Observability of radio reflections from exoplanet ionospheres with next generation radio telescopes

Abstract: Much has been learned about exoplanets and their atmospheres in the last three decades with the help of highly sensitive optical telescopes. Limited observations using X-ray telescopes have revealed the presence of ionospheres with very high density plasma around the hot Jupiter HD189733b. Owing to high density, the cutoff frequency of this plasma would lie in the range of few GHz. As the planet goes around the star, we suggest it might be possible to capture the stellar radio emission reflected from the ionosphere of the planet. We find that the reflected spectrum has a slope which is representative of the plasma density profile of the ionosphere and has a cutoff frequency. After investigating the reflection and free-free absorption process in the ionosphere, we find that this reflected signal, though feeble, can be captured by very sensitive radio telescopes operating in the low frequency range. We estimate the reflected signal from the ionosphere of a hot Jupiter and find that the flux ratio of the planet to the star are about $\sim 0.01\%$. In the view of development of facilities like Square Kilometer Array, it might be possible to capture the reflected radio signal from the ionosphere and constrain the thermal state of the ionosphere.