Given the information we have seen in Monster's Inc. and Monster's University, we cannot make any assumptions about whether the "Door bridging technology" would be capable of reaching distant planets (assuming they had doors or suitable portals to connect to).
- In and of itself, the Door bridging technology appears to be highly advanced (by Human standards) but we simply don't know enough about HOW it works to determine its range, efficacy or limits in target-choosing capacity to know if interplanetary or interstellar ranges are within its capacity.
Since it is implied that Monstropolis IS on Earth we must first establish the parameters for interactions between the two groups:
At some point in the past, Monsters were forced to seek refuge away from Humanity and have hidden themselves on an island Humans are not able to find. Humanity has come to accept Monsters are a myth and as such remain unaware of them in the modern day. Why the two factions separated or how monsters came to exist in the first place is never discussed.
What is known (at least by all Monsters) is that Monsters and Humans never mix and it would seem today Monsters both fear discovery by Humans and have created a mythology to ensure Monsters never desire to freely mix with Humans, i.e. the myth of human children (and by proxy, Humans in general) are toxic and dangerous to Monsterdom.
Given that what we have seen of the Human world appears to mimic our own, Monstropolis either employs sophisticated stealth technology capable of hiding it from Human technology like spy satellites, or it exists in such a way it is only capable of being reached using the Door technology.
Why did the Monsters create their unusual technologies in the first place? Technology development is an answer to a problem in a society. Why did Monsters create a technology to harvest screams (fear) to power their society? Was the technology or ability to harvest fear unique to Monsters?
The ability to harvest and convert "Scream" and later "Laughter" as viable energy sources indicate the Monsters have a level of technology far beyond Human norms. Neither has energy potential value in the Human World.
We are forced to make assumptions about the door bridging technology based on its specific and highly regulated use in Monstropolis.
The Door technology is only used to reach the Human World. It isn't used to reduce travel time within Monstropolis. There is mass transit including busses, trains and personal cars. This indicates either the technology is too energy intensive for local work, requires sophisticated targeting, or is absolutely only able to reach target doors in the Human World.
This raises a few other questions such as: Why are humans unable to find the Monster's World? Such advanced technology surely has, if nothing else, a heat and/or light emission capable of being seen from orbit. So how does Monstropolis stay hidden from Human's curious nature?
What do we know of the Door Bridging Technology? Turns out, not much:
The Monster technology that is never explained in detail is:
the targeting system (how are doors with children of the right age found?) How is the signature for the Bridging Door, linked to the Child's Door?
the bridging system, is the technology in which the Door is placed to open a link between the two worlds. We must presume the creation of the link uses less power than it is able to recover, because if it didn't, they would use something else instead.
We are never taught or told what a unit of Scream or Laughter is equal to in the Human World in terms of horsepower or ability to do work. (Insert your preferred unit of energy here) So we don't know if the Monsters are sitting on an energy gold mine or just gathering enough energy to keep themselves and their society hidden from Humanity.