Can exoplanets be found using neutrino detectors? I read that KamLAND can detect geoneutrinos produced by thorium and uranium decay in Earth's crust.
Could a larger detector detect neutrinos from other planets in the solar system or perhaps even exoplanets?
The answer is presumably no.
Let's assume that the nearest possible exoplanet is in the Proxima Centauri system, and it presents an electron antineutrino luminosity identical to that of Earth. The flux from the exoplanet at the KamLAND detector would then be weaker than the flux from Earth by a factor of roughly
$$\left(\frac{\text{Radius of Earth}}{4\;\text{light-years}}\right)^2\approx2\times10^{-20}$$
thanks to the inverse-square law. So the intrinsic signal strength is likely to be essentially zero.
More concretely: the KamLAND collaboration's 2013 measurement of the Earth's geoneutrino flux was approximately $\sim3.4\times10^6\;\text{cm}^{-2}\text{s}^{-1}$, which isn't a lot considering the low probability that a given neutrino will interact with the detector. Indeed, the group reported only $116^{+28}_{-27}$ events in their period of monitoring - and some of that was during a time where nearby nuclear reactors were off and therefore not producing neutrino noise!
Any signal from an exoplanet would be tiny and easily lost in the background of all of the other possible neutrino sources.