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Stargazers in ancient Greece noticed something extraordinary in the night sky. They observed that a few bright objects moved among the constellations. They called these objects planetes, meaning “wanderers.”
We still call them planets. But our understanding of these celestial bodies has grown since ancient times. Now we know that planets just look like they are wandering through the night sky. They are actually orbiting the sun.
But what exactly is a planet? Scientists are still debating that question. Recently, the International Astronomical Union (IAU) determined that a planet must meet three conditions. It must be spherical and orbit the sun. Also, its orbit must be free of other planets and objects.
Scientists have identified eight planets in our solar system. But there are many other planets farther out in space. So far, experts have identified about 900 planets beyond our solar system. They estimate there are trillions more.
How Were Planets Formed?Experts believe that the universe formed after a massive explosion. It’s called the Big Bang. They say the explosion occurred some 13.7 billion years ago. It released huge amounts of dust and gas. Energy from the blast cooked the dust and gas together. Bits of dust formed together into clumps. The clumps grew larger over time. Gravity held them together. Meanwhile, one clump of gas began to produce its own energy. Over time, it became our sun.
The remaining dust and gas began to swirl around the new sun. Eventually, this dust and gas became the planets of our solar system. A solar system is made up of a star and the planets that orbit it.
Our solar system has four inner planets. Mercury, Venus, Earth, and Mars are closest to the sun. They are called terrestrial planets and are largely made of rock and metal. The four outer planets are Jupiter, Saturn, Uranus, and Neptune. They are called gas giants. They formed in an extremely cold area of the solar system.
The outer planets are made up of gases that swirl around in cold, windy storms. The outer planets also have ring systems. Saturn, for example, has an enormous ring. It can hold a billion Earths. How do rings form? When small moons and comets get too close to a gas giant, gravity rips them apart. The bits of rock and ice then begin orbiting the planet. Eventually, they form rings.
Earth is the only planet in our solar system that is known to support life. It is just the right distance from the sun to have water and oxygen. The planet is also protected by an atmosphere. It is made up of layers of gases. The atmosphere protects us from being harmed by the sun’s rays.
Redefining PlanetsPluto is located in the distant reaches of our solar system. It was once considered a planet. But astronomers began to question this. Why? They discovered similar objects nearby, in a region called the Kuiper Belt. They decided that if those similar objects were too small to be considered planets, Pluto should not be considered a planet either. In 2006, the IAU removed Pluto’s planetary status. It was reclassified as a dwarf planet.
Over the past 50 years, experts have also discovered that stars other than the sun have planets orbiting them. These are called exoplanets. Some of them lie within the “habitable zone.” That is the region around a star in which a planet could support life. As scientists gain deeper understanding of the universe, the debate over how to define a planet is sure to continue.
We still call them planets. But our understanding of these celestial bodies has grown since ancient times. Now we know that planets just look like they are wandering through the night sky. They are actually orbiting the sun.
But what exactly is a planet? Scientists are still debating that question. Recently, the International Astronomical Union (IAU) determined that a planet must meet three conditions. It must be spherical and orbit the sun. Also, its orbit must be free of other planets and objects.
Scientists have identified eight planets in our solar system. But there are many other planets farther out in space. So far, experts have identified about 900 planets beyond our solar system. They estimate there are trillions more.
How Were Planets Formed?Experts believe that the universe formed after a massive explosion. It’s called the Big Bang. They say the explosion occurred some 13.7 billion years ago. It released huge amounts of dust and gas. Energy from the blast cooked the dust and gas together. Bits of dust formed together into clumps. The clumps grew larger over time. Gravity held them together. Meanwhile, one clump of gas began to produce its own energy. Over time, it became our sun.
The remaining dust and gas began to swirl around the new sun. Eventually, this dust and gas became the planets of our solar system. A solar system is made up of a star and the planets that orbit it.
Our solar system has four inner planets. Mercury, Venus, Earth, and Mars are closest to the sun. They are called terrestrial planets and are largely made of rock and metal. The four outer planets are Jupiter, Saturn, Uranus, and Neptune. They are called gas giants. They formed in an extremely cold area of the solar system.
The outer planets are made up of gases that swirl around in cold, windy storms. The outer planets also have ring systems. Saturn, for example, has an enormous ring. It can hold a billion Earths. How do rings form? When small moons and comets get too close to a gas giant, gravity rips them apart. The bits of rock and ice then begin orbiting the planet. Eventually, they form rings.
Earth is the only planet in our solar system that is known to support life. It is just the right distance from the sun to have water and oxygen. The planet is also protected by an atmosphere. It is made up of layers of gases. The atmosphere protects us from being harmed by the sun’s rays.
Redefining PlanetsPluto is located in the distant reaches of our solar system. It was once considered a planet. But astronomers began to question this. Why? They discovered similar objects nearby, in a region called the Kuiper Belt. They decided that if those similar objects were too small to be considered planets, Pluto should not be considered a planet either. In 2006, the IAU removed Pluto’s planetary status. It was reclassified as a dwarf planet.
Over the past 50 years, experts have also discovered that stars other than the sun have planets orbiting them. These are called exoplanets. Some of them lie within the “habitable zone.” That is the region around a star in which a planet could support life. As scientists gain deeper understanding of the universe, the debate over how to define a planet is sure to continue.
Spaceplace.nasa:
What is a planet?
A planet must do three things: it must orbit a star, it must be big enough to have enough gravity to force a spherical shape, and it must be big enough that its gravity cleared away any objects of a similar size near its orbit.
The planets in our solar system didn’t appear out of nowhere. Neither did the sun. They were all part of a big cloud of gas and dust. Gravity collected lots of material in the centre to create the sun. The left over stuff swirled around the forming sun, colliding and collecting together. Some would have enough gravity to attract even more gas and dust, eventually forming planets. Scientists spent a lot of time arguing over what a planet actually is. In 2006, they came up with a definition. They said a planet must do three things. The first thing might seem obvious—it has to orbit around the sun. Second, it must be big enough to have enough gravity to force it into a spherical shape . And third, it must be big enough that its gravity cleared away any other objects of a similar size near its orbit around the Sun.
What about planets in other places?This definition is very much focused on our own solar system. But there are also planets in places that are not our solar system. These planets are called exoplanets. They can be found circling around stars, just like the planets here in our own solar system. Does that mean that all planets form the same way? Are all planets made from a star’s leftovers?
That depends on who you talk to. What happens if a small cloud of gas floating out in the middle of nowhere forms a sphere because it has gravity? Is that a planet, too? After all, Jupiter is a big sphere of gas, and both are just a mass of stuff that wasn’t quite big enough to form a bright, fiery star.
Big planet or tiny star? Clouds of gas that don’t have enough material to form a bright star collect into spheres all the time. Most of the time these clouds form a type of star called a brown dwarf. They are pretty big compared to most planets, but they are not big enough to turn into the kind of star that makes lots of energy and gives off light.
But scientists recently discovered an even smaller gassy object in the middle of nowhere (read more about it here). It appears redder than most brown dwarfs, and is likely much younger than most, too. This object could have formed just like a brown dwarf—from a small cloud of gas. Or maybe it was created around a star and it somehow got flung off into space. Some scientists are calling this object a planet. Others think that it can only be a planet if it formed around a star. They think that if it just formed from a cloud of gas, then it’s nothing more than a not-quite-star. Science is full of arguments like this. That’s what makes it so interesting. What do you think? Do all planets, even exoplanets, need to form around stars?
Watch this video:
A planet must do three things: it must orbit a star, it must be big enough to have enough gravity to force a spherical shape, and it must be big enough that its gravity cleared away any objects of a similar size near its orbit.
The planets in our solar system didn’t appear out of nowhere. Neither did the sun. They were all part of a big cloud of gas and dust. Gravity collected lots of material in the centre to create the sun. The left over stuff swirled around the forming sun, colliding and collecting together. Some would have enough gravity to attract even more gas and dust, eventually forming planets. Scientists spent a lot of time arguing over what a planet actually is. In 2006, they came up with a definition. They said a planet must do three things. The first thing might seem obvious—it has to orbit around the sun. Second, it must be big enough to have enough gravity to force it into a spherical shape . And third, it must be big enough that its gravity cleared away any other objects of a similar size near its orbit around the Sun.
What about planets in other places?This definition is very much focused on our own solar system. But there are also planets in places that are not our solar system. These planets are called exoplanets. They can be found circling around stars, just like the planets here in our own solar system. Does that mean that all planets form the same way? Are all planets made from a star’s leftovers?
That depends on who you talk to. What happens if a small cloud of gas floating out in the middle of nowhere forms a sphere because it has gravity? Is that a planet, too? After all, Jupiter is a big sphere of gas, and both are just a mass of stuff that wasn’t quite big enough to form a bright, fiery star.
Big planet or tiny star? Clouds of gas that don’t have enough material to form a bright star collect into spheres all the time. Most of the time these clouds form a type of star called a brown dwarf. They are pretty big compared to most planets, but they are not big enough to turn into the kind of star that makes lots of energy and gives off light.
But scientists recently discovered an even smaller gassy object in the middle of nowhere (read more about it here). It appears redder than most brown dwarfs, and is likely much younger than most, too. This object could have formed just like a brown dwarf—from a small cloud of gas. Or maybe it was created around a star and it somehow got flung off into space. Some scientists are calling this object a planet. Others think that it can only be a planet if it formed around a star. They think that if it just formed from a cloud of gas, then it’s nothing more than a not-quite-star. Science is full of arguments like this. That’s what makes it so interesting. What do you think? Do all planets, even exoplanets, need to form around stars?
Watch this video: