Cold ions trapped in the vicinity of conductive surfaces experience heating of their oscillatory motion. Typically, the rate of this heating is orders of magnitude larger than expected from electric field fluctuations due to thermal motion of electrons in the conductors. This effect, known as anomalous heating, is not fully understood. One of the open questions is the heating rate's dependence on the ion-electrode separation. We present a direct measurement of this dependence in an ion trap of simple planar geometry. The heating rates are determined by taking images of a single 172 Yb + ion's resonance fluorescence after a variable heating time and deducing the trapped ion's temperature from measuring its average oscillation amplitude. Assuming a power law for the heating rate vs. ion-surface separation dependence, an exponent of -3.79 ± 0.12 is measured.Electric field noise in close proximity to metal surfaces is an important issue in various fields of experimental physics, such as measuring weak forces in scanning probe microscopy [1,2] or for Casimir effect studies [3, 4], gravitational wave detection [5], and experiments on the gravitational properties of charged particles [6]. In experiments with cold trapped ions such noise results in excitation (also termed heating) of the ions' motional degrees of freedom [1]. In realizations of quantum information processing based on trapped ions, this heating can become a major source of decoherence [1,8,9].Experiments have shown that the observed heating rate is orders of magnitude greater than would be caused by thermal motion of electrons in the conductors (i.e. Johnson noise) [10,11]. This high heating rate is mostly associated with surface contamination and surface imperfections, as surface treatment is known to be able to reduce the heating rate significantly [12,13]. However, its mechanism is not fully understood, and thus this effect is referred to as anomalous heating; a recent review of experimental and theoretical studies of this phenomenon is given in [1]. A comparison of experiments, employing different types and sizes of ion traps, shows that the anomalous heating rate grows fast as the ion-electrode separation decreases [1]. Therefore, anomalous heating is particularly prominent for microfabricated planar ion traps [14], where this separation can be as small as tens of micrometers. Microfabricated traps are central for the realization of scalable quantum information processing with trapped ions [14][15][16][17][18][19][20][21][22][23][24], and, therefore, in addition to its fundamental interest, it is of particular importance to characterize and understand anomalous heating.Though electric field noise-induced heating of ion motion has been studied in many experimental and theoretical works over the last years [1], one of the still open questions regarding anomalous heating is its dependence on the ion-electrode separation. In addition to being of practical use for ion trap design, knowing this dependence can confirm or contradict various existing theoret...