A Y-type star is a star within the Y-class, a spectral class indicating a star with a noticeable absorption line at wavelength 1.55 μm and a surface temperature under 700 K, which encompasses the coolest brown dwarfs (Y dwarfs). The spectral energy distribution peaks in the mid-infrared range. Other than that, their spectra is not well-characterized. As of 2013, seventeen bodies are known that fit the criteria. Such cool brown dwarfs are not that different from T-dwarfs or hot planets and discussion remains regarding whether this classification is useful: it is presumably for objects classified as brown dwarfs based upon their mass that fall within this temperature range.