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Astrometry covers a parameter space that cannot be reached by RV or transitmethods to detect terrestrial planets on wide orbits. In addition, highaccuracy astrometric measurements are necessary to measure the inclination ofthe planet's orbits. Here we investigate the principles of an artefact of theastrometric approach. Namely, the displacement of the photo-centre due toinhomogeneities in a dust disc around the parent star. Indeed, theory andobservations show that circumstellar discs can present strong asymmetries. Wemodel the pseudo-astrometric signal caused by these inhomogeneities, askingwhether a dust clump in a disc can mimic the astrometric signal of anEarth-like planet. We show that these inhomogeneities cannot be neglected whenusing astrometry to find terrestrial planets. We provide the parameter spacefor which these inhomogeneities can affect the astrometric signals but stillnot be detected by mid-IR observations. We find that a small cross section ofdust corresponding to a cometary mass object is enough to mimic the astrometricsignal of an Earth-like planet. Astrometric observations of protoplanetarydiscs to search for planets can also be affected by the presence ofinhomogeneities. Some further tests are given to confirm whether an observationis a real planet astrometric signal or an impostor. Eventually, we also studythe case where the cross section of dust is high enough to provide a detectableIR-excess and to have a measurable photometric displacement by actualinstruments such as Gaia, IRAC or GRAVITY. We suggest a new method, whichconsists in using astrometry to quantify asymmetries (clumpiness) in innerdebris discs that cannot be otherwise resolved.

Effects of disc asymmetries on astrometric measurements - Can they mimic planets?