Seismology provides a powerful tool for probing planetary interiors(1,2), but it has been considered inapplicable to tectonically inactive planets where earthquakes are absent. Here, however, we show that the atmospheres of solid planets are capable of exerting dynamic pressure on their surfaces, thereby exciting free oscillations with amplitudes large enough to be detected by modern broad-band seismographs. Order-of-magnitude estimates of these forces give similar amplitudes of a few nanogals for the Earth, Venus and Mars despite widely varying atmospheric and ambient conditions. The amplitudes are also predicted to have a weak frequency dependence. Our analysis of seismograms, recorded continuously from 1992 to 1993 at 13 globally distributed stations, shows strong evidence for continuously excited fundamental-mode free oscillations on the Earth. This result, together with other recent studies(3-5), is consistent with our estimate of atmospheric forcing and we therefore propose that it may be possible to detect atmospheric excitation of free oscillations on Venus and Mars as well.