Could a star become a planet? I am asking this because the gas giants are ¨Failed Stars¨ and they are classified as planets in our solar system.

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    $\begingroup$ Your question in the title is "Could a star become a planet" and the question in the body is "Could a planet become a star". Which question to you mean to ask? $\endgroup$
    – James K
    Jan 31 at 19:22
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    $\begingroup$ @Joseph Casey Brown dwarfs are not classified as planets in our solar system becuae they aren't classified as planets and aren't in our solar system.. There are no known brown dwarfs in our solar system. Brown dwarfs ae not classified as planets or as stars. Brown dwarfs are over about 13 times the mass of jupiter and are massive enough to fuse deuterium for at least part of their life times. Objects more than about 75 to 80 times the mass of Jupiter are stars. $\endgroup$ Jan 31 at 21:23
  • $\begingroup$ @JamesK I meant in the title, sorry my mind has been crazy today $\endgroup$
    – user37892
    Jan 31 at 22:10
  • $\begingroup$ I am editing to revert @fasterthanlight's edit (there are no brown dwarfs in our solar system) and to make the question in the body be in line with the question raised in the title. $\endgroup$ Feb 1 at 10:10
  • $\begingroup$ Are black holes, neutron stars, and white dwarfs that orbit a currently fusing star planets? Once a brown dwarf has consumed all of its deuterium, does it become a planet? Per the IAU definition of a planet, there are only eight planets in the entire universe. One reason the IAU restricted it's definition of what qualifies as a planet to objects that orbit the Sun was to avoid the fuzzy boundaries between super-Jupiters and brown dwarfs, and between brown dwarfs and red dwarfs. Classifying black holes, neutron stars, and white dwarfs as planets seems ludicrous, but they aren't "stars". $\endgroup$ Feb 1 at 10:37

A star could become a planet, but this almost never happens. It would probably have to lose almost all of its mass early in its life, perhaps by a black hole. Then, the exposed core would have to be stopped from becoming degenerate and further mass loss would have to continue. It is possible, but it is not realistic and will not happen in a human lifetime, even if you observed every star in the universe (including white dwarfs).

A brown dwarf definitely can become a planet. A brown dwarf early in its life acts like a star, fusing deuterium and reaching temperatures of almost 2000 K. But as it ages it cools down and stops fusing deuterium. Then it acts like a planet.

In theory, a brown dwarf directly could become a planetary-mass object through mass loss. This could happen from a giant impact or another body accreting from it.

Contrarily, a planet (brown dwarf) can become a star if it accretes enough mass. For example, the most massive brown dwarfs have masses of about ~80 Jupiters, while the least massive stars have a mass of about ~60 Jupiters (there is an overlap). Therefore, a "planet" could become a star under special circumstances.

As @User123 said in a comment, all stars start as a "planet," without burning hydrogen. While it accretes more material, fusion ignites in its core and it becomes a star.

  • $\begingroup$ We could say, that even Sun started out as a planet: without burning hydrogen and helium. So the answer is pretty obvious. $\endgroup$
    – User123
    Jan 31 at 19:29
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    $\begingroup$ @User123 Thanks for the feedback :) However, isn't there a term called a "fusor" which is more general than a star? A fusor is any object capable of fusion but does not fuse. See en.wikipedia.org/wiki/Fusor_(astronomy) $\endgroup$ Jan 31 at 19:34
  • $\begingroup$ Actually at first there was only a small "rock", with another small colliding. So we can say that it started as a "planet" (in parentheses). By the way, thank you for the new word I have never heard. $\endgroup$
    – User123
    Jan 31 at 19:36
  • $\begingroup$ @fasterthanlight Your statement that the lowest mass star has a mass of about 90 Jupiters might be incorrect. This list dictionary.sensagent.com/List%20of%20least%20massive%20stars/… of the lowest mass stars includes many stars with masses lower than 90 Jupiters, someless massive than the most massive brown dwarfs on the list. The lowest mass star on the list is listed with a mass of only 52 Jupiters. If that is correct, there are stars with much less than 90 Jupiter's mass. $\endgroup$ Jan 31 at 21:30
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    $\begingroup$ This is a great answer, but since it has no supporting links whatsoever it's hard to judge the veracity of the information. It's best if we included sources for facts rather than hoping our memory is 100% correct, that way future readers will be able to better judge the accuracy and to read further. (at)MAGoldring has provided one such link already, consider citing it as one such source? Thanks! $\endgroup$
    – uhoh
    Jan 31 at 23:14

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