How many planets are there in this solar system?

Question

So, in school (that's a long time ago) they have been teaching us there are 9 planets in our solar system.

1. Mercury
2. Venus
3. Earth
4. Mars
5. Jupiter
6. Saturn
7. Uranus
8. Neptune
9. Pluto

But every now and then I keep reading stories about another "dwarf planet" (Eris, discovered in 2005) that - depending on what source tells the story - is another planet according to the astronomical definition, while other sources say that it isn't a planet. Some even say Pluto isn't a planet anymore either.

The result: I'm confused due to the contradicting stories. Even Wikipedia isn't clear about Eris and only writes (emphasis mine):

NASA initially described it as the Solar System's tenth planet.

Initially? So, is it a 10th planet or not? Fact is, there is another "something" out there and it surely seems to look like a planet. Yet, some people keep stating there are 9 planets in our solar system, while others say there are more than 9 planets, and then again there are people stating that the latest definition of "planet" has kicked out Pluto too so there are actually fewer than 9 planets in our solar system.

Trying to get a definite, official, and astronomically correct answer I can actually rely on, I'm therefore asking: How many planets are there in this solar system?

EDIT

The "Definition of planet" at Wikipedia doesn't really help either, as it states:

Many astronomers, claiming that the definition of planet was of little scientific importance, preferred to recognize Pluto's historical identity as a planet by "grandfathering" it into the planet list.*

_{* Dr. Bonnie Buratti (2005), "Topic — First Mission to Pluto and the Kuiper Belt; "From Darkness to Light: The Exploration of the Planet Pluto"", Jet Propulsion Laboratory. Retrieved 2007-02-22.}

So, if you link somewhere to provide proof, it would be great if you could point me to a more trusted source than Wikipedia. Ideally, an astronomical trusted source and/or paper.

Answer 1

In addition to Undo's fine answer, I would like to explain a bit about the motivation behind the definition.

When Eris was discovered, it turned out to be really, really similar to Pluto. This posed a bit of a quandary: should Eris be accepted as a new planet? Should it not? If not, then why keep Pluto? Most importantly, this pushed to the foreground the question

what, exactly, is a planet, anyway?

This had been ignored until then because everyone "knew" which bodies were planets and which ones were not. However, with the discovery of Eris, and the newly-realized potential of more such bodies turning up, this was no longer really an option, and some sort of hard definition had to be agreed upon.

The problem with coming up with a hard definition that decides what does make it to planethood and what doesn't is that nature very rarely presents us with clear, definite lines. Size, for example, is not a good discriminant, because solar system bodies come in a continuum of sizes from Jupiter down to meter-long asteroids. Where does one draw the line there? Any such size would be completely arbitrary.

There is, however, one characteristic that has a sharp distinction between some "planets" and some "non-planets", and it is the amount of other stuff in roughly the same orbit. This is still slightly arbitrary, because it's hard to put in numbers exactly what "roughly" means in this context, but it's more or less unambiguous.

Consider, then a quantity called the "planetary discriminant" µ, equal to the ratio of the planet's mass to the total mass of other bodies that cross its orbital radius and have non-resonant periods (so e.g. Neptune doesn't count as sharing Pluto's orbit) up to a factor of 10 longer or shorter (to rule out comets, which has little effect in practice). This is still a bit arbitrary (why 10?) but it's otherwise quite an objective quantity.

Now take this quantity and calculate it for the different bodies you might call planets, comparing it to both the objects' mass,

and their diameter,

or with an arbitrary horizontal axis, in order of decreasing discriminant,

Suddenly, a natural hard line emerges. If you look only at the mass and the diameter of the objects (shown in the insets above the plots), then there is a pretty continuous spread of values, with bigger gaps between the gas giants and the terrestrial planets than between Mercury and Eris/Pluto. However, if you look at the planetary discriminant, on the vertical axis, you get a very clear grouping into two distinct populations, separated by over four orders of magnitude. There's a finite set of bodies that have "cleared their orbits", and some other bodies which are well, well behind in that respect.

This is the main reason that "clearing its orbital zone" was chosen as a criterion for planethood. It relies on a distinction that is actually there in the solar system, and very little on arbitrary human decisions. It's important to note that this criterion need not have worked: this parameter might also have come out as a continuum, with some bodies having emptier orbits and some others having slightly fuller ones, and no natural place to draw the line, in which case the definition would have been different. As it happens, this is indeed a good discriminant.

For further reading, I recommend the Wikipedia article on 'Clearing the neighbourhood', as well as the original paper where this criterion was proposed,

What is a planet? S Soter, The Astronomical Journal 132 no.6 (2006), p. 2513. arXiv:astro-ph/0608359.

which is in general very readable (though there are some technical bits in the middle which are easy to spot and harmless to skip), and from which I took the discriminant data for the plots above.

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^{Edit: I must apologize for having included, in previous versions of this post, an incorrect plot, caused by taking data from Wikipedia without verifying it. In particular, the planetary discriminant for Mars was wrong (1.8×105 instead of 5.1×103), which now puts it below Neptune's instead of just below Saturn's, but the overall conclusions are not affected. The Mathematica code for the graphics is available at Import["http://goo.gl/NaH6rM"]["https://i.stack.imgur.com/CQA4T.png"].}

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... and, as a final aside: Pluto is awesome. It was visited in July 2015 by the New Horizons probe, which found a world that was much more rich, dynamic, and active than anyone expected, including what appear to be churning lakes of solid nitrogen ringed by mountains of water ice, among other marvels.

(Note the image has been colour-enhanced to bring out the variety of surface materials; the true-colour version of this image is here.) I, personally, don't feel it's at all necessary to 'grandfather' Pluto into the list of planets to really feel the awe at the amazing place it is - it's perfectly OK for it to be a cool place with cool science, that is also not a planet.

Answer 2

Since we're talking about terminology, we need to remember that none of this really matters, outside of clarity when communicating. Still, some people tend to have rather strong opinions on it, thus confusion about how many planets are really in the solar system arises.

The people

The most trusted source in Astronomy would have to be the people that set the generally accepted rules. The IAU (International Astronomical Union) has been in existence since 1919 and is comprised of 10814 Individual Members in 93 different countries worldwide. Of those countries, 73 are National Members.

The key activity of the IAU is the organization of scientific meetings. Every year the IAU sponsors nine international IAU Symposia. The IAU Symposium Proceedings series is the flagship of the IAU publications. Every three years the IAU holds a General Assembly, which offers six IAU Symposia, some 25 Joint Discussions and Special Sessions, and individual business and scientific meetings of Divisions, Commissions, and Working Groups. The proceedings of Joint Discussions and Special Sessions are published in the Highlights of Astronomy series. The reports of the GA business meetings are published in the Transactions of the IAU - B series.

The definition

At the 2006 IAU General Assembly in Prague, the accepted definition of a planet was debated vigorously. The outcome of the meeting was the currently accepted definition of a planet:

A celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

What about Pluto?

With this in mind, the group decided on Pluto's fate. From page 2 of this official resolution document:

The IAU further resolves:

Pluto is a "dwarf planet" by the above definition and is recognized as the prototype of a new category of Trans-Neptunian Objects.

The reason given in the FAQ on this page:

Q: Why is Pluto now called a dwarf planet?

A: Pluto now falls into the dwarf planet category on account of its size and the fact that it resides within a zone of other similarly-sized objects known as the transneptunian region.

Basically, they decided that it isn't officially a planet anymore because it didn't match criteria (c): has cleared the neighbourhood around its orbit. It hasn't done this, because it 'resides within a zone of other similarly-sized objects'. Therefore, it hasn't cleared its neighborhood.

Soo... what about the number of planets?

Q: Based on this new definition, how many planets are there in our Solar System?

A: There are eight planets in our Solar System; Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. Mnemonic: My Very Educated Mother Just Served Us Nachos.

But that's if you don't count Dwarf planets - if you do count them, you end up with five more:

• Ceres
• Pluto
• Eris
• Makemake
• Haumea

So there are 8 planets in the solar system if you don't count Dwarfs, 13 if you do.

Answer 3

The correct answer is 8 (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune).

Pluto is not longer a planet since 2006 when the IAU adopted a formal definition of planet

Answer 4

Well, it depends on how you define a planet.

According to IAU (InternationalAstronomical Union), an object should satisfy three conditions for it to be called a planet

1. It should revolve around the sun.
2. It should have enough gravity to form itself into a sphere.
3. It should have cleared space around itself (it should be a dominant object in that area)

Pluto was demoted as a planet because it failed the third criteria. Currently, there are only Eight planets that satisfy the three conditions

1. Mercury
2. Venus
3. Earth
4. Mars
5. Jupiter
6. Saturn
7. Uranus
8. Neptune

There are five dwarf planets officially recognised by IAU

1. Pluto
2. Ceres
3. Eris
4. Haumea
5. Makemake

Answer 5

According to a geophysical planet definition, as well as the IAU planets and dwarf planets, the largest moons are also planets: the Moon, Io, Europa, Ganymede, Callisto, Titan and Triton.

https://en.wikipedia.org/wiki/Geophysical_definition_of_planet

Although other moons of Saturn and Uranus and also Charon look round to me also.

Answer 6

This is not exactly what you asked, but I'm writing it anyway because it's relevant to the title question.

Strictly speaking, the answer is at least 8, possibly more. The "8" are Mercury / Venus / Earth / Mars / Jupiter / Saturn / Uranus / Neptune, as detailed in the other answers. These celestial bodies are unambiguously planets. Others (like Pluto, Ceres ...) are generally treated as dwarf planets, at least by professional astronomers.

The "possibly more" part is because the data isn't good enough to rule out the existence of other large planets around the Sun. See the Planet Nine article on Wikipedia. It's even possible that there is a "Planet Ten". These planets are hypothetical, however, and it will take a direct detection to convince everyone that they definitely exist.

Answer 7

The other answers told how many planets are recognized, how many dwarf planets that aren't IAU-recognized planets are recognized and one that indeed includes Eris. In a month it will be fifteen years since the infamous decision for an extremely vague and therefore stretched definition that was trying hard to keep the number of planets as low as possible for some reason. The majority among ~4% of the IAU who attended the meeting feared if they accepted Ceres, Pluto and Eris, there would eventually be "hundreds" of planets.

Instead of the current allegedly "official" position, I will portray a history of what should have been done if there wasn't any panic about "hundreds of planets" and if one were to go by logic and science according to how planets and asteroids have been defined pre-2006. Let's see if the fear was even justified.

Since 1930 there were 9 planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. This number was subjectively correct from 1930 to 2004 (not 06). In 2004 the Hubble telescope shot the first image of Ceres sharp enough to recognize features and its spherical shape:

At that time Ceres was considered the largest asteroid, and even today some still call it an asteroid for some reason (though they probably mean "asteroid belt object"). An asteroid is an irregular body, and Hubble proved that Ceres is near-perfectly spherical and therefore indeed a planet as it was recognized in the first half of the 19th century. This should have sufficed pre-2006 to reclassify it a planet again and even if you don't stretch the current definition too much it may be a planet as it's the largest and most massive object of the main belt. Whatever, Hubble proved Ceres was a planet, not an asteroid, and following the publication of the shot, Ceres should have gained planethood back. This wasn't done which was the first falsification in the number and recognition of planets.

If they had done so they would consequently have had to give planethood to the larger body Sedna in expectance that it must be a planet too. So if they did what would have been logical according to the then-understanding of celestial bodies, they would have accepted Ceres and Sedna as planets and there would be 11 planets now.

Later the year, 2005 FY9, later named Makemake, has been discovered. 2005 FY9 is larger than Ceres and Sedna and therefore should have got planethood as well, and it would get it probably along with Quaoar which is larger than Ceres as well, and Orcus the "anti-Pluto" which has about the same size as Ceres and an orbit similar to Pluto. Hence there would be 14 planets now.

In 2004/05 Haumea was discovered which was named a dwarf planet by the IAU in 2008. However, Haumea is a Jacobi ellipsoid (rugby ball shape), and back then a planet needed to be "spherical", not to "overcome rigid body forces to pull itself into hydrostatic equilibrium". Therefore, rather than gaining planet status it would have been considered the largest asteroid, thereby replacing Ceres which got planethood back in 2004. If Haumea was recognized a planet, Vesta would have to be recognized too which isn't perfectly spherical either but more so than Haumea, and is internally differentiated, and Pallas and Varuna perhaps too.

In 2007, 2007OR10, later named Gonggong, has been discovered. Of course it would have been recognized a planet too as it is larger than Ceres, Orcus and Sedna. There would now be 15 planets.

In 2017 and 18, the VLT shot sharp images of 10 Hygiea proving a near-spherical shape:

Published in 2019, many consider Hygiea the smallest dwarf planet now. Hygiea would have beeen recognized a planet under the expectation that subsequent study would prove it to be one, maybe a probe would have been sent to study it from close up. As you see, planets are obviously considered more "important" than bodies unrecognized as planets. There would be 16 planets now. There would be 20 planets if you add Haumea, Vesta, Pallas and Varuna. If Hygiea turned out to actually be spherical enough and differentiated, one may someday add 2002MS4 and Salacia as well so there would be 22 planets. In 2014 and 16 a hypothetical super-Earth or ice giant has been proposed to explain the very eccentric orbits of 6 TNOs; if you add it there are 23 planets (or 17 planets if you don't count the six controversial ones).

And no more planets would come unless we discover yet another Earth-sized or larger planet beyond the hypothetical one, or discover a dwarf planet that has an orbit similar to Sedna and comes to perihelion, or send a space probe to a planet candidate (however, the other "dwarf planet candidates" are unlikely to be near-perfectly spherical or differentiated). WISE/neoWISE ruled out the possibility of Saturn- and Jupiter-sized planets to a certain distance.

The fear of "hundreds of planets" was unnecessary and is disproved. And even if there would have been, I don't see anything wrong with it. Non-professionals would only have to learn the eight largest ones and Ceres and Pluto for historical reasons perhaps.