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The nature, size and orientation of the dominant structural components in theMilky Way's inner ~4 kpc - specifically the bulge and bar - have been thesubject of conflicting interpretations in the literature. We present adifferent approach to inferring the properties of the long bar which extendsbeyond the inner bulge, via the information encoded in the Galaxy's X/peanut(X/P)-shaped structure. We perform a quantitative analysis of the X/P featureseen in WISE wide-field imaging at 3.4 $\mu$m and 4.6 $\mu$m. We measure thedeviations of the isophotes from pure ellipses, and quantify the X/P structurevia the radial profile of the Fourier n=6 harmonic (cosine term $B_6$). Inaddition to the vertical height and integrated `strength' of the X/Pinstability, we report an intrinsic radius of $R_{\Pi;{\rm int}} = 1.67\pm0.27$kpc, and an orientation angle of $\alpha = {37^\circ}^{+7^\circ}_{-10^\circ}$with respect to our line-of-sight to the Galactic Centre. Based on X/Pstructures observed in other galaxies, we make three assumptions: (i) thepeanut is intrinsically symmetric, (ii) the peanut is aligned with the longGalactic bar, and (iii) their sizes are correlated. Thus the implication forthe Galactic bar is that it is oriented at the same $37^\circ$ angle and has anexpected radius of ~4.2 kpc, but possibly as low as ~3.2 kpc. We furtherinvestigate how the Milky Way's X/P structure compares with other analogues,and find that the Galaxy is broadly consistent with our recently establishedscaling relations, though with a moderately stronger peanut instability thanexpected. We additionally perform a photometric decomposition of the MilkyWay's major axis surface brightness profile, accounting for spiral structure,and determine an average disc scale length of $h = 2.54\pm0.16$ kpc in the WISEbands, in good agreement with the literature.

Quantifying the (X/peanut)-shaped Structure of the Milky Way - New Constraints on the Bar Geometry