航天局研究：引力波天文台 LISA 有望在双白矮星系统里找到全新一类系外行星
马里兰州 Greenbelt — 美国国家航空航天局（NASA）11月10日发布研究介绍说，正在研制中的引力波天文台 LISA 有望在双白矮星系统里揭示一类全新的系外行星：两项由 NASA 天体物理学家主导、发表在《天体物理学杂志》上的研究，详细说明了行星如何把自己的印记留在双白矮星发出的引力波信号里。
LISA 全称激光干涉仪空间天线（Laser Interferometer Space Antenna），由欧洲航天局主导、计划在 2030 年代中期发射，NASA 提供望远镜等关键硬件与工程和科学支持。它由三艘航天器组成、跟随地球绕太阳运行，构成边长约 250 万公里的巨大三角形阵列，测量航天器之间小于氦原子宽度的距离变化，捕捉经过的引力波。
研究第一作者、马里兰大学巴尔的摩分校与 NASA 戈达德太空飞行中心的博士后研究员 Casey McGrath 说，双白矮星系统预计是 LISA 最常见的信号源，由两颗白矮星在紧密轨道上互相绕转；如果一颗系外行星也在这个系统里运行，它的存在会印在引力波信号上、能被研究人员找出来。其原理与用光找行星的径向速度法相似：行星的引力让双星系统产生微小的反冲运动，给引力波信号加上多普勒调制，在主频率两侧形成边带，边带里就带着行星存在的消息。
研究估算，银河系里轨道周期不到一小时的双白矮星系统约有 1,000 万个，LISA 预计能分辨其中约 10,000 个系统。在理想条件下，LISA 能在数千光年外探测到木星质量的行星。迄今只在少数孤立白矮星周围发现过系外行星，双白矮星周围还没有发现过，这类探测将代表一个全新的行星群体。
第二篇论文研究了如何确认这类行星：双白矮星系统里的行星发出的红外光比例高于恒星本身的预期，NASA 的 James Webb 太空望远镜凭红外灵敏度能在至少 500 光年外探测到这种红外过量。论文合著者、密歇根大学助理研究科学家 Mary Anne Limbach 说，LISA 首次探测到系外行星后，用 Webb 做联合确认、证明引力波信号对应的是真行星而不是假信号，将非常关键。
（来源：美国国家航空航天局）
NASA Study: Gravitational-Wave Observatory LISA Could Reveal a New Population of Exoplanets Around Double White Dwarfs
GREENBELT, Maryland — NASA said on November 10 that two studies led by agency astrophysicists show in new detail how the LISA gravitational-wave observatory, now in development, could reveal a new and unique population of exoplanets throughout the galaxy, imprinted on the signals from tightly orbiting double white dwarf systems. The papers were published Tuesday in The Astrophysical Journal.
LISA, the Laser Interferometer Space Antenna, is led by ESA (European Space Agency) and slated for launch in the mid-2030s, with NASA contributing telescopes, other critical hardware, and engineering and scientific support. The observatory will consist of a trio of spacecraft following Earth as it orbits the Sun, forming a triangular array stretching 1.6 million miles (2.5 million kilometers) on each side, measuring changes in their relative distances smaller than the width of a helium atom.
Casey McGrath, a postdoctoral researcher at the University of Maryland, Baltimore County and NASA's Goddard Space Flight Center in Greenbelt, Maryland, and author of the first paper, said double white dwarf systems are expected to be the most common source for LISA. If an exoplanet also orbits within one of these systems, its presence becomes imprinted on the gravitational wave signal. The method is similar to the radial velocity technique used with light: the planet's gravity gives the binary a small reflex motion, adding a Doppler modulation to the gravitational signal that appears as sidebands on either side of the primary frequency, and the sidebands carry the message of the planet's presence.
Astronomers estimate the Milky Way contains 10 million double white dwarf systems with orbital periods of less than an hour, and expect LISA to resolve about 10,000 individual systems. Under ideal circumstances LISA could detect a sub-Jupiter exoplanet, and a Jupiter-mass planet in such systems up to thousands of light-years away. Exoplanets have been detected around a few isolated white dwarfs, but none around a binary white dwarf, so LISA detections would represent a distinct new population.
The second paper investigates confirmation using NASA's James Webb Space Telescope: a planet in a double white dwarf system produces a larger portion of infrared light than expected from the stars themselves, and Webb can detect that infrared excess out to at least 500 light-years. Mary Anne Limbach, an assistant research scientist at the University of Michigan and a co-author, said validating some of the first detections will be critical to establish that the gravitational signals correspond to genuine planets rather than false positives.
(Source: NASA)
来源：美国国家航空航天局
发布：GSM·美国（发布链接）
图片来源：图：LISA 引力波天文台概念图（ESA／NASA，Wikimedia Commons）。 / Image: Artist concept of the LISA gravitational-wave observatory (ESA/NASA, via Wikimedia Commons).
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