* Mass: 1.8986 x 10^27 kg (317.8 Earth masses)

* Volume: 1.4313 x 10^15 km³ (1,321 Earth volumes)

* Density: 1.326 g/cm³

* Surface gravity: 24.79 m/s² (2.54 g)

* Temperature: -110 °C at the cloud tops

* Composition: Mostly hydrogen and helium, with trace amounts of other elements

* Magnetic field: Strongest in the Solar System, with a dipole moment of 1.8 x 10^20 T m³

* Moons: 79 known moons, with the four largest being Io, Europa, Ganymede, and Callisto

51 Pegasi b

* Mass: 0.47 ± 0.07 Jupiter masses

* Volume: Unknown

* Density: Unknown

* Surface gravity: Unknown

* Temperature: 1,254 ± 46 K (981 ± 46 °C)

* Composition: Unknown, but likely mostly rock and gas

* Magnetic field: Unknown

* Moons: None known

As you can see, 51 Pegasi b is much smaller and hotter than Jupiter. It also has a much lower density, suggesting that it is composed of different materials. Additionally, 51 Pegasi b is not a gas giant like Jupiter, but rather a "hot Jupiter" that orbits very close to its star.