Extrasolar planets

The first known extrasolar planet in orbit around a Sun-like star was discovered in 1995. This object, as well as over two dozen subsequently detected extrasolar planets, were all identified by observing periodic variations of the Doppler shift of light emitted by the stars to which they are bound. All of these extrasolar planets are more massive than Saturn is, and most are more massive than Jupiter. All orbit closer to their stars than do the giant planets in our Solar System, and most of those that do not orbit closer to their star than Mercury is to the Sun travel on highly elliptical paths. Prevailing theories of star and planet formation, which are based on observations of the Solar System and of young stars and their environments, predict that planets should form in orbit about most single stars. However, these models require some modifications to explain the properties of the observed extrasolar planetary systems.

1995年，人类首次发现围绕类太阳恒星（Sun-like star）运行的系外行星（extrasolar planet）。该行星与后续二十余颗已探测到的系外行星，均通过观测其宿主恒星发射光线的多普勒频移（Doppler shift）周期性变化得以识别。所有此类系外行星的质量均大于土星，其中多数质量超过木星。它们的公转轨道均较太阳系（Solar System）内巨行星更贴近宿主恒星；而那些轨道并未比水星距太阳更近的系外行星，其运行轨迹大多呈高度椭圆形态。基于太阳系、年轻恒星及其所处环境的观测所构建的恒星与行星形成主流理论预测，绝大多数孤立恒星周围应当会形成行星。然而，要解释已观测到的系外行星系统的各项特性，这些理论模型需要进行一定修正。