How Many Planets Are There?

Before we try and answer the question of how many planets are there, we first need to know what a planet is – what is the definition of a planet? The following is a formal resolution(definition) by the International Astronomical Union: A planet is a celestial body that is in orbit around the Sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium, a nearly round shape, and has cleared the neighborhood around its orbit.

For the purpose of this article, the definition of a planet will extend outside our own Solar System to the rest of the Milky Way Galaxy, and far beyond even that going throughout the entire Universe.

The quick answer is that there are 8 planets in our Solar System, according to the official definition by the International Astronomers Union. These planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

In years past the answer would have been 9 planets – this is the answer many of us learned in school. But in recent years Pluto has been demoted from ‘planet’ status to that of a dwarf planet, due in large part to new information we have received from the New Horizons space probe, which flew about 7800 miles above the surface of Pluto on July 14th, 2015 collecting some remarkable data, including photos. The Hubble Space Telescope also gave us some remarkable photos of Pluto, better than any previous pictures from Earth-based telescopes.

Of the 8 planets in the Solar System, there are 4 terrestrial planets – Mercury, Venus, Earth, and Mars. Following is a brief synopsis of each:

Being the closest planet to the Sun, and also the smallest, Mercury makes a complete revolution around the Sun every 88 days at a mean distance of about 35 million miles.

Venus is the second closest planet to the sun making a complete revolution about every 225 days at a mean distance of about 67.3 million miles. Venus is very close to the Earth in size with a diameter about 5.5 percent less and a mass about 18.5 percent less.

Earth, the planet on which we all live, is the third planet from the Sun at a mean distance of about 93 million miles and making a complete revolution around every 365 days. It is the only planet known to have life, although there is a significant possibility for life on Mars.

Mars is the fourth planet from our Sun at a mean distance of about 141.7 million miles; the orbit of miles is very elliptical so there is a large variance between closest and furthest distance from the Sun. Mars is the planet in our Solar System with the best chance for life outside of our own Earth.

Of the 8 planets in the Solar System, there are 4 gas giant planets – Jupiter, Saturn, Uranus, and Neptune. A brief synopsis of each follows:

Jupiter is the largest planet in the Solar System and the fifth planet from the Sun at a mean distance of 484 million miles making a complete revolution about every 11.6 years. It has two and one-half the mass of all the other planets in the Solar System combined and could hold 1300 Earths inside its volume – the Giant Red Spot of Jupiter alone could swallow 3 Earths.

The sixth planet from the Sun is Saturn at a mean distance of about 890 miles and making a complete revolution around the Sun around every 29.5 years. Saturn is the only planet in the Solar System with a magnificent ring structure that is easily seen from a telescope on Earth.

The seventh planet from the Sun is Uranus, a cold bluish gas giant with orbits the Sun at a mean distance of 1.79 billion miles about every 84 years.

Neptune, an icy blue gas giant, is the eighth planet from the Sun at a mean distance of about 2.8 billion miles making a complete revolution about every 164.8 years.

Pluto, once considered a full planet, is now classified as a dwarf planet and is the ninth planet from the Sun after Neptune. It orbits the Sun at a mean distance of about 3.67 billion miles and makes a complete revolution around the Sun about every 248 years. Pluto is quite small compared to the 8 full sized planets in the Solar System with a diameter only about two-thirds that of the Earths Moon.

The Kuiper Belt is a circumstellar disc, a pancake or ring-shaped accumulation of matter – gas, dust, planetesimals, asteroids, and so on, that is in the outer region of our Solar System. It is now known for certain to have at least two dwarf planets, Eris and Farout, and may very well have more.

Eris is a dwarf planet in the far reaches of the Solar System well beyond Pluto in a region known as the Kuiper Belt. It has a mean distance from the Sun of about 6.29 billion miles although it’s orbit is highly elliptic and this distance will vary greatly. It is the second largest planet(in volume) in the Solar System behind Pluto, although it is about 27 percent more massive than Pluto. It is believed to make a complete revolution around the Sun about every 558 years.

A dwarf planet with the unofficial nickname of Farout, officially known as 2018 VG18 was discovered on November 10th, 2018. It orbits the Sun in the Kuiper Belt region of the Solar System at a mean distance of 11.8 billion miles – this is over 3 times the distance of Pluto from the Sun and almost twice that of Eris. Farout is believed to make a complete revolution around the Sun about every 929 years and has a diameter of only about 310.7 miles, making it far smaller than either Pluto or Eris.

The Milky Way is our home galaxy – the galaxy which contains our own Solar System in the far region of the Orion Arm of the galaxy. It is estimated to be somewhere in the range of 100 thousand light-years in diameter with an average thickness(taking into account the central bulge) of about 1000 light-years. The Milky Way Galaxy is believed to contain somewhere in the range of 100 to 400 billion stars.

We simply don’t know the exact number since there is no way to count the individual stars. Also, there is such a wide range of stars with varying sizes and masses which makes a precise number impossible at this time. So, for the time being, we can only estimate the total number of stars.

That being said, we are quite certain at the present time that there is an absolutely huge number of planets orbiting many, if not most, of these stars. We will explore this in more detail in the next couple of sections.

The definition of an exoplanet is simply a planet which is outside our Solar System, whether it is orbiting another star, star remnant, or just a rogue planet drifting in the void of interstellar space.

The first exoplanet was detected in 1988 and later confirmed in 2012; today in 2019 there are 4023 confirmed exoplanets in 3005 different star systems – 656 of these star systems are known to have multiple planets. There are probably dozens of these planets which have favorable conditions for some sort of life. With our improving detection technology, this number of planets will just keep growing.

The number of planets in the Milky Way Galaxy is impossible to know precisely, for reasons which have already been discussed. As we have seen, estimates generally range from 100 billion to 400 billion stars, so the total number of planets has to be gigantic, just in our galaxy alone.

If we take this estimated range of the number of stars in our galaxy and make some extrapolations, based on what has already been observed and discovered, then we can estimate that there are a total number of planets in the Milky Way Galaxy somewhere in the range of 800 billion to 3.2 trillion – this is a stunningly large number! But how many of these planets are habitable, that is which have conditions that are favorable to life? This is what we will look at in the next section.

Now that we’ve seen what the potential is for the total number of planets in the Milky Way Galaxy, somewhere in the range of 800 billion to 3.2 trillion, we have to ask the question how many of these are likely to be habitable, favorable for the evolution of some kind of alien life forms?

Of course, we know that this has to just be an estimate but based on observations made with our modern technology and advanced statistical analytic techniques this number of habitable worlds would seem to fall in a range of 500 million at the low end to tens of billions at the higher end. But that’s not the end of it; the Universe contains a huge number of galaxies and we will look at the potential number of planets in the whole Universe next.

The Universe is a place of gigantic proportions – the observable part is a sphere with a diameter estimated at about 93 billion light years – the radius is, of course, half that for a sphere, which means if we had the optical means to do so we could look out in the Universe for 46.5 billion light years in all directions from Earth.

We call this the observable Universe because there is a part of the Universe we can never hope to see because of the light speed barrier. Galaxies and other celestial objects beyond this 46.5 billion light year radius are moving away from us at speeds greater than that of light, so light from them can never reach us and they can never be observed by us.

The point of this discussion is that any estimates we make of the number of planets in the Universe will be understood to mean the observable Universe, beyond this limit we can’t even guess – it will forever remain a mystery.

Our Earth is one of only 8 full sized planets in our Solar System, our Solar System is only a tiny speck in our Milky Way Galaxy(a very average sized galaxy at that – there are some that are far larger), our Milky Way Galaxy is one of only 54 galaxies in the Local Group of galaxies, the Local Group is only a tiny part of the Virgo Supercluster of galaxies containing over 100 galaxy groups(1300 to 2000 galaxies) with a diameter of about 110 million light years, and the Virgo Supercluster is but a tiny speck in the entire observable Universe, which is now believed to contain upwards of 2 trillion galaxies, and this number could very well increase further. This Universe may be a speck itself in some gigantic multiverse, but that is a discussion beyond the scope of this article.

This is a very big question with really no answer – we can only extrapolate and make an estimate, which is really more of a guess, based on observations we have already made about the number of planets in the area of our galaxy as well as the total number of galaxies in the observable Universe.

Let’s use the figure of 2 trillion galaxies in the entire observable Universe and then make an assumption that the average number of stars in these 2 trillion galaxies is about 100 billion – this may be on the low side but we’ll be a little bit conservative with our estimate.

Now let’s assume a very conservative estimate of 800 billion planets per galaxy. If we multiply this figure time 2 trillion galaxies we come to a gigantic number of 160 sextillion planets – that is 160 billion trillion planets – a number so huge that it is beyond our comprehension.

Keep in mind this is really a conservative estimate and also different types of galaxies in different parts of the Universe will be in various stages of their evolution so the probability of planets in these various galaxies will vary by a significant amount. But I think the point of this is that rather than coming up with a precise number of planets, which is of course not possible anyway, it gives us the realization that the total number of planets in the Universe is incredibly gigantic.

How many habitable planets are there in the Universe? First of all, we will use our previous estimations to arrive at an answer, or really more of a guess, to this question. We arrived at a conservative estimate of 160 sextillion, or 160 billion trillion, planets in the entire observable Universe. Now let’s assume that only one out of every one-thousand planets is habitable – that would give us an estimate of 160 quintillion, or 160 million trillion, habitable planets in the whole Universe.

What if we make a less conservative estimate of one out of every ten planets being habitable – that would give us a much larger estimate of 16 sextillion, or 16 billion trillion, habitable planets in the Universe. This is an astonishingly large number but could be closer to the truth if we extrapolate from our own Solar System. After all, out of the 8 planets in our Solar System, one planet – Earth – is clearly habitable which comes to 12.5 percent. And that’s not even counting Mars, which is on the borderline of habitability.

The point of this discussion is not to come to a precise number of habitable planets in the Universe but to simply illustrate that whatever the actual number might be, it is super gigantic in its scope and what it implies.

The implication is that the Universe may very well be brimming full of habitable planets with a wide range of alien life forms, some of which may be highly intelligent, possibly well beyond our comprehension. In the years, decades, and centuries ahead humans should be prepared to be more and more amazed!