如果你開始記事的時間早於2006年，那你很可能記得當時天文學界發生的一件“破圈”大事。當年，國際天文學聯合會（International Astronomical Union，IAU）決定開除冥王星的“行星籍”，將其從太陽系第九大行星降級為矮行星，這顆行星於1930年由天文學家克萊德·湯博（Clyde Tombaugh）發現，長期被視為太陽系的第九大行星。對於很多喜歡冥王星的天文學愛好者而言，這無疑是個令人震驚和悲傷的噩耗。與此同時，學術界對此也展開了長期爭論。
與之相伴而生的是“第九行星”假說，該假說由天文學家康斯坦丁·巴特金（Konstantin Batygin）和邁克·布朗（Mike Brown）於2016年提出，他們希望透過該假說解釋一些海王星外天體（TNOs）的異常軌道聚集現象。“第九行星”假說認為，在海王星軌道之外的遙遠區域可能存在一顆大小約為地球5~10倍的太陽系行星，其引力影響了某些海王星外天體的軌道。但截至目前，這顆行星依舊停留在理論層面，沒人見過它真正的樣子，也無法確定它是否真的存在。
考慮到這片區域距離地球實在太遠，天文學家將發現第九行星的希望寄託在一些探測器或天文望遠鏡上。2015年7月，美國“新視野”號（New Horizons）探測器飛掠冥王星，深入太陽系邊緣，留下了一張冥王星的高畫質影像和珍貴的天文資料和資訊。2025年，一個即將投入使用的新望遠鏡或許可以幫助我們“看到”更多天體，揭開有關第九行星的真相。
自2006年以後，許多人可能都模糊地聽說過第九行星。這是一個非常引人入勝的概念，但當我們談論第九行星時，我們實際上在談論什麼？是一顆下落不明、即使存在也未必能保住行星地位的天體嗎？
這確實是個瘋狂的想法。除了大家耳熟能詳且已經有許多影像記錄的八大行星，以及前文那顆有爭議的冥王星之外，在我們所處的太陽系中可能還存在著一整顆我們從未見過的巨大行星。有人會覺得驚訝，與大海撈針不同，我們怎麼可能在宇宙中錯過一位行星“鄰居”呢？
當年，冥王星被降級是一個轟動性的事件，這次降級來自國際天文學聯合會對行星定義的修訂與明確。而冥王星的“遭遇”也在某種程度上啟發了天文學家提出“第九行星”的假說。
2005年，美國加州理工學院（Caltech）天文學家邁克·布朗的團隊在太陽系中發現了一塊比冥王星更大，且距離我們非常遠的“大石頭”，併為之暫時定名為齊娜星（Xena），這就是後續顛覆冥王星第九大行星地位的鬩神星（Eris）。它與太陽的距離大約是68AU（Astronomical Unit，天文單位，地球到太陽的平均距離）。在邁克·布朗宣佈了有關鬩神星的發現後，天文學家們突然意識到，如果這顆石頭比冥王星還大，那它豈不是也算行星？然而彼時，學界對行星的定義卻並不明確，“行星”（Planet）一詞源自希臘語中的“漫遊者”，最初被定義為在夜空中相對於固定恆星背景移動的天體。這更像一種鬆散的共識，或是一種直覺。
直到2006年，在布拉格舉行的國際天文學聯合會大會上，天文學家才嘗試明確這一詞彙的正式定義，用標準的語言描述它。經過激烈的辯論，行星定義的新版本逐漸成形，草案在大會閉幕式上經由國際天文學聯合會成員投票透過。
這一定義沿用至今，其中包括三點內容：首先，天體必須圍繞其主恆星執行，就像地球和木星繞太陽執行一樣；其次，它必須足夠大，具有足夠的質量使其自身重力克服剛體力，從而呈現流體靜力平衡（近乎圓形）的形狀，也就是說，必須是大致呈球形的，如果它小到只是個塊狀、長條形或奇形怪狀的物體，像小行星那樣，那它就不是行星；第三個條件是它必須有足夠的引力，能夠將其他岩石或小行星清除出它所在的軌道。
冥王星被除名的原因正是第三點，它所在的軌道附近有一大堆別的小石塊及天體，這些物體也圍繞著太陽運轉。因此，在2006年國際天文學聯合會的決議中，他們正式宣告太陽系只有八大行星。
與此同時，天文學家還為冥王星和鬩神星，以及從這一時期開始大量發現的一大批類似天體量身打造了一個新的定義：矮行星（dwarf planet）。矮行星也是圍繞太陽執行的天體，其體積足以使自身形成近乎圓形的形狀，但無法清除其軌道上的其他碎片。
如果我們沒有一個嚴格的定義，太陽系行星的名單將隨著天文學的發展被越拉越長，這將阻礙我們對宇宙的認知。接著，這個故事開始引向第九行星的概念。
“第九行星”假說的提出者和最堅定支持者，正是將“第九大行星”冥王星從太陽系行星除名的人——邁克·布朗。在發現鬩神星，迫使國際天文學聯合會修改行星定義後，邁克·布朗想到，他曾在2003年與來自雙子星天文臺（Gemini Observatory）和耶魯大學（Yale University）的同事們發現過一顆名為塞德娜（Sedna）的神秘天體。
發現它時，賽德娜距離太陽約90 AU，是當時已知最遙遠、最寒冷的太陽系天體之一。後來，天文學家們又發現了一些類似的天體，並將其歸類為極端海王星外天體（Extreme Trans-Neptunian Objects，ETNOs）。它們位於太陽系邊緣，所有軌道完全或大部分位於海王星軌道外側，離太陽很遠，執行軌跡也與太陽系內的天體截然不同，而這可能正是第九行星存在的有力佐證。
極端海王星外天體的特別之處，一方面在於其軌道的距離跨度極大，呈一個極扁的橢圓形。以塞德娜為例，它的近日點為76 AU，遠日點則為約936 AU，繞太陽一週的公轉週期約為10,500年（一說11,400年）。另外，與太陽系中的其他行星相比，它們的軌道傾角（軌道平面與太陽系黃道面的夾角）也普遍較高。黃道面是地球繞太陽公轉軌道所在的平面，也是太陽系中關鍵的基準平面，八大行星、月球和小行星的軌道平面均與黃道面接近，傾角較小（傾角最大的水星為7°），形成近似共面的結構。
恆星誕生後會留下大量殘骸，這些碎片會在一個類似甜甜圈的圓盤狀軌道上繞恆星執行，最終，這些物質會形成恆星-行星系統中的行星，它們的軌道也應當大致位於同一平面上。因此，以邁克·布朗為代表的一批天文學家認為，可能存在某種未發現的遙遠大質量天體，其引力擾動塑造了塞德娜等極端海王星外天體的高傾角軌道。而這顆天體，就是傳說中的“第九行星”。
十多年來，即使到今天還沒能真正觀測到第九行星，天文學家依然試圖透過多方面的證據表明其確實存在。但除了“第九行星”假說，賽德娜之所以如此特殊，還有一些可能的解釋。
一種較為簡單的可能性是，對於這些離我們太過遙遠的天體或太陽系邊緣區域，人們的瞭解還不夠充分。這是一處難以抵達和觀測的地帶，我們發現的這些奇怪天體可能只是碰巧有些特別，實際上那裡可能存在著大量軌道更為普通的天體，它們和我們熟知的八大行星的執行軌跡並無二致。
還有一些解釋，比如也許在很久以前的某個時期，一顆流浪行星（rogue planet，又稱星際行星）或路過的恆星掠過太陽系，將塞德娜等天體從原有軌道拖至今天這個奇怪的軌道。天文學家甚至提出，在太陽系邊緣附近可能隱藏著一個黑洞，都怪黑洞將這些天體拉入了奇怪的位置。畢竟我們也不清楚太陽系邊緣和外圍到底有什麼，這是一片未知之地。
如今，這個錯綜複雜的宇宙謎團終於要迎來一位新的“解題者”：即將投入使用的西蒙尼巡天望遠鏡（Simonyi Survey Telescope）。它位於智利北部帕穹山伊爾佩恩峰的薇拉·魯賓天文臺（Vera C. Rubin Observatory），將搭載全球最大最清晰的32億畫素數碼相機。經過十年的建設，1月15日，天文臺成功使用總畫素達1.44億的除錯相機（ComCam）完成了一系列全系統測試。接下來，這臺世界最大的數碼相機將被安裝到西蒙尼巡天望遠鏡的頂部，從今年7月開始對南部夜空開展為期十年的時空遺珍巡天（Legacy Survey of Space andTime，LSST）專案。
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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