Weighing an Alien Planet
Step on a tilted scale and it probably won't read your weight right. Planet hunters have a similar problem: When they detect a planet tugging on its star, they often don't know how tilted the planet's orbit is—and so they can't measure the world's exact mass. Now, as astronomers report online today in Nature, infrared spectra from the Very Large Telescope in Chile have unveiled the orbital tilt of one of the first planets ever found, allowing the researchers to derive its true mass. Located 51 light-years from Earth, this planet—named Tau Bo?tis A b—is a "hot Jupiter" that whirls around its sun every 3 days, 7 hours, and 30 minutes. The astronomers discovered carbon monoxide in the planet's air. Coupled with the orbital period, the poison gas's Doppler shift reveals that the planet's orbit is tilted 45° to our line of sight and that the world itself weighs six times more than Jupiter . Furthermore, the spectra of the carbon monoxide molecules indicate that unlike many other hot Jupiters, the planet's atmosphere has no temperature inversion; instead, temperature drops with increasing altitude. The reason? Tau Bo?tis A spins fast and emits ultraviolet radiation, which destroys compounds in the planet's upper air that would otherwise absorb starlight, heat up, and cause the temperature to rise with altitude. | 如果登上翘起的磅秤，它很可能不能够正确显示你的体重。搜寻行星的人员遇到了相同的问题：遇到一颗行星拖拉其恒星时，他们通常不知道该行星的轨道有多么倾斜，因此他们无法测量出这个星球的准确质量。现在，正如天文学家们今天在《自然》杂志在线版上报道的那样，智利甚大望远镜的红外光谱揭示了最初发现的一颗行星的轨道倾斜度。该行星距离地球51光年，被称为“Tau Bo?tis A b”，是一颗“热木星”，每3天7小时30分绕其恒星一周。天文学家们在这颗行星的大气中发现了一氧化碳。一氧化碳的多普勒频移结合轨道周期显示，这颗行星的轨道对于我们的视线来说倾斜45°，它本身比木星重六倍。此外，一氧化碳分子的光谱表明：跟许多其他的热木星不同，该行星的大气不存在逆温现象；相反，那里的温度是随着高度的上升而下降的。原因何在？恒星Tau Bo?tis A旋转很快，释放出的紫外线破坏了行星大气上层的化合物，因此无法吸收星光而升温，也不能致使气温随着高度的上升而上升 |
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