Perhaps we should consider a planet to be a body with a round shape and not big enough to start nuclear fusion in its core. An asteroid should be a body too small for its gravity to round it.
We could try to put radius measures to make a clasification. This is useful to delimit dust and molecules floating in space from asteroids.
But on big planets, it's harder to use radius measurements. We could find a very heavy planet, with a diameter roughly like Jupiter's, but with a much greater mass. This planet would ignite helium3 in its core and be considered a brown dwarf, a failed star. And there are dead stars like white dwarves, smaller than Jupiter. A neutron star is 10 miles in diameter and it isn't a star by definition, no fusion is taking place in its core.
In the last years astronomers discovered groups of free floating planets. They are bigger than Jupiter, but not big enough to start helium3 fusion, to be catalogued as brown dwarves. I'm sure a planet can be born around stars and then kicked out from a star system (most likely Sedna was kicked out on its current orbit by Jupiter or by other big planet) or can form in space, outside star systems. I don't think it is a bottom line for a cloud of dust or gas to collapse, if it's dense enough. A supernova explosion can form shock waves in a cloud, so it may be possible for a planet to form. Because of that, we should not consider a planet to necessarely orbit a star.
So, let's count the bodies in our solar system big enough to be round, and consider them as planets - no matter they are around the Sun or around other planets.
There are different types of planets: star planets (planets revolving around a star), moon planets (planets revolving around another planet, free planets (free floating planets in space).
There are some problems even with these definitions:
The moon planet definition: Our Moon floats around Earth. It's a system of two planets, one bigger than the other. But, as the Moon floats around Earth, it drags our planet in a smaller circle. Had the Moon been bigger, it would had dragged Earth around it. Sometimes in the near future we'll find two planets having roughly the same mass, orbiting around a star and around each other. They are star planets or moon planets?
The star planet definition: Where is our solar system boundary? We could consider than any planet or asteroid that floats around our star is a part of our solar system. Gravity is a force that bounds bodies that has mass, and weakens by distance. But it never dissapear. It might be not so obvious, but you and I are bound to our galactic black-hole, Andromeda Galaxy and quasars billions of light years distance from us. That's why, there could be bodies out there, one or two light years from our Sun, and gravitationaly bound to it. The nearest star system is Alpha Centaury, at 4.5 light years from us. But this system is in a very specific place in the sky. There are other nearby stars, like Sirius etc. Nevertheless, we could find a planet or asteroid perhaps at 10 light years from us, in other parts of the sky, that is more gravitationaly bound to our Sun than to other stars, because our Sun is its nearest star. This planet would make an orbit around our Sun in millions of years and eventually will be captured by another star that passes near-by. So, where is our solar system boundary? One definition marks this place at the point where stellar wind from other stars is stronger than our Sun's own wind. If this is the boundary, then Sedna surely passes out of the boundary at least 95% of its year which is 10.500 terrestrial years (it has a very eccentric orbit, from 6 billion miles to roughly 50 billions miles distance from our Sun )
The free float definition: Obviously, there are no free float bodies in our Galaxy. All the planets move around stars or around the galactic center which is packed with billions of stars and a huge black-hole.(at the core, there are some 10 millions stars around the black hole, in a s
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Perhaps we should consider a planet to be a body with a round shape and not big enough to start nuclear fusion in its core. An asteroid should be a body too small for its gravity to round it.
We could try to put radius measures to make a clasification. This is useful to delimit dust and molecules floating in space from asteroids.
But on big planets, it's harder to use radius measurements. We could find a very heavy planet, with a diameter roughly like Jupiter's, but with a much greater mass. This planet would ignite helium3 in its core and be considered a brown dwarf, a failed star. And there are dead stars like white dwarves, smaller than Jupiter. A neutron star is 10 miles in diameter and it isn't a star by definition, no fusion is taking place in its core.
In the last years astronomers discovered groups of free floating planets. They are bigger than Jupiter, but not big enough to start helium3 fusion, to be catalogued as brown dwarves. I'm sure a planet can be born around stars and then kicked out from a star system (most likely Sedna was kicked out on its current orbit by Jupiter or by other big planet) or can form in space, outside star systems. I don't think it is a bottom line for a cloud of dust or gas to collapse, if it's dense enough. A supernova explosion can form shock waves in a cloud, so it may be possible for a planet to form.
Because of that, we should not consider a planet to necessarely orbit a star.
So, let's count the bodies in our solar system big enough to be round, and consider them as planets - no matter they are around the Sun or around other planets.
There are different types of planets: star planets (planets revolving around a star), moon planets (planets revolving around another planet, free planets (free floating planets in space).
There are some problems even with these definitions:
The moon planet definition:
Our Moon floats around Earth. It's a system of two planets, one bigger than the other. But, as the Moon floats around Earth, it drags our planet in a smaller circle. Had the Moon been bigger, it would had dragged Earth around it. Sometimes in the near future we'll find two planets having roughly the same mass, orbiting around a star and around each other. They are star planets or moon planets?
The star planet definition:
Where is our solar system boundary? We could consider than any planet or asteroid that floats around our star is a part of our solar system. Gravity is a force that bounds bodies that has mass, and weakens by distance. But it never dissapear. It might be not so obvious, but you and I are bound to our galactic black-hole, Andromeda Galaxy and quasars billions of light years distance from us.
That's why, there could be bodies out there, one or two light years from our Sun, and gravitationaly bound to it. The nearest star system is Alpha Centaury, at 4.5 light years from us. But this system is in a very specific place in the sky. There are other nearby stars, like Sirius etc. Nevertheless, we could find a planet or asteroid perhaps at 10 light years from us, in other parts of the sky, that is more gravitationaly bound to our Sun than to other stars, because our Sun is its nearest star. This planet would make an orbit around our Sun in millions of years and eventually will be captured by another star that passes near-by. So, where is our solar system boundary? One definition marks this place at the point where stellar wind from other stars is stronger than our Sun's own wind. If this is the boundary, then Sedna surely passes out of the boundary at least 95% of its year which is 10.500 terrestrial years (it has a very eccentric orbit, from 6 billion miles to roughly 50 billions miles distance from our Sun )
The free float definition:
Obviously, there are no free float bodies in our Galaxy. All the planets move around stars or around the galactic center which is packed with billions of stars and a huge black-hole.(at the core, there are some 10 millions stars around the black hole, in a s