With the finding of the first alien world around another sunlike star almost 25 years ago, astronomers began what would become known as the “exoplanet revolution.”
As the rate of discovery increased and new data poured in, it became clear that the cosmos is awash in planets—big planets, small planets, planets broiled by their stars or frozen on the outskirts of their systems, and, by far, planets that are unlike anything we have in our solar system in terms of size and orbit.
Humanity has gone from knowing almost no worlds beyond our solar system to have dozens in our catalog in just a quarter-century.
Despite all of this development, we still don’t know much about the true nature of most of these worlds, let alone their potential for life.
We are unlikely to visit any exoplanet, let alone numerous unless there is a breakthrough in physics that allows practical interstellar travel, so solid answers to our core questions about them have long appeared out of reach.
New technologies and collaborations, on the other hand, are pushing the exoplanet revolution even further—not to the stars, but the depths of cutting-edge plasma physics laboratories.
Scientists from several fields are bringing some of their loftiest concerns about exoplanets down to Earth using football-field-sized lasers, warehouse-sized electromagnets, and other extreme devices, forsaking telescopes to acquire deeper, more direct glimpses into the hearts of foreign worlds.
Scientists must first grasp a planet’s deep innards, where churning flows of liquid rock and metal may generate tremendous magnetic fields and send continents in motion, whether it orbits our sun or a faraway star.
Researchers investigating planetary interiors, like scuba divers diving deep beneath the waters, must learn to deal with the enormous pressures they encounter at depth.
Unlike what a diver discovers, however, inside planets, pressures are so intense that stuff takes on strange new forms.
According to Raymond Jeanloz, a planetary scientist at the University of California, Berkeley, pressures within a Jupiter-sized planet are 70 million times higher than on Earth’s surface.
“Matter reacts in ways we don’t fully comprehend under those conditions.”
That’s when the massive electromagnets and big lasers come in.
SUPER-EARTHS ARE BEING SQUEEZED
Yingwei Fei, a researcher at the Carnegie Institution for Science, is interested in learning more about the universe’s most common planet species: super-Earths.
These worlds, which are two to ten times the mass of our planet, are extraordinarily rare in our solar system.
However, many independent exoplanet surveys have revealed that our galaxy is teeming with them.
Because super-Earths are so plentiful, they would become the main target in astronomers’ centuries-long quest to uncover extraterrestrial life and set Earthly biology in a cosmic context if even a small fraction of them were proved to be habitable.
“That is why there is such a strong urge to comprehend these worlds,” Fei explains.
“Super-Earths are the twinkle in every astronomer’s eye these days,” Jeanloz says poetically.
However, because of their enormous masses, super-Earths’ core pressures are on the order of 10 million atmospheres, which is several times higher than our own planet’s core.
To learn more about super-Earths, Fei and his colleagues needed to figure out how to investigate the matter under such tremendous pressures.
He explains, “We needed to try something different.”
Planetary scientists typically pressurize stuff by squeezing small samples of rock or metal between two diamonds.
However, these “diamond anvil cell” devices can only reach a few million atmospheres, which is far too low for a super-crush. Earth’s Fei and his team took their rock samples to Sandia National Laboratories’ powerful “Z machine” to acquire a bigger squeeze.
This is the world’s largest Z-pinch—a soup-can-sized wire cage hanging on top of a warehouse-sized array of capacitor banks—a plasma physics apparatus initially designed to explore nuclear fusion.
A tsunami of electric current bursts from the batteries and into the wires when you turn on the Z machine.
The nanosecond-long current pulse generates intense magnetic fields that violently compress the wire, subjecting anything inside to a force—or rather, a “pinch”—approaching that of a thermonuclear weapon detonating.
This was precisely the kind of crush Fei and his colleagues needed to pressurize a small sample of bridgmanite, a mineral prevalent in Earth’s lower mantle, to super-Earth pressures.
Fei and his team traveled to Sandia and blew up (or “blew in”) the valuable samples after months of painstakingly developing and producing bridgmanite-filled “targets.”
The results revealed that the material acted strangely, refusing to melt until it reached temperatures far higher than those found at Earth’s inner pressures.
Melted, flowing material is required for the formation of planetary magnetic fields, which may be required to protect a planet’s biosphere from harmful radiation bursts from its host star.
Fei’s findings were hailed as a significant step forward since astronomers are eager to learn if super-Earths have such a protective magnetic field.
The combination of the huge fusion plasma machine and planetary science, according to Fei, indicates a way to the future.
He claims that “only huge lasers and Z machines will be able to get us to the pressures we need to directly replicate the interior conditions of big planets.”
MAGMA OCEANS AND MOONS
Experiments like this show how scientists can collaborate across disciplines to progress exoplanet study.
However, interdisciplinarity has its own set of issues. It’s not easy to get researchers from diverse fields to understand one another.
A plasma physics experimentalist’s training and culture are considerably different from that of a planetary scientist.
Learning the different languages used by each profession for the same physical process can be a challenge.
To make matters even more challenging, understanding super-Earths and other huge planets necessitate the collaboration of not only plasma physicists and planetary scientists, but also exoplanet-observing astronomers and condensed matter physicists working with materials under severe pressures.
That’s a large group to invite to a party.
Getting various disciplines to collaborate, according to Sarah T. Stewart, a planetary scientist at the University of California, Davis, will be critical to continued development.
“We’ve been simulating the structure of huge planets using what I’d call ‘best guess’ science for a time now,” she explains.
That is, planetary scientists have several strong theoretical theories about how the matter might behave at really high pressures, but they haven’t had any data to back them up.
“The challenge with using the data in a meaningful way is everyone has to talk to each other,” she says, as more—and often surprising—evidence from lab-based proxies for planetary interiors arrive.
That is, in part, what prompted researchers from seven different institutions to launch the Center for Matter at Atomic Pressures, which was just established (CMAP).
CMAP’s purpose is to develop the deep, long-term, and interdisciplinary collaborations needed to overcome the blind spots in scientists’ growing map of matter under extreme conditions, with a five-year schedule and approximately $13 million in funding from the National Science Foundation.
(Full disclosure: I am a member of the CMAP collaboration as an astrophysicist.) CMAP, which is based at the University of Rochester’s Laboratory for Laser Energetics (LLE), uses the massive OMEGA laser system to squeeze matter into new, extreme states. The OMEGA laser, like the Z machine, is primarily used to investigate fusion energy.
The laser, which is the size of a football field and has 60 high-intensity beams, is used to blast hydrogen pellets until they reach conditions similar to those found in the sun. It’s a method that can be used to model conditions within a super-Earth.
The OMEGA laser can thus gain direct access to conditions in these worlds’ cores that may determine their potential to support life.
The laser can also provide views inside Jupiter-sized worlds or the explosive aftermaths of planetary collisions to CMAP scientists.
At the University of Rochester’s Laboratory for Laser Energetics, the OMEGA Laser system is in use.
Materials squeezed in the focus of OMEGA’s 60 beams can achieve conditions similar to those seen within stars or deep within planetary interiors.
Eugene Kowaluk and the University of Rochester Laboratory for Laser Energetics are to thank for this.
Stewart and her LLE teammates are currently utilizing the OMEGA laser to investigate the collision of the young Earth with a Mars-sized body that is hypothesized to have generated our planet’s moon.
The problem is that at the time of the impact, Earth’s surface had not yet cooled from the heat created by the planet’s formation, and it was covered in magma oceans that are difficult to simulate effectively.
Stewart explains, “We want to know how that magma ocean would soak up the energy of the impact between the Earth and another globe.”
Stewart and her collaborators pounded samples of liquified rock with lasers to recreate the shock waves caused by a planetary impact after figuring out how to get them into the OMEGA laser chamber, which was no simple task.
Even though the data from this experiment has yet to be analyzed, Stewart claims that there are already some surprises.
The researchers thought that liquified material would be easier to squeeze than solid rock, but the “compressibility” they found in the studies exceeded their expectations.
Such unexpected outcomes are exactly the kind of thing that could be exploited to vastly improve pre-existing models of planetary impacts that generate moons.
SATURN’S HELIUM RAIN AND JUPITER’S JELLIFIED HEART
Researchers are also utilizing CMAP to develop a systematic view of how atoms behave at extreme pressures to support such niche investigations.
“There is a fundamental physics aspect to all of this,” adds Jeanloz, who has pioneered the use of plasma physics equipment for planetary science in recent decades.
“At pressures of millions of atmospheres, the energy required to squeeze an atom is comparable to that required to form a chemical bond.
That means the fundamental chemical characteristics of matter will transform at high pressures.”
At normal pressures, oxygen atoms in rocks, for example, act as insulators, preventing electricity from flowing through them.
However, deep within a massive planet, oxygen atoms will begin to behave like metals, with their nuclei pressure bound in place but electrons free to travel.
This effectively suggests that the concept of a “rocky core” for massive planets like Jupiter is most likely a pure fabrication.
“We should be thinking of the center of huge planets as some kind of metalized oxygen jelly,” Jeanloz says, rather than being rocklike.
At giant-planet pressures, even the simplest atoms can be perplexing.
Researchers from France and the United States (including Jeanloz) employed numerous gigantic lasers at LLE and Lawrence Livermore National Laboratory’s National Ignition Facility to explore the phenomenon of “helium rain” in Saturn and Jupiter in a recent report published in Nature.
Deep within gas-giant worlds, nearly incomprehensible pressure compresses hydrogen and helium into metallic fluids similar to mercury.
These fluids mix nicely in the outer reaches of a planet’s deep interior, but atomic theory predicts that they will “unmix” like water and oil in the depths.
“Because helium is heavier [than hydrogen], when the mixture is separated, helium falls downhill, producing heat,” Jeanloz explains. Saturn may release more heat radiation than it absorbs from the Sun because of this “helium rain.”
To put this notion to the test, Jeanloz and his colleagues utilized diamond anvils to manufacture “pre-compressed” samples with various hydrogen-to-helium ratios.
“At normal temperature and pressure, the elements don’t like to mix,” Jeanloz explains.
The scientists created liquified and well-mixed samples by squeezing hydrogen and helium hard enough before bringing them into the laser target chamber, allowing the enormous lasers to more easily trigger tremendous shock waves within them to mimic conditions deep inside the gas giant planets.
The team confirmed the core characteristics of the helium rain theory while also discovering minor new details that hint where more theoretical elaboration will be necessary by comparing pure hydrogen and helium samples to the pre-compressed mixtures.
Exploring the exotic, planet-sculpting chemistry of helium rain or abandoning the traditional but flawed notion of rocky cores within Jupiter-sized worlds are just two examples of the vast and largely uncharted territory that can be explored when plasma physics labs, astronomers, and planetary scientists collaborate and start from the ground up.
Exoplanet science is in desperate need of the creative alchemy that occurs when various societies learn to communicate with one another as it pushes further into uncharted territory.
Stewart explains, “Geophysicists are used to thinking about different mineral phases with extremely specific crystal structures.” “But you can’t think that way at the pressures we’re interested in with CMAP.
We don’t even have the terminology to articulate what could happen yet, but that’s something we’re working on.”
Stewart, Jeanloz, and Fei are creating a new lexicon that reflects far more than a group of scientists experimenting with new cooperation. Instead, it marks the start of one of science’s newest and most intriguing areas.
A whole new window on the cosmos has been opened by combining the laser-driven, high-tech study of matter’s collective behavior at atomic scales with the telescopic research of its global properties on the planetary scale.
And this one-of-a-kind synthesis of micro and macro might just be the best and only way to figure out when the planetary table for life’s banquet is set.
THE AUTHOR IS : Adam Frank is a professor of astrophysics at the University of Rochester who blogs at 13.8 at BigThink.com and the author, most recently, of Light of the Stars: Alien Worlds and the Fate of the Earth.
