每日一词：太阳系外行星（Extrasolar planets）

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太阳系外行星，或称系外行星，是一个围绕着太阳以外的恒星的世界。自1990年代中期以来，在我们的银河系中已经发现了500多颗系外行星，这表明其他行星在整个宇宙中很常见。
这些世界中的大多数都是通过 "径向速度 "技术探测到的。天文学家利用多普勒效应来测试一颗恒星是否由于一颗看不见的绕行行星的引力而来回摇摆。其他猎取行星的技术包括 "过境 "法——当一颗黑暗的行星从它前面经过时，寻找一颗恒星的轻微变暗。少数的系外行星已经被直接成像了。
迄今为止发现的许多系外行星都与太阳系的行星非常不同。有些是 "热木星"，即在短短几天内绕着恒星旋转的巨型行星，另一些是 "超级地球"，即质量是地球数倍的岩石世界。令人惊讶的是，一些系外行星围绕中子星运行。最重要的是找到与地球类似的、围绕太阳等 "正常 "恒星运行的潜在宜居行星。
<hr/>An extrasolar planet, or exoplanet, is a world that circles a star beyond our Sun. More than 500 exoplanets have been discovered in our galaxy since the mid-1990s, suggesting that other planets are common throughout the universe.
Most of these worlds have been detected by the 'radial velocity' technique. Astronomers use the Doppler effect to test whether a star is wobbling back and forth due to the gravity of an invisible orbiting planet. Other planet-hunting techniques include the 'transit' method-looking for a slight dimming of a star when a dark planet passes in front of it. A handful of exoplanets have been imaged directly.
Many exoplanets found so far are very unlike the planets of the solar system. Some are 'hot Jupiters', giant planets that zoom round their stars in just a few days, others are 'super-Earths', rocky worlds several times as massive as Earth. Surprisingly, some exoplanets orbit neutron stars. The holy grail is to find potentially habitable planets similar to Earth orbiting 'normal' stars like the Sun.