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They're not. There's no real scientific consensus, but most scientists agree that given the immensity of the universe, it's almost arrogant to assume we are the only denizens inside of it.
If you're still interested, look up the Drake Equation on Wikipedia, or watch this video of Carl Sagan explaining it: [See related link]
What else can I help you with?
Why are other planets there in the solar system?
Other planets other than earth is put in the solar system for humans to explore and do researches on them.
Can any other planets have living things?
It is highly unlikely that any other planet in our solar system will. That does not preclude the possibility of life being elsewhere in the universe.
Are the planets after Pluto in a different solar system?
No, the planets after Pluto are still within our solar system. After Pluto, there is Eris, Haumea, Makemake, and several other dwarf planets and minor planets that are part of our solar system. Beyond these, there is the Kuiper Belt and the Oort Cloud which are also part of our solar system.
What are the planets out of your solar system?
Planets outside our solar system are called exoplanets. Thousands of exoplanets have been discovered orbiting other stars in the Milky Way galaxy. They come in a variety of sizes and compositions, and many are quite different from the planets in our own solar system.
Can astronauts travel to distant planets in the solar system?
They could, but as of now, no astronauts have travelled to any other planets in the solar system.
How money planets are there in your solar system?
There are eight planets and five dwarf planets in our solar system.
Why are other planets there in the solar system?
Other planets other than earth is put in the solar system for humans to explore and do researches on them.
Is it possible that other planets exist within your solar system?
It is very unlikely there are unknown planets in our system,, although beyond the kuiper belt, at the very far edge of our solar system some believe there may be another planet. It is referred to as "Planet X".
Can any other planets have living things?
It is highly unlikely that any other planet in our solar system will. That does not preclude the possibility of life being elsewhere in the universe.
What planet have aliens?
ALL OF THeM!! But seriously: We know of no planet except Earth with life on it in our solar system, and the conditions on other planets forbid advanced life. There are planets belonging to other suns, but they are very far away, and we have no information about life on them. It is extremely unlikely, but not impossible, that aliens from other solar systems have visited our solar system.
Are the planets after Pluto in a different solar system?
No, the planets after Pluto are still within our solar system. After Pluto, there is Eris, Haumea, Makemake, and several other dwarf planets and minor planets that are part of our solar system. Beyond these, there is the Kuiper Belt and the Oort Cloud which are also part of our solar system.
What are the planets out of your solar system?
Planets outside our solar system are called exoplanets. Thousands of exoplanets have been discovered orbiting other stars in the Milky Way galaxy. They come in a variety of sizes and compositions, and many are quite different from the planets in our own solar system.
Can astronauts travel to distant planets in the solar system?
They could, but as of now, no astronauts have travelled to any other planets in the solar system.
Atom and solar system equal?
The atom and the solar system are not equal. The solar system contains the sun and the other planets.
What are other important features of your solar system?
planets
Is venus and other planets in our solar system are luminous?
No.
Is it true that there is other planets out of your solar system?
Yeah
