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寻找“第九行星”

组装中的西蒙尼巡天望远镜
@Rubin Observatory/NSF/AURA, CC BY 4.0, via Wikimedia Commons

如果你开始记事的时间早于2006年，那你很可能记得当时天文学界发生的一件“破圈”大事。当年，国际天文学联合会（International Astronomical Union，IAU）决定开除冥王星的“行星籍”，将其从太阳系第九大行星降级为矮行星，这颗行星于1930年由天文学家克莱德·汤博（Clyde Tombaugh）发现，长期被视为太阳系的第九大行星。对于很多喜欢冥王星的天文学爱好者而言，这无疑是个令人震惊和悲伤的噩耗。与此同时，学术界对此也展开了长期争论。

与之相伴而生的是“第九行星”假说，该假说由天文学家康斯坦丁·巴特金（Konstantin Batygin）和迈克·布朗（Mike Brown）于2016年提出，他们希望通过该假说解释一些海王星外天体（TNOs）的异常轨道聚集现象。“第九行星”假说认为，在海王星轨道之外的遥远区域可能存在一颗大小约为地球5~10倍的太阳系行星，其引力影响了某些海王星外天体的轨道。但截至目前，这颗行星依旧停留在理论层面，没人见过它真正的样子，也无法确定它是否真的存在。

考虑到这片区域距离地球实在太远，天文学家将发现第九行星的希望寄托在一些探测器或天文望远镜上。2015年7月，美国“新视野”号（New Horizons）探测器飞掠冥王星，深入太阳系边缘，留下了一张冥王星的高清图像和珍贵的天文数据和信息。2025年，一个即将投入使用的新望远镜或许可以帮助我们“看到”更多天体，揭开有关第九行星的真相。

自2006年以后，许多人可能都模糊地听说过第九行星。这是一个非常引人入胜的概念，但当我们谈论第九行星时，我们实际上在谈论什么？是一颗下落不明、即使存在也未必能保住行星地位的天体吗？

这确实是个疯狂的想法。除了大家耳熟能详且已经有许多影像记录的八大行星，以及前文那颗有争议的冥王星之外，在我们所处的太阳系中可能还存在着一整颗我们从未见过的巨大行星。有人会觉得惊讶，与大海捞针不同，我们怎么可能在宇宙中错过一位行星“邻居”呢？

当年，冥王星被降级是一个轰动性的事件，这次降级来自国际天文学联合会对行星定义的修订与明确。而冥王星的“遭遇”也在某种程度上启发了天文学家提出“第九行星”的假说。

2005年，美国加州理工学院（Caltech）天文学家迈克·布朗的团队在太阳系中发现了一块比冥王星更大，且距离我们非常远的“大石头”，并为之暂时定名为齐娜星（Xena），这就是后续颠覆冥王星第九大行星地位的阋神星（Eris）。它与太阳的距离大约是68AU（Astronomical Unit，天文单位，地球到太阳的平均距离）。在迈克·布朗宣布了有关阋神星的发现后，天文学家们突然意识到，如果这颗石头比冥王星还大，那它岂不是也算行星？然而彼时，学界对行星的定义却并不明确，“行星”（Planet）一词源自希腊语中的“漫游者”，最初被定义为在夜空中相对于固定恒星背景移动的天体。这更像一种松散的共识，或是一种直觉。

直到2006年，在布拉格举行的国际天文学联合会大会上，天文学家才尝试明确这一词汇的正式定义，用标准的语言描述它。经过激烈的辩论，行星定义的新版本逐渐成形，草案在大会闭幕式上经由国际天文学联合会成员投票通过。

这一定义沿用至今，其中包括三点内容：首先，天体必须围绕其主恒星运行，就像地球和木星绕太阳运行一样；其次，它必须足够大，具有足够的质量使其自身重力克服刚体力，从而呈现流体静力平衡（近乎圆形）的形状，也就是说，必须是大致呈球形的，如果它小到只是个块状、长条形或奇形怪状的物体，像小行星那样，那它就不是行星；第三个条件是它必须有足够的引力，能够将其他岩石或小行星清除出它所在的轨道。

冥王星被除名的原因正是第三点，它所在的轨道附近有一大堆别的小石块及天体，这些物体也围绕着太阳运转。因此，在2006年国际天文学联合会的决议中，他们正式宣告太阳系只有八大行星。

与此同时，天文学家还为冥王星和阋神星，以及从这一时期开始大量发现的一大批类似天体量身打造了一个新的定义：矮行星（dwarf planet）。矮行星也是围绕太阳运行的天体，其体积足以使自身形成近乎圆形的形状，但无法清除其轨道上的其他碎片。

如果我们没有一个严格的定义，太阳系行星的名单将随着天文学的发展被越拉越长，这将阻碍我们对宇宙的认知。接着，这个故事开始引向第九行星的概念。

“第九行星”假说的提出者和最坚定支持者，正是将“第九大行星”冥王星从太阳系行星除名的人——迈克·布朗。在发现阋神星，迫使国际天文学联合会修改行星定义后，迈克·布朗想到，他曾在2003年与来自双子星天文台（Gemini Observatory）和耶鲁大学（Yale University）的同事们发现过一颗名为塞德娜（Sedna）的神秘天体。

发现它时，赛德娜距离太阳约90 AU，是当时已知最遥远、最寒冷的太阳系天体之一。后来，天文学家们又发现了一些类似的天体，并将其归类为极端海王星外天体（Extreme Trans-Neptunian Objects，ETNOs）。它们位于太阳系边缘，所有轨道完全或大部分位于海王星轨道外侧，离太阳很远，运行轨迹也与太阳系内的天体截然不同，而这可能正是第九行星存在的有力佐证。

极端海王星外天体的特别之处，一方面在于其轨道的距离跨度极大，呈一个极扁的椭圆形。以塞德娜为例，它的近日点为76 AU，远日点则为约936 AU，绕太阳一周的公转周期约为10,500年（一说11,400年）。另外，与太阳系中的其他行星相比，它们的轨道倾角（轨道平面与太阳系黄道面的夹角）也普遍较高。黄道面是地球绕太阳公转轨道所在的平面，也是太阳系中关键的基准平面，八大行星、月球和小行星的轨道平面均与黄道面接近，倾角较小（倾角最大的水星为7°），形成近似共面的结构。

恒星诞生后会留下大量残骸，这些碎片会在一个类似甜甜圈的圆盘状轨道上绕恒星运行，最终，这些物质会形成恒星-行星系统中的行星，它们的轨道也应当大致位于同一平面上。因此，以迈克·布朗为代表的一批天文学家认为，可能存在某种未发现的遥远大质量天体，其引力扰动塑造了塞德娜等极端海王星外天体的高倾角轨道。而这颗天体，就是传说中的“第九行星”。

十多年来，即使到今天还没能真正观测到第九行星，天文学家依然试图通过多方面的证据表明其确实存在。但除了“第九行星”假说，赛德娜之所以如此特殊，还有一些可能的解释。

一种较为简单的可能性是，对于这些离我们太过遥远的天体或太阳系边缘区域，人们的了解还不够充分。这是一处难以抵达和观测的地带，我们发现的这些奇怪天体可能只是碰巧有些特别，实际上那里可能存在着大量轨道更为普通的天体，它们和我们熟知的八大行星的运行轨迹并无二致。

还有一些解释，比如也许在很久以前的某个时期，一颗流浪行星（rogue planet，又称星际行星）或路过的恒星掠过太阳系，将塞德娜等天体从原有轨道拖至今天这个奇怪的轨道。天文学家甚至提出，在太阳系边缘附近可能隐藏着一个黑洞，都怪黑洞将这些天体拉入了奇怪的位置。毕竟我们也不清楚太阳系边缘和外围到底有什么，这是一片未知之地。

如今，这个错综复杂的宇宙谜团终于要迎来一位新的“解题者”：即将投入使用的西蒙尼巡天望远镜（Simonyi Survey Telescope）。它位于智利北部帕穹山伊尔佩恩峰的薇拉·鲁宾天文台（Vera C. Rubin Observatory），将搭载全球最大最清晰的32亿像素数码相机。经过十年的建设，1月15日，天文台成功使用总像素达1.44亿的调试相机（ComCam）完成了一系列全系统测试。接下来，这台世界最大的数码相机将被安装到西蒙尼巡天望远镜的顶部，从今年7月开始对南部夜空开展为期十年的时空遗珍巡天（Legacy Survey of Space andTime，LSST）项目。

这个项目将每三天对整个南半球夜空扫描一次，持续十年……[查看全文]

Largest Camera in the World Will Look for Planet Nine

Rachel Feltman: For Scientific American’s Science Quickly, I’m Rachel Feltman. Unless you’re really on the low end of our listener age bell curve, chances are you grew up learning about our solar system’s nine planets. Of course, unless you’ve been living under a rock since 2006, you also know that now we only have eight planets. Sorry, Pluto fans.

But maybe you’ve also heard rumblings about the mysterious Planet Nine. This hypothetical extra planet has been popping in and out of the news for more than a decade. Thanks to a new observatory set to come online in 2025, the truth about Planet Nine could finally be within reach.

Here to tell us more is Clara Moskowitz, senior editor for space and physics at Scientific American.

Thanks so much for coming on to chat today.

Clara Moskowitz: Thank you for having me.

Feltman: So, starting with basics, I feel like a lot of people have heard vaguely of Planet Nine. It’s a very evocative concept, but when we talk about Planet Nine, what are we actually talking about?

Moskowitz: So we’re talking about this potential planet—nobody knows if it actually exists or not—that might live in our own solar system.

So if you think about it, it’s a wild idea that there could be this whole other planet in our solar system that we’ve never seen. You know, we obviously have these eight planets that we’re really familiar with. Then, of course, there’s the contested situation with Pluto. So it’s like, how could we have missed a whole other world in our cosmic neighborhood? And we’re not talking about something teeny tiny, either. This Planet Nine that might be out there is between, like, five and 10 times the size of Earth. So, you know, it would be a major member of the solar system if it’s there.

Feltman: Yeah, well, I have a lot of questions about that. But I think this is also a great moment to pause and—maybe for folks whose immediate reaction to Planet Nine is something like “We already have a ninth planet. It’s Pluto. How dare they?”—would you remind us what it is that got Pluto so contentiously demoted and why this theoretical Planet Nine would still have planetary status if it does exist?

Moskowitz: Right. So actually, the two stories are related because the whole story of Pluto is what kind of led us to come up with this idea that there might be a Planet Nine. But let me rewind and explain how that’s the case.

The situation with Pluto began to become dicey in 2005 when astronomers discovered this big rock out in the solar system called Eris. Now Eris, turns out, is actually larger than Pluto, but it’s really far out there, which is why we hadn’t seen it before. It’s about 68 times as far from the sun as Earth. So when we found Eris, all of a sudden, astronomers started thinking, “If this thing is bigger than Pluto, then it’s got to be a planet, too, right?” You know, “What exactly is our definition of a planet?” And they realized we didn’t quite have one, a formalized one.

Feltman: [Laughs] It was more of a vibe, more of a state of mind.

Moskowitz: Exactly, exactly. So then astronomers got talking, and it’s really this group called the International Astronomical Union that makes the rules, and they decided that we needed new rules for what qualifies as a planet. So in 2006—this is right after the discovery of Eris—they were forced to come up with rules for what counts as a planet.

So there’s three things. The body has to orbit a star, right? That makes sense, clearly. It has to be sufficiently massive for gravity to make it basically sphere-shaped. You know, it has to be round. If it’s small enough that it’s all chunky and oblong and funky, like asteroids, that’s not a planet. And then the third condition was that it has to clear its own orbit, meaning that it has to have enough gravity that it’s kind of pushed any other rocks or asteroids out of its orbit.

And it was actually that third thing that got Pluto kicked out because Pluto shares its neighborhood with a bunch of other rocks that kind of circle the sun along with it. So it really just isn’t big enough. So Pluto became what we now call a dwarf planet, along with Eris and along with a whole bunch of other objects similar to Eris that started to be discovered around this time.

Feltman: Yeah. And I know a lot of people found that really emotional, but it does seem like we would have had to have this, like, really long, ever growing roster of planets if we hadn’t settled on that firm definition. So I do get why it had to happen.

Moskowitz: Exactly. And then this is where the story starts leading toward the idea of Planet Nine because then they found this object called Sedna.

Sedna is another sort of, you know, similarly sized, really-far-out-there object. The closest it ever gets to the sun is 76 times the Earth-sun distance. And then they found other objects like this.

But the weird thing about these is that they’re on these crazy orbits. The orbits are so stretched out and so distant, and they later found out they also seem to be tilted at this weird angle compared to all of the other planets in the solar system. So they’re just odd, but there’s a bunch of them like this. And scientists can’t really explain how you get all these objects on these extreme, weird, long orbits unless there was something hidden out there guiding them—kind of shaping their paths with its own gravity. And that hidden something would have to be pretty large.

Feltman: So now that scientists have, you know, spent more than a decade thinking about the possibility of Planet Nine, what evidence do we have to support that something like this actually exists?

Moskowitz: So the evidence is all in these other objects, these extreme trans-Neptunian objects that we see, and the fact that their orbits are so odd. Like I said, they’re just super large, super long orbits of a wholly different magnitude than the planets in our solar system.

So that’s one thing, and then the other thing is the tilt of their orbits. So in our solar system, we have what’s called an ecliptic. It’s a plane, and all of the eight planets orbit in this plane, roughly flat. So you can picture them all kind of orbiting in the same flat line. Meanwhile these other weird rocks way out there are orbiting at this total tilt. And it’s like, that’s hard to explain with physics because we think the planets would have all formed kind of together from this circumstellar disk that would have been circling the sun after the sun was born.

So after a star is born, you still have a lot of debris left over that didn’t quite make it into the star itself. And all that junk starts orbiting the star—the baby star—in this doughnutlike disk, and then that material eventually becomes the planets of the star’s planetary system. So it makes sense that they would all be in a plane, and it doesn’t make sense that you’d have this significant population of other things circling way far outside and at a total angle to the plane. For that to happen, you kind of need some other large object influencing everything, which is where we come to this idea of Planet Nine.

Feltman: Okay, so scientists have found these things that maybe suggest a Planet Nine, but what other explanations could there be?

Moskowitz: Right, so, you know, we haven’t seen the planet, so we don’t really know it’s there. One simple possibility is that we still just don’t know that much about these extreme, faraway objects or this realm of the solar system because it’s very hard to see. And it’s possible that this handful of weird objects that we have found just happen to be weird and that there’s actually a ton more objects out there with a lot more normal orbits—and that we’re just looking at, sort of, the outliers and seeing this trend and grabbing for an explanation, but perhaps, you know, it’s a lot more mundane than we thought.

There’s other explanations, too, like maybe a rogue planet or a passing star kind of flew by the solar system at some point and dragged Sedna and these other objects into their weird orbits a long time ago. So maybe they were influenced by some passing object that’s no longer part of our solar system. Astronomers have even proposed that, you know, there could be a hidden black hole out there in the further reaches of the solar system, pulling these things into weird places. It’s kind of, you know, this total hidden realm where we have no idea really what’s out there.

Feltman: So what kind of work is going on to answer these questions and, you know, try to figure out what’s out there?

Moskowitz: The very exciting thing about this story is that it’s a big mystery that we’re pretty much guaranteed to solve one way or the other soon because we have this giant new telescope coming online this year called the Vera Rubin Observatory.

It’s got the largest camera in the world, and it’s in Chile, at the top of a mountain, and it’s turning on this year. It’s supposed to have its first light in July.

And this thing is going to change everything. The way the Rubin Observatory is going to work is that it’s going to scan the sky every couple of days...[full transcript]

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