Predictions and observations for the exoplanet τ Boötis b. Gray area: emission below 10 MHz is not detectable for ground-based observations (ionospheric cutoff). Lower solid line (light orange): typical spectrum of Jupiter’s radio emissions at 15.6 pc distance. Two upper solid line (orange): Jupiter’s radio emission scaled for values expected for τ Boötis b according tomodels R (left) and NR (right) from Grießmeier (2017). For Jupiter, the radio flux increases during periods of high activity (frequently by one order of magnitude, and exceptionally by two orders of magnitude, see e.g. Zarka et al. 2004; Zarka 2004); the same variability is assumed for exoplanetary emission, as indicated by the vertical black arrows. Dashed lines and triangles (light blue) show the theoretical sensitivity limit of the radio-telescopes UTR-2, LOFAR (low band), VLA, LOFAR (high band), and GMRT (for 1 h of integration time and a bandwidth of 4 MHz, or an equivalent combination). Numbered lines and points (green): sensitivity achieved in previous observations of τ Boötis. Care must be taken when comparing the theoretical sensitivity limit to previous observations. The sensitivity limits of the radio telescopes as well as most upper limits were calculated for continuous emission, whereas the values for observations2 and 9 take into account the expected “bursty” nature of the emission. References: (1) Farrell et al. (2003); (2) Ryabov et al. (2004); (3) Lazio et al. (2004); (4) Shiratori et al. (2006); (5) Lazio & Farrell (2007); (6) Stroe et al. (2012); (7) Hallinan et al. (2013); (8) Lynch et al. (2018); (9) this article, Sect. 5.1.