Astronomers discover Earth-like planet
We now know of well over 200 extra-solar planets, the vast majority of which are thought to be gas-giant planets like Jupiter and Saturn. As it happens, this parent star, Gliese 581, is thought to have at least two other planets. The first is a Neptune-like planet orbiting in a very close 5-day orbit, whilst the second is about twice the size of Earth, but occupies a cooler 84-day orbit where water would be in its frozen form - ice. Compare these periods to Mercury's orbit of 88 days.
Note that the distance of the habitable zone (where temperatures are between 0-100 degrees celsius) is very dependent on the type of star. In our own Solar System, the Sun's habitable zone (HZ) lies way out at Earth's 365 day orbit, whereas for the much cooler Gliese 581, the HZ is around 10 times closer.
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