January 30, 20255 min readHow Many Planets Are in the Solar System?The number of planets that orbit the sun depends on what you mean by planet, and thats not so easy to defineBy Phil Plait edited by Lee BillingsAll eight of the solar systems major planets appear in this illustration, but the actual number of planetary bodies orbiting our star is far greater. Digital Vision/Getty ImagesWhen I was a kid, there were nine planets.Now we know of thousands! But that includes exoplanets, alien worlds that orbit alien suns. Only eight planets call our solar system home. Or there might still be nine if some astronomers are correct about an as-yet undiscovered large body out past Neptune. On the other hand, if youre nostalgic (and a tad antiestablishment), there are still nine now, but then, to be fair, youd have to include the so-called dwarf planets, in which case there may be thousands orbiting the sun alone.Planets are a mess. Or perhaps more to the point, our way of thinking about them is.On supporting science journalismIf you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.Although What is a planet? seems like a simple question, it turns out its anything but.You could go by the ancient meaning of the word. Planetes means wanderers in ancient Greek, referring to the fact that as the planets and Earth orbit the sun, those dots of light in the sky appear to move relative to the background stars. This meaning, then, would also include the sun and moon, which most people would agree arent actually planets. It would also include comets and asteroids, and by now you can see that this may be too broad a definition.Perhaps a more modern take is called for. But thats fraught with trouble, too.In the early 2000s the term planet wasnt terribly controversial. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto all claimed that moniker. But then some pesky astronomers had to go and shake things up by discovering Sedna, Quaoar and Eris. These are bodies of ice and rock that orbit the sun well past Neptune and Pluto, and theyre large, all more than 1,000 kilometers wide. In fact, with a width of more than 2,300 km, Eris rivals Pluto in size and is even somewhat more massive. Are these, then, planets as well?When the existence of Eris was announced in 2005, one of its discoverers, astronomer Mike Brown, even called it the tenth planet in accompanying NASA press materials. At that time it was thought to be slightly larger than Plutoand if we called Pluto a planet, then Eris deserved planethood as well. Even when better observations downgraded Eris to slightly sub-Pluto in size, the issue remained. How many planets are there in the solar system?The growing confusion triggered the International Astronomical Union, or IAUa global group of astronomers that is the official keeper of cosmic nomenclatureto create an exploratory committee to propose potential definitions for the word. Several proposals were put forward, most of which were unsatisfactory one way or another. Among them, for example, was a proposal that would have included Ceres, the largest asteroid in the main asteroid belt between Mars and Jupiter, as a planet. Its smaller than what most people would think of as a planet, however; many planetary scientists now consider Ceres to be a protoplanet, a large object that almost reached planetary stature in the early days of the solar system but ran out of material to grow.Eventually, in 2006, the IAU settled on a three-part definition: a planet orbits the sun, is sufficiently massive to form itself into a round shape by gravity and has cleared the neighborhood around its orbit, which means its the most gravitationally dominant body there. (This term did initially cause some confusion because it could be misinterpreted to mean that the planet can sweep its orbit completely clean of any and all other bodies, which is impossible.) If a body fulfills the first two conditions but not the third, its called a dwarf planet.This means Pluto is not considered a major planet like Mercury or Earth but instead a dwarf planet. Some astronomers didnt like this definition because of Plutos diminished statusand certainly, even to this day, many members of the public remain vocally displeased about Plutos demotion as well.Over the years other definitions have been proposed, some better than others. One, in my opinion, seemed backward-defined just to include Pluto. But bluntly speaking, such reverse-engineering is how we got to our current divisive eight-planet situation. If you want to say Pluto is a planet, then you really should at least include Eris as well, and once youve set that particular precedent, youll find it necessary to include a lot of other dwarf planets. Without a firm lower limit on size or mass, were forced to say the solar system has hundreds of planetsmaybe thousands. Objects need to be at least 400 km or so in size to round out via self-gravity, and theres still lots of room for large numbers of them to lurk undiscovered in the darkness beyond Neptune.Now, theres no scientific reason we cant have thousands of planets in the solar system, but cultural considerations are a different issue. An overabundance of planets would seem to dilute the term itself, so most space scientists stand firm at either eight or nine.Or should that be nine or 10? Astronomers are pursuing this idea right now. Many smaller outer solar system objects appear to have orbits that are aligned in certain mysterious ways; the simplest explanation is that their motions are influenced by another, as-yet-unseen large body that is many times the mass of Earth and orbits more than 30 billion km from the sun. Temporarily (and tongue-in-cheekingly) called Planet Nine, this putative worlds potential location in the sky is not clear. Different teams of astronomers propose different hiding places for Planet Nine. The advent of large survey telescopes such as the Vera Rubin Observatory is set to narrow that search considerably or even to find the purported planet. Should it be discovered, whether you want to call it the ninth or 10th or 1,000th planet is up to you.But have a care before you make up your mind. Id argue that the main reason weve struggled for so long to come up with a definition for planet is that its not a definable thing. As Brown put it in his 2010 book How I Killed Pluto and Why It Had It Coming, planet is more of a concept, a generalization, than a definition. Hes right. There are no hard and fast lines we can draw around the concept of a planet to make it definable and distinct from other classes of objects. Whether decided wholly or in part by mass, size, shape, orbit or seemingly any other conceivable property, the criteria for planethood appear inescapably arbitrary.Such slipperiness is a universal theme in the natural sciences: definitions are a human conceit, not a natural occurrence. Look at colors; we have definitions for what the wavelength cutoff is for orange versus red, but that boundary was a choice, not something observed. Blue light and red light are two flavors of the same thing, just with different wavelengths. And all the colors in between literally lie along a spectrum; one blends smoothly into the next with no sharp steps to discern between them.Its not hard to come up with many more examples: sex, gender, political affiliation, species, and more. All of these exist on a spectrum. The differences are obvious between examples at opposite ends of a spectrum, tempting you to put them in a binary category, but when you compare any two samples close together on that spectrum, the differences are far harder to tease out. So where do you draw the line?Humans like putting things in clear-cut categories, but in general, nature isnt so picky. Acknowledging that can make life a lot easier and help us understand the universeand ourselvesbetter.