National Research Council Canada, Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, British Columbia V9E 2E7, Canada.
Nature. 2010 Dec 23;468(7327):1080-3. doi: 10.1038/nature09684. Epub 2010 Dec 8.
High-contrast near-infrared imaging of the nearby star HR 8799 has shown three giant planets. Such images were possible because of the wide orbits (>25 astronomical units, where 1 au is the Earth-Sun distance) and youth (<100 Myr) of the imaged planets, which are still hot and bright as they radiate away gravitational energy acquired during their formation. An important area of contention in the exoplanet community is whether outer planets (>10 au) more massive than Jupiter form by way of one-step gravitational instabilities or, rather, through a two-step process involving accretion of a core followed by accumulation of a massive outer envelope composed primarily of hydrogen and helium. Here we report the presence of a fourth planet, interior to and of about the same mass as the other three. The system, with this additional planet, represents a challenge for current planet formation models as none of them can explain the in situ formation of all four planets. With its four young giant planets and known cold/warm debris belts, the HR 8799 planetary system is a unique laboratory in which to study the formation and evolution of giant planets at wide (>10 au) separations.
对附近恒星 HR 8799 的高对比度近红外成像显示出三颗巨行星。之所以能够获得这样的图像，是因为所成像的行星具有宽轨道（>25 个天文单位，其中 1 au 是地球-太阳距离）和年轻（<100 Myr），它们仍在辐射在形成过程中获得的引力能，因此仍然很热很亮。在系外行星领域，一个重要的争议领域是，是否存在质量大于木星的外行星（>10 au）是通过一步引力不稳定性形成的，或者更确切地说，是通过涉及核心积累然后积累主要由氢和氦组成的大量外部包层的两步过程形成的。在这里，我们报告了第四颗行星的存在，它位于内部，质量与其他三颗行星大致相同。这个系统有了这颗额外的行星，对当前的行星形成模型构成了挑战，因为它们都无法解释所有四颗行星的原位形成。HR 8799 行星系统拥有四颗年轻的巨行星和已知的冷/暖碎片带，是研究宽距（>10 au）分离处巨行星形成和演化的独特实验室。
