I have a question where I have to calculate the orbital inclination of a planet, so that it isn't a exoplanet but a stellar component. The mass of the star it is orbiting is 1.1 $M_☉$. Does the exoplanet has a special mass if it is a stellar component (for example 0.5 $M_☉$) or is there another relation?

The question was made out of a few sub-questions. It is about 51 Peg b. In the first question, I had to calculate the half long ax (=0.05284 AU), the eccentricity (=0.013) and the $mass*sin(i)$ (=0.500 $M_J$) In the second question, I had to calculate the inclination at which the mass of the planet is 4 $M_J$ (=7.18°) and the inclination at which it wasn't a planet, but a stellar component.

  • $\begingroup$ So you're asking what happens if instead of an exoplanet orbiting a star, it is another star that is orbiting? Orbital inclination relative to what? The line of sight? $\endgroup$
    – usernumber
    Mar 7, 2020 at 10:55
  • $\begingroup$ Highly confused/confusing. Needs much clarification. $\endgroup$
    – ProfRob
    Mar 7, 2020 at 11:19
  • $\begingroup$ The Q seems to involve radial velocity based minimum mass and substellar objects. $\endgroup$
    – Mike G
    Mar 7, 2020 at 12:31
  • $\begingroup$ @MikeG Yes, There is a radial velocity graph given. The graph is the same as in this site $\endgroup$
    – BOB
    Mar 7, 2020 at 12:58
  • 1
    $\begingroup$ There are a couple of potentially interesting questions in here: where the dividing line between stars and substellar objects is (as far as I can tell there isn't an existing question there), and whether the approximation that leads to writing the minimum mass as $m \sin i$ remains valid in that mass range. Which bit in particular are you interested in? $\endgroup$
    – user24157
    Mar 7, 2020 at 13:39

1 Answer 1


The term "stellar component" refers to a part ("component") of a system that is in the form of one or more stars.

For example, the Alpha Centauri system has three stellar components: Rigil Kentaurus, Toliman and Proxima Centauri. The planet Proxima Centauri b is also a component of the Alpha Centauri system but it is not a stellar component because it isn't a star.

Another example: the stellar component of a galaxy refers to the fraction of the galaxy that is in the form of stars, as opposed to dust, gas, dark matter, etc.

In the case of your question, it is asking what the inclination would be if the companion were a star (i.e. a stellar component of a binary star system).


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