Welcome to Bedtime Astronomy. Explore the wonders of the cosmos with our soothing Bedtime Astronomie 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.
Welcome back. We're about to jump into one of the biggest, uh most frustrating puzzles in astronomy today.
It really is. It's a genuine mystery.
We found what over five thousand planets outside our solar system something like that.
Thousands, yes, hot Jupiter's super earths, you name it. We're a getting really good at finding planets.
Especially the big ones, the gas giants. And if you look at our own solar system, Jupiter Saturn, they're just swarming with moons.
Absolutely, They're like miniature solar systems in their own right.
So the big question, the one that vexes everyone, is where are all the exo moons?
Exactly statistically they should be everywhere, billions of them. Yet as of today, we have zero confirmed exomoons, not a single one.
An absence of evidence, as they say, but that's not evidence of absence. It just means we might be looking for them in the wrong way.
And that's precisely what a new paper we've been digging into suggests. It's by Thomas Winterholder and his.
Colleagues, right, it's on the ARCSIFF preprint server, so you can go check it out.
And it lays out this really clear verdict. It basically says, look, the moons are there. The problem isn't the universe. The problem is us. Our technology is for this specific task crippled.
So that's our mission today. We're going to unpack why our current methods are failing us, what some are calling the transit.
Trap, and then we'll get into the exciting part, the proposed solution.
A completely new technological path forward. The paper details how we could build something capable of finding Earth sized moons up to what was it, two hundred parsecs away?
Two hundred parsex that's about six hundred and fifty light years. So we're not just talking about our immediate stellar neighborhood.
We're talking about a serious, wide scale survey of our corner of the galaxy. So okay, let's start at the beginning the transit trap. Why are our best planet hunting tools failing so badly at finding moons?
It all comes down to the workhourse of exoplanet detection, the transit method. It's given us thousands of planets, and.
It's I mean, it's an elegantly simple idea. Right, You scare at a star.
You stare at a star and you wait for it to blink exactly.
You watch for a tiny, tiny dip in its brightness, and if that dip happens regularly, like clockwork.
Then you can be pretty sure something a planet is passing in front of it. From our point of view, it's blocking a little bit of the starlight.
It's been incredibly successful missions like Kepler Tess. They've used this to build our entire catalog of worlds.
But when you try to apply that same logic to finding a moon orbiting that planet, the whole thing just falls apart.
Why. I mean, a moon would block life too, wouldn't it?
It would? But the problem is alignment. For a planet, you need three things lined up, the star, the planet, and us here on Earth.
Okay, that's already a pretty rare alignment it is.
Now for a moon, you need four things to line up. The star, the planet, the Moon, and us, and not just lined up, but at the exact moment the Moon is also transiting the star.
So the Moon has to be in the right place in its own little orbit around the planet at the exact same time the planet is in the right place in its giant orbit around the star precisely.
And the signal you get is incredibly complex. It might be a tiny dip before or after the main planet's transit. It might even just subtly change the shape of the planet's dip.
And you're trying to pick that out from hundreds of light years away.
You're trying to distinguish that from all the other noise the star it sell flickers, you have instrument noise. It's a statistical nightmare. The odds of that perfect celestial choreography happening are just astronomically low.
But that's not even the real killer, is it. There's a much deeper, more fundamental problem with using transits.
That's right, and this is the core of the issue. It's a physics constraint. We need to talk about something called the hill sphere.
The hillsphere. Okay, break that down for us.
The hill sphere is the best way to think of it is as a planet's gravitational zone of influence. It's a bubble of space around the planet where its own gravity is the boss.
So inside that bubble, the planet's gravity is stronger than the star's gravity.
Strong enough to hold on to something. Yes, if an object like a moon is inside the hillsphere, the planet can keep it in a stable long term orbit, and if it's outside the stars gravity wins. It'll either rip the moon away or destabilize its orbit until it's ejected from the system. So the size of that hillsphere is absolutely critical. It dictates how much room a planet has to keep its moons.
Okay, so a bigger hillsphere means more stable moons, or at least the potential for them exactly.
And here is the paradox. Transit method. The one we've been using works best for planets that are very close to their star.
Right because they orbit quickly, they transit all the time, so we get lots of data points and can confirm them easily the hot Jupiter's precisely.
But what happens when a planet is very close to its star.
The star's gravity is much much stronger.
There, overwhelmingly strong, and that immense gravitational pull from the star shrinks the planet's hillsphere down to almost nothing.
So let me get this straight. The planets that are easiest for us to find with our best method are in the.
Very region where it's gravitationally hardest for them to hold onto moons in the first place.
We've been looking in the absolute worst place we have.
We've perfected a tool that is biased towards finding planets and environments that are actively hostile to the very things we're looking for. It's a fundamental mismatch.
It makes you wonder, was this a huge oversight? Why do we spend decades on a method that was doomed to fail for moon hunting.
It wasn't really an oversight so much as a necessary first step. We had to prove exoplanets were common first. The transit method was the fastest, cheapest way to do that. It gave us the numbers, the low hanging fruit, but that fruit.
Was growing in a very barren orchard.
So to speak, a barren orchard for moons. Yes, we had to build the catalog of planets first. But now that we know they're everywhere. We need a new tool. We need to shift our focus to planets much much farther away from their stars.
And there's a really famous example that just drives this point home right. The whole story with Kepler sixteen twenty.
Five b oh, Yes, the great exomoon candidate that wasn't. For a moment we thought we had one.
The whole community was buzzing. The initial data from Kepler showed this gas giant and there was this weird, extra little dip in the light after the main.
Transit, and the main transit seemed to start a bit early, which hinted that the planet was being tugged on by something a wabble. The signs pointed to a big moon, maybe the size of Neptune.
It felt like we were right on the edge of a historic discovery.
We were, so they pointed the Hubble Space telescope at it to get a better look to confirm it, and the data was messy. It was ambiguous. Some analyses still saw the moon's signal, others said it was gone. It just sort of dissolved under closer scrutiny, So.
It's likely just instrument noise or some weird stellar activity that mimicked the moon.
That's the consensus. Now, yes, yeah, the discovery was essentially debunked, and the whole saga is the perfect illustration of the problem. Even in a best case scenario, the transit signal is so faint, so close to the noise that it's almost impossible to be sure.
So if the transit method is a bust, we need a whole new approach. And this is where the new paper gets really interesting.
This is where we stop looking for a shadow and we start looking for gravity itself.
The technique is called astrometry.
Astrometry it's one of the oldest branches of astronomy, but we're now applying it with a level of precision that is just mind boggling.
So astronometry is basically just measuring the precise position of things in the sky and their movement.
Yes, when we use it to find a planet, we're not looking at the planet at all. We're looking at its star, and we're watching the star wobble Exactly. A planet's gravity pulls on its star just as the star pulls on the planet. They both orbit a shared center of mass, so the star isn't stationary. It makes these tiny little circles in the sky. If we can measure that wobble, we know a planet is there.
Okay, so that's for planets. How do we apply that to moons?
We just shift our focus. Instead of watching the star wobble, we watch the planet wobble.
Ah, because the moon is pulling on the planet.
Right, The planet and its moon are also orbiting their own little center of mass, their Berry center. So as the planet moves through its huge orbit around the star, it's not tracing a smooth, clean arc.
It's doing a little quarkscrew pattern, tiny wobble on top of its main orbit.
A tiny subtle dance. And if we can measure that dance, we've found the moon that's leading it.
Now, this feels like a much better approach, and it directly solves the Hillsphere problem, doesn't it.
It completely flips the scar. The beautiful thing about astrometry is that it works best for planets that are far away from their star. Why is that Well, a more distant orbit is a longer orbit, which gives you more time to measure the wabble and separate it from other signals. But more importantly, planets far from their star are free from the star's intense gravitational mettling, which means they have huge,
stable hill spheres. They have all the room in the world to capture and hold on to large families of moons for billions of years.
So astrometry naturally points us to the most promising systems. It's bias towards the gravitationally friendly neighborhoods exactly.
It resolves the paradox that crippled the transit method. It lets us finally look in the right place.
Which brings up the obvious question, if the strometry is so much better, why haven't we been doing this all along?
Ah, because of what the paper calls the technological deficit. The principle is sound, but the reality of the measurement is the problem.
The wobble is just too small to see with what we have now.
Almost infinitesimally small. When you're looking at a planet hundreds of light years away, the precision required is just staggering.
Let's talk scale. What can our best telescopes do right now?
Our current champion is the very large telescope in our barometer, the VLTI down in Chile. It's an incredible machine and that's not.
One telescope, right, It combines the light from multiple one correct.
It links its four main telescopes together to act as one giant virtual telescope, and with that it can resolve an angular wobble of about fifty micro arc seconds fifty ens.
Okay, you have to put that number in perspective for us. What is a micro arc second?
Okay, So imagine a single degree in the sky. Divide that by three thy six hundred. That's an arcsecond already tiny. Now divide that arc second by another million, that's a micro arcsecond one ears.
So fifty micro arc seconds is already measuring a movement that is just an absurdly small fraction of a degree.
It is. The common analogy is that it's like being able to spot a single human hair from two miles away. That's what fifty a's resolution gets you. It's a breath taking achievement of engineering.
But it's not good enough.
It's not even close. The wobble induced by an Earth sized moon orbiting a gas giant two hundred parsecs away is far, far smaller than that fifty a's limit.
And the VLTI gets that resolution by having its telescopes spread out over a distance a baseline of about two hundred meters.
Right about the length of two football fields. But to find these moons we need to bridge a monumental gap. We're not talking about a small improvement.
We're talking about a leap, and the paper by winter Holder is very specific about how big that leap needs to be.
It is. It calculates that to find what they call a reasonable number of Earth sized exomoons out to that two hundred parsec distance, you need a resolution of around one micro arc second one as.
So from fifty down to one, that's a fiftyfold increase in precision.
A fiftyfold jump. That's not an incremental upgrade. That requires a fundamental rethinking of how we build an observatory.
Soans is seeing a hair from two miles away. What's the analogy for one curation?
The one I've heard is it's the equivalent of measuring the width of a small coin, say a nickel. If that coin we're sitting on the surface of the Moon.
You're trying to measure the width of a nickel on the Moon from Earth.
That's the angular scale we're aiming for. It's measuring a tiny movement of an object that is itself more than a million times farther away than the moon. This is an immense challenge.
Jo, How do you do it? How do you get a fifty fold increase in resolution?
You have to go back to the basic physics of interferometry. There's a simple equation. Your resolution is equal to the wavelength of the light you're observing, divided by your baseline.
Okay, so resolution equals wavelength over baseline.
Right, we can't really change the wavelength. That's just the starlight we're looking at. So if you want to increase your resolution by a.
Factor of fifty, you have to increase your baseline by a factor of fifty.
Exactly, if the vlti is two hundred meters baseline gets you fifty coins, then to get to one oins, you need a baseline that's fifty times longer.
Two hundred meters times fifty is ten thousand meters, ten kilometers.
Several kilometers. Yes, this is the heart of the proposal. We have to stop thinking in terms of hundreds of meters and start thinking in terms of kilometers. This is why they call it a kilometric baseline interferometer.
So you're not building one giant telescope, you're building an array of smaller telescopes spread out over an entire valley.
Maybe that's the concept. A whole network of light collecting stations spread out over kilometers, all linked together optically with a precision that is almost hard to comprehend. They have to act as a single coherent instrument.
How is that even possible? Keeping the light from mirrors kilometers apart perfectly in sync.
That's the insane engineering challenge. It involves a thing called optical delay lines, basically long vacuum tunnels with mirrors that can move to precisely adjust the path length of the light from each station.
So the light from a faraway mirror has to travel further, and these tunnels come for that. So all the light waves arrive at the central hub at the exact same instant to be combined to.
Within a fraction of a wavelength of light. Yes, you have to compensate for everything, thermal expansion, seismic vibrations, atmospheric turbulence. It pushes engineering to its absolute limit.
You mentioned LGO before the gravitational wave detector that's also on a kilometer scale. How is this different.
It's a great comparison for the scale, but the function is totally different. LAGO isn't a telescope. It uses lasers inside its tunnels to measure the stretching of space time itself.
It's detecting ripples in the fabric of the universe.
Right this proposed interferometer would be a true telescope. It would be collecting faint starlight from distant objects using these separated mirrors. Its goal is to measure the position and movement of a physical object, not a distortion in space time.
In an instrument this precise can't just scan the sky randomly. It would be incredibly inefficient. It needs a target.
List, yes, and this is where the synergy with other p become so important. This interferometer isn't a discovery tool, it's a measurement tool. It needs a partner, and.
That partner is the extremely large telescope, the ELT.
The ELT it's under construction right now in Chile, set for completion around twenty twenty eight. It's going to have a thirty nine meters primary mirror, the biggest eye on the sky we've ever built, and.
Its job is different. Its job is just to collect as much light as possible.
Its job is to take the first direct pictures of these very faint, very distant gas giants, the ones orbiting far from their stars, the ones with the big hill spheres that we think are the best candidates for hosting moons.
So the ELT finds the planets. It does the heavy lifting of identifying the targets. It says, okay, look over there, there's a promising gas giant.
Precisely, and once the ELT gives us a target, this kilomer scale interferometer can then stare at that one planet. It doesn't waste time searching. It just locks on and monitors its position over months and.
Years, watching for that tiny one micro arc second wobble.
That's the tag team. The ELT is the spotter, the interferometer is the sniper. It's an incredibly efficient way to tackle the problem.
So what does this all mean. We're talking about a multi billion dollar project, a decade of engineering to build this planetary wobble detector. Why yeah, White chase moons so hard when we have thousands of.
Planets because exomoons might fundamentally change our search for life in the universe.
It's about habitability, It's.
About redefining what habitable even means it gives us a pathway to life that has almost nothing to do with the traditional Goldilocks zone.
Okay, the Goldilock zone being that narrow band around a star where it's not too hot not too cold for liquid water to exist on a planet's surface.
Right, it's all based on energy from the star. But for moons orbiting a gas giant, the star is almost irrelevant. They can be far outside the Goldilocks zone, in the frozen depths of their Solar system and still be habitable.
And this is because of tidal heating.
Tidle heating. It's the real game changer, and we see it right here in our own backyard.
Europa and Enceladus our.
Two most promising candidates for life beyond Earth. Europa at Jupiter and Seladus at Saturn. They are both ice worlds, far from the Sun's warmth. They should be frozen solid.
But they're not. We're pretty sure they both have huge liquid water oceans beneath their ice shells.
And that liquid water is kept warm not by the Sun but by the immense gravity of the planets they orbit.
It's just gravity doing the heating.
Think about the sheer mass of Jupiter as Europa orbits it, Jupiter's gravity is constantly pulling and stretching the little Moon. The side of Europa closest to Jupiter is pulled much harder.
Than the far side, so the Moon is being flexed like squeezing a stress ball, over and over exactly.
And that constant flexing and friction in its rocky core generates a tremendous amount of heat. It's like a planetary scale engine churning away in the Moon's interior, and that.
Heat is what keeps those subsurface oceans liquid.
For billions of years. It's a much more stable and long last energy source than the light from a distant star. It creates this tiny local habitable zone right around the gas giant itself.
So if we find a big moon around a big exit planet, it could have liquid water, even if the whole system is in a deep freeze.
It dramatically expands the amount of real estate in the galaxy where life could get a foothold. It tells us we should maybe stop looking for warm stars and start looking for massive, gravitationally active planets.
Now, the paper is careful to include a reality check here. We all dream of finding another Europa. But that's not what this new telescope would be looking for, is it.
That's a very important caveat. Yes, finding a true analog to Europa or Enceladus is still far beyond our reach. They're just too small.
Europa is only about a quarter the size of Earth.
Right, the gravitational wabble they would induce on Jupiter or Saturn is far far smaller than the one is we're aiming for. We wouldn't be able to see them.
So what is the realistic goal? What are we actually hoping to find?
We're hoping to find the giant versions of these worlds. The paper's target is Earth sized moons or even larger orbiting gas.
Giants, a super Europa.
A super Europa, a world with the mass of Earth or Mars, would be heavy enough to produce that detectable one micro arc second signal in its parent planet, and.
An Earth sized moon would have some major advantages for habitability.
Right, huge advantages. It would have enough gravity to hold onto a thick atmosphere if it's in the right place to experience strong tidal heating. It could have vast liquid oceans, maybe even on its surface.
A world like that it would immediately become the number one candidate for the first confirmed habitable exo world.
It would be the Holy Grail, the ultimate Prize. It's a huge technical leap, but the potential reward completely changes our place in the universe.
So let's just wrap up the grand plan here. We have a cosmic mystery, no confirmed moons.
We have a flawed method transit that looks in the wrong place, and gravitationally hostile environments.
The proposed solution is a pivot to astrometry. But to make it work, we need a fiftyfold jump in precision down to one microar second.
Which is physically impossible without a fiftyfold increase in our telescope's baseline.
So we have to build a kilometric baseline interferometer spread telescopes out over several kilometers to create a single hyper precise instrument.
And of course, a project of that scale comes with a price tag to match.
A few billion dollars. Yeah, in the same ballpark as the ELT itself.
At least.
This is big science. It requires international cooperation, decades of.
Planning, and a huge battle for funding. What are the real world chances of something like this getting built?
The biggest hurdle is convincing the world's funding agencies that this is the most important next step. But its greatest advantage is its timing. The ELT comes online around twenty twenty eight, and.
It's going to start fighting these perfect target planets exactly.
The ELT is going to deliver a catalog of distant gas giants, and the scientific community is going to be clamoring for a way to study them in more detail. The need for this precision follow up tool will become undeniable, So.
The argument will be that the ELT is only half the solution. You need the innerferometer to unlock its full potential.
That's the case you'll have to make. It's the logical next step. First you build the world's biggest camera to find the objects. Then you build the world's biggest ruler to measure them.
A ruler designed to look in the one place we've never been able to look before. Yeah, the stable tightly heated zones far from any star.
It will finally pull the search for eximoons out of the realm of speculation and put it firmly into the realm of observational confirmed science.
Okay, we have covered a lot of ground here, from gravitational bubbles to kilometer long telescopes. But we want to leave you with one final thought, something to really chew on.
We're talking about building technology on the scale of a small city to measure a gravitational nudge from an object hundreds of light years away. It's an incredible endeavor.
But think about the implication if it turns out that the most commonplace for life in the galaxy isn't on a pleasant earth like planet soaking up gentle sunlight.
What if it's on a moon being constantly crushed and stretched and heated from within by the raw mechanical power of its parent planet's gravity.
What does that do to our whole concept of a habitable zone.
Does the real search for life in the universe begin only when we stop looking for the gentle energy of light and start looking for the violent, yet life sustaining power of gravity.
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