Discovery of the first 系外行星 candidate in X-ray binary M51-ULS-1 in the spiral galaxy Messier 51 (M51), also called the Whirlpool Galaxy using transit technique by observing dips in brightness at X-ray wavelengths (instead of optical wavelengths) is pathbreaking and a game changer because it overcomes the limitation of observation of dips in brightness at optical wavelengths and opens up the avenue for the search of 系外行星 in external galaxies. Detection and characterisation of planets in external galaxies has significant implications for search for extra-terrestrial life.
“但是每个人都在哪里？” 早在 1950 年夏天，费米就脱口而出，思考为什么尽管外星生命存在的可能性很大，但没有证据表明太空中存在任何外星生命 (ET)。 这条著名路线过去四分之三世纪，仍然没有地球以外任何地方存在任何生命的证据，但搜索仍在继续，该搜索的关键组成部分之一是探测太阳系以外的行星及其可能存在的特征生命的签名。
超过4300岁； 系外行星 在过去的几十年中发现了可能有也可能没有适合维持生命的条件。 所有这些都是在我们家中发现的 星系。 没有 系外行星 已知是在银河系外发现的。 事实上，没有证据支持行星系统存在于任何外部星系的想法。
科学家们现在报告发现了一种可能的 系外行星 首次在外部星系中候选。这颗太阳系外行星位于螺旋星系梅西耶 51 (M51)，也称为漩涡星系，距母星系约 28 万光年 银河.
Usually, a planet is detected by way of observing eclipse it produces when it transits in front of its star while orbiting around thus blocking the light emanating from the star (transit technique). This event is observed as temporary dimming of the star. Search for an 系外行星 involves searching for dips in the light of a star. The other method of detection of planets is by radial velocity measurements. All 系外行星 have been detected using these techniques in our home galaxy at relatively short intra-galactic distances in the range of 3000 light years.
However, searching for dips in the light at larger inter-galactic distances to detect 系外行星 outside the Milky Way is an uphill task because an external galaxy occupies a small area in the sky and the high density of stars does not allow study of an individual star in sufficient details to enable detection of signatures of a planet. As a result, search at optical wavelength in an external galaxy was not feasible until now and no 系外行星 outside our home galaxy could be discovered. The latest research is pathbreaking and a game changer because it seemingly overcomes this limitation by observing dips in brightness at X-ray wavelengths (instead of optical wavelengths), and opens up the avenue for the search of 系外行星 in other galaxies.
X-ray binaries (XRBs) in the external galaxies are considered to be ideal for the search of 系外行星. These (i.e., XRBs) are a class of binary stars made up of a normal star and a collapsed star like white dwarf or a 黑洞. 当恒星足够近时，由于重力，来自正常恒星的物质会从正常恒星拉向致密恒星。 结果，致密恒星附近的吸积物质变得过热并在 X 射线中发光，表现为明亮的 X 射线源 (XRS)。
为了探测围绕 X 射线双星 (XRB) 运行的行星，研究小组在三个外部星系 M51、M101 和 M104 中寻找从明亮 X 射线双星 (XRB) 接收到的 X 射线亮度的下降.
该团队最终专注于 X 射线双星 M51-ULS-1，它是 M51 星系中最亮的 X 射线源之一。 观察到钱德拉望远镜接收到的 X 射线亮度下降。 对亮度下降的数据进行了各种可能性的检查，发现适合由一颗行星（最有可能是土星的大小）进行凌日。
This study is also novel for carrying out the search of 系外行星 successfully for the first time at X-ray wavelength. On the broadest level, this landmark discovery of 系外行星 outside our home galaxy expands the scope of search of 系外行星 to other external galaxies, which has implications for the search of extra-terrestrial intelligent life.
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来源：
- Di Stefano, R.、Berndtsson, J.、Urquhart, R. 等。 通过 X 射线传输探测到的外部星系中可能的行星候选者。 自然天文学（2021）。 DOI： https://doi.org/10.1038/s41550-021-01495-w. 也可在网上购买 https://chandra.harvard.edu/photo/2021/m51/m51_paper.pdf. 预印本可在 https://arxiv.org/pdf/2009.08987.pdf
- 美国宇航局。 钱德拉在另一个星系中看到了可能存在行星的证据。 可在线获取 https://chandra.harvard.edu/photo/2021/m51/
- 美国宇航局。 科学 – 物体 – X 射线双星。 可在线获取 https://imagine.gsfc.nasa.gov/science/objects/binary_stars2.html
- 施维特曼 E.，江 N.， 等 2018. 系外行星生物特征：对远程可探测生命迹象的回顾。 天体生物学卷。 18, No. 6。1 年 2018 月 XNUMX 日在线发布。DOI： https://doi.org/10.1089/ast.2017.1729
