  The atmosphere of Titan, the largest moon of Saturn, is rich in organic
molecules, and it has been suggested that the moon may serve as an analog for
the pre-biotic Earth due to its highly reducing chemistry and existence of
global hazes. Photochemical models of Titan have predicted the presence of
propadiene (historically referred to as allene), CH$_{2}$CCH$_{2}$, an isomer
of the well-measured propyne (also called methylacetylene) CH$_{3}$CCH, but its
detection has remained elusive due to insufficient spectroscopic knowledge of
the molecule - which has recently been remedied with an updated spectral line
list. Here we present the first unambiguous detection of the molecule in any
astronomical object, observed with the Texas Echelle Cross Echelle Spectrograph
(TEXES) on the NASA Infrared Telescope Facility (IRTF) in July 2017. We model
its emission line near 12 $\mu$m and measure a volume mixing ratio (VMR) of
(6.9 $\pm$ 0.8) $\times$10$^{-10}$ at 175 km, assuming a vertically increasing
abundance profile as predicted in photochemical models. Cassini measurements of
propyne made during April 2017 indicate that the abundance ratio of propyne to
propadiene is 8.2$\pm$1.1 at the same altitude. This initial measurement of the
molecule in Titan's stratosphere paves the way towards constraining the amount
of atomic hydrogen available on Titan, as well as future mapping of propadiene
on Titan from 8 meter and larger ground based observatories, and future
detection on other planetary bodies.
