PLANETARY SYSTEMSNew finds on Earth, such as colonies of bacteria deep underground, have suggested that organisms can thrive even if sealed off from the sun, by living on chemical rather than solar energy. Therefore the possible environments for life in space, once restricted to the relatively narrow slice of the solar system inside the martian orbit, have multiplied. In fact, the so-called habitable zone, which is defined by the presence of water and energy, has reached Pluto and even into interstellar space and may now include exotic objects such as distant moons or even rogue planets lost between the stars.