Ross 128 b is one of the nearest roughly Earth-mass planets known, orbiting a red dwarf about 11 light-years away. A team led by Xavier Bonfils announced it in 2017 after more than a decade of measurements with HARPS, a spectrograph at the European Southern Observatory's La Silla Observatory in Chile. HARPS uses the radial velocity method: as a planet orbits, its gravity makes the star move slightly toward and away from us, and that motion shifts the star's light.
Its minimum mass is about 1.35 times Earth's, and it orbits every 9.9 days. Its star is so dim that, even that close in, the planet receives only about 1.38 times the light Earth gets from the Sun. The discovery team estimated an equilibrium temperature, the temperature set by starlight alone, of between 213 and 269 kelvin depending on how much light the planet reflects. Studies place it near the inner edge of the habitable zone, the range of distances where liquid water could last on a surface.
What sets Ross 128 apart is that it is a quiet star. Many red dwarfs flare violently, and those outbursts can strip a planet's atmosphere. Measurements from the K2 mission and from ground-based surveys show Ross 128 spins slowly and has weak magnetic activity, so an atmosphere on its planet is more likely to survive. When it was found, it was the closest temperate planet known around such a quiet star.
In 2018 a team led by Diogo Souto measured the chemistry of Ross 128 itself using infrared spectra from the APOGEE survey, and found its makeup close to the Sun's. Their modelling suggests the planet is a mix of rock and iron and, if its makeup matches its star's, that it has a larger core than Earth's. That conclusion rests on assumptions, because the planet does not pass in front of its star: K2 data showed no transit, so its size has never been measured. The discovery team noted that future extremely large telescopes may be able to separate its light from the star's and search its atmosphere for oxygen.