  High-frequency gravitational waves (HFGWs) carry a wealth of information on
the early Universe with a tiny comoving horizon and astronomical objects of
small scale but with dense energy. We demonstrate that the nearby planets, such
as Earth and Jupiter, can be utilized as a laboratory for detecting the HFGWs.
These GWs are then expected to convert to signal photons in the planetary
magnetosphere, across the frequency band of astronomical observation. As a
proof of concept, we present the first limits from the existing low-Earth-orbit
satellite for specific frequency bands and project the sensitivities for the
future more-dedicated detections. The first limits from Juno, the latest
mission orbiting Jupiter, are also presented. Attributed to the long path of
effective GW-photon conversion and the wide angular distribution of signal
flux, we find that these limits are highly encouraging, for a broad frequency
range including a large portion unexplored before.
