Is there another Earth just 12 light-years away?
The Tau Ceti planets were discovered by looking at the radial velocity of the parent star. As planets orbit around their host star, their gravitational pull causes the star to wobble back and forth, and it's the size of this wobble that reveals how massive the orbiting objects are. Tau Ceti-e is estimated to be around 4 times the mass of the Earth, which would only make it around 1.6 times the size (radius) of Earth, assuming a similar density. With such a low mass, it is unlikely to be a gas giant planet like Jupiter, but rather a rocky planet like Earth.
The host star, Tau Ceti, is very similar to the Sun in many ways, but is thought to be much older. And like the Sun, it is the only star in the system, which means that the orbits of its planets are likely to be more stable, i.e. not distorted by the gravitational pull of two of more stars.