NASA reported the discovery of an Earth-size planet, named TOI 700 d (Source: Wikimedia Commons)On Tuesday, NASA reported the discovery of an Earth-size planet, named TOI 700 d, orbiting its star in the habitable zone. A habitable zone, also called the Goldilocks zone, is the area around a star where it is not too hot and not too cold for liquid water to exist on the surface of surrounding planets.Obviously, our Earth is in the Suns Goldilocks zone. If Earth were where the dwarf planet Pluto is, all its water would freeze; on the other hand, if Earth were where Mercury is, all its water would boil off.Life on Earth started in water, and water is a necessary ingredient for life as we know it. So, when scientists search for the possibility of alien life, any rocky exoplanet in the habitable zone of its star is an exciting find.The newest such planet was found by NASAs Transiting Exoplanet Survey Satellite (TESS) mission, which it launched in 2018. Very few such Earth-size planets have been found so far, including some by NASAs Kepler mission, and this one is the first such discovery by TESS. The find was confirmed by the Spitzer Space Telescope, which sharpened the measurements that TESS had made, such as orbital period and size.Explained: What is Goldilocks zone?MoreSource: NASA's Goddard Space Flight CenterTOI 700 d measures 20% larger than Earth. It orbits its star once every 37 days and receives an amount of energy that is equivalent to 86% of the energy that the Sun provides to Earth. The star, TOI 700, is an M dwarf located just over 100 light-years away in the southern constellation Dorado, is roughly 40% of our Suns mass and size, and has about half its surface temperature.Two other planets orbit the star TOI 700 b, which is almost exactly Earth-size, probably rocky, and which completes an orbit every 10 days, and TOI 700 c, the middle planet, which is 2.6 times larger than Earth, is probably gas-dominated, and orbits every 16 days. TOI 700 d is the outermost planet, and the only one in the stars habitable zone. NASA said future missions may be able to identify whether the planets have atmospheres and, if so, even determine their compositions.Don't miss from Explained | Irans counterattack: where and how big?