According to the Wikipedia article Triple point of water, the triple point of water occurs at
The single combination of pressure and temperature at which liquid water, solid ice, and water vapor can coexist in a stable equilibrium occurs at exactly 273.16 K (0.01 °C; 32.02 °F) and a partial vapor pressure of 611.657 pascals (6.11657 mbar; 0.00603659 atm).
Shown in the illustration below (from the Wikipedia article):
Crucially, to put this into a context, the 611.73 pascals quoted in the article corresponds to the
minimum pressure at which liquid water can exist. At pressures below the triple point (as in outer space), solid ice when heated at constant pressure is converted directly into water vapor in a process known as sublimation.
Potentially useful information to determine if an exoplanet can possibly have water (if all other conditions are suitable).
What are the spectroscopic signatures that differentiate triple state water from water at 1 atm?