In mathabx.dcl
we find
\DeclareMathSymbol{\Sun} {0}{mathb}{"40}
\DeclareMathSymbol{\Mercury} {0}{mathb}{"41}
\DeclareMathSymbol{\Venus} {0}{mathb}{"42}
\mathabx@aliases\Venus\girl
\DeclareMathSymbol{\Earth} {0}{mathb}{"43}
\DeclareMathSymbol{\Mars} {0}{mathb}{"44}
\mathabx@aliases\Mars\boy
\DeclareMathSymbol{\Jupiter} {0}{mathb}{"45}
\DeclareMathSymbol{\Saturn} {0}{mathb}{"46}
\DeclareMathSymbol{\Uranus} {0}{mathb}{"47}
\DeclareMathSymbol{\Neptune} {0}{mathb}{"48}
\DeclareMathSymbol{\Pluto} {0}{mathb}{"49}
\DeclareMathSymbol{\varEarth} {0}{mathb}{"4A}
\DeclareMathSymbol{\leftmoon} {0}{mathb}{"4B}
\mathabx@aliases\leftmoon\Moon
\DeclareMathSymbol{\rightmoon} {0}{mathb}{"4C}
\DeclareMathSymbol{\fullmoon} {0}{mathb}{"4D}
\DeclareMathSymbol{\newmoon} {0}{mathb}{"4E}
OK, now we want to see what mathb
refers to, which we find in mathabx.sty
\DeclareFontFamily{U}{mathb}{\hyphenchar\font45}
\DeclareFontShape{U}{mathb}{m}{n}{
<5> <6> <7> <8> <9> <10> gen * mathb
<10.95> mathb10 <12> <14.4> <17.28> <20.74> <24.88> mathb12
}{}
\DeclareSymbolFont{mathb}{U}{mathb}{m}{n}
\DeclareFontSubstitution{U}{mathb}{m}{n}
Now it's just a matter of refactoring to just use the font as a text one.
\documentclass{article}
\DeclareFontFamily{U}{mathb}{}
\DeclareFontSubstitution{U}{mathb}{m}{n}
\DeclareFontShape{U}{mathb}{m}{n}{
<-5.5> mathb5
<5.5-6.5> mathb6
<6.5-7.5> mathb7
<7.5-8.5> mathb8
<8.5-9.5> mathb9
<9.5-11> mathb10
<11-> mathb12
}{}
\newcommand{\mathbsymbol}[1]{{\usefont{U}{mathb}{m}{n}\symbol{#1}}}
\DeclareRobustCommand{\Sun}{\mathbsymbol{"40}}
\DeclareRobustCommand{\Mercury}{\mathbsymbol{"41}}
\DeclareRobustCommand{\Venus}{\mathbsymbol{"42}}
\DeclareRobustCommand{\Earth}{\mathbsymbol{"43}}
\DeclareRobustCommand{\Mars}{\mathbsymbol{"44}}
\DeclareRobustCommand{\Jupiter}{\mathbsymbol{"45}}
\DeclareRobustCommand{\Saturn}{\mathbsymbol{"46}}
\DeclareRobustCommand{\Uranus}{\mathbsymbol{"47}}
\DeclareRobustCommand{\Neptune}{\mathbsymbol{"48}}
\DeclareRobustCommand{\Pluto}{\mathbsymbol{"49}}
\DeclareRobustCommand{\varEarth}{\mathbsymbol{"4A}}
\DeclareRobustCommand{\leftmoon}{\mathbsymbol{"4B}}
\NewCommandCopy{\Moon}{\leftmoon}
\DeclareRobustCommand{\rightmoon}{\mathbsymbol{"4C}}
\DeclareRobustCommand{\fullmoon}{\mathbsymbol{"4D}}
\DeclareRobustCommand{\newmoon}{\mathbsymbol{"4E}}
\begin{document}
\Sun\Mercury\Venus\Earth\Mars\Jupiter\Saturn\Uranus\Neptune\Pluto\Moon
\leftmoon\rightmoon\fullmoon\newmoon
\end{document}
