Kepler-296 d is the largest of five planets around Kepler-296, a pair of red dwarfs about 550 light-years away. It is the third planet out, crossing in front of its star every 19.9 days, and NASA's Kepler space telescope measured its radius at about 2.1 times Earth's. It was found together with its four siblings by watching for the small dips in starlight as each planet passed in front of the star.
Its size depends on which star it orbits, and that was an open question for a while. Kepler-296 is two red dwarfs so close together on the sky that Kepler saw them as one point of light. A transit dims that combined light, and the planet's size is worked out from how deep the dip is, so a planet circling the fainter star would have to be larger to make the same dip. In 2015 a team led by Thomas Barclay used the shapes of the five planets' transits to show that they almost certainly orbit the brighter star, Kepler-296 A. The planets themselves had been confirmed in 2014, in a study led by Jason Rowe that validated hundreds of Kepler multi-planet systems.
Its mass has not been measured; the NASA Exoplanet Archive's figure of about 5 Earth masses is an estimate from its size. Without a measured mass, it is not known whether it is a large rocky planet or one with a thick envelope of gas. It receives about three times the light Earth gets from the Sun, warmer than the habitable zone, the range of distances where liquid water could last on a surface. Just beyond it, Kepler-296 e and f orbit within or close to that zone.