  We have found a multi-scale steady solution of the Boussinesq equations for
Rayleigh-B\'enard convection in a three-dimensional periodic domain between
horizontal plates with a constant temperature difference by using a homotopy
from the wall-to-wall optimal transport solution given by Motoki et al. (J.
Fluid Mech., vol. 851, 2018, R4). The connected steady solution, which turns
out to be a consequence of bifurcation from a thermal conduction state at the
Rayleigh number $Ra\sim10^{3}$, is tracked up to $Ra\sim10^{7}$ by using a
Newton-Krylov iteration. The exact coherent thermal convection exhibits scaling
$Nu\sim Ra^{0.31}$ (where $Nu$ is the Nusselt number) as well as multi-scale
thermal plume and vortex structures, which are quite similar to those in the
turbulent Rayleigh-B\'enard convection. The mean temperature profiles and the
root-mean-square of the temperature and velocity fluctuations are in good
agreement with those of the turbulent states. Furthermore, the energy spectrum
follows Kolmogorov's -5/3 scaling law with a consistent prefactor, and the
energy transfer to smaller scales in the wavenumber space agrees with the
turbulent energy transfer.
