The last four lines,its longer, more traditionally structured lines, affects a type of objectivity. From the objective standpoint, the star is nothing special, especially next to the planet Saturn. However, even in the midst of expressing himself objectively, the speaker takes a moment to insist he continues to pine for his star.
Then it stops like a bird; like a flower, hangs furled:
They must solace themselves with the Saturn above it.
What matter to me if their star is a world?
Mine has opened its soul to me; therefore I love it.
In the last line, In fact, the single use of the word "love" in the very last line is one of several factors that suggest the poem's primary theme is not love.