Introduction

Natural philosophers had hypothesized centuries ago that other planetary systems orbited the many stars in the night sky, that the Solar System was not unique. Up until 1995, however, there was no reproducible astronomical evidence to support this visionary viewpoint (Boss 1998). One decade later, a new field of astronomy has been born, with rapid observational progress that threatens to far outpace theoretical efforts to keep up. Major discoveries continue to appear on roughly a monthly basis, an unprecedented level of advancement in any field of science. While we have yet to find a true Solar System analogue, the planetary systems discovered so far leave little doubt that we will soon be discovering planetary systems that will be hospitable to the existence of Earth-like planets, the ultimate goal of this entire field of research.

Surprisingly, the very first planetary-mass bodies discovered outside our Solar System were roughly Earth-mass planets orbiting the pulsar PSR 1257+12 (Wolszczan & Frail 1992). These objects must have formed after their host star underwent a supernova explosion, as the explosion and accompanying stellar-mass loss would likely have removed any pre-existing planets. The fact that Earth-mass bodies later managed to form in a disk around the neutron star was taken as a strong proof of the resiliency of the planet-forming process of collisional accumulation of solids into larger bodies, even in a hostile environment. Nevertheless, the paucity of evidence for planetary systems around normal, main-sequence stars caused significant concern even after the pulsar planets were announced in 1992—where were the gas giant planets?

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