  Thermal emission spectra of hot Jupiters have led to key constraints on
thermal inversions (or 'stratospheres') in their atmospheres with important
implications for their atmospheric processes. Canonically, thermal inversions
in hot Jupiters have been suggested to be caused by species such as TiO and VO
which have strong visible opacity to absorb incident starlight. We explore two
new avenues for thermal inversions in hot Jupiters, exploring both the visible
and infrared opacities in their atmospheres. Firstly, by exploring a range of
metal-rich species we find that four species (AlO, CaO, NaH and MgH) provide
visible opacities comparable to TiO/VO and can cause strong inversions with
reasonable abundances. Secondly, we show that a low infrared opacity caused by
a low H$_2$O abundance, e.g. through a C/O~1, can also lead to strong thermal
inversions even with low abundances of the visible absorbers mentioned above.
We find that increasing the C/O ratio towards unity requires almost 2 orders of
magnitude lower abundances for the visible absorbers in order for an inversion
to form. Finally, we explore the thermal inversion in WASP-121b and find that
it can be explained by all the visible absorbers listed above for different C/O
ratios. Our study demonstrates the importance of both the refractory and
volatile species in governing the physicochemical processes in hot Jupiter
atmospheres. Spectroscopic observations in the visible have the potential to
detect the newly proposed refractory species that can cause thermal inversions
in addition to TiO and VO.
