The emergent spectra of close-in, giant exoplanets (‘hot Jupiters’) are expected to be distinct from those of self-luminous objects with similar effective temperatures because hot Jupiters are primarily heated from above by their host stars rather than internally from the release of energy from their formation¹. Theoretical models predict a continuum of dayside spectra for hot Jupiters as a function of irradiation level, with the coolest planets having absorption features in their spectra, intermediate-temperature planets having emission features due to thermal inversions and the hottest planets having blackbody-like spectra due to molecular dissociation and continuum opacity from the H⁻ ion^2,3,4. Absorption and emission features have been detected in the spectra of a number of individual hot Jupiters^5,6, and population-level trends have been observed in photometric measurements^{7,8,9,10,11,12,13,14,15}. However, there has been no unified, population-level study of the thermal emission spectra of hot Jupiters as there has been for cooler brown dwarfs¹⁶ and transmission spectra of hot Jupiters¹⁷. Here we show that hot Jupiter secondary eclipse spectra centred around a water absorption band at 1.4 μm follow a common trend in water feature strength with temperature. The observed trend is broadly consistent with model predictions for how the thermal structures of solar-composition planets vary with irradiation level, but is inconsistent with the predictions of self-consistent one-dimensional models for internally heated objects. This is particularly the case because models of internally heated objects show absorption features at temperatures above 2,000 K, whereas the observed hot Jupiters show emission features and featureless spectra. Nevertheless, the ensemble of planets exhibits some degree of scatter around the mean trend for solar-composition planets. The spread can be accounted for if the planets have modest variations in metallicity and/or elemental abundance ratios, which is expected from planet formation models^18,19,20,21.

预计近距离的巨型系外行星（“热木星”）的出现光谱与具有相似有效温度的自发光天体的光谱不同，因为热木星主要由其主恒星从上方加热，而不是在内部从释放中加热从他们的形成¹的能量。理论模型预测热木星的白天光谱连续谱是辐照水平的函数，其中最冷的行星在其光谱中具有吸收特征，中温行星由于热反转而具有发射特征，而最热的行星由于热逆而具有类似黑体的光谱从 H ^-离子^2,3,4到分子解离和连续不透明度. ^{在许多单独的热木星5,6}的光谱中检测到吸收和发射特征，并在光度测量中观察到种群水平趋势^{7,8,9,10,11,12,13,14,15}。然而，还没有对热木星的热发射光谱进行统一的人口水平研究，因为对较冷的褐矮星¹⁶和热木星^{17的透射光谱进行了研究}. 在这里，我们表明以 1.4 μm 的吸水带为中心的热木星二次食光谱遵循水特征强度随温度的共同趋势。观察到的趋势与太阳成分行星的热结构如何随辐照水平变化的模型预测大体一致，但与内部加热物体的自洽一维模型的预测不一致。尤其如此，因为内部加热物体的模型在 2,000 K 以上的温度下显示出吸收特征，而观察到的热木星显示出发射特征和无特征的光谱。然而，行星的集合在太阳成分行星的平均趋势周围表现出一定程度的分散。^18,19,20,21。