距今将近100年，汤博在发现冥王星后，直到1992年才出现下一个「行星级」新发现，但那颗不在太阳系，而是在其他恒星周围运行的系外行星。此后天文学家已编目超过6,000颗系外行星，例如距离约725光年的Kepler-10 b。即使最近的系外行星也在4光年以上（约36兆英里），几乎不可能亲访，但它们关系到「宇宙中是否只有我们」这个问题。

直接用望远镜「看见」系外行星很难，关键在角分辨率与亮度对比。哈伯的分辨率约0.05角秒，约等于1/72,000度；理想情况下它能在约5900亿公里处分辨出木星大小的目标，这只相当于0.06光年，远小于最近恒星比邻星的4.25光年。再加上行星反射光远弱于恒星直射光，淹没在星光中。

因此主力方法改为「看恒星的反应」：其一是径向速度／多普勒法，利用光谱蓝移与红移测出恒星因行星引力而「抖动」的速度与周期，但因光速约3×10^8 m/s，位移讯号通常极小，需多年监测。其二是凌日法，行星遮挡使亮度出现小幅、可重复的下降；作为对照，日全食可让地面光照约降低1,000倍。方法也有选择偏差：更容易找到近星的大行星（热木星）；类地行星往往至少要3年才能累积3次凌日，而冥王星类的250年轨道几乎无法侦测。已知6,000颗中除1颗外都在银河系；在「数兆」其他星系背景下，粗估全宇宙行星总量约100 sextillion（10^23）。

Nearly 100 years after Pluto’s discovery, the next “new planet” milestone arrived in 1992—not in our solar system but around another star. Since then, astronomers have cataloged 6,000+ exoplanets, including Kepler-10 b at about 725 light-years. Even the nearest exoplanets are >4 light-years away (~36 trillion miles), so the payoff is not travel but statistics: how common are potentially Earth-like worlds?

Direct imaging is constrained by resolution and glare. Hubble’s resolving power is ~0.05 arcsecond (~1/72,000 of a degree); in principle it could separate a Jupiter-sized planet at ~590 billion km, only ~0.06 light-year—far short of Proxima Centauri at 4.25 light-years. Planets are also intrinsically dim next to their stars, like trying to see Jupiter in daytime against the Sun.

So the dominant techniques measure stellar effects. Radial-velocity (Doppler) searches track tiny blue/red shifts as a star “wobbles”; because c≈3×10^8 m/s, the fractional shifts are small and often require years of data to infer a planet’s period, mass (with a stellar-mass estimate), and orbital scale. Transit searches look for repeatable dips in a light curve; for scale, a total solar eclipse can cut light by ~1,000×, while exoplanet dips are much smaller but measurable. Both methods are alignment-limited and biased toward big, close-in planets (“hot Jupiters”): an Earth analog may need ~3 years to catch 3 transits, and a Pluto analog with a ~250-year orbit is effectively out of reach. With ~6,000 found (all but one in the Milky Way) amid “trillions” of other galaxies, a back-of-the-envelope guess puts the true total near 100 sextillion (10^23) planets.