When NASA launched the Kepler spacecraft in 2009, it triggered an age of exoplanet discovery. Many surprises lay in wait for astronomers, as they quickly understood that there were exotic types of exoplanets with no counterparts in our own Solar System. One of those types was mini-Neptunes, or sub-Neptunes.
Sub-Neptunes are smaller than Neptune but larger than Earth, and as time went on—and as Kepler’s successor TESS was launched—it became clear that these planets are extremely common. In fact, they’re the most common type of exoplanet out there. Scientists got to work to understand the nature of these planets.
What evolutionary path did they follow? What’s their composition? Why are they so close to their host stars, and why doesn’t our Solar System have any?
As observational data poured in, one consensus began to emerge. Since it’s pretty clear that all rocky planets begin as magma oceans, planetary scientists think this is true of sub-Neptunes. And since they also think that these planets have very dense hydrogen-dominated atmospheres, these planets could efficiently trap heat, allowing the magma ocean phase to persist long after other rocky planets like Earth had cooled.
But new research may overturn that understanding. It’s titled “Not All Sub-Neptune Exoplanets Have Magma Oceans,” and it’s published in The Astrophysical Journal Letters. The lead author is Professor Eliza Kempton from the Department of Astronomy & Astrophysics at the University of Chicago.
“At the bottom of it, quite literally, we’re trying to understand what these objects are, because they don’t exist in our solar system.” – Prof. Eliza Kempton, University of Chicago.
“The evolution and structure of sub-Neptunes may be strongly influenced by interactions between the outer gaseous envelope of the planet and a surface magma ocean,” the authors write. “However, given the wide variety of permissible interior structures of these planets, it is unclear whether conditions at the envelope–mantle boundary will always permit a molten silicate layer or whether some sub-Neptunes might instead host a solid silicate surface.”
*Most of the new exoplanets found so far have sizes in between Earth and Neptune. This schematic compares Neptune, Earth, super-Earths and mini-Neptunes. Image Credit: NASA/ESA/CSA and STScI*
In their work, the researchers modelled the internal structure of sub-Neptunes across a wide range of atmospheric and bulk properties. In their model, all planets have an iron core, a silicate mantle, and a mixed H/He/H2O envelope. The researchers used an Earth-like iron-to-silicate ratio, with 1/3 iron and 2/3 silicate by mass. The researchers varied five different inputs: planet mass, pressure at the radiative–convective boundary, photospheric temperature, envelope mass fraction, and MMW, or mean molecular weight. The mean molecular weight is basically the average mass of all the molecules and atoms that make up the atmosphere.
They also used case studies. One of them is based on GJ 1214 b, a mini-Neptune that’s been extensively studied. “Observing campaigns from the ground as well as using Hubble Space Telescope (HST) and JWST have determined that this planet hosts a hazy, high-MMW atmosphere,” the authors write. That’s important because it means that its atmosphere has molecules larger than hydrogen and helium. That, in turn, means that its atmosphere is heavier than previously thought, and also far larger than Earth’s.
So GJ 1214 b, and potentially other mini-Neptunes, could have heavy enough atmospheres to create high temperature and high pressure conditions. The pressure bearing down on the surface could be high enough to force the magma to transition to solid.
“Our primary goal in this Letter was to determine whether all sub-Neptunes should be expected to host magma oceans or whether a subset of such planets should present solid surfaces,” the authors explain. As a result of the GJ 1214 b case study and their models, they determined that not all sub-Neptunes have magma oceans.
There are three reasons why some of these planets have solid surfaces.
“The envelope mass fraction and MMW most strongly impact whether a planet hosts a magma ocean,” the authors write. These factors can create high pressures at the envelope-mantle boundary, forcing a phase change from liquid magma to solid surface. “Planets with a high envelope mass fraction and a high MMW are therefore likely to have a solid silicate surface with no magma ocean,” the researchers explain.
The second reason concerns temperature. Cold temperatures combined with high-MMW envelopes and low envelope mass fractions can also create solid surfaces. But for temperatures at the envelope boundary to be low enough, there must be a very thin envelope. That means these planets might be closer to super-Earths than mini-Neptunes.
The third reason involves the pressure at the radiative-convective boundary. It’s the depth boundary in a planet’s atmosphere where changes from being dominated by convection below to being radiation-dominated above. It’s a proxy for an exoplanet’s age because young planets are hotter inside while older planets have had more time to cool. An exoplanet with a lower pressure at the radiative-convective boundary is likely younger, and is more likely to have a magma ocean.
*While we know mini-Neptunes exist in great numbers, we have no real images of them. All we have are artist’s impressions like this one. Image Credit: By Pablo Carlos Budassi – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=112487881*
“This really upends a paradigm about these planets, which is interesting because there are so many of them in the universe,” lead researcher Kempton said in a press release. “At the bottom of it, quite literally, we’re trying to understand what these objects are, because they don’t exist in our solar system.”
“It’s an either-or,” said Kempton. “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.”
These results have implications for the JWST’s ongoing observation of mini-Neptunes. “Once the bulk and atmospheric properties of a sub-Neptune have been determined, our results can provide an indication of whether said planet is likely to presently host a magma ocean,” the researchers write. They note that they’ve already demonstrated that for several previous and future JWST targets. “Given that high-MMW atmospheres have already been observed for several sub-Neptunes, there is a real possibility that a large fraction of this population does not currently possess a magma ocean in contact with its gaseous envelope.”
The age of exoplanets has shown us how wrong we were. In our forced ignorance, we assumed that what we saw around us was normal. All we knew was our Solar System, and inevitably, assumptions crept into our understanding. This is how solar systems form and this is what they look like.
How wrong we were. The massive numbers of mini-Neptunes illustrates the depth of our ignorance. But there’s no shame in ignorance, unless we cling to it long after evidence banishes it.
“Before we found any exoplanets, we had a nice neat story about how solar systems form based on how our solar system formed. We thought that would apply to other solar systems,” explained study co-author Matthew Nixon. “By following that logic, other solar systems should look like ours. But they don’t.”
We’re only in the early stages of understanding how planets form and evolve, aided by the JWST and its ability to probe exoplanet atmospheres. There will likely be many more assumptions and understandings overturned as the JWST and other future observatories continue their work.
As is often true, much of our desire to understand exoplanets relates back to our yearning to understand our own planet, how it formed and evolved, how life arose, and how we got here.
“It gets back to why are we here—how did Earth come to be?” said Nixon. “This is a really fundamental piece for us to understand both other planets and our own.”