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The Nine Planets is an encyclopedic overview with facts and information about mythology and current scientific knowledge of the planets, moons, and other objects in our solar system and beyond.

The 9 Planets in Our Solar System

The Five Dwarf Planets

Other Objects in The Solar System

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The Nine Planets of The Solar System | Eight Planets Without Pluto

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An overview of the history, mythology and current scientific knowledge of the planets, moons and other objects in our solar system.

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In real life, we can't pick up a planet and put it on a scale. However, scientists do have ways to figure out how much a planet weighs. They can calculate how hard the planet pulls on other things. The heavier the planet, the stronger it tugs on nearby objects-like moons or visiting spacecraft. That tug is what we call gravitational pull.

What does gravity have to do with weight?

When you stand on a scale, what it's actually doing is measuring how hard Earth's gravity is pulling on you.

If you were to step onto a scale on another planet, it would say something different than it does here. That's because the planets weigh different amounts, and therefore the force of gravity is different from planet to planet.

For example, if you weigh 100 pounds on Earth, you would weigh only 38 pounds on Mercury. That's because Mercury weighs less than Earth, and therefore its gravity would pull less on your body. If, on the other hand, you were on heavy Jupiter, you would weigh a whopping 253 pounds!

How do scientists use gravitational pull as a scale?

In order to figure out how heavy a planet is, scientists need to know two things: how long it takes nearby objects to orbit the planet and how far away those objects are from the planet. For example, the closer a moon is to its planet, the stronger the planet will tug on it. The time it takes an object (whether it's a moon or spacecraft) to orbit a planet depends both on its distance from the planet and how heavy the planet is.

Why do scientists usually talk about mass rather than weight?

An object's weight is dependent on its mass and how strongly gravity pulls on it. The strength of gravity depends on how far away one object is from another. That's why the same object weighs different amounts on different planets. It's sometimes easier to compare planets using a measurement that isn't quite so complicated. That's why scientists and engineers often measure an object's mass -how much matter the object contains-rather than its weight.

Mass stays the same regardless of location and gravity. You would have the same mass on Mars or Jupiter as you do here on Earth.

What is the mass of Earth?

We know that Earth has a mass of approximately 5,970,000,000,000,000,000,000,000 kilograms. That's a really big number!

How do you write a shorter version of a very big number? Exponents!

The Earth's mass is 5,970,000,000,000,000,000,000,000 kilograms. That is a lot of mass! Here is a shorter way of writing that big number: 5.97 x 10 ²⁴ kg. That little 24 is called an exponent. An exponent of a number is how many times to use that number as a multiplier. So, in other words:

5,970,000,000,000,000,000,000,000 is the same as...

5.97 x 10 x 10 x 10 x 10 x 10 x 10 x 10 x 10 x 10 x 10 x 10 x 10 x 10 x 10 x 10 x 10 x 10 x 10 x 10 x 10 x 10 x 10 x 10 x 10

which can be shortened to...

What is the mass of the other planets in our solar system?

The table below lists all the planets in our solar system in order from least massive to most massive. You can also find the mass of each planet in kilograms, and how the mass of each planet compares to that of Earth.

Planets (in order of least massive to most massive)

Mass
(in kilograms)

Each planet's mass relative to Earth

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In real life, we can't pick up a planet and put it on a scale. However, scientists do have ways to figure out how much a planet weighs. They can calculate how hard the planet pulls on other things. The heavier the planet, the stronger it tugs on nearby objects-like moons or visiting spacecraft.

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Asteroid: Rocks floating around in space. Some are the size of a pick-up truck. Others are hundreds of miles across.

Atmosphere: The gases held by gravity around Earth and around other planets. "Atmosphere" can also be used to talk about gases around stars.

Atom: The basic building block of matter. It is made of protons, neutrons, and electrons. There are many different kinds of atoms. For example, the simplest atom with one proton and one electron is a hydrogen atom. An atom with six protons, six neutrons, and six electrons is a carbon atom.

Aurora australis: Bright glows and bands of light that appear in the skies at night near the South Pole. They are also called the southern lights.

Aurora borealis: Bright glows and bands of light that appear in the skies at night near the North Pole. They are also called the northern lights.

Black hole: A place in space where matter and light cannot escape if they fall in.

Comet: An icy rock that lets off gas and dust, which may form tails when it is flying close to a sun.

Constellation: A group of stars in the sky. They're often named after an animal, object, or person. The stars form certain patterns based on where you are. We have one view of stars here on Earth, but from another solar system or galaxy, the constellations would look different.

Corona: The outer atmosphere of a star.

Cosmos: The universe seen as an orderly, harmonious whole.

Crater: A large, bowl-shaped dent in the ground. They can be caused by an explosion or the impact of a meteorite.

Dwarf planet: Objects that are round and orbit the sun, just like planets do. But unlike planets, dwarf planets are not able to clear their path around the sun. That means there are other objects orbiting at roughly the same distance from the sun. A dwarf planet is much smaller than a planet (smaller even than Earth's moon), but it is not a moon. Pluto is the best known of the dwarf planets.

El Niño: A weather condition that sometimes occurs in the Pacific Ocean. It is so big that it affects weather all over the world. It starts with unusually warm water in the Pacific Ocean near the equator.

Electromagnetic Spectrum: The name for all the different kinds of light and energy in the universe. This includes radio waves, microwaves, infrared radiation, visible light, ultraviolet light, X-rays, and gamma rays.

Equator: The circle around a planet or moon that is the same distance from its north pole and its south pole.

Exoplanet: A planet that freely floats between stars or one that orbits a star outside our solar system.

Galaxy: A collection of thousands to billions of stars held together by gravity. The galaxy we live in is called the Milky Way.

Gas: A loose collection of atoms moving around each other.

GPS: This stands for Global Positioning System. It's a system that uses satellites, ground stations, and receivers to tell you exactly where you are on Earth.

Gravity: A force that pulls matter together.

Greenhouse gas: Gases in the atmosphere that trap heat from the sun. Some greenhouse gases are carbon dioxide, methane, water vapor, and nitrous oxide.

Kuiper Belt: A donut-shaped ring of icy objects beyond the orbit of Neptune. Pluto is the best known of these icy worlds.

Light year: It's not a year, or an amount of time at all. It's the distance light travels in one year. It's the same as 5,878,499,810,000 miles (or 9,460,538,400,000,000 meters). When things are very far away, it's easier to talk about their distance in light years than millions or billions or trillions of miles.

Mass: The amount of matter something is made of.

Matter: The stuff that everything is made of. Atoms are a tiny bit of matter. Big planets have lots of matter. Even you are made of matter!

Meteor: The streak of light caused when a meteoroid enters a planet's atmosphere and starts to burn from the heat of friction.

Meteorite: A meteoroid that lands on the surface of a planet.

Molecule: The smallest unit of a substance that still acts like the main substance. A molecule can be a single atom or a group of atoms. Water is a substance, and one molecule of water is made of two hydrogen atoms and one oxygen atom, which we write as H2O. That molecule of water still has the same properties as a glass of water. But if you split it into hydrogen and oxygen, it's not water any more.

Moon: A natural object that travels around a bigger natural object. Planets can have moons. Dwarf planets can have moons. Even some asteroids have moons! Astronomers usually call them satellites or natural satellites.

Nebula: A cloud of dust or gas found between stars.

Neutron star: A very dense star made mostly of neutrons. It has very powerful gravitational force nearby because the whole mass of a star is pulled into one object just a few miles across.

Oort Cloud: A spherical shell around our solar system. It may contain more than a trillion icy bodies. Long-period comets (which take more than 200 years to orbit the sun) come from the Oort Cloud.

Orbit: The curved path that a planet, satellite, or spacecraft moves as it circles around another object.

Ozone layer: A part of Earth's atmosphere that absorbs lots of the sun's ultraviolet radiation. It is made of a gas called ozone, which is a molecule of three oxygen atoms.

Particle: A tiny amount or small piece of something.

Planet: A large body in outer space that circles around the sun or another star.

Pulsar: An object, thought to be a rapidly rotating neutron star. It releases short pulses of radio waves and other electromagnetic radiation.

Radioactive: How we describe some atoms that are unstable. They change into different kinds of atoms and release lots of energy.

Satellite: An object that orbits another object. A moon is actually a satellite. We also say satellite to refer to spacecraft people build that orbit Earth, other planets, moons, asteroids, or other objects out in space.

Solar flare: A burst of energy and particles from the sun. It releases gases, radiation waves, and magnetic storms.

Solar Panel: A piece of material that can capture sunlight and turn it into electricity.

Solar system: A set that includes a star and all of the matter that orbits it, including planets, moons, asteroids, comets, and other objects.

Solar wind: The constant stream of particles and energy emitted by the sun.

Space weather: The conditions in space that can affect Earth, satellites, and space travel. Space weather is mostly the result of solar wind and solar storms.

Spacecraft: A vehicle used for traveling in space.

Speed of Light: Light is the fastest thing in the universe. It travels 186,282 miles (299,792,458 meters) every second.

Star: A ball of shining gas, made mostly of hydrogen and helium, held together by its own gravity. Turning hydrogen into helium creates the energy that makes stars shine.

Sun: The star in the center of our solar system.

Supernova: The explosion of a star that makes it as bright as a whole galaxy.

Tectonics: Big movements of Earth's crust. Tectonics creates mountain ranges, deep sea trenches, volcanoes, and earthquakes that can cause destructive tsunami waves that can cross oceans.

Universe: All of space and time, and everything in it. It's everything ever!

Volcano: A mountain or hill that has an opening where lava, rock fragments, or gas erupt from deep inside a planet or moon.

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Asteroid: Rocks floating around in space. Some are the size of a pick-up truck. Others are hundreds of miles across. Atmosphere: The gases held by gravity around Earth and around other planets. "Atmosphere" can also be used to talk about gases around stars. Atom: The basic building block of matter.

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Stellarium Web is a planetarium running in your web browser. It shows a realistic star map, just like what you see with the naked eye, binoculars or a telescope.

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The first U.S. asteroid sample, delivered by the OSIRIS-REx spacecraft to Earth on Sept. 24, has arrived at its permanent home at NASA's Johnson Space Center in Houston, where it will be cared for, stored, and distributed to scientists worldwide.

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OSIRIS-REx, the first U.S. mission to collect an asteroid sample in 2020, delivered the sample to Earth on Sept. 24, 2023.

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NASA’s Solar System Interactive (also known as the Orrery) is a live look at the solar system, its planets, moons, comets, and asteroids, as well as the real-time locations of dozens of NASA missions.