 2006 (see further information below).
To read more on this, see Is Pluto a Planet?
Wait, wait, didn’t I hear something about a ‘Planet 9’?
Yes, NASA’s astrophysicists have recently presented evidence that a huge and, as yet, undiscovered Planet 9 (or ‘Planet X’) probably exists at the far reaches of our solar system.
To read more on this, see our piece on Planet 9.
2. How many stars are in the Milky Way galaxy?
Given that we can’t be completely certain how many planets there are in the Solar System, it shouldn’t be too much of a surprise that once we look beyond that our estimates get much vaguer. But we are going to have a go anyway.
So, Earth and the planets of the solar system revolve around the Sun. Which is one star within our galaxy, the Milky Way.
Whilst our galaxy and sun seem huge to us, the current best estimate is that the Milky Way contains between 100 billion and 400 billion stars (source).
Estimates vary considerably as it is extremely hard to calculate and so you can find different estimates out there (including this one estimating a trillion stars in the Milky way).
We can also expect this figure to be updated with more discoveries made by the Hubble telescope and TESS exploration mission in the coming years.
3. How many planets are in the Milky Way?
If we then take the higher-end figure of 400 billion stars in the Milky Way, how many planets does this mean?
We know that our Sun has at least 8 planets, but the most recent analysis in Nature journal is that, on average, each star of the Milky Way hosts one planet.
Therefore we can take the estimate that there are 400 billion planets in the Milky Way.
4. How many planets, stars, and galaxies are there in the Local Group?
The Milky Way galaxy then resides within what is known as the Local Group.
The Local Group is made up of 54 galaxies, and this includes our Milky Way. It is 10 million light-years across – to put this in context, understand that one light year is about 9.5 trillion kilometers (5.9 trillion miles), so times that by 10 million to get the distance from one side to the other.
Just as we can only guess the number of planets in the Milky Way, estimates get even rougher as we move beyond our own galaxy.
A crude estimate would be to take the number of planets thought to be in the Milky Way and times it by 54, so this would be 54 times 400 billion, which makes an estimate of 21.6 trillion planets.
However, the same analysis from Nature journal cited above proposes that each star in the universe hosts, on average, 1.6 planets. This would then give us an estimate of 34.6 trillion planets in the in the Local Group.
To help you get some perspective, see this zoomable image of the Andromeda galaxy taken by the Hubble space telescope. Each dot is a sun and solar system of its own. Trying zooming in slowly and get a feel for the size of the galaxy and universe!
5. How many planets are there in the Virgo Supercluster?
The Local Group then resides in the Virgo Supercluster, which is the next step on the cosmic address of the Earth.
The Virgo Superstructure comprises at least 100 galaxy groups and it has a diameter of around 110 million light-years.
Again, only very crude estimates could be made of the number of stars in the supercluster, let alone planets, but if you take the estimate of the number of planets in the Local Group (34.6 trillion) and times it by 100 (the number of galaxy groups) you get an estimate of over 3.4 thousand trillion planets in the Virgo Supercluster.
6. How many planets are there in the universe?
The Virgo Supercluster is then one of about 10 million superclusters (or two trillion galaxies) in the observable universe (source).
To keep going with our estimates, this would make the number of planets in the universe 10 million times 3.4 thousand trillion (the estimate of the number of planets in the Virgo Supercluster). This now gets way beyond our mathematical capabilities (!), and into numbers that are truly hard to comprehend.
One further thing is that the two trillion galaxies are only for what we call the observable universe. This is the area of the universe where light from far-off objects has had time to reach Earth. This therefore means there is a limit to how far we can see in any direction and the actual universe may in fact be many times the size of the observable universe. Right now we have no way of knowing.
One mind-blowing illustration of the size of space is this image known as the Hubble Ultra-Deep Field (HUDF) image of a small area of space containing around 10,000 galaxies (each of these colored specks is a galaxy like our Milky Way (which, remember, are estimated to each contain 400 billion stars and 640 billion planets, on average):
This gif gives you some perspective on how small the area of space is in which this shot is taken:
6. Conclusion – how many planets are there?
So, in conclusion, the best answer we have to the question ‘how many planets are there?’ is:
- 8 planets in the solar system (maybe 9)
- 400 billion planets in the Milky Way
- 34.6 trillion planets in the Local Group
- 3.4 thousand trillion planets in the Virgo Supercluster
- 10 million times 3.4 thousand trillion planets in the observable universe
If you want to see how much of this you took in, take the quiz!
What is the definition of a planet?
In 2006 the International Astronomical Union (IAU) defined that, in the Solar System, a planet is a celestial body which:
- is in orbit around the Sun,
- has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
- has “cleared the neighborhood” around its orbit.
A body fulfilling only the first two of these criteria is classified as a “dwarf planet”. That is an object that is neither a planet nor a natural satellite, but is in direct orbit of a star, and is massive enough for its gravity to crush it into a spherical shape, but has not cleared the neighborhood of other material around its orbit.
With Pluto being part of the Kuiper Belt of other objects, it does not meet the third criteria for being a planet and is therefore a dwarf planet.
Read more about this on the IAU website here: https://www.iau.org/public/themes/pluto/
See this NASA video on dwarf planets:
What are planets made of?
The four inner planets of the Solar System – Mercury, Venus, Earth and Mars – are terrestrial planets composed primarily of rock and metal.
For the outer planets, Jupiter and Saturn are gas giants and Uranus and Neptune are ice giants. All four are composed mainly of hydrogen and helium but the more distant two have more ice.
What are planets of the solar system named after?
The planets of the solar system have names derived from Greek or Roman mythology. This is because the Greeks and Romans were the first astronomers to observe and record these planets and so named them. The tradition was later continued by other astronomers when Uranus, Neptune, and Pluto were discovered.
- Mercury is the god of commerce, travel and thievery in Roman mythology.
- Venus is the Roman goddess of love and beauty.
- Earth is the only planet whose English name does not derive from Greek/Roman mythology. The name derives from Old English and Germanic.
- Mars is the Roman god of War.
- Jupiter was the King of the Gods in Roman mythology.
- Saturn is the Roman god of agriculture.
- Uranus is the ancient Greek deity of the Heavens.
- Neptune, was the Roman god of the Sea.
As Pluto was previously classified as a planet it is also named this way – Pluto is the Roman god of the underworld in Roman mythology.
If you’d like to read more about the planets, check out our guide to astronomy for beginners and these space facts pieces: