The cores of larger exoplanets that are rocky should stay warmer than those of small planets, according to an article published in Science. This is great news for interstellar space explorers, as a molten core may be necessary for the development of life on the planet.
The study of this feature of exoplanets needed an experiment that involved massive lasers as well as an extremely thin piece of iron placed under extreme pressure. “We’re discovering a lot of planets, and one of the main questions asked by people is: Are these planets inhabitable?” says Rick Kraus a physicist with Lawrence Livermore National Laboratory who conducted the research.
For the purpose of answering this query scientists don’t usually begin with the idea of a planet’s core. Instead, they consider whether the world is at the proper distance from its star or if it’s got water. However, Kraus along with his colleagues were looking for alternative methods to determine whether the planet is habitable.
They looked into the planet’s capacity to create a magnetosphere, a magnetic field that shields the planet from solar radiation similar to the one that Earth is for us. It’s an opportunity to study the possibility of living there, Kraus says. Our current way of life would not be possible without the magnetosphere of Earth.
Magnetic fields result from the formation of the molten cores of planets. Earth has a core that is made mostly of iron and split into a solid core and an outer core of liquid. The Earth’s magnetic field is created by the convection of liquid iron, which is how it spins in the cooler, dense areas of the liquid sink to the bottom while hotter areas rise as if they were wax in the lava lamp.
The study of the core of an exoplanet in the laboratory is challenging because there aren’t many ways to replicate such extreme temperatures and pressures. It is also the very first study to determine the melting temperature of iron at higher pressures than those in the Earth’s core, Kraus declares. To reach these temperatures the team needed large lasers, and specifically those at the National Ignition Facility in Lawrence Livermore National Lab, where large lasers are their specialty.
In the experiment, the lasers destroyed a multilayered piece of iron. Beryllium layers an element in the metal and a few filters made up the exterior of an extremely thin iron “sandwich” and a chunk of lithium fluoride that was transparent formed the remaining portion, Kraus says. The beryllium layer on the outside heated to thousands of degrees in “a fraction of one billionth of a second” Kraus claims and the inside of the sandwich was cooked into an intense plasma. It then expanded and cooled, causing an intense shockwave through the sample.
This mimics the conditions of a piece of iron that is hot as it descends into the planet’s core that is molten. “You’re shock-inducing the hell out by this,” claims Peter Driscoll Geophysicist at Carnegie Science who models the core of Earth as well as other planets, and did not participate in the study. He adds it was a challenging process to research. The sample is destroyed, therefore the researchers must gather their data at once. The process produced only “a few numbers of data points” the researcher says, however “these types of experiments are very beneficial.”
When the sample reached the pressures at which it was at its highest, another instrument was used to determine if the iron remained liquid or solid at specific times to aid researchers in focusing the behavior of iron at high temperatures and pressures.
The team discovered the following “as you increase pressure, temperatures increase quickly,” Kraus says. Exoplanets are exoplanets. This means that the bigger they become the longer it will take for their cores to harden. Super-Earths with masses between four and six times Earth’s weight will require the longest time to solidify the time, he claims. The team estimates it will take six billion years to allow Earth’s cores to solidify the core, whereas cores found in massive exoplanets with similar compositions as Earth could take as much as thirty percent more time.
“While it may sound kind like a natural inclination,” this wasn’t really a possibility with the many variables Kraus states.
The measurement of how much iron melts under extreme conditions is vital as it tells you when and how the planet’s core will harden, Driscoll says. At the point where there is a border between Earth’s solid inner core and its liquid outer core, researchers aren’t able to determine exactly what temperature it is, though it’s believed to be close to the surface of the sun approximately 10,000degF.
The problem with applying the results to exoplanets is the fact that super-Earths may have other elements that iron within their cores and this could alter their melting temperature by a nebulous quantity, Driscoll says. It is also difficult to determine how fast exoplanets cool due to the mantle, which is the layers of rock that is hot that surrounds that core area, playing a major impact on how quickly the core cools. The mantles of exoplanets can consist of “pretty everything,” he says.
Venus, Driscoll says, is the best illustration of this disconnection. On paper, its structure is quite like Earth’s, however, it does not have the magnetic field as well as plate tectonics.
Other elements can determine whether a magnetosphere will develop in the same way, for instance, how well the material within the core allows the flow of electricity or heat through it. However, these properties are hard to determine Even on Earth-based scientists have only managed to determine how heat flows throughout Earth’s core during the last decade. However, Driscoll says, that is “the following thing we should look after.”
