Tau Ceti g is the innermost of the four planets reported around Tau Ceti, the closest single Sun-like star to us, about 12 light-years away. It orbits every 20 days and has a minimum mass of about 1.75 Earth masses, which made it one of the lowest-mass planets found around a star like the Sun.
It was one of two new signals announced in 2017 by a team led by Fabo Feng at the University of Hertfordshire, together with Tau Ceti h, while the same study refined the previously suspected planets e and f. They used the radial velocity method, which picks up the slight back-and-forth motion a planet's gravity gives its star. The signals in this study are as small as 30 centimetres per second, and the team found them by removing a kind of noise in the data that changes with the colour of the light. What sets it apart from its siblings is that it was the one signal the team also found independently in data from Keck Observatory, a separate telescope and instrument.
Being so close to its star, it is far too hot for liquid water: the archive estimates its equilibrium temperature, set by starlight alone, at about 640 kelvin. The discovery paper presents all four as planet candidates. None has been seen passing in front of the star, so their sizes and true masses are unknown. Measuring signals this small is still close to the limit of what current instruments can do, and the team estimated that instrumental noise sets a floor of about 20 centimetres per second for the HARPS spectrograph used in much of the data.