Tom Thumb is a star in the constellation of the Big Dipper. Specifically, Alcor, a barely-visible star right in the middle of the dipper’s handle, piggybacking on a star called Mizar.
According to Le Petit Poucet et la Grande Ourse, or “Tom Thumb and the Great Bear,” by Gaston Paris, there's a little-heard-of tradition in Wallonia regarding this constellation. Here, its name is “Chaûr-Pôcè."
The idea is that the four stars on the right are the wheels of a cart, and the three in line on the left are horses. Sitting on the horse in the middle is tiny Alcor. This is the driver, Pôcè – or Poucet, i.e. Tom Thumb. (This is sourced from Grandgagnage’s etymological dictionary.) Apparently the tradition is that Poucet is hanging beneath the belly of the second horse, trying to reattach the harness.
I’ve seen other sources refer to this constellation as a wain or wagon, and there was an Arabic tradition of calling Alcor and neighboring star Mizar the horse and rider, but this is the only time I’ve heard Tom Thumb in astronomy.
Paris is basing all this in theories that all myths are connected and furthermore they all have astronomical ties. He links it further, by a few leaps, to the myth of baby Hermes stealing the oxen. This seems a bit much. (Like Tom Thumb being Tam Lin.)
However, many variants of the Thumbling folktale do include him riding horses and donkeys (usually perched in an ear) or driving a plow pulled by horses or oxen. This, coupled with how hard it is to see Alcor, makes the name seem quite appropriate.
This website is based on my research into folklore.