How many planets they are

That would bring the total number of planets to ten. There's no turning back, textbooks would need to be changed. In order to settle the dispute, the International Astronomical Union met in , and argued for, and against Pluto's planethood.

Some astronomers advocated widening the number of planets to twelve, including Pluto, its moon Charon, the Asteroid Ceres, and the newly discovered Eris. In the end, they changed the definition of what makes a planet, and sadly, Pluto doesn't make the cut:.

A planet needs enough gravity to pull itself into a sphere. Okay, spherical. Pluto's fine there too. For example, planet Earth accounts for a million times the rest of the material in its orbit, while Pluto is just a fraction of the icy objects in its realm. There's Ceres, the first asteroid ever discovered, and the smallest of the dwarf planets.

The surface of Ceres is made of ice and rock, and it might even have a liquid ocean under its surface. NASA 's Dawn mission is flying there right now to give us close up pictures for the first time. Haumea, named after the Hawaiian goddess of fertility, is about a third the mass of Pluto, and has just enough gravity to pull itself into an ellipsoid, or egg shape. Even though it's smaller, it's got moons of its own.

Makemake, a much larger Kuiper belt object, has a diameter about two-thirds the size of Pluto. It was discovered in by Mike Brown and his team. So far, Makemake doesn't seem to have any moons. Eris is the most massive known dwarf planet, and the one that helped turn our definition of a planet upside-down.

It even has a moon: Dysnomia. Want an easy way to remember the eight planets, in order? Just remember this mnemonic: my very excellent mother just served us noodles. For all you currently writing angry tweets to Mike Brown, hold on a sec. Changing Pluto's categorization is an important step that really needed to happen.

The more we discover about our Universe, the more we realize just how strange and wonderful it is. When Pluto was discovered 80 years ago, we never could have expected the variety of objects in the Solar System. Categorizing Pluto as a dwarf planet helps us better describe our celestial home. So, our Solar System now has eight planets, and five dwarf planets.

Explore further. More from Astronomy and Astrophysics. Use this form if you have come across a typo, inaccuracy or would like to send an edit request for the content on this page. For general inquiries, please use our contact form. October 19, By: Monica Young September 28, Solar System. By: Jure Japelj September 21, Imaging Foundations with Richard Wright. September 16, August 16, July 15, Beyond the Printed Page. Groups Why Join? Astronomy Day.

The Complete Star Atlas. Astronomers estimate that there are billions of worlds beyond our solar system. Astronomers estimate that there is roughly one exoplanet per star in our galaxy. Of course, some stars have many planets — our own Sun has eight. And some stars have none.

But if a star lives long enough, forming planets seems to be the rule, rather than the exception. When it comes to exoplanets that have been measured or counted in some way, the numbers are much smaller. The running counter of known exoplanets — as of this writing — stands at 4, confirmed worlds.

Would you like to learn more about exoplanets and other solar systems? Check out our free downloadable eBook: Our search for extrasolar planets. This may be the first exoplanet found orbiting three stars. Astronomers identify a new class of habitable planet. Are there any planets outside of our solar system? TESS needs your help finding new worlds.