Context. It is common practice to claim the detection of a signal if, for a certain statistical significance metric, the signal significance exceeds a certain threshold fixed in advance. In the context of exoplanet searches in radial velocity data, the most common statistical significance metrics are the Bayes factor and the false alarm probability (FAP). Both criteria have proved useful, but do not directly address whether an exoplanet detection should be claimed. Furthermore, it is unclear which detection threshold should be taken and how robust the detections are to model misspecification. Aims. The present work aims at defining a detection criterion which conveys as precisely as possible the information needed to claim an exoplanet detection, as well as efficient numerical methods to compute it. We compare this new criterion to existing ones in terms of sensitivity, and robustness to a change in the model. Methods. We define a detection criterion based on the joint posterior distribution of the number of planets and of their orbital elements called the false inclusion probability (FIP). In the context of exoplanet detections, it provides the probability of presence of a planet with a period in a certain interval. Posterior distributions are computed with the nested sampling package polychord. We show that for FIP and Bayes factor calculations, defining priors on linear parameters as Gaussian mixture models allows to significantly speed up computations. The performances of the FAP, Bayes factor and FIP are studied with simulations as well as analytical arguments. We compare the methods assuming the model is correct, then evaluate their sensitivity to the prior and likelihood choices. Results. Among other properties, the FIP offers ways to test the reliability of the significance levels, it is particularly efficient to account for aliasing and allows to exclude the presence of planets with a certain confidence. We find that, in our simulations, the FIP outperforms existing detection metrics. We show that planet detections are sensitive to priors on period and semi-amplitude and that letting free the noise parameters offers better performances than fixing a noise model based on a fit to ancillary indicators.The exoplanet detection process usually consists in assessing sequentially whether an additional planet should be included in the model. Planet detections are typically claimed based on one of two approaches. The first one is the computation of a periodogram, that is a systematic scan for periodicity on a grid of frequencies. This is followed by the computation of a false alarm probability (FAP) to assess the significance of a detection. There are several definitions of the periodogram, corresponding to different assumptions on the data (e.g.