  Despite many methods developed to find young massive planets in
protoplanetary discs, it is challenging to directly detect low-mass planets
that are embedded in discs. On the other hand, the core-accretion theory
suggests that there could be a large population of embedded low-mass young
planets at the Kelvin-Helmholtz (KH) contraction phase. We adopt both 1-D
models and 3-D simulations to calculate the envelopes around low-mass cores
(several to tens of $M_{\oplus}$) with different luminosities, and derive their
thermal fluxes at radio wavelengths. We find that, when the background disc is
optically thin at radio wavelengths, radio observations can see through the
disc and probe the denser envelope within the planet's Hill sphere. When the
optically thin disc is observed with the resolution reaching one disc scale
height, the radio thermal flux from the planetary envelope around a 10
M$_{\oplus}$ core is more than 10 % higher than the flux from the background
disc. The emitting region can be extended and elongated. Finally, our model
suggests that the au-scale clump at 52 au in the TW Hydrae disc revealed by
ALMA is consistent with the envelope of an embedded 10-20 $M_{\oplus}$ planet,
which can explain the detected flux, the spectral index dip, and the tentative
spirals. The observation is also consistent with the planet undergoing pebble
accretion. Future ALMA and ngVLA observations may directly reveal more such
low-mass planets, enabling us to study core growth and even reconstruct the
planet formation history using the embedded "protoplanet" population.
