How Do Lava Worlds Become Earth-Like, Living Planets?

Earth was once entirely molten. Planetary scientists call this phase in a planet’s evolution a magma ocean, and Earth may have had more than one magma ocean phase. Earth cooled and, over 4.5 billion years, became the vibrant, life-supporting world it is today.

Can the same thing happen to exo-lava worlds? Can studying them shed light on Earth’s transition?

Planet-hunters like the Kepler Spacecraft and TESS have found thousands of worlds around other stars. Many of these worlds orbit their stars very closely, so close that they’re heated to extreme temperatures. A lot of these planets are gas giants, but a significant number are rocky, and the extreme heat keeps them molten, or at least partially molten. At least half of these super-heated rocky worlds are capable of maintaining magma on their surfaces.

There’s nothing like a lava world in our Solar System. The closest is Jupiter’s moon Io. But it’s volcanically active, which isn’t the same as a magma ocean. Studying lava worlds gives scientists a glimpse into Earth’s molten past, and luckily, they’re not hard to find.

A new study looked at hot rocky super-Earths, how their magma oceans affect our observations, and how they also influence their evolutionary paths.

The study is “Fizzy Super-Earths: Impacts of Magma Composition on the Bulk Density and Structure of Lava Worlds,” and it was published in The Astrophysical Journal. The lead author is Kiersten Boley, a graduate student in astronomy at The Ohio State University.

“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.”

“Being able to trap a lot of volatile elements within their mantles could have greater implications for habitability.”

Kiersten Boley, lead author, Ohio State University.

The team used exoplanet modelling software to simulate Super-Earths that orbit their stars very closely. These planets are called ultra-short period (USP) planets. They simulated multiple evolutionary pathways for a planet similar to Earth but with surface temperatures between 2600 and 3860 F (1426 and 2126 C.) Within this range, a planet’s solid mantle would melt into magma depending largely on its composition.

Their work produced three classes of magma oceans, each with different mantle structures: a mantle magma ocean, a surface magma ocean, and one consisting of a surface magma ocean, a solid rock layer, and a basal magma ocean.

This figure from the study shows the three types of mantle structures in the simulations. The researchers found that the mantle may be a mantle magma ocean, a surface magma ocean and solid rock layer, or a MOSMO structure (i.e., Surface Magma Ocean (MO)–Solid Rock Layer (S)–Basal Magma Ocean (MO)). Image Credit: Boley et al. 2023.

The research shows that mantle magma ocean planets are less common than the other two, but not by much. But when it comes to evolutionary pathways that might lead to habitable planets, it’s the planet’s composition that’s more important than its mantle structure. In lava worlds without atmospheres, the composition dictates how effective the magma is at trapping volatiles. That’s critical when it comes to life as we know it.

For a planet to one day express life, it needs an atmosphere with critical components like carbon and oxygen. Earth life is based on carbon, and oxygen is key to complex life here on Earth. So a magma planet with ample carbon and oxygen in its magma could eventually off-gas these critical materials into a planet’s burgeoning atmosphere if it held onto one.

Water, as we all know, is also critical to life, and some of the simulated planets had massive reserves of water. According to the study, a basal magma planet four times more massive than Earth—a Super-Earth—can trap over 130 times more water than in all of Earth’s oceans. The same planet can also trap 1,000 times more carbon than there is in Earth’s crust and mantle.

“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,” said Boley.

The study also looks at planet density and what it can tell us from a distance when we observe lava worlds. The magma and the volatiles determine a planet’s density, and temperature plays a large role in the volatile content.

This figure from the research shows the bulk density differences between modelled planets at two different surface temperatures. The main point is that with greater mass, the density difference between a magma ocean and an equivalent-mass solid planet decreases. That’s largely due to higher pressure and greater magma compression. “Broadly speaking, all models find that low-mass planets exhibit the largest fractional inflation due to magma, compared to higher-mass planets,” the authors explain. Image Credit: Boley et al. 2023.

To understand the nature of magma planets and how they might evolve, astronomers need to know how magma oceans affect the properties they can observe from a distance. But the study actually shows that when it comes to lava worlds, measuring their densities might not be the best way to understand them. Not, at least, when they’re being compared to solid exoplanets. That’s because the magma ocean doesn’t have a pronounced effect on density. In fact, according to this research, the presence of a magma ocean can’t explain low-density magma ocean planets.

The researchers came to other conclusions about magma oceans. For a planet of a given mass, there’s a range of temperatures in which a planet can have a basal magma ocean that could hold a lot of volatiles. And in their models, they injected H2O and CO2 into the magma of some planets and found that it made very little difference in the density.

Earth as viewed from the cabin of the Apollo 11 spacecraft. Earth was once a magma ocean, a hellish place hostile to life. Now it’s a beautiful, benign ocean world covered in life. Credit: NASA

What does this all amount to? The study’s objective was to determine if a planet’s bulk density indicates a magma ocean and if volatiles are trapped in the interior. Did it accomplish that? Sort of.

It narrows down the observable characteristics that can tell planetary scientists about magma worlds. The density fluctuations aren’t large enough in most cases to reveal much about the planet and if it might contain enough volatiles like carbon and oxygen to eventually form a life-supporting atmosphere. Instead, the results show that researchers should focus on other things like fluctuations in an exoplanet’s surface density.

The researchers write that they “cannot attribute the extremely low densities of some likely lava worlds primarily to magma. Instead, models addressing hot, relatively low-density planets should consider an atmosphere or smaller core-mass fraction in addition to magma.”

So it’s complicated, and while there are some answers here, it really leads to more questions.

“This work, which is a combination of earth sciences and astronomy, basically opens up exciting new questions about lava worlds,” said Boley.

Earth eventually cooled down, and as it cooled, it released volatiles from the magma and formed an atmosphere. Now only its core remains molten, and the molten core makes life possible by generating our protective magnetosphere. Might something similar happen on some magma ocean planets?

Most of the magma oceans we find are USPs and are very close to their stars. These planets will likely never cool enough to solidify if they maintain their close separation. But our detection methods are biased toward planets close to their stars. As planet-finding methods get better, we may find young magma planets further away from their stars, maybe in the habitable zones. Or, for some reason, some of the magma ocean USPs could migrate outward.

The magma ocean planets we’ve found are very close to their stars. Image Credit: NASA

Almost half of the rocky planets we’ve found around other stars could maintain magma on their surfaces. So there are probably billions of these planets in the Milky Way alone. It’s possible that one of them, probably as yet undiscovered, is very similar to Earth, with ample carbon and oxygen sequestered in its magma.

It’s possible that astronomers one day spot an Earth analogue among these lava worlds, but one that’s billions of years behind.

Evan Gough

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