Stern S Alan
Southwest Research Institute, Boulder, Colorado 80302, USA.
Astrobiology. 2003 Summer;3(2):317-21. doi: 10.1089/153110703769016406.
Like all low- and moderate-mass stars, the Sun will burn as a red giant during its later evolution, generating of solar luminosities for some tens of millions of years. During this post-main sequence phase, the habitable (i.e., liquid water) thermal zone of our Solar System will lie in the region where Triton, Pluto-Charon, and Kuiper Belt objects orbit. Compared with the 1 AU habitable zone where Earth resides, this "delayed gratification habitable zone" (DGHZ) will enjoy a far less biologically hazardous environment - with lower harmful radiation levels from the Sun, and a far less destructive collisional environment. Objects like Triton, Pluto-Charon, and Kuiper Belt objects, which are known to be rich in both water and organics, will then become possible sites for biochemical and perhaps even biological evolution. The Kuiper Belt, with >10(5) objects > or =50 km in radius and more than three times the combined surface area of the four terrestrial planets, provides numerous sites for possible evolution once the Sun's DGHZ reaches it. The Sun's DGHZ might be thought to only be of academic interest owing to its great separation from us in time. However, approximately 10(9) Milky Way stars burn as luminous red giants today. Thus, if icy-organic objects are common in the 20-50 AU zones of these stars, as they are in our Solar System (and as inferred in numerous main sequence stellar disk systems), then DGHZs may form a niche type of habitable zone that is likely to be numerically common in the Galaxy.
和所有低质量及中等质量恒星一样，太阳在其后期演化过程中会变成一颗红巨星，并在数千万年的时间里保持太阳光度。在这个主序后阶段，我们太阳系的宜居（即液态水存在的）热区将位于海卫一、冥王星 - 卡戎以及柯伊伯带天体的轨道区域。与地球所在的1天文单位宜居带相比，这个“延迟满足宜居带”（DGHZ）的生物危害环境要小得多——来自太阳的有害辐射水平更低，碰撞环境的破坏性也小得多。像海卫一、冥王星 - 卡戎以及柯伊伯带天体这类已知富含水和有机物的天体，届时将有可能成为生物化学甚至生物进化的场所。柯伊伯带拥有半径大于或等于50千米且数量超过10⁵个的天体，其总面积是四颗类地行星表面积总和的三倍多；一旦太阳的延迟满足宜居带到达这里，就会有众多可能发生进化的场所。太阳的延迟满足宜居带可能会因为与我们在时间上相隔甚远而被认为仅仅具有学术意义。然而，如今银河系中约有10⁹颗恒星正处于明亮红巨星阶段。因此，如果像在我们太阳系中那样（以及在众多主序恒星盘系统中所推断的那样），冰质有机天体在这些恒星20至50天文单位的区域很常见，那么延迟满足宜居带可能会形成一种特殊类型且在银河系中数量可能众多的宜居带。
