Statistical D/H ratios in Solar System comets, protoplanetary discs, and low-mass Class 0 protostars for methanol (red), water (blue), and HCN or CH₃ CN (green). References: 1P/Halley: Eberhardt et al. (1995); C/1996 B2 (Hyakutake): Bockelée-Morvan et al. (1998); C/1995 O1 (Hale-Bopp): Meier et al. (1998); Crovisier et al. (2004); 8P/Tuttle: Villanueva et al. (2009); C/2007 N3 (Lulin): Gibb et al. (2012); C/2002 T7 (LINEAR): Hutsemékers et al. (2008); 153P/Ikeya-Zhang: Biver et al. (2006); C/2009 P1(Garradd): Bockelée-Morvan et al. (2012); C/2012 F6 (Lemmon): Biver et al. (2016); C/2014 Q2 (Lovejoy): Biver et al. (2016); 67P/C-G: Altwegg et al. (2015); 103P/Hartley 2: Hartogh et al. (2011); 45P/HMP: Lis et al. (2013); All discs: Huang et al. (2017); IRAS 16293: Persson et al. (2014); Jørgensen et al. (2018); Calcutt et al. (2018); IRAS 2A: Coutens et al. (2014), this work; IRAS 4A: average values derived by Taquet et al. (2013b) and Persson et al. (2014, M. Persson priv. comm.), this work; IRAS4B: Persson et al. (2014).