* Atmosphere: Gliese 521b's atmosphere is likely to be very different from Earth's. It is thought to be composed mostly of hydrogen and helium, with trace amounts of other gases. Earth's atmosphere, on the other hand, is composed mostly of nitrogen and oxygen, with trace amounts of other gases.

* Temperature: Gliese 521b is much hotter than Earth. Its surface temperature is estimated to be around 2,000 degrees Fahrenheit (1,100 degrees Celsius). Earth's surface temperature, on the other hand, is around 59 degrees Fahrenheit (15 degrees Celsius).

* Size: Gliese 521b is about 1.5 times the size of Earth. This means that it has a surface area that is about 2.25 times greater than Earth's.

* Orbit: Gliese 521b orbits its star every 42 days. Earth orbits the Sun every 365 days.

* Distance from star: Gliese 521b is about 6 million miles (10 million kilometers) from its star. Earth is about 93 million miles (150 million kilometers) from the Sun.

* Tidal locking: Gliese 521b is tidally locked to its star, meaning that one side of the planet always faces the star and the other side always faces away from the star. This would cause extreme temperature differences between the two sides of the planet. Earth is not tidally locked to the Sun.

* Habitability: Gliese 521b is not considered to be habitable for life as we know it. It is too hot, too large, and too close to its star.