  The motivation to build spaceborne interferometric arrays for low-frequency
radio astronomy is widely recognised because frequencies below the ionospheric
cutoff are inaccessible for ground-based radio telescopes. We discuss the
theoretical possibilities to use low-frequency spacecraft arrays to detect
signals from magnetized extrasolar planets, including earthlike ones. A major
uncertainty that prohibits us from knowing if it is possible to detect
exoplanet cyclotron maser signals is the incomplete knowledge of the properties
of the interstellar plasma.
  We also present some ideas of how to construct efficient and affordable
space-based radio telescopes. We discuss two possibilities, a log-periodic
antenna in the spin plane and a two-spacecraft concept where one spacecraft
holds a large parabolic wire mesh reflector and the other one contains the
receiver. In the latter case, the effective area could be of the order of 1
km$^2$. The purpose of the paper is to stress once more the importance of
spaceborne low-frequency measurements by bringing in the intriguing possibility
of detecting earthlike exoplanet radio emissions and to demonstrate that
building even very large low-frequency antennas in space is not necessarily too
expensive.
