Earth and the Habitable Zone
Earth is in the middle of the Solar System‘s habitable zone, with Venus and Mars on either side. The exact limits of the Sun‘s CHZ is unclear. However, the best estimate to date of the Sun's habitable zone for an Earth-sized planet is between 120 million and 240 million km (75 million and 150 million miles). Nevertheless, this is one reason why scientists are interested in finding life on Mars; it would extend the size of the habitable zone.
The habitable zone clearly depends on the size, and luminosity of the star. In general, hot, luminous stars, in other words, those with spectral types G3–G9, F, A, B, and O (our Sun is a G2V), all have wide habitable zones. For example, to enjoy Earth-like temperatures around Sirius, a star that is about 26 times more luminous than the Sun, a planet, like Earth, would have to orbit at about the distance Jupiter is from the Sun. On the other hand, if the Sun was replaced by a star with lesser luminosity, like Epsilon Indi, (it is a K4.5 spectral classification and has one-tenth the luminosity of the Sun), the Earth would need to be in a Mercury like orbit to receive the same level of warmth.
See also: Super-Earths - Can They Sustain Life?