The planetary nervous system is a large-scale distributed research platform that provides real-time social sensing services as a public good. Existing Big Data systems threaten social cohesion as they are designed to be closed, proprietary, privacy-intrusive and discriminatory. In contrast, the Planetary Nervous System is an open, privacy-preserving and participatory platform designed to be collectively built by citizens and for citizens. The planetary nervous system is enabled by Internet of Things technologies and aims at seamlessly interconnecting a large number of different pervasive devices, e.g. mobile phones, smart sensors, etc. For this purpose, several universal state-of-the-art protocols and communication means are introduced. A novel social sensing paradigm shift is engineered: Users are provided with freedom and incentives to share, collect and, at the same time, protect data of their digital environment in real-time. In this way, social sensing turns into a knowledge extraction service of public good. The social sensing services of the planetary nervous system can be publicly used for building novel innovative applications. Whether you would like to detect an earthquake, perform a secure evacuation or discover the hot spots of a visited city, the Planetary Nervous system makes this possible by collectively sensing social activity of participatory citizens.