This paper will argue for a conception of intrinsic value which, it is hoped, will do justice to the following issues:

1. (1) that Nature need not and should not be understood to refer only to what exists on this planet, Earth;

2. (2) that an environmental ethics informed by features unique to Earth may be misleading and prove inadequate as technology increasingly threatens to invade and colonize other planets in the solar system;

3. (3) that a comprehensive environmental ethics must encompass not only our attitude to Earth, but to other planets as well—in other words, it must not simply be an Earthbound but virtually an astronomically bounded ethics.

What is unique about Earth? That it has water and an atmosphere which supports life. Its atmosphere preserves a constant 0·03% of carbon dioxide, 1·7 ppm of methane, 21% of oxygen, 79% of nitrogen, a surface temperature of 13°C. Water covers roughly two thirds of its surface. By contrast, planets like Venus and Mars which have no life and no water (at least today) have no methane either, but respectively 96·5% and 95% of carbon dioxide, 3·5% and 2·7% of nitrogen, a mere trace and 0·13% of oxygen, and surface temperatures of 459°C and -53°C (see Lovelock, 1988, p. 9).