According to a new study, sweeping molten oceans have a large influence on the observed properties of hot rocky Super-Earths, such as their size and evolutionary path. Lava worlds, massive exoplanets with sparkling skies and roiling volcanic seas known as magma oceans, are unlike the planets in our solar system.
To date, nearly half of all rocky exoplanets discovered are capable of retaining magma on their surfaces, most likely because these planets orbit their host stars in less than ten days. Being so close causes the planet to be bombarded by harsh weather and forces surface temperatures to the extreme, making it all but inhospitable to life as we know it today.
In a new study, scientists discovered that these vast molten oceans have a significant impact on the observed properties of hot rocky Super-Earths, such as their size and evolutionary path.
Their findings, published recently in The Astrophysical Journal, show that oceans of magma can cause lava-rich planets without atmospheres to be slightly denser than similarly sized solid planets, as well as impact the structure of their mantles, the thick inner layer that surrounds a planet’s core.
When planets initially form, particularly for rocky terrestrial planets, they go through a magma ocean stage as they’re cooling down. So lava worlds can give us some insight into what may have happened in the evolution of nearly any terrestrial planet.
Dr. Boley
Even so, because these objects are notoriously understudied, it can be difficult to characterize the fundamental workings of lava planets, according to Kiersten Boley, lead author of the study and a graduate student in astronomy at The Ohio State University.
“Lava worlds are very odd, very interesting things and because of the way we detect exoplanets, we’re more biased to finding them,” said Boley, whose research revolves around understanding what essential ingredients makes exoplanets unique and how tweaking those elements, or in the case of lava worlds, their temperatures, can completely change them.
One of the most well-known of these mysterious burning worlds is 55 Cancri e, an exoplanet about 41 light-years away that scientists describe as home to both sparkling skies and roiling lava seas.
While there are objects in our solar system that are extremely volcanically active, such as Jupiter’s moon Io, there are no true lava planets that scientists can study up close and personal. However, studying how the composition of magma oceans contributes to the evolution of other planets, such as how long they stay molten and why they eventually cool down, can provide insights into Earth’s own fiery past, according to Boley.
“When planets initially form, particularly for rocky terrestrial planets, they go through a magma ocean stage as they’re cooling down,” said Boley. “So lava worlds can give us some insight into what may have happened in the evolution of nearly any terrestrial planet.”
Using the exoplanet interior modeler software Exoplex and data collected from previous studies to construct a module that included information on several types of magma compositions, researchers simulated several evolutionary scenarios of an Earth-like planet with surface temperatures from between 2600 and 3860 degrees Fahrenheit — the melting point at which the planet’s solid mantle would turn to liquid.
The team was able to deduce from their models that the mantles of magma ocean planets can take one of three forms: one in which the entire mantle is completely molten, one in which a magma ocean lies on the surface, and a third sandwich-esque model in which a magma ocean lies on the surface, a solid rock layer in the middle, and another layer of molten magma that lies closest to the planet’s core.
The results indicate that the second and third forms are slightly more common than fully molten planets. Depending on the composition of magma oceans, some atmosphere-free exoplanets are better than others at trapping volatile elements – compounds such as oxygen and carbon necessary to the formation of early atmospheres – for billions of years.
The study, for example, notes that a basal magma class planet four times the mass of Earth can trap more than 130 times the mass of water currently present in Earth’s oceans, as well as about 1,000 times the amount of carbon currently present in the planet’s surface and crust.
“When we’re talking about the evolution of a planet and its potential to have different elements that you would need to support life, being able to trap a lot of volatile elements within their mantles could have greater implications for habitability,” Boley said in a statement.
Lava planets are a long way from becoming habitable enough to support life, but understanding the processes that help these worlds get there is critical. Nonetheless, when compared to solid exoplanets, this study shows that measuring their density isn’t the best way to characterize these worlds because a magma ocean neither significantly increases nor decreases its planet’s density, according to Boley. Instead, their findings suggest that scientists should concentrate on other terrestrial parameters, such as fluctuations in a planet’s surface gravity, to test their theories about how hot lava worlds work, especially if future researchers intend to use their data to aid in larger planetary studies.
