Would this not be a passive construction?
Full context from Eine kurze Geschichte der Zeit by Stephen Hawking:
Im Falle des Merkur jedoch, der – von allen Planeten der Sonne am nächsten – der stärksten Gravitationswirkung unterworfen ist und eine entsprechend längliche Umlaufbahn hat, prognostiziert die allgemeine Relativitätstheorie, daß die lange Achse der Ellipse sich mit einer Geschwindigkeit von ungefähr einem Grad pro zehntausend Jahren um die Sonne drehen müßte.
I don't like the German translation and would choose instead:
der stärksten Gravitationswirkung unterliegt
Unterwerfen has a strong action connotation, which is not appropriate here, and which is also part of the reason, why unterworfen wird is not a good choice here.
Nobody does anything to expose Mercure to gravitation, it is simply a state (applicable since Mercure exists), why Zustandpassiv (and therefore unterworfen ist) is a convincing grammatical choice.
German has two different types of passive voice:
I show you with shorter sentences:
Der Planet ist der Schwerkraft unterworfen.
The planet is subject to gravity.
This is the description of a state (state = Zustand in German). Nothing is happening here. This is just the description of how it is.
Der Planet wird der Schwerkraft unterworfen.
The planet is subjected to gravity.
This is the description of an process (process = Vorgang). Here we are watching how something is changing. The planet was not subjected to gravity in the past, but it will be in the future, and now this change is going on.
