\usepackage{marvosym} allows you to write M_\Sun or r_\Earth to have variables annotated with celestial body symbols. It would be surely nice to have in the mark-up.
The list of planet symbols included is as follows:
\Sun \Moon \Mercury \Venus \Mars \Jupiter \Saturn \Uranus \Neptune \Pluto \Earth
Would it be much trouble to include this package in our set, so that this would work?:
$M_\Sun = 1.989e30 kg$
Agreed, I believe we could use that. It shouldn't be too much trouble, we already have some MathJax extensions enabled, like mhchem (load it with \require{mhchem} in the first MathJax line, then invoke each time you need it with \ce{} within which you can write empirical formulas and some other chemical notations).
In the meantime, we can use those Unicode characters that are built into HTML and universally supported in browsers, and built-in MathJax symbols:
☉): $☉$☿): $☿$♀): $♀$⊕, or \oplus): $\oplus$☾): $☾$♂): $♂$♃): $♃$♄): $♄$♅ is supported): $♅$♆): $♆$♇): $♇$Some other symbols of interest:
☽): $☽$☄): $☄$☆ or ★ or \star): $☆★\star$☊): $☊$☋): $☋$☌): $☌$☍): $☍$♈): $♈$♉): $♉$♊): $♊$♋): $♋$♌): $♌$♍): $♍$♎): $♎$♏): $♏$♐): $♐$♑): $♑$♒): $♒$♓): $♓$All these should display correctly in all browsers, but please do let me know if that isn't the case. But yes, agreed, this would have been a whole lot simpler if we could type in names of celestial bodies instead of copying them from somewhere.
Always handy: MathJax basic tutorial and quick reference
$M_♃$ produces $M_♃$.
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\text{}around the symbol name, and adding a package name? I don't use LaTeX outside of the MathJax on the site, so the answer in the link confuses me... $\endgroup$