We report the discovery of four relatively massive ( 2–7 M _ { J } ) transiting extrasolar planets . HAT-P-20b orbits the moderately bright V=11.339 K3 dwarf star GSC 1910-00239 on a circular orbit , with a period P = 2.875317 \pm 0.000004 d , transit epoch T _ { c } = 2455080.92661 \pm 0.00021 ( \mathrm { BJD _ { UTC } } ) , and transit duration 0.0770 \pm 0.0008 d. The host star has a mass of 0.76 \pm 0.03 M _ { \sun } , radius of 0.69 \pm 0.02 R _ { \sun } , effective temperature 4595 \pm 80 K , and metallicity [ Fe / H ] = +0.35 \pm 0.08 .
The planetary companion has a mass of 7.246 \pm 0.187 M _ { J } , and radius of 0.867 \pm 0.033 R _ { J } yielding a mean density of 13.78 \pm 1.50 g cm ^ { -3 } , which is the second highest value among all known exoplanets . HAT-P-21b orbits the V=11.685 G3 dwarf star GSC 3013-01229 on an eccentric ( e = 0.228 \pm 0.016 ) orbit , with a period P = 4.124481 \pm 0.000007 d , transit epoch T _ { c } = 2454996.41312 \pm 0.00069 , and transit duration 0.1530 \pm 0.0027 d. The host star has a mass of 0.95 \pm 0.04 M _ { \sun } , radius of 1.10 \pm 0.08 R _ { \sun } , effective temperature 5588 \pm 80 K , and metallicity [ Fe / H ] = +0.01 \pm 0.08 .
The planetary companion has a mass of 4.063 \pm 0.161 M _ { J } , and radius of 1.024 \pm 0.092 R _ { J } yielding a mean density of 4.68 _ { -0.99 } ^ { +1.59 } g cm ^ { -3 } .
HAT-P-21b is a border-line object between the pM and pL class planets , and the transits occur near apastron . HAT-P-22b orbits the bright V=9.732 G5 dwarf star HD 233731 on a circular orbit , with a period P = 3.212220 \pm 0.000009 d , transit epoch T _ { c } = 2454930.22001 \pm 0.00025 , and transit duration 0.1196 \pm 0.0014 d. The host star has a mass of 0.92 \pm 0.03 M _ { \sun } , radius of 1.04 \pm 0.04 R _ { \sun } , effective temperature 5302 \pm 80 K , and metallicity [ Fe / H ] = +0.24 \pm 0.08 .
The planet has a mass of 2.147 \pm 0.061 M _ { J } , and compact radius of 1.080 \pm 0.058 R _ { J } yielding a mean density of 2.11 _ { -0.29 } ^ { +0.40 } g cm ^ { -3 } .
The host star also harbors an M-dwarf companion at a wide separation .
Finally , HAT-P-23b orbits the V=12.432 G0 dwarf star GSC 1632-01396 on a close to circular orbit , with a period P = 1.212884 \pm 0.000002 d , transit epoch T _ { c } = 2454852.26464 \pm 0.00018 , and transit duration 0.0908 \pm 0.0007 d. The host star has a mass of 1.13 \pm 0.04 M _ { \sun } , radius of 1.20 \pm 0.07 R _ { \sun } , effective temperature 5905 \pm 80 K , and metallicity [ Fe / H ] = +0.15 \pm 0.04 .
The planetary companion has a mass of 2.090 \pm 0.111 M _ { J } , and radius of 1.368 \pm 0.090 R _ { J } yielding a mean density of 1.01 \pm 0.18 g cm ^ { -3 } .
HAT-P-23b is an inflated and massive hot Jupiter on a very short period orbit , and has one of the shortest characteristic in-fall times ( 7.5 _ { -1.8 } ^ { +2.9 } Myr ) before it gets engulfed by the star .