Astronomers Confirm Discovery of the Youngest Known Exoplanet Ever
Astronomers Confirm Discovery of the Youngest Known Exoplanet Ever
天文学家确认发现迄今为止最年轻的系外行星
How are planets born, and how do they grow? This is an essential question in astronomy, one that researchers try to answer by observing “young planets.” That’s easier said than done. All the planets in our solar system are the same age, 4.6 billion years old. Astronomers therefore need to look elsewhere to find planets that have formed more recently—making exoplanets enticing research grounds.
行星是如何诞生并成长的？这是天文学中的一个核心问题，研究人员试图通过观测“年轻行星”来寻找答案。但这说起来容易做起来难。我们太阳系中的所有行星年龄相同，均为 46 亿岁。因此，天文学家必须将目光投向别处，寻找近期形成的行星，这使得系外行星成为了极具吸引力的研究领域。
Still, the vast majority of the approximately 6,000 exoplanets confirmed to date are also billions of years old. That’s not because there aren’t youthful planets out there, but old planets are easier to discover using common techniques such as the transit method, in which astronomers detect the slight dimming of a star’s light as a planet passes in front of it.
尽管如此，迄今为止已确认的约 6,000 颗系外行星中，绝大多数也已有数十亿年的历史。这并不是因为宇宙中不存在年轻的行星，而是因为年老的行星更容易通过常用技术被发现，例如“凌日法”——即天文学家通过探测行星从恒星前方经过时引起的微弱光线变暗来发现行星。
Newly formed exoplanets are often buried within disks of gas and dust, the leftover building blocks from a planet’s formation, also known as a protoplanetary disk. The difficulty of just locating such a new celestial body is part of what makes the discovery of the exoplanet Elias 2-24b—now the youngest ever confirmed—so exciting.
新形成的系外行星通常被埋藏在气体和尘埃盘中，这些是行星形成后留下的“建筑材料”，也被称为原行星盘。仅仅是定位这样一个新天体就非常困难，这也是发现 Elias 2-24b（目前已确认的最年轻系外行星）如此令人兴奋的原因之一。
The research team, led by Andrea Bernardi at Diego Portales University in Chile, identified the exoplanet by focusing on seven protoplanetary disks where there were notable gaps in the dense matter, since it is believed that growing planets carve out such voids. They reanalyzed “coronagraph” data, in which the central star is blocked out, making the dim reflected light from surrounding celestial bodies stand out.
由智利迭戈波塔利斯大学（Diego Portales University）的安德烈亚·贝尔纳迪（Andrea Bernardi）领导的研究团队，通过聚焦七个在致密物质中存在明显空隙的原行星盘，成功识别出了这颗系外行星，因为人们认为正在成长的行星会“开辟”出这样的空隙。他们重新分析了“日冕仪”数据，通过遮挡中心恒星，使周围天体反射的微弱光线得以显现。
Scientists had known about a gap in the disk surrounding the star Elias 2-24 for nearly a decade. The European Southern Observatory’s Very Large Telescope (yes, that’s its official name) had detected a point of light within this gap, sparking an ongoing debate over whether it was truly a planet. The researchers used data collected in 2018 and 2020 at the W.M. Keck Observatory in Hawaii to compare the dot’s positions over the years. They concluded that this bright spot was not an unrelated background star or noise from the equipment, but rather a celestial object gravitationally bound to Elias 2-24—in other words, a planet.
科学家们近十年来一直知道恒星 Elias 2-24 周围的圆盘中存在一个空隙。欧洲南方天文台的“甚大望远镜”（Very Large Telescope，没错，这就是它的正式名称）曾在该空隙内探测到一个光点，引发了关于它是否真的是一颗行星的持续争论。研究人员利用 2018 年和 2020 年在夏威夷凯克天文台（W.M. Keck Observatory）收集的数据，对比了该光点多年来的位置。他们得出结论：这个亮点并非无关的背景恒星或设备噪声，而是一个受引力束缚在 Elias 2-24 周围的天体——换句话说，就是一颗行星。
Elias 2-24b is located approximately 450 light-years from Earth. It orbits its star at a distance of approximately 55 astronomical units (one astronomical unit being the average distance between Earth and the sun). Its mass is estimated to be at most 1.9 to 4.0 times that of Jupiter, though researchers believe its volume is likely to be much smaller than our local gas giant. Since its host star is less than 1 million years old, Elias 2-24b is estimated to be similarly young.
Elias 2-24b 距离地球约 450 光年。它以约 55 个天文单位（一个天文单位为地球到太阳的平均距离）的距离绕其恒星运行。据估计，其质量最多为木星的 1.9 到 4.0 倍，尽管研究人员认为其体积可能比我们太阳系的这颗气态巨行星要小得多。由于其宿主恒星的年龄不到 100 万年，因此估计 Elias 2-24b 也同样年轻。
Until now, the youngest exoplanets confirmed through direct observation were those orbiting the stars PDS 70 and WISPIT 2, both of which are over 5 million years old. This new discovery breaks those records by a significant margin.
在此之前，通过直接观测确认的最年轻系外行星是围绕 PDS 70 和 WISPIT 2 恒星运行的行星，它们的年龄都超过了 500 万年。这一新发现大幅刷新了这些纪录。
And Elias 2-24b has a few attributes that require reevaluating some planetary formation models. One of the standard theories is known as the “core accretion model,” which posits that giant planets are formed when a solid core grows more massive and attracts surrounding gas. The time required for formation varies depending on the conditions of the calculation, but according to NASA, it should take about 5 million years for a Jupiter-sized planet to form. That’s also given Jupiter’s distance from the sun, approximately 5 astronomical units. For a planet with a much farther removed orbit like Elias 2-24b, accruing this mass should take even longer.
Elias 2-24b 具备的一些特性要求我们重新评估现有的行星形成模型。其中一个标准理论被称为“核吸积模型”，该理论认为巨行星的形成过程是：一个固体核心变得越来越大，并吸引周围的气体。形成所需的时间因计算条件而异，但据美国国家航空航天局（NASA）称，一颗木星大小的行星形成大约需要 500 万年。这还是基于木星距离太阳约 5 个天文单位的前提。对于像 Elias 2-24b 这样轨道距离远得多的行星来说，积累到如此质量应该需要更长的时间。
However, Elias 2-24b doesn’t seem to be following these rules. Despite being 55 astronomical units from its host star, it’s already a giant and behaving like a planet after a remarkably short 1 million years. The research team believes this planet was formed through core accretion, which would mean that this model can produce giant planets much faster than previously assumed.
然而，Elias 2-24b 似乎并不遵循这些规则。尽管它距离宿主恒星 55 个天文单位，但在短短 100 万年后，它就已经成为了一颗巨行星，并表现得像一颗成熟的行星。研究团队认为这颗行星是通过核吸积形成的，这意味着该模型产生巨行星的速度比之前假设的要快得多。
Lucas Siesa, a professor at Diego Portales University and coauthor of the paper, pointed out in a press release, “Elias 2-24 b shows us that even our best planet-formation models are still missing some important processes.”
迭戈波塔利斯大学教授、该论文的合著者卢卡斯·西萨（Lucas Siesa）在新闻发布会上指出：“Elias 2-24b 向我们表明，即使是我们最先进的行星形成模型，仍然遗漏了一些重要的过程。”
What can improve these models is finding more young planets, which may soon be more possible than ever. The Nancy Grace Roman Space Telescope, launched by NASA this past August, is equipped with an even more powerful coronagraph than the one the researchers used to locate Elias 2-24b. Astronomers therefore hope to soon detect more young planets as well as other planets obscured by their bright stars.
能够改进这些模型的方法是发现更多年轻的行星，而这在不久的将来可能会比以往任何时候都更容易实现。NASA 于今年 8 月发射的“南希·格蕾丝·罗曼太空望远镜”（Nancy Grace Roman Space Telescope）配备了比研究人员用于定位 Elias 2-24b 的设备更强大的日冕仪。因此，天文学家希望很快能探测到更多的年轻行星，以及其他被明亮恒星遮蔽的行星。
“This is just the beginning of a new era of discovery,” Siesa said. “It’s incredible that with modern technology, we are actually able to see planet formation in action, and Roman will take planet hunting to the next level.”
“这仅仅是一个新发现时代的开始，”西萨说，“令人难以置信的是，借助现代技术，我们实际上能够亲眼目睹行星的形成过程，而罗曼望远镜将把行星搜寻工作提升到一个新的水平。”
This story originally appeared on WIRED Japan and has been translated from Japanese.
本文最初发表于《连线》日本版，由日语翻译而来。