About 25 years ago astronomers kicked off what would come to be called the “exoplanet revolution” with the discovery of the first alien world orbiting another sunlike star. As the pace of discovery quickened and new data came pouring in, it became clear that the cosmos is awash in planets—big planets, small planets, planets broiled by their stars or frozen in the outskirts of their systems and, overwhelmingly, planets that in size and orbit are unlike anything we have in the solar system. In the span of just a quarter-century, humankind went from knowing essentially no worlds beyond our solar system to having thousands in our catalogs. Yet even with all this progress, we still remain in the dark about the true nature of most of these worlds—and their possibilities for life. Absent some breakthrough in physics that allows practical interstellar travel, it appears unlikely we will ever visit any exoplanet, let alone several, so definitive answers to our fundamental questions about them have long seemed beyond our reach.
Now, however, new technologies and collaborations are taking the exoplanet revolution a surprising step further—not out to the stars but rather into the depths of cutting-edge plasma physics laboratories. Using football-field-sized lasers, warehouse-sized electromagnets and other extreme machines, scientists from across disciplines are bringing some of their loftiest questions about exoplanets down to Earth, abandoning telescopes to gain deeper, more direct views into the hearts of alien worlds.
To really understand a planet—any planet, whether it orbits our sun or some distant star—scientists must understand its deep interior, where churning flows of liquid rock and metal can generate powerful magnetic fields and set continents in motion. But like a scuba diver sinking far below the waves, researchers studying planetary interiors must learn to deal with the high pressures they find at depth. Unlike what a diver finds, however, pressures inside planets are so high that matter itself takes on baffling new forms. “Inside a Jupiter-sized planet, pressures reach 70 million times more than what we experience at the surface of the Earth,” says Raymond Jeanloz, a planetary scientist at the University of California, Berkeley. “At those pressures, matter behaves in ways we don’t really understand.”
That’s where the giant lasers and gargantuan electromagnets come in.
Putting the Squeeze on Super-Earths
Yingwei Fei, a researcher at the Carnegie Institution for Science, wants to understand the most abundant species of planet in the universe: super-Earths. Our solar system is remarkably bereft of these worlds, which weigh in at two to 10 times the mass of our own planet. But multiple independent exoplanet surveys have shown our galaxy to be positively packed with them. Because these planets are so common, if even a small fraction of super-Earths were found to be habitable, they would become a primary target in astronomers’ centuries-spanning quest to discover extraterrestrial life and place earthly biology in a cosmic context. “That’s why there is a big push to understand these worlds,” Fei says. Jeanloz puts it more poetically: “These days, super-Earths are the twinkle in every astronomer’s eye.” With their larger masses, however, typical pressures inside a super-Earth are on the order of 10 million atmospheres—several times higher than in our own planet’s core. To understand super-Earths, Fei and his fellow researchers had to find a way to study matter at those extreme pressures. “We needed to do something unconventional” he says.
The usual way planetary scientists push matter to high pressures is to squeeze tiny samples of rock or metal between two diamonds. But these “diamond anvil cell” devices can only reach a few million or so atmospheres, far too low for the crush inside a super-Earth. To get a bigger squeeze, Fei and his team took their rock samples to the formidable “Z machine” at Sandia National Laboratories. This machine is the world’s largest Z-pinch, a plasma physics device originally designed to study nuclear fusion—basically a soup-can-sized wire cage sitting on top of a warehouse-sized series of capacitor banks. When you flip a switch on the Z machine, a tsunami of electric current surges from the batteries and into the wires. The nanosecond-long current pulse generates powerful magnetic fields that are so strong that they violently implode the wire, subjecting anything placed inside to a force—or rather a “pinch”—approaching that of a detonating thermonuclear warhead.
This was exactly the kind of crush Fei and his collaborators needed to get a small sample of bridgmanite, a mineral abundant in Earth’s lower mantle, up to super-Earth pressures. After months of painstakingly designing and fabricating bridgmanite-filled “targets,” Fei and his team traveled to Sandia and blew up (or “blew in”) the precious samples. The data showed the mineral behaving in unexpected ways, failing to melt until it reached much higher temperatures than what occurs at Earth-interior pressures. Melted, flowing material is a prerequisite for making planetary magnetic fields, which may themselves be necessary for shielding a planet’s biosphere from outbursts of damaging radiation from its host star. Because astronomers are keen to know if super-Earths host such a protective magnetic field, Fei’s results were seen as an important advance.
For Fei, the marriage of the giant fusion plasma machine with planetary science represents a path to the future. “Only the big lasers and Z machines will be able get us to the pressures we need to directly simulate the interior conditions of big planets,” he says.
Of Moons and Magma Oceans
Such experiments demonstrate the power of scientists crossing disciplinary lines to drive advances in exoplanet science. Interdisciplinarity comes with its own problems, however. Getting researchers from widely different fields to understand one another is not easy. The training and the culture of a plasma physics experimentalist is very different from that of a planetary scientist. Just learning each discipline’s differing terminology for the same physical process can be a barrier. To make matters even more complicated, the effort to understand super-Earths and other massive planets requires not just plasma physicists and planetary scientists but also exoplanet-observing astronomers and condensed matter physicists studying materials at extremely high pressures. That’s a lot of people to invite to the party.
For Sarah T. Stewart, a planetary scientist at the University of California, Davis, getting these disciplines to work together will be crucial to further progress. “We’ve been going along for a while now in modeling the structure of big planets using what I’d call ‘best guess’ science,” she says. That is, planetary scientists have several sound theoretical ideas about how matter might behave at really high pressures, but they have lacked data. Now that more—and often surprising—data are coming in from lab-based proxies for planetary interiors, she says, “the problem with using the data in a meaningful way is everyone has to talk to each other.”
In large part, that is what spurred researchers from seven different institutions to recently form the Center for Matter at Atomic Pressures (CMAP). With a five-year timeline and nearly $13 million in funding from the National Science Foundation, CMAP’s goal to is to build the deep, long-term and interdisciplinary collaborations needed to overcome the blind spots in scientists’ emerging map of matter under extreme conditions. (Full disclosure: I am an astrophysicist who is part of the CMAP collaboration.) Based at the Laboratory for Laser Energetics (LLE) at the University of Rochester, CMAP takes advantage of the giant OMEGA laser system to squeeze matter into new, extreme states. Like the Z machine, the OMEGA laser is primarily an instrument for exploring fusion energy. The football-field-sized laser, with its 60 high-intensity beams, is used to blast pellets of hydrogen until they reach conditions similar to those within the sun. It is a process that can also be used to simulate conditions inside a super-Earth. The OMEGA laser can thus provide direct access to conditions in the hearts of these worlds that may set their ability to host life. The laser can also give CMAP scientists views inside Jupiter-sized worlds or even the fiery aftermaths of planetary collisions.
Right now, for example, Stewart and her LLE collaborators are using the OMEGA laser to probe the smashup between the young Earth and a Mars-sized body that is thought to have formed our planet’s moon. The problem they face is that, at that time of the impact, Earth’s surface had yet to cool from the heat generated by the planet’s formation, and it was covered in magma oceans that are difficult to accurately model. “We want to know how that magma ocean might soak up the energy of the collision between the Earth and another world,” Stewart says.
After finding a way to get samples of liquified rock into the OMEGA laser chamber, which was no easy task, Stewart and her collaborators bombarded the samples with lasers to simulate the shock waves produced by a planetary impact. Although the data from this experiment have not yet been analyzed, Stewart says there are already surprises. The researchers had predicted that liquified material would be easier to squeeze than solid rock, but the “compressibility” they observed in the experiments was even greater than expected. Such unanticipated results are exactly the sort of thing that could be used to dramatically improve preexisting models for moon-forming planetary impacts.
Jupiter’s Jellified Heart and Saturn’s Helium Rain
To underpin such niche experiments, researchers are also using CMAP to improve the systematic view of how atoms behave at extreme pressures.
“There really is a fundamental physics aspect of all this,” says Jeanloz, who has been at the vanguard of using plasma physics machines for planetary science during the past few decades. “When you get to pressures of millions of atmospheres, the energy squeezing the atom is comparable to energies typical of a chemical bond. That means, at these pressures, the basic chemical properties of matter are going to shift.” At normal pressures, the oxygen atoms in rocks, for example, act like insulators that cannot conduct electricity. Deep inside a large planet, however, the oxygen atoms will start behaving as a metal, with their nuclei pressure locked in place but their electrons becoming free to flow. This essentially means that the idea of a “rocky core” for really big planets such as Jupiter is probably a complete misnomer. “Rather than being rocklike,” Jeanloz says, “we should really be thinking of the center of big planets as some kind of metallized oxygen jelly.”
Even the simplest elements provide puzzles at giant-planet pressures. In a recent paper published in Nature, researchers in France and the U.S. (including Jeanloz) used multiple giant lasers at LLE and Lawrence Livermore National Laboratory’s National Ignition Facility to study the phenomenon of “helium rain” in Saturn and Jupiter. Deep within gas-giant worlds, almost unfathomable pressure squeezes hydrogen and helium alike into metallic fluids more akin to mercury. These fluids mix well together in outer portions of such a planet’s deep interior, but in the depths, the tenets of atomic theory predict they will “unmix” like water and oil. “Since the helium is heavier [than the hydrogen], when the unmixing occurs helium falls downward, which produces heat,” Jeanloz says. This “helium rain” may be why Saturn emits more heat radiation than it absorbs from the Sun.
To test this theory, Jeanloz and his colleagues first used diamond anvils to create “pre-compressed” samples possessing varying ratios of hydrogen to helium. “The elements don’t like to mix at room temperature and pressure,” Jeanloz explains. By squeezing hydrogen and helium hard enough before they were brought into the laser target chamber, the team produced samples that were already liquified and well-mixed, allowing the massive lasers to more readily induce powerful shock waves within them to replicate conditions deep inside the gas giant planets. Comparing pure samples of hydrogen and helium to the precompressed mixtures, the team confirmed the basic details of the helium rain theory while also observing subtle new details suggesting where further theoretical elaboration will be required.
Exploring the exotic, planet-sculpting chemistry of helium rain or abandoning the classic-but-flawed notion of rocky cores within Jupiter-sized worlds are only two examples of the vast and still largely uncharted territory that can be explored when plasma physics labs, astronomers and planetary scientists collaborate and start from first principles. The creative alchemy that occurs when different communities learn to talk to one another is exactly what exoplanet science needs now as it pushes further through its unexplored frontiers. “Geophysicists are used to thinking about separate mineral phases with very specific crystal structures,” Stewart says. “But at the pressures we’re interested in with CMAP, you can’t think that way anymore. We don’t even have the vocabulary yet to describe what can happen, but that’s what we are hoping develop.”
The new vocabulary Stewart, Jeanloz and Fei are building represents a lot more than just some scientists trying out a new collaboration. Instead it is the opening of one of the newest and most exciting frontiers in science. By merging the laser-driven, high-tech study of matter’s collective behavior at atomic scales with the telescopic exploration of its global properties on the planetary scale, an entirely new window on the universe has been opened. And this unique synthesis of the micro and macro may be just be our best and only route toward understanding when a planetary table for life’s banquet is set.