As of assuming two very close planets to be one
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We don’t really know it precisely.
We do know the shapes of other galaxies and have figured which type of galaxy ours is and what it likely looks like.
We have some idea about spiral arms and bar in the center, but it is really difficult to get a precise idea what exactly it would look like from outside, when we can only see it from the inside a large parts of it are behind other parts.
Planets are not really involved in this.
We can determine the position of stars in the sky and estimate their distance for most stars to some extend and extrapolate from that.
From listening to the radio emissions of neutral hydrogen gas (HI), we can put a map of the gas on the sky.
But, when we investigate the change in frequency of that emission because of the Doppler shift effect, we can get a cross section of relative speed and relative intensity for each of those points.
We can also tell the polarisation of the radio emission as well.
Combining all of these with a bit of lateral thinking, we can get a fairly good model for the overall shape of things.
Then, comparing to the same investigations of other galaxies we can see which of those match closely to our model, and we can get a fairly good educated guess on how accurate our model actually is.
By counting the really faint stars in the sky we can get a duplication of our position with a statistical analysis, that we can get another match with the statistical distribution of globular clusters in the galaxy.
Adding all of these up and comparing, that’s how we see where we are and what the approximate shape appears to be.
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