New study revises our picture of the most common types of planets in the galaxy
Exoplanets smaller than Neptune are thought to have a molten rocky surface, but recent work suggests this might not always be the case. Image courtesy/Credit: L. Kreidberg, G. Bacon/NASA, ESA; J. Bean/U. Chicago; H. Knutson/Caltech
Thanks to better telescopes, like the James Webb Space Telescope, we now know our solar system isn’t unique — there are millions of other planets in our galaxy.
But we’re still teasing out clues about what they are actually like.
One mystery is a kind of planet called a “mini-Neptune.” These are smaller than Neptune and seem to be very common. They’re made of rock and metal, with thick atmospheres full of hydrogen, helium and maybe water and other molecules. Strangely, we don’t have any planets like this in our solar system, so scientists are still trying to figure them out.
But a new study, published in the Astrophysical Journal Letters and co-led by Matthew Nixon at Arizona State University, adds a new wrinkle to our best picture yet of these distant worlds.
Scientists used to think these planets were covered in oceans of molten rock, but Nixon and his team found that many might actually have solid surfaces.
Even so, these planets wouldn’t be friendly places to visit. Their solid surfaces only exist because the thick atmosphere exerts such a strong force. All that air above creates super high temperatures and pressures.
“The pressure can be so high that the rock turns solid again,” said Nixon, 51 Pegasi b Fellow in the School of Earth and Space Exploration. “It’s a bit like how carbon turns into diamond deep beneath the Earth’s surface.”
Mass and magma
Planets outside our solar system, called exoplanets, are so far away that our telescopes can’t see them directly. Instead, scientists look for tiny dips in a star’s light when a planet passes in front of it.
But scientists have found clever ways to gather information. For example, they can guess what’s in a planet’s air by looking at the light that passes through it. They can also figure out how heavy a planet is by watching how it pulls on its star.
Finding so many mini-Neptunes surprised scientists who saw them around nearby stars, given their total absence from our own neighborhood.
Because these planets are so hot and have thick atmospheres, scientists thought they had seas of molten rock on their surfaces, like early Earth. Some even believed these magma oceans could “eat” parts of the atmosphere, stopping the planet from growing bigger.
But digging deeper into the data, a team of researchers, including Nixon, Bodie Breza (who was then an undergraduate student at the University of Maryland) and University of Chicago Professor Eliza Kempton, realized the story might be more complicated.
The group first realized the potential twist while analyzing a planet called GJ 1214 b, which orbits a distant star in the constellation Ophiuchus. Recent data from the James Webb Space Telescope suggests this planet’s atmosphere may contain larger molecules than simple hydrogen and helium, which implies the atmosphere would be heavier than previously thought — much, much larger than Earth’s thin shell.
This surprised the scientists, so they made computer models of different kinds of planets. They found that many mini-Neptunes once thought to be lava worlds might actually have solid surfaces.
“It’s an either-or,” Kempton said. “You can have this 'the-floor-is-lava' scenario or a solid surface, and you’re going to have to take into account a number of other factors about a planet’s atmosphere to try to figure out which regime it falls under.”
Revising the story
These mini-Neptune planets are of special interest to scientists because of their sheer numbers and what they imply about how planets form.
“Before we found any exoplanets, we had a nice, neat story about how solar systems form based on how our own solar system formed. We thought that would apply to other systems,” Nixon said. “Following that logic, other solar systems should look like ours. But they don’t.”
Scientists, therefore, want to understand how mini-Neptunes form and what they look like now to build a more complete picture of how planets form in general. This can guide, among other things, the search for habitable planets.
The School of Earth and Space Exploration is leading the charge in understanding these strange worlds.
“What excites me about working on this research at SESE is the breadth of expertise — not just in astronomy, but also in geology and mineral physics, which are really fundamental for our understanding both of other planets and of our own,” Nixon said.
This study is supported by funding from the Alfred P. Sloan Foundation, NASA and the Heising-Simons Foundation.
This article was written by Louise Lerner of the University of Chicago, with contributions from Kim Baptista of ASU’s School of Earth and Space Exploration.
More Science and technology
Lessons on maintaining your humanity in the world of AI technology
AI is not human. But it does a good job of acting like it.It is capable of replicating how we speak, how we write and even how we solve problems.So it’s easy to see why many consider it a threat, or…
When you’re happy, your dog might look sad
When people are feeling happy, they’re more likely to see other people as happy. If they’re feeling down, they tend to view other people as sad. But when dealing with dogs, this well-established…
New research by ASU paleoanthropologists: 2 ancient human ancestors were neighbors
In 2009, scientists found eight bones from the foot of an ancient human ancestor within layers of million-year-old sediment in the Afar Rift in Ethiopia. The team, led by Arizona State University…