  The search for habitable exoplanets in the Universe is actively ongoing in
the field of astronomy. The biggest future milestone is to determine whether
life exists on such habitable exoplanets. In that context, oxygen in the
atmosphere has been considered strong evidence for the presence of
photosynthetic organisms. In this paper, we show that a previously unconsidered
photochemical mechanism by titanium(IV) oxide (titania) can produce abiotic
oxygen from liquid water under near ultraviolet (NUV) lights on the surface of
exoplanets. Titania works as a photocatalyst to dissociate liquid water in this
process. This mechanism offers a different source of a possibility of abiotic
oxygen in atmospheres of exoplanets from previously considered
photodissociation of water vapor in upper atmospheres by extreme ultraviolet
(XUV) light. Our order-of-magnitude estimation shows that possible amounts of
oxygen produced by this abiotic mechanism can be comparable with or even more
than that in the atmosphere of the current Earth, depending on the amount of
active surface area for this mechanism. We conclude that titania may act as a
potential source of false signs of life on habitable exoplanets.
