Can a PhD from a non-TU9 German university become a professor in a TU9 university?
Of course you can. Otherwise their pool for hiring would be awfully small.
But don't take my word for it. To provide some evidence we can look at a department at one of the TU9 universities. For example the Computer Science Department of TU Darmstadt (mainly because I'm somewhat familiar with it)
If we look at the list of full professors and eliminate those that have non-German doctorates, we are left with 18 Professors, 9 of which did not receive their degree from one of the TU9. (Though in practical terms almost none of them did, as the TU9 was only established in 2003.)
- Carsten Binning TU Darmstadt
Christian Bischof Cornell- Johannes Buchmann Universität zu Köln
Dieter W. Fellner TU Graz- Marc Fischlin Goethe-Universität Frankfurt am Main
Johannes Fürnkranz TU Wien- Iryna Gurevych Universität Duisburg-Essen
- Reiner Hähnle Universität Karlsruhe
- Matthias Hollick TU Darmstadt
Stefan Katzenbeisser TU Wien- Kristian Kersting Universität Freiburg
- Andreas Koch TU Braunschweig
- Heiko Mantel Universität des Saarlandes
- Mira Mezini Universität Siegen
- Max Mühlhäuser Universität Karlsruhe
Jan Peters University of Southern California- Christian Reuter Universität Siegen
Stefan Roth Brown University- Ahmad-Reza Sadeghi Universität des Saarlandes
- Thomas Schneider Ruhr-Universität Bochum
Neerja Suri University of Massechusetts, Amherst- Orkar von Stryk Technische Universität München
- Michael Waidner Universität Karlsruhe
- Karsten Weihe TU Berlin
- Felix Wolf RWTH Aachen
Yes, you can become professor at a TU9 university with a PhD from a non-TU9 university.
- There is no formal difference between PhDs from different universities in Germany.
- There is far from universal agreement in Germany that TU9 are the 'better' or 'elite' universities. Their main distinguishing factor is their focus on engineering subjects. The German 'Universities of Excellence' (Exzellenzunis) would be a better approximation of `German elite universities', but even for them the institution granting the PhD would not matter.
- In general the differences in reputation between universities in Germany is smaller than e.g. in the US or in France
- I know of several concrete cases of TU9 professors with non-TU9 PhDs.