An exoplanet called LP 791-18 d, an Earth-size world about 90 light-years away, is seen in an undated artist’s rendering. A more massive planet in the system, shown as a small blue dot on the right, exerts a gravitational tug that may result in internal heating and volcanic eruptions, like on Jupiter?s moon Io. NASA Goddard Space Flight Center/Chris Smith (KRBwyle)/Handout via REUTERS