Mobile objects (or mobile agents) attract the attention of many researchers in many network-related research areas. Despite the broad attraction, the implementation schemes of mobile object systems are inclined to adopt programming-language- centric approach; that is, the functionality of mobile object computing is so tightly incorporated into the design of a brand-new programming language that users are obliged to use the single programming language. Besides the programming-language-centricity, there is another trend in most current mobile object systems: bytecode-interpretation- basis, whereby the state of objects and/or computation activity can be handled in a straightforward and neat way, the platform-heterogeneity can be easily dealt with, and security-handling is not so difficult in principle.We, however, can enumerate several reasons why the above-mentioned currently dominant approach is not sufficient. First, it goes without saying that a lot of software libraries written in conventional programming languages exist, and that in each application area there is a language that can be used willingly. If mobile object computing is a very special area and if it is sufficient that only a limited number of applications can deal with it, limiting programming languages is not so problematic. But we think that a few number of application areas are likely to enjoy the benefits of mobile object computing. For example, some people might think that their demonstration program written in their favorite programming languages could run as applets on the Internet. Second, the bytecode-interpretation-basis has to suffer inefficiency in execution performance. Just-in-time compiler technologies are improving their performance, but there are some issues that just-in-time compiler can hardly overcome. For example, Java treats string as objects based on its language design principle, thus all the accesses to the contents of a string require ordinary method-calls. On the other hand, however, some programming languages such as C or C++ can manipulate strings very efficiently, since such languages permit the user programs to directly access the contents of a string.We are designing and implementing the PLANET mobile object system that has quite a different approach [2]. PLANET is designed to be open, which means that the system is designed as a middleware between the operating system layer and the programming language system layer and is designed to be neutral to the designs of programming languages and bytecodes. Furthermore, it is designed to be used in an open network environment such as the Internet, whereby open we mean users and services of the network are not fixed but are independent of each other. The functions that PLANET provides include unloading memory segments which contains a mobile object from a virtual address space, transferring that memory segments to another computer site efficiently, loading it into a virtual address space there, and continuing its execution under rigorous but flexible protection control.