At some point in the future, human beings will likely need to find another planet to live on in order to continue as a species.

This means either creating a planet that’s habitable or finding and migrating to one that’s already out there.

But how many habitable planets are there out in space?

Like most scientific issues, this is a simple question with an extremely complicated answer. 

Read on to find out how many confirmed and estimated habitable planets there are in our solar system, galaxy, and the universe.

 

What makes a planet habitable?

First of all, we need to establish what a habitable planet is, and what it’s not.

By “habitable,” we mean habitable to human beings, which seems simple but is actually quite complex.

Take Mars, for example. If we have the proper suits, shelters, and equipment, we could live on Mars. 

So does Mars count as a “habitable planet”? No, it does not – at least not for the purpose of this discussion as we can’t just land and walk around there.

For this article, we are talking about planets where, if it were at all possible to get there, human beings could live without suits, air-tight shelters, and advanced terraforming (the process of altering a planet to make it habitable.) 

what makes a planet habitable

Artists’s impression of possibly habitable planet HD 189733b (Credit: NASA, ESA, M. Kornmesser).

 

How many habitable planets are there in our solar system?

There is only one habitable planet in our solar system: Earth.

Although some planets, or planetary moons (like Jupiter’s Europa), could support some form of life such as bacteria or single-cell organisms, no other planet or celestial body in the solar system is capable of supporting humans. 

This means we need to look outside of our solar system, towards planets that revolve around a star other than our Sun.

 

How are planets discovered?

For generations, astronomers, philosophers, and other deep thinkers have theorized about the existence of planets located outside our solar system.

As early as the 16th century, there was written speculation on this topic, and Isaac Newton, perhaps the most important contributor to physics in history, theorized accurately that our Sun is a star, and that the stars you can see in the sky would therefore also have orbiting planets.

The problem for finding planets is that these bodies don’t reflect enough light to be detectable through typical observation methods – such as looking through telescopes, so more creative techniques are required.

As often happens, advancements in science had to wait for advancements in technology; now that we have that technology, we can find numerous planets.

There are four main techniques for discovering planets:

  1. Looking for wobbling stars
  2. Observing shadows
  3. Taking photos
  4. Detecting starlight that has been bent by gravity

 

1. Looking for stars that wobble

This technique is known as “radial velocity” or “astrometry”. 

This is where a planet orbiting a star creates a pulling effect, causing the star to “wobble” as the planet circles. This wobble is usually small, as the planet is often tiny compared to its star.

However, using highly sensitive technology, this can be observed if the planet is large enough. Once a wobble is detected, researchers are cued to the existence of a planet.

The wobble technique is mostly effective for finding large planets. To find small planets the “transit” method is used.

 

2. Observing shadows

The “transit” method is where the shadow of a planet on a star is seen.

As a planet passes in front of a star, it blocks a tiny portion of light from the observer, so the star will appear slightly (and we mean slightly!) dimmer.

Using advanced technology, astronomers can detect these changes in light and can even calculate the size of the planet based on the amount of light being blocked.

They can also use the transit time to estimate how far the exoplanet is from the sun, which is a crucial part of habitability.

 

3. Taking photos

This might seem obvious, but it is not as straightforward as it sounds.

Because these potential planets are so far away and reflecting so little light they are very hard to see. In addition, they are close to their own stars which are exceptionally bright.

Therefore, it’s like staring into the sun trying to watch a plane pass in front of it (don’t try this!).

Advanced techniques and equipment have to be used to block the light of the star so as to make it possible to see the objects that are orbiting around it.

 

4. Detecting starlight that has been bent by gravity

Lastly, planets can be discovered by a technique known as “gravitational microlensing”.

This is where light from a star can be seen to be bent. The cause of this bend can be the gravity of a unseen planet. Thereby alerting us to its presence, even if we cannot see it.

 

How many planets have been discovered beyond our solar system?

Using these methods, there have been thousands of planets already discovered, with more being added every year.

As of August 2019, there have been 4,043 confirmed planets discovered outside of our solar system (source).

There have been 4,043 confirmed planets discovered outside of our solar systemClick To Tweet

But of all these planets that have been confirmed, how many are actually habitable? And what makes a planet habitable to humans.

habitable planets found

NASA’s count of different types of habitable planet found.

 

What makes a planet habitable?

In order to be habitable, the conditions on and around the planet need to be in line with the below six parameters:

  1. Distance: First of all, the planet needs to be in the “habitable zone”. This means that the planet should be the right distance from the star; too close and it will be too hot, too far and it will be too cold.
  2. Stability: The star itself also needs to be stable and should not have severe increases in the light, radiation, and heat that it expels.
  3. Gravity: To be habitable, a planet can’t be too large or too small, as this will affect the gravity. If there is not enough gravity (too small), lifeforms could spin out into the cosmos and the planet would not hold an atmosphere. If gravity is too powerful (the planet is too large), humans would be unable to function in the oppressive weight.
  4. Atmosphere: Even if gravity is just right, there also needs to an atmosphere in the first place, which is not a given.
  5. Rotation: It’s also believed that a habitable planet needs to rotate on its axis like Earth to provide relative stability and consistency.
  6. Core: It must have a molten core, which produces a magnetic field that protects life from solar flares. (A molten core would also be a source of thermal energy, another major advantage.)
conditions needed for life on a planet

Artist’s impression of WASP-107b (Credit: ESA/Hubble, NASA, M. Kornmesser).

 

So…how many “habitable” planets are there?

All of this brings us back to our fundamental question: how many habitable planets are there?

In our solar system, there is one habitable planet, Earth.

In the Milky Way Galaxy (our home), there have been over 4000 planets discovered so far. These have been detected by the techniques outlined above but none are confirmed as habitable as we don’t have enough information about them yet.

We therefore need to make estimates based on what information we have to make a guess as to the possible number of habitable planets in space.

Based on the data available, the current best guess is that there are between 5 and 10 billion potentially habitable planets in our galaxy

There are between 5 and 10 billion potentially habitable planets in our galaxy.Click To Tweet

This was the conclusion reached by a team of researchers at Penn State University in August 2019. They looked at numerous factors such as size, orbit times, and distance to the nearest star of planets discovered so far and extrapolated from that.

This is a huge number but bear in mind that there are thought to be around 400 billion planets in the Milky Way in total.

Therefore, this means that we are estimating between 1% and 2.5% of the planets in our galaxy are habitable.

In the known universe, there are estimated to be around 20 sextillion planets (that’s 2 plus 23 zeros). To make a calculation of how many habitable planets there might be then, we can take the estimate of 2.5% from the total number of planets (20 sextillion).

That comes to 200 quintillion, or 200,000,000,000,000,000,000.

 

Conclusion: habitable planets in numbers

To wrap up then, our conclusions are that there is:

  • 1 habitable planet in our solar system
  • 5 to 10 billion habitable planets in our galaxy
  • 200 quintillion habitable planets in the known universe

Over to you. Please let us know your thoughts on this and whether there is other interesting data out there on this topic.