A giant planet orbiting the 'extreme horizontal branch' star V 391 Pegasi

摘要

超过200个已知太阳系外行星中大多数绕与我们的太阳相似的主星序 恒星运行。当它们核心的氢耗尽时，它们将变成红色的巨型天体(“红 色巨星”)，能够轻易到达并吞没比较靠里面的行星。在大约50亿年之 后，太阳也将发生这种情况。在此之后，地球的命运就是不确定的， 但围绕一颗“后红色巨星”阶段的恒星运转的一颗行星的发现表明， 地球在那个时刻之后也许能够以某种形式存在下去。该行星在距V 391 Pegasi大约1.7天文单位(AU)的轨道上运行。“红色巨星”阶段的 最大半径约为0.7 Au，该行星在主星序阶段的距离为1 AU。这表明， 轨道距离不到2 Au的行星(按照定义，这样的距离也包括太阳在内) 能够经受住“红色巨星”扩展而生存下来。%After the initial discoveries fifteen years ago, over 200 extrasolar planets have now been detected. Most of them orbit main-sequence stars similar to our Sun, although a few planets orbiting red giant stars have been recently found. When the hydrogen in their cores runs out, main-sequence stars undergo an expansion into red-giant stars. This expansion can modify the orbits of planets and can easily reach and engulf the inner planets. The same will happen to the planets of our Solar System in about five billion years and the fate of the Earth is matter of debate. Here we report the discovery of a planetary-mass body (Msini = 3.2M_(Jupiter)) orbiting the star V391 Pegasi at a distance of about 1.7 astronomical units (au), with a period of 3.2 years. This star is on the extreme horizontal branch of the Hertzsprung-Russell diagram, burning helium in its core and pulsating. The maximum radius of the red-giant precursor of V 391 Pegasi may have reached 0.7 au, while the orbital distance of the planet during the stellar main-sequence phase is estimated to be about 1 au. This detection of a planet orbiting a post-red-giant star demonstrates that planets with orbital distances of less than 2 au can survive the red-giant expansion of their parent stars.