After hearing that it has been confirmed that we found seven similar exoplanets like earth. I thought exoplanets were dead stars. After doing a little bit of research I found out that exoplanets are planets that are orbiting a star, NOT the sun. But isn't a star like the Sun? Can someone please explain this to me?

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    $\begingroup$ ANOTHER star than the Sun. It's as simple as that. "Sun" is just the name of one star. $\endgroup$
    – LocalFluff
    Feb 23, 2017 at 11:58
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    $\begingroup$ "Planets" are in our solar system. "Exoplanets" are in other star systems (or possibly associated with no star system at all). $\endgroup$
    – ProfRob
    Feb 23, 2017 at 12:36
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    $\begingroup$ The 2018 meeting of the IAU may see two competing redefinitions of what constitutes a planet. One will remove the concept of roundishness as being completely superfluous and will change the qualification of orbiting the Sun to orbiting a star. This will make all of the exoplanets discovered to date "planets". The other will focus solely on the concept of roundishness. This alternative will preclude calling those exoplanets "planets" (we can't observe how round they are, at least not yet) but will add hundreds of objects to the list of planets in the solar system. $\endgroup$ Feb 25, 2017 at 14:50

1 Answer 1


Not all planets are exoplanets, but all exoplanets are planets.

When we differentiate the two, we use "planets" to refer to the eight planets revolving around our Sun. The Greek prefix "exo" (outside) should indicate that exoplanets orbit other stars.

In reality, you can call exoplanets "planets", and you'll often hear "Astronomers have found x planets around star y". The word "exoplanet" is used interchangeably with planets from other planetary systems.

  • $\begingroup$ A better way to put this: Exoplanets are planet-like objects that orbit some star other rather than the Sun.The current definition of "planet" means that there are eight planets in the entire universe. The qualification that planets orbit the Sun is one of the two parts of the definition of "planet" that needs to be modified or removed. This needs to be modified. The part that needs to go, much to the consternation of Alan Stern, is the concept of roundishness. An object that orbits a star, that hasn't undergone fusion, and that has "cleared its neighborhood" should be deemed to be a planet. $\endgroup$ Feb 25, 2017 at 14:33
  • $\begingroup$ @DavidHammen I guess, if we want to be pedantic. In practice people rarely acknowledge that difference and use exoplanets and planets interchangeably for one's outside the Solar System. $\endgroup$ Feb 28, 2017 at 13:35

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