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