Exoplanets

The nearest exoplanet is about 4 light-years away, but from a single speck of light, scientists can peer into their atmospheres - and recreate pieces of these distant worlds here on Earth.

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  What if you could walk on a planet beyond the solar system? Molten rock stretches toward the horizon beneath your feet, and the air around you wobbles with heat like a mirage stretching across a desert land. Vapor rises from the magma. You look up through the haze.

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  High above the surface, some of it may cool enough to condense into lava droplets for a moment before falling back toward the planet below. The star overhead is enormous, roughly 70 times wider in the sky than the sun appears from Earth. And it's barely moving. Travel toward the night side of the planet, and eventually that star slips below the horizon. The landscape grows darker and cooler. But cooler is a relative term on a planet where temperatures are measured in the thousands of degrees. Before a single Earth day has passed, this planet has already completed an entire trip

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  around its star. Here, a calendar year lasts less than 18 hours. This is the exoplanet 55 Cancri E, and it exists roughly 41 light-years away from Earth.

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  An exoplanet is a planet that orbits a star outside the solar system. And what astronomers have found is that possibly every star has its own planetary system.

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  On exoplanet GJ1214B, descend far enough beneath the atmosphere, and water may stop behaving like anything you recognize. Pressure becomes so immense that water is forced into exotic forms of ice at temperatures that should melt it away. On this planet, ice doesn't melt or freeze. It burns hot. Drift toward TOI 3757B, and an entirely different kind of world fills your view. It stretches almost as wide as Jupiter, but there's surprisingly little mass filling all the space.

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  Its atmosphere has swollen outward into an enormous planet so light for its size that NASA compares its density to a marshmallow. And yet, none of these planets have ever been touched by human hands. No one has stood on their surface, drilled into their interiors, or returned with a sample. They remain trillions of miles away, suspended in space. At Lawrence Livermore National Laboratory, scientists are piecing together these unreachable exoplanets from multiple perspectives, capturing clues from the outside with powerful telescopes, and recreating pieces of their hidden interiors with powerful lasers here on Earth.

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  Welcome to the Big Ideas Lab, your exploration inside Lawrence Livermore National Laboratory. Hear untold stories, meet boundary-pushing pioneers, and get unparalleled access inside the gates. From national security challenges to computing revolutions, discover the innovations that are shaping tomorrow today.

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  Growing up, I asked my parents to buy a telescope for my birthday.

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  Federica Capari is a research scientist at Lawrence Livermore National Laboratory.

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  I could watch the moon, and one night I was able to spot Jupiter. It was really tiny, it was so far away, but I spotted Jupiter and the Galilean moons, and I was so excited about it that I couldn't even go back to sleep after watching it.

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  Through the Omega Laser Facility and the National Ignition Facility, scientists at Lawrence Livermore are recreating the extreme pressures and temperatures found deep inside planets.

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  Now doing experiments that can tell us something about the interior structure of these planets that I was watching when I was in high school, it's really exciting.

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  They are even studying planets outside our solar system: exoplanets. For Federica, it's quite literally the dream.

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  One night, I remember I was dreaming of watching outside the window, and there was another big planet nearby. But it wasn't surprising, it was just the planet I was living on. Essentially, I was already dreaming of exoplanets before even knowing that exoplanets existed.

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  Inside some of those interiors, pressures and temperature can reach extremes far beyond anything possible at Earth's core.

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  If I were to talk to my grandma about the work that I'm doing, I think the first thing that would surprise her is really the concept of pressure. The pressure at the center of the Earth is 3.5 million atmospheres, meaning 3.5 million times higher pressure than we are experiencing right now on the surface of the Earth. The temperature of the Earth's core is about 6,000 Kelvin, which means that it is six times hotter than lava. And these are not even the most extreme conditions that you can find in the universe. Bigger planets have much higher pressures and temperatures.

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  But how do scientists recreating these environments even know what exoplanets are like at all, let alone what their conditions are on the inside? While Federica's team works from the inside out, astronomers in observatories around the globe are approaching them from the outside in.

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  The powerful telescopes that have been built, they are able to detect those planets. It's pretty amazing.

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  With the world's largest optical telescopes, scientists are searching the blackness of the heavens for something almost impossibly faint: a planet buried beneath the light of its star.

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  We're detecting light from that exoplanet, and we can calibrate our instrument in some way to compare that light to some known light intensity.

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  Benjamin Gerard is a research scientist at Lawrence Livermore specializing in exoplanet detection.

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  From that, we can use models to infer how massive the planet is. This is a picture of the actual exoplanets. You can see their light, and there's nothing more real than that.

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  Even the nearest of these planets is unimaginably far away. Our closest exoplanet is Proxima Centauri B. It sits about four light-years away from Earth. So even moving at the speed of light, the journey to our closest known exoplanet would take more than four years. The farthest exoplanets scientists can directly image are around 300 light-years away, 1.8 quadrillion miles. The light reaching a telescope from that distance began its journey roughly 300 years ago. And at the fastest speed that humans are able to travel through space, it would take more than 100,000 years to get there.

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  But that's like a fraction of a percent of the size of the galaxy.

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  It may take centuries of travel for the light to arrive at the telescope. But even then, that light is complicated. It doesn't come from just the exoplanet.

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  The primary challenge is that the planet is so much dimmer than its host star. Jupiter-like planet would be about a billion times fainter than its host star. And so your instrument sensitivity, which we call contrast, it's a normalized sensitivity relative to the brightness of the host star, would need to be better than a billion. And that is very, very hard to do.

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  Separating any planet from a host star's glare is already an extraordinary achievement. But there's still a gap between the planets we can image today and the much fainter planets scientists want to see.

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  There's a lot of physics limitations that make achieving a contrast ratio of 10 to the minus 10 very challenging, even in space. That's not been done. And so the exoplanets that we have seen are actually much brighter than that. They're about 100,000 or a million times fainter than their host star.

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  So astronomers look for exoplanets that don't require such extreme contrast.

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  The Newborns.

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  The reason they're so bright is because these planets are young and they're emitting infrared light. They've recently formed and they're glowing hot in the infrared. These planets where imaging are like 100 million years old or younger, so relatively young on a solar system time scale.

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  Among the youngest planets, astronomers can usually only pick out the brightest ones. And even those can nearly vanish beside the stars they orbit. So inside the telescope's imaging system, astronomers deliberately erase the brightest thing in the frame.

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  There are a number of ways to do that, but a standard one is with an instrument called a coronagraph. Essentially, it's an instrument that blocks out the brighter photosphere of the sun to see the fainter but more diffuse corona of the sun and create an eclipse inside the instrument. And that same concept applies to exoplanet imaging.

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  A coronagraph was originally built to study the sun, but the same idea works for exoplanets. Create an artificial eclipse inside the telescope so the star's glare falls away and much fainter objects nearby can be seen clearly.

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  The star's so bright, it swamps any fainter exoplanetary signal, but if you can block out that bright part in an optimal way, you can more easily see that much fainter signal.

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  But erasing the star still doesn't leave a completely clean picture of an exoplanet. Before that distant exoplanet can reach the telescope, it has one final obstacle to cross: Earth's atmosphere. Layers of moving air bend and ripple what astronomers are trying to see, creating a phenomenon called atmospheric turbulence. A distortion corrected with adaptive optics and laser guide star technology developed at Lawrence Livermore, covered in episode 36 of the Big Ideas Lab, Laser Guide Star. Benjamin's work adds another layer of correction. Adaptive optics measure those distortions as they happen and tell tiny motors underneath

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  the telescope mirror to move, correcting the incoming image in real time.

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  What I have been working on kind of since my PhD and now with this instrument at Lake Observatory is that if you take coronographic images fast enough at high enough speed to freeze the scattered starlight from the atmosphere, the leftover imperfect adaptive optics correction, you can then correct that scattered starlight a lot better than you could with a conventional adaptive optics system.

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  Think of standard adaptive optics as the first cleanup pass for a telescope. Benjamin's system builds on that with a second step made for imaging exoplanets.

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  It's inherently an enhanced correction. It leverages that to create a much more sensitive wavefront sensor way of measuring the turbulence in an adaptive optics system to feed back to the existing deformable mirrors in that system to make much better correction at high speed, so around 100 or 1,000 times a second.

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  Different molecules absorb and emit energy at specific wavelengths, leaving behind their own recognizable signature. Once the interference is stripped away, the remaining light becomes an outline of a planetary description.

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  Somewhat uniquely and very powerfully, we can spread that light out into a spectrograph and measure the different wavelengths of light or colors of light from that exoplanet. And from that, we can learn about its chemistry. So we can see different molecular components in the atmosphere of that exoplanet and compare that to ones in our solar system and understand what's going on in its atmosphere.

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  Water in one place, methane in another, once scientists match the signatures, the image of an exoplanet begins to take shape. One exoplanet astronomers have caught in its childhood is 51 Aradani B. It's nearly 100 light years away and only tens of millions of years old. It orbits far from its star, much like the scale of Saturn's orbit in our own solar system, with an atmosphere rich in methane and temperatures hot enough to melt lead. The planet is still cooling from its formation, releasing enough heat in the infrared for astronomers to pick it up. The exoplanet was found with the Gemini Planet Imager.

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  It was a $25 million instrument funded primarily by the National Science Foundation in the mid 2000s.

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  A massive undertaking, with instrument design led by Lawrence Livermore National Laboratory.

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  This was an enormous effort coordinated across at least 10 different institutions in building the instruments, which Livermore led, and operating it and processing the data for science. I would say close to 100 students and postdocs and faculty across the US and Chile to ultimately produce these results, like detection of the new exoplanet 51 Aradani B.

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  51 Aradani B is only the beginning. Scientists have identified thousands of exoplanets that exist beyond our solar system. Some are scorching hot, others are frozen, and many of them are unlike anything we've encountered in our own solar neighborhood. And with every improvement in technology, scientists can reach a little farther into that unknown. A planet that once disappeared beside its star becomes visible, its atmosphere reveals its chemistry, and its mass, temperature, and orbit all fall into place like connecting

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  puzzle pieces, allowing scientists to continue to ask, what else is out there? And how else can we study it?

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  What interests me so much now is that this can be built on further to get these 100 times more sensitive regimes to see planets like Earth, those that could harbor life.

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  A telescope can tell scientists a lot about an exoplanet's orbit, atmosphere, and temperature, but some of the conditions that make a planet habitable are hidden thousands of miles beneath the surface.

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  For example, the Earth is an habitable planet, and there are many specifics of its interior structure that made it habitable. The existence of plate tectonic, the existence of a magnetic field, they are all related to the planet's interior structure.

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  Earth's interior gives scientists planetary signposts they can connect to larger habitability questions.

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  The convective motion in the mantle that generate plate tectonics are due to the fact that the rocks inside the mantle have the right geological properties to make it flow on geological time scale. The fact that we have a liquid iron core surrounding the inner core generate a magnetic field that shield the Earth from cosmic rays and solar wind that are dangerous for life and make the Earth habitable. So we are looking for these correlations between how the interior structure of a planet is and some feature that may make life possible on that surface.

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  But no telescope can see through thousands of miles of rock and pressure, and the inside of a planet stays hidden. So scientists use mass, size, and density as clues in a cosmic game of guess who to complete the picture.

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  There are a few data that we have from astronomical observation. We know the mass of the planet, we know the size of the planet, and from these two measurements, we can get a density profile for the planet. And then we say, okay, if the density has to be 10 gram per cc at that pressure, then one material has that density at that pressure. And then they run simulations and modeling to come up with a potential composition for the planet that may match that density profile.

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  Once observations and modeling narrow the possibilities, Federica and her team can begin building one of those possible interiors here on Earth. A density points to a material while a model predicts the conditions, and that material goes into a target chamber. Then, a laser fires.

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  We focus high-power lasers onto the sample, and this sample gets really hot, and the layers of the sample that interact with the laser are vaporized, creating this plasma that expands in the direction the laser is coming from. And by conservation of momentum, this expansion launches compression waves in the opposite direction. It is kind of a rocket effect. If you want to launch a rocket, you expel propellant towards the ground, and the rocket lifts off. So here is the same. We are pushing the lasers against the material. The material is vaporized,

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  and this plasma that is generated pushes waves into the material that compress it and heat it at the same time. In these few billionths of a second, the sample goes from ambient to extremely high pressure, temperature, and densities.

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  For a few billionths of a second, the sample is no longer behaving like something on Earth. It's behaving like matter buried deep inside another world. One piece of an exoplanet recreated Earthside.

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  In the blink of an eye, you could do 10 million of these experiments, if you could repeat one after the other, which we cannot, but it's just to give you an idea of how fast that is. And in this very short amount of time, we need to run as many diagnostics as we can, as many measurements as we can, to learn as much as we can about what is happening to this sample. Getting the results that you want the first time is very rare.

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  But Federica and her team persist, iteratively learning from each experiment until they get it right. One place to see this method at work is with ice giants. Uranus and Neptune are the two ice giants in our own solar system, and astronomers have already found exoplanets thought to resemble them. Ice giants are thought to contain mixtures of water, methane, and ammonia. Under the enormous pressures deep inside those planets, even something as familiar as water can become almost unrecognizable.

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  What we know is that if these planets are actually made of these ice mixtures, then these mixtures are likely in this superionic phase. And a superionic phase of water is where the oxygens form a solid lattice, so behaves like a solid, while the hydrogen behave like a liquid.

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  The most common material known to mankind turned completely upside down.

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  It's a very weird substance where the oxygens form something that looks like the ice cube that we have in the refrigerators in terms of holding a shape.

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  It may hold its shape like a familiar ice cube, but it's truly otherworldly.

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  Superionic ice is very hot because it forms at 2,000 Kelvin, and it's also very dense. An ice cube of superionic ice will sink into liquid water. It would not float like an iceberg. But at the same time, it's not fully solid because the hydrogen moves like if they were liquid. And so these hydrogen motions create a very high conductivity. The superionic ice that formed these planets then may change the property of the planets because this high mobility may create properties that are very different than fluid water.

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  Federica and her team set out to prove that this strange state of matter could actually form under those conditions. That process began with a tiny amount of liquid water.

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  That was one of the most challenging experiments I've ever worked on. It was hard from the beginning, just making the sample, confining very small amount of liquid water in a way that you can bring the target into the laser chamber and having the water staying there for sufficiently long that you can actually fire the lasers and measure something. As water gets heated and compressed, its properties changed so much that it was also very difficult to time the experiment right.

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  Even if the lasers could create the right conditions, the material still had to respond before those conditions disappeared.

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  It's not just overcoming the technical challenges, but also it told us something about the science because going from a liquid into a solid phase in such a short amount of time, it's not clear that the experiment was long enough to allow that nucleation.

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  It's the process of growing a solid phase out of a liquid, and the team had only a few billionths of a second to do it. The challenges and variables take time, but success happens instantaneously.

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  Seeing the data that showed us that we had made a solid out of a liquid in a few billionths of a second, that was so exciting.

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  Experiment successful.

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  The team didn't just recreate the conditions. The signature confirmed they had created the exotic phase they were looking for.

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  Analyzing the data and seeing that that signature actually corresponded to the evidence for this exotic phase of water ice that is called superionic, that was also the next level of excitement.

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  Water is only one ingredient inside an ice giant. The team had pushed many other planetary materials to the same extremes, looking for the next piece of the interior to recreate. When they tested carbon-rich materials, another possibility revealed itself.

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  The composition of the ice giant planet is not pure water. They are expected to be a mixture of methane, ammonia, and water mainly. So they are somehow mixed together. We don't exactly know how. And one hypothesis is that if you compress these mixtures, the carbon that comes from the methane may actually be isolated from this mixture, forming diamonds.

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  The carbon deep inside ice giants could literally crystallize into diamonds. And because those diamonds are denser than the materials surrounding them, they wouldn't stay suspended. They would fall as diamond rain within the interior of the planet.

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  Building all these pieces together can give us insight of how complex, how extraordinary these interior compositions might be. And that can inform theories for stellar systems formation.

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  This method isn't just helping reconstruct strange Neptune-like planets. It's also helping scientists understand terrestrial exoplanets, including the kinds of places that may be more relevant to habitability and more similar to Earth. For rocky exoplanets, the ingredients start to look more like home: iron, silicates, and oxides. But increase the size of the planet, and the pressure rises with it. Deep below the surface, these materials can rearrange, flow differently, and build an interior unlike anything inside Earth.

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  And the hope is that one day we can fully reconcile all the data with all the observation and come up with a model that we actually believe is the right one.

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  The more these measurements come together, the more complete a planet's picture becomes. But there are still pieces missing. In the lab, Federica wants to move beyond testing ingredients one at a time and get closer to the complex mixtures that would actually exist inside a planet.

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  I think if we could do an experiment on some more realistic composition of the planet, that would be really good because right now we have done experiments on water, and we know that there's no pure water inside a planet. We have done experiment on iron, and there's no pure iron inside the Earth core of the core of a bigger planet.

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  Meanwhile, telescopes are getting bigger. A new generation of extremely large telescopes, some with mirrors roughly 30 to 40 meters across, are being built to separate planets from their stars at even more precise scales. For Lawrence Livermore, Benjamin's work on adaptive optics and Federica's research on planetary interiors are dual-purpose. They go beyond characterizing planets and into defending our own.

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  By understanding how to characterize planetary building blocks, we are developing a better understanding of high-energy density physics that informs national security questions, stockpile stewardship, and also new techniques, new instruments to really characterize materials at extreme pressure and temperature conditions.

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  We use adaptive optics in a lot of areas related to national security. This issue of compensating for atmospheric turbulence and blocking out something bright next to something dim remain beneficial for national security as well as exoplanet imaging.

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  Humanity may never set foot on exoplanets. But scientists at Lawrence Livermore keep narrowing the distance between what they can observe and what they can understand. Light reveals the outside. Laser experiments recreate pieces of the inside, and together, they close the distance, transforming mystery into discovery.

• 28:48Speaker 00

  Thank you for tuning in to Big Ideas Lab. If you loved what you heard, please let us know by leaving a rating and review. And if you haven't already, don't forget to hit the follow or subscribe button in your podcast app to keep up with our latest episode. Thanks for listening.