Astronomers find Earth-sized exoplanet, LP 791-18 d, 90 light-years away.

LP 791-18 d shows potential for frequent volcanic activity like Jupiter's moon.

 Discovered using NASA's TESS and Spitzer Space Telescope, along with ground observatories.

 LP 791-18 d is tidally locked, with one side always facing the star.

Volcanic activity could sustain an atmosphere, allowing water to condense on the cooler side.

 Orbits a small red dwarf star in Crater constellation, slightly larger and more massive than Earth.

 System previously had two known planets, LP 791-18 b and c.

 Gravitational interaction with LP 791-18 c causes elliptical orbit and volcanic activity.

LP 791-18 d near inner edge of habitable zone where liquid water could exist.

 James Webb Space Telescope approved for observing LP 791-18 c, LP 791-18 d a candidate for atmospheric studies.