different math fonts in xelatex using unicode-math

Question

I have expect two different types of math fonts required in $ ... $. My MWE is:

\documentclass{book}
\usepackage{tabularx,booktabs}

\usepackage{unicode-math}
\defaultfontfeatures{Mapping=tex-text}
\setmainfont{Calibri}
\setmathfont{Calibri}


\begin{document}
\chapter{Title here}

From the Egyptian pyramids to modern architecture, points, lines and   angles
are everywhere. $a^\circ$ Geometry is a $\alpha + \beta$ very visual    element of mathematics where the
designs of buildings and the orbits of planets can be studied using basic
objects like points, lines and circles.

\begin{table}[htb!]
\centering
\begin{tabular}{p{2cm}|p{10cm}}
\toprule
\textbf{Variabili} & $\mathbf{Descrizione}$ \\
\midrule
$\alpha + \beta$ & $a+b$\\
\bottomrule
\end{tabular}
\end{table}

Thales (624--546~BCE) is known to be the founder of Greek geometry. He was an astronomer and $a^\circ$ philosopher, and $\alpha + \beta$ eclipse.
\end{document}