A Young Solar System Reveals How Comets Could Seed Planets With Water

0:00Welcome to Bedtime Astronomy. Explore the wonders of the cosmos with our soothing Bedtime Astronomy podcast. Each episode offers a gentle journey through the stars, planets, and beyond, perfect for unwinding after a long day. Let's travel through the mysteries of the universe as you drift off into a peaceful slumber under the night sky. You know, it's funny how we look at a glass of water or like the vast oceans that cover 70% of our planet and we just kind of accept it as this completely ordinary part of the world. Oh, absolutely. We take it totally for granted every single day. Right. But we rarely stop to think about the sheer mechanical absurdity of that water even being here because if you roll the clock back about 4 and a half billion years to when the Earth was first forming, our planet was essentially this glowing hot molten rock. Yeah, turbulent searing hot furnace, just a chaotic, violent environment.

1:01Exactly. So you're forced to ask this fundamental question, where did Earth's oceans actually come from? How on Earth did this delicate life-sustaining liquid arrive on a newly formed planet without just, you know, instantly boiling off into the infinite void of space? It's wild to think about and it remains one of those compelling paradoxes in planetary science. Because a young terrestrial planet is, well, it's incredibly hostile. The surface temperatures are extreme. You have this young sun blasting the inner system with intense solar winds. Right. Which sounds like a terrible place for water. It is the absolute last place you would expect to find volatile substances like liquid water just hanging around. Because any original water present during the Earth's violent accretion phase, it would have been vaporized, just completely stripped away. So mathematically, you're required to find a delivery mechanism. The planet had to be restocked somehow. Precisely. There had to be a cosmic supply chain that brought the water in much later after the surface

2:01began to cool down. And it had to arrive in a format that actually protected that water during transit. And uncovering that specific supply chain is our central mission today. We are looking at a recent ground-breaking discovery and astronomy that essentially functions as like a time machine. It allows us to watch this restocking process happen in real time. Which is incredibly rare. Yeah, it's amazing. We are traveling to a distant infant planetary system called PDS-70. But analyzing the activity around this specific star, we're basically watching a live-action prequel to our own solar system. Getting a chance to witness firsthand how massive icy comets might act as these heavy-duty cosmic delivery trucks actively transporting water across the void to forming planets. Okay. Let's unpack this. Because to really understand the physics of this delivery mechanism, we first need to get a clear picture of the destination itself, right? We have to travel 370 light years away and really set the scene at PDS-70. So 370 light years, I mean, it's a substantial distance for a human.

3:03Yeah. But in the grand architectural scale of the Milky Way, it's practically in our galactic backyard. Just down the street, cosmically speaking. Exactly. The light we observe from PDS-70 left that system. Every around the time Galileo was building his very first telescopes. But what we see when that light finally hits our modern instruments is a system that is an absolute newborn. Right. PDS-70 is only about 5 million years old. Which is just a cosmic blink of an eye. For context, if our 4.6 billion year old solar system is, say, a middle aged adult, PDS-70 is a newborn that was delivered just a few hours ago. It is still heavily entrenched in its planetary infancy. Yeah. And the central star itself is actually quite similar to our own sun, just slightly cooler and a bit less massive. Yeah. But because it's only 5 million years old, it's still surrounded by this massive, circumsteller disc. That's the swirling chaotic halo of primordial gas and dust, right? The stuff that hasn't been cleared out or absorbed yet. Right. Exactly. But within that messy disc, massive structural development is already underway.

4:04We have definitively identified at least two massive gas giants in there. Kind of like our Jupiter and Saturn. Very similar, yeah. And they are actively orbiting that central star, literally carving massive gaps in the dust cloud as they move. Okay. So we have a relatively cool sunlight star, a messy disc of primordial dust, and two massive gas giants already throwing their weight around. But the major mystery, the spark that ignited this specific investigation actually happened back in 2023. Oh, right. The water vapor detection. Yes. Astronomers pointed their instruments at PDS-70 and detected the distinct signature of water vapor. And they didn't just find it out in the freezing outer edges where you would expect ice to safely survive. No, they found it right in the inner part of the system, relatively close to the star. Which is the critical inner region where terrestrial rocky planets like Earth typically form. So think of this young 5 million year old system like a massive, messy, cosmic construction site. I like that analogy. The raw building materials are scattered everywhere.

5:06But that inner zone right next to the stellar furnace is incredibly hot, swept by stellar winds and just highly turbulent. Very hostile. Right. So if volatile materials like water are out there, how are they surviving? Or more accurately since planets form in these hot, turbulent inner regions, how does water survive or get there in the first place without boiling away? What's fascinating here is that catching a 5 million year old system in this specific transitional phase is just immensely valuable to astrophysics. Only astronomers are stuck playing historical detective. Right looking at old evidence. Exactly. We look at mature, stable systems like our own and try to run the computer models backward. Or we look at formless, dark, molecular clouds and try to guess how they'll eventually collapse into a star. So a PDS 70 basically removes the guesswork. It really does. It provides a live action snapshot of active planetary development. It gives us the laboratory we need to literally test our theories about how volatile substances

6:07maneuver through a young chaotic environment. It's like the difference between analyzing a finished skyscraper to guess how the plumbing was installed versus just pulling up a lawn chair and watching the cranes drop the pipes into the foundation in real time. Yeah, that is a perfect way to look at it. Now the 2023 detection was a monumental clue, but it was really only half the puzzle. Detecting water vapor near the star proved that the water successfully arrived in the inner system. It told us the destination, but not the journey. Right, it provided zero mechanical data on how it actually got there. The water vapor is just the resulting exhaust. To track the actual transport mechanism to find those cosmic delivery trucks you mentioned earlier, researchers had to engage in some incredibly clever observational physics. And that leads us to the breakthrough from a team of researchers at Lund University in Sweden. This was led by astronomy researcher Aline Novee who published their findings in nature communications. And absolutely brilliant piece of detective work. The brilliance of their approach was that to track the water they completely stop looking

7:09for water. Instead, they hunted for a totally different element, which was sodium. Yeah, because tracking water vapor directly as it moves dynamically through a system is notoriously difficult. Water molecules just don't always leave the cleanest, most visible signatures against the really complex backdrop of a star's light. What comments do? Right. Exactly. Comments of those ancient dirty snowballs made of ice, rock and organic dust. They carry a wide cocktail of elements and they carry a significant amount of sodium. In the realm of astrophysics, sodium is an exceptional tracer element. Because it shows up so clearly. Yeah, it's a atomic structure interacts with star light in highly specific, aggressively visible ways. So Novee's and her team went back to the archives. They analyzed highly detailed spectroscopic observations of PDS-70 taken back in 2018. And when they analyzed the light from the star, they found distinct variations in sodium gas. Moving incredibly fast, too. Right. Clocking in at several kilometers per second relative to the star.

8:11But the crucial detail wasn't just the speed. It was the behavior. The sodium gas wasn't a permanent stationary cloud. It was appearing, lingering, and then completely disappearing over the course of just a few nights. And that rapid transient appearance and disappearance is the behavioral smoking gun. It perfectly mirrors the mathematical models of how a comet behaves when it aggressively passes in front of a star on a highly elliptical orbit. And it just moves rapidly out of view. Okay, wait. Let me play the steptic for a second because we need to bridge a gap here for the listener. We do not have a photograph of a comet with a glowing tail flying around PDS-70. No, our telescope just aren't that powerful yet. Right. So wait, if we can't actually see the icy comets, how does disappearing sodium gas prove that comets are flying around? Why don't we just be looking at, I don't know, a random glowing clump of gas floating through the system? It's a fair question. But the mechanics of the observation rule out a random clump. It really comes down to understanding the physics of a process called sublimation within

9:14a vacuum. Okay, break that down for us. So imagine the life cycle of this comet. It forms in the extreme, freezing, dark outer edges of the planetary system. Out there, the water, the sodium, and the other volatiles are all locked together in a rock hard frozen state. Right. But as this icy body gets pulled inward, it approaches the intense heat of the central star. And because space is a vacuum, this isn't like leaving an ice cube on a hot sidewalk where it slowly melts into a puddle? No, not at all. The lack of atmospheric pressure changes the physics entirely. In the vacuum of space, ice cannot exist in a liquid state. So as the comet heats up rapidly from the star's thermal radiation, the ice skips the liquid phase entirely. It just vaporizes. It violently transitions from a solid directly into a massive expanding cloud of gas. That transition is sublimation. It basically rubs. And when that comet erupts, it violently ejects the sodium that was trapped within its ice. Oh, wow. So it creates this massive, expanding halo of sodium gas that physically positions itself

10:17between our telescopes here on Earth and the central star of PDS 70. Exactly. As we used to see it is called absorption spectroscopy. Basically every element on the periodic table absorbs specific wavelengths or colors of light. Right, like a fingerprint. Yeah. So when the star's full spectrum of light shines through that erupting cloud of sodium gas, the sodium atoms act like a sponge. They absorb very specific wavelengths. When that starlight finally reaches our telescopes, we run it through a spectrograph, which spreads the light out into a rainbow. And because the sodium absorbed its specific colors, there are literally black lines missing from that rainbow. It creates a measurable barcode. You got it. It's almost like throwing a highly specific colored filter in front of a camera lens for just a few nights. And then violently ripping it away as the comet moves past. That is the exact mechanism. We are not seeing the physical three mile wide rock of the comet itself. We are detecting the massive temporary shadow cast by its erupting gas cloud. That is so cool.

11:18And the speed of the shadow, combined with the fact that it disappears after a few days, perfectly aligns with the velocity and trajectory of an inward falling comet. Here's where it gets really interesting. Finding this sodium shadow isn't just a neat observational trick. This is the very first time astronomers have ever found evidence of exocommets. Comments operating outside our solar system, orbiting a relatively cool, sunlight star. The magnitude of that simply cannot be overstated. We've detected signatures of exocommets before, but almost exclusively around much hotter, brighter, more massive stars. Big beta-pictorus, right? Right, beta-pictorus. Where the thermal environment is so extreme that the sublimation is incredibly violent and just much easier to spot. But detecting this around a sunlight star brings the phenomenon right to our own doorstep. It proves the mechanism happens in systems that look just like ours. And as Novee's pointed out, this is the absolute youngest system where exocommet activity has ever been proposed. We're watching the cosmic delivery system boot up for the very first time in an inace

12:21and solar system. The trucks are officially on the highway. Exactly. And the discovery of the traffic inevitably forces us to examine the physics of the highway itself. Which is the next major hurdle. Because, okay, we have the spectroscopic evidence of these sublimating comets right near the hot star. But comets are born in the freezer. Right. Out in the dark. The extremely cold outer rim of a planetary system where water can safely exist as a solid chunk of ice. So how do you strip an icy body of its orbital momentum so drastically that it plunges toward the star? Because they don't just catch a breeze and float inward. Exactly. Orbits are governed by inertia. To bridge the immense gap between the freezing outer system and the hot inner system, you need an engine. You need an immense amount of force to alter their trajectory. Those orbits in the outer system are generally quite stable. I mean, unless a kinetic impact or a massive gravitational disruption occurs, an icy body will happily orbit in the dark outer rim for billions of years.

13:22And this brings us back to the LUN university team. Because they didn't stop at just analyzing the light shadows. To find that gravitational engine, they ran highly complex, end-body orbital simulations of the entire PDS-70 system. Which takes a lot of computing power. Oh, yeah. These simulations involve plugging in the estimated masses, velocities, and gravitational fields of all the known objects, and then just fast-forwarding the math over millions of years to see how things physically interact. And the key players in those simulations are the two massive gas giants we established at the beginning. Right. Because the gas giants are already fully formed. They possess immense mass, which means they wield incredible gravitational influence over the rest of the disk. The simulations reveal that those gas giants are not just passively orbiting. The best way to visualize the mechanics of PDS-70 is to picture the entire system as a massive cosmic pinball machine. I love that visual. Right. The comets are the pinballs slowly drifting around in the cold outer rim.

14:23But those two massive gas giants are acting exactly like the flippers. As a comet's orbit slightly overlaps with the territory of a gas giant, it enters that massive gravitational field. The gravity from a Jupiter-sized planet is an incredibly potent kinetic tool. When a smaller object approaches a massive planet, it doesn't just quietly get captured. Depending on the angle, it can undergo a gravitational assist, basically a slingshot effect. It just whips it around. Yes. The planet's gravity violently alters the comet's momentum and vector. It aggressively yanks these distant icy comets out of their stable orbits and violently flings them inward, turning an outer-room ice cube into an inward bound projectile aimed right at the inner system. If we connect this to the bigger picture, the implications for planetary formation are absolutely staggering. What we are observing is that gravitational bullying by gas giants is a highly efficient vital transport mechanism. They're doing the heavy lifting. The gas giants are the heavy machinery of the supply chain. They are actively harvesting water, sodium, and other volatile building blocks from the

15:27frozen sterile wastelands of the outer system and channeling them directly into the inner zone. The exact region where terrestrial rocky planets are trying to form. Because like we said, the inner region is far too hot for water to just condense and stay there on its own during the early stages. But if you constantly bombard that inner construction site with massive icy comets flung inward by the gas giants, you're fundamentally overriding the thermal limits of that zone. You're forcefully delivering the water precisely where it needs to be for a cooling rocky planet to eventually capture it. It is a brilliantly chaotic process. It's violent, random, and incredibly effective. So we've solved the mystery of the moving water in PDS 70. We have broken down the spectroscopy of the sodium shadows and we've modeled the cosmic pinball machine. But if we pull back, the crucial question is, what does a 370 late-year distant pinball game actually have to do with us?

16:27It matters deeply. This analyzing PDS 70 is functionally identical to looking into a mirror that reflects our own deep geological past. Alexander Stockwell Murphy, an astronomer at Lund University who collaborated on this research, highlighted this connection perfectly. She pointed out that this exact transport process, massive gas giants, slinging icy comets inward, is highly reminiscent of the dominant theories regarding our own early solar system. Because the origin of Earth's water is still a rigorously debated topic in both geology and astronomy. We established earlier that the early Earth was a molten hellscape, totally incapable of holding surface water. So how did the Pacific Ocean get here? We needed a delivery. Now, to be fair to the broader science, water-rich asteroids from the relatively nearby asteroid belt are strongly considered as a primary source for Earth's oceans. True, they are a huge piece of the puzzle. But this observational data from PDS 70 massively strengthens the theory that early comets played a critical, simultaneous role. It provides tangible evidence that our own gas giants, specifically Jupiter and Saturn,

17:31likely acted as our local pinball flippers. They were the gravitational engines. Yeah, flinging ancient icy comets inward from the Kuiper belt and the Orte Cloud, delivering the water that now makes up the Atlantic Ocean, the rain falling outside your window, and even the water inside your own cells. And that observation provides empirical physical backing to what was mostly a threat theoretical mathematical model of our own history. We are no longer just running computer simulations, guessing that Jupiter might have flung comets at early Earth. We're seeing it happen. Exactly. We are aiming our telescopes at another young system and verifying that the mechanism actively happens in nature. So what does this all mean? If we have proven the delivery mechanism exists, the next logical step feels like we need to see the package actually arrive. That's the holy grail, yeah. But looking for a tiny, dark, rocky planet buried inside a dusty, turbulent disc, 370 light years away, seems physically impossible with our current technology.

18:32How do we actually prove the interplanets are catching this water? The future of this specific investigation requires a massive leap in optical technology. And Jen's Hoymakers, another astronomy researcher at Lund, laid out the precise roadmap for this. Okay, what's the plan? What's your breakthrough relies on an unprecedented piece of hardware called the extremely large telescope or ELT, which is currently under construction in the high deserts of Chile? When astronomers name things, they really don't mess around. The ELT. The naming is highly accurate, I promise you. Oh. To understand why we need it, you have to look through the limitations of our current telescopes. When Novea as an her team found the comets, they relied on detecting expanding gas clouds because the comets themselves are just too small. Right, the shadows. Yeah. Rocky planets in the early stages of formation are also relatively small and they emit very little light. So when you try to find a small dark rock hidden within the incredibly bright glare of the central star and all that thick, dusty mess of a young planetary disc, current telescopes

19:32simply do not have the angular resolution or the light gathering power to separate the planet from the background noise. To put this into perspective for you, the James Webb Space Telescope, which is currently the most powerful observatory humanity has ever built, has a primary mirror that is about 6.5 meters across. Which is huge for space. It is. But the extremely large telescope is going to have a primary mirror that is over 39 meters across. That is a single telescope mirror, roughly the size of a baseball diamond. The surface area difference is just exponential. That massive light bucket, combined with advanced adaptive optics, which physically deformed the mirror thousands of times a second to cancel out the blurring effect of Earth's atmosphere that will provide a resolution that we have never had access to before. Unbelievable. Furthermore, the ELT is highly optimized for infrared observation. Which means it isn't just looking for visible light, right? It's looking for heat signatures. It can pierce straight through the cold dust of the primordial disc and detect the faint, thermal glow of a rocky planet that is still hot from the friction of its own formation.

20:35Precisely. If that unprecedented capability, once the ELT becomes operational late in this decade, astronomers will point it right back at PDS-70. To look for the plants themselves. Exactly. We have already proven the water is being transported. We know the icy comets are flying inward. The ELT will finally allow us to pierce the veil of the inner system and see if there are hidden, newly forming rocky terrestrial planets sitting right there actively being bombarded and catching this incoming water. We could literally watch an alien world acquiring its very first oceans in real time. To synthesize this entire journey for you, we started by questioning the origin of Earth's oceans, looking at the water in our own backyard. And that simple question led us to analyze the physics of disappearing sodium shadows 370 light years away. By decoding those spectral shadows, we uncovered a massive, chaotic gravitational transport system driven by giant planets acting as cosmic flippers, forcibly delivering the chemical building

21:36blocks of life to the inner solar system. The change of discovery is phenomenal. But taking a step back, the mechanics of this transport system leave us with a deeply profound, almost unsettling implication about the chaotic look required for planet formation. Uncettling in what way? Well, analyze the precise tolerances of the mechanism we just outlined. The delivery of water to the inner system relies entirely on the immense gravitational strength and a specific orbital positioning of those outer gas giants. If the gas giants in PDS-70 had formed to just a little less mass, their gravitational reach wouldn't be strong enough to violently alter the comet's trajectories. Or if they had formed in slightly different, more distant, oracle tracks, their gravitational fuels might not overlap with the icy comet reservoirs in the exact right way. Ah, I see. The flippers would completely miss the pinballs. Exactly. If the architecture of that system was tweaked by even a fraction of a percent, those gas giants wouldn't fling those icy comets inward. The comets would remain safely permanently frozen in the outer rim.

22:39And the inner system, where the rocky planets are attempting to form, would be completely starved of alladils, who would remain totally and permanently dry. Just bear in the sterile rock forever. Which fundamentally challenges how we view our place in the universe. We tend to search for water worlds with a baseline assumption that oceans are a natural expected byproduct of standard planetary evolution. Like it's just meant to happen. Right. But the mechanics of PDS-70 demonstrate how incredibly fragile and conditional the process truly is. It forces us to ask, is a life-sustaining water-rich planet like Earth actually a cosmic inevitability? Or are we simply the incredibly fortunate winners of a completely random, high stakes game of gravitational pinball? Think about it. If our own Jupiter had been just a little bit smaller or formed a little further out, worth might just be another dry, dead rock floating in the dark. That is a staggering perspective to leave you with. Every single time you look at the ocean or simply turn on your kitchen tap, you might just be looking at the jackpot of a chaotic cosmic lottery that played out billions of

23:42years ago. Thank you for joining us for this exploration today. Keep looking up, keep asking questions, and we will catch you next time.