  The formation of asteroids, comets and planets occurs in the interior of
protoplanetary disks during the early phase of star formation. Consequently,
the chemical composition of the disk might shape the properties of the emerging
planetary system. In this context, it is crucial to understand whether and what
organic molecules are synthesized in the disk. In this Letter, we report the
first detection of formic acid (HCOOH) towards the TW Hydrae protoplanetary
disk. The observations of the trans-HCOOH 6$_{(1,6)-5(1,5)}$ transition were
carried out at 129~GHz with ALMA. We measured a disk-averaged gas-phase t-HCOOH
column density of $\sim$ (2-4)$\times$10$^{12}$~cm$^{-2}$, namely as large as
that of methanol. HCOOH is the first organic molecules containing two oxygen
atoms detected in a protoplanetary disk, a proof that organic chemistry is very
active even though difficult to observe in these objects. Specifically, this
simplest acid stands as the basis for synthesis of more complex carboxylic
acids used by life on Earth.
