Sub-Neptune planets are going to be occupying scientists for a long time. They’re the most common type of planet yet discovered, and they have no counterpart in our own Solar System. The media buzz about K2-18b that we looked at last time focused solely on the possibility of a biosignature detection. But this world, and another that I’ll discuss in just a moment, have a significance that can’t be confined to life. Because whether or not what is happening in the atmosphere of K2-18b is a true biosignature, the presence of a transiting sub-Neptune relatively close to the Sun offers immense advantages in studying the atmosphere and composition of this entire category.
Are these ‘ hycean’ worlds with global oceans beneath an atmosphere largely made up of hydrogen? It’s a possibility, but it appears that not all sub-Neptunes are the same. Helpfully, we have another nearby transiting sub-Neptune, a world known as TOI-270 d, which at 73 light years is even closer than K2-18b, and has in recent work from the Southwest Research Institute provided us with perhaps the clearest data yet on the atmosphere of such a world. This work may prompt re-thinking of both planets, for rather than oceans, we may be dealing with rocky surfaces under a hot atmosphere.
TOI-270 d exists within its star’s habitable zone. The primary here is a red dwarf in the constellation Pictor, about 40 percent as massive as the Sun. Three planets are known in the system, discovered by the TESS satellite by detection of their transits.
SwRI’s Christopher Glein is lead author of the paper on this work. He and his team are working with data from the James Webb Space Telescope, collected by Björn Benneke and reported in a 2024 paper that was startling in its detail (citation below). Seeing TOI-270 d as a possible “archetype of the overall population of sub-Neptunes,” the Benneke paper describes it as a planet in which the atmosphere is not largely hydrogen but enriched throughout, blending hydrogen and helium with heavier elements (the term is ‘miscible’) rather than formed with a stratified hydrogen layer at the top.
Image: SwRI’s Christopher Glein. Credit: Ian McKinney/SwRI.
Glein acknowledges the appeal of planets with the potential for life, the search for which drives much of the energy in exoplanet research. And the new data offer much to consider:
“The JWST data on TOI-270 d collected by Björn Benneke and his team are revolutionary. I was shocked by the level of detail they extracted from such a small exoplanet’s atmosphere, which provides an incredible opportunity to learn the story of a totally alien planet. With molecules like carbon dioxide, methane and water detected, we could start doing some geochemistry to learn how this unusual world formed.”
TOI-270 d, in other words, offers up plenty of detail, with carbon dioxide, methane and water readily detected, allowing as Glein notes the possibility of doing a geochemical analysis to delve into not just the atmosphere’s composition, but how this super-Earth formed in the first place. We have to begin with temperature, for the gases that showed up in the JWST data were at temperatures close to 550 degrees C. Hotter, in other words, than the surface of Venus, a fact that we need to reckon with if we’re holding out hope for global oceans. For at these temperatures gases do some interesting things.
Here the term is ‘equilibration process.’ At a certain level of the atmosphere, pressures and temperatures are high enough that gases reach chemical equilibrium – they become a stable mix. Going higher means both temperature and pressure drop, thus slowing reaction rates. But it is possible for gases to move upward faster than their chemical reactions can adjust to the change, which ‘freezes’ in the composition that was set at the equilibrium level. The mixture ‘quenches,’ in the terminology, and at that point the chemical ratios can no longer change. Finding out where this happens allows scientists to interpret what they see in data taken much higher in the atmosphere.
We are left with the chemical signature of the deep atmosphere where equilibrium occurs. We draw these inferences from the data taken by JWST from the upper atmosphere, offering a broader view of the atmosphere’s composition throughout.
The paper analyzes the balance between methane and carbon dioxide in terms of this quenching, as the relative amounts of the two gases become ‘frozen’ as they move upward. Working out where the balance would occur in a hydrogen-rich atmosphere allowed the team to work out that the freeze out occurred at temperatures between 885 K and 1112 K, with pressures ranging from one to 13 times Earth sea-level pressure. All this points to a thick, hot atmosphere, and one with a persistent conundrum.
For while models suggest that we should find ammonia in the atmosphere of temperate sub-Neptunes, it fails to appear. A nitrogen-poor atmosphere, the authors believe, is possibly the result of nitrogen being sequestered in a magma ocean. The speculation points to a world that is anything but hycean – no water oceans here! We may in fact be observing a planet with a thick atmosphere rich in hydrogen and helium that is well mixed with “metals” (elements heavier than helium), all of this over a rocky surface.
Image: An SwRI-led study modeled the chemistry of TOI-270 d, a nearby exoplanet between Earth and Neptune in size, finding evidence that it is a giant rocky world (super-Earth) surrounded by a deep, hot atmosphere. NASA’s JWST detected gases emanating from a region of the atmosphere over 530 degrees Celsius — hotter than the surface of Venus. The model illustrates a potential magma ocean removing ammonia (NH3) from the atmosphere. Hot gases then undergo an equilibration process and are lofted into the planet’s photosphere where JWST can detect them. Credit: SwRI / Christopher Glein.
The paper also notes the lack of carbon monoxide, explaining this by a model showing that CO would have frozen out even deeper in the atmosphere. Both modeling and data offer an explanation for TOI-270 d but also point to alternatives for K2-18 b. The modeling of the latter as an ocean world is but one explanation. Photochemical models show how difficult it is to produce and maintain enough methane under such conditions. Furthermore, K2-18 b likely receives too much stellar energy to maintain surface liquid water, due to greenhouse heating and limited atmospheric circulation. Thus the paper’s conclusion on K2-18b:
Because a revised deep-atmosphere scenario can accommodate depleted CO and NH3 abundances, the apparent absence of these species should no longer be taken as evidence against this type of scenario for TOI-270 d and similar planets, such as K2-18 b. Our results imply that the Hycean hypothesis is currently unnecessary to explain any data, although this does not preclude the existence of Hycean worlds.
This is a deep, rich analysis drawing plausible conclusions from clearer data than we had previously acquired from transiting sub-Neptunes. The question of water worlds under hydrogen atmospheres remains open, but the galvanizing nature of this paper is that it points to forms of analysis that until now we’ve been able to do only in our own Solar System. I think the authors are connecting dots in very useful ways here, pointing to the progress in exoplanetary science as we go ever deeper into atmospheres.
From the paper. The italics are mine:
Our overall philosophy was to develop modeling approaches that are rooted as much as possible in empirical experience. This experience includes fumaroles on Earth that constrain quench temperatures between redox species in hot gases, and making planets out of meteorites and cometary material to understand how different elements can reach different levels of enrichment in planetary atmospheres. Our approaches were simple, perhaps too simple in some cases if the goal is to accurately pinpoint the composition, present conditions, and history of the planet. If, instead, the goal is to suggest new ways of thinking about geochemistry on exoplanets that maintain focus on key variables and how they can be connected to observational data, as well as large-scale links between what we observe and how the atmosphere might have originated, then a different path to progress can be taken.. The latter is the point of view we pursued.
The paper is Glein et al., “Deciphering Sub-Neptune Atmospheres: New Insights from Geochemical Models of TOI-270 d,” accepted at the Astrophysical Journal (preprint). The Benneke et al. paper is “JWST Reveals CH4, CO2, and H2O in a Metal-rich Miscible Atmosphere on a Two-Earth-Radius Exoplanet,” currently available as a preprint.
This is another good reasoned paper on these types of planets and is a good counterpoint to all the hype we’re being bombarded with on the internet about the “life” detected on K2-18 b. Furthermore, I think these guys are right.
The hycean model may well describe such planets that are further out from their stars. After all, that graph show hycean planets being habitable up to 10 AU out from G-type stars. So there is still plenty of possibilities for life-bearing hycean planets.
The Benneke paper has a very nice NIR spectrum of the atmosphere of TOI-270d in which they matched carbon disulfide (CS2) to the IR spectrum. It is a pity that the Madhusudhan team used the MIR spectrum for K2-18b so that we cammot compare the spectra. We don’t know which 20 compounds they tested to match teh MIR spectrum, only that they matched DMS/DMDS to it.
Hycean worlds with an ocean, or a rocky surface with lava. Both worlds in teh HZ (just) but hot. It is reminiscent of our beliefs about Venus. Once it was a tropical world, with possibly soda water carbonated oceans, populated by creatures like giant reptiles. Then reality stepped in and it became of lethal hot, dry, acidic world with a surface that could melt lead. Our last hopes for life on Venus are in the temperate altitude in the atmosphere. Will these sub-Neptune worlds follow a similar fate?
Very interesting though I can’t agree with some of the conclusions of this paper. First carbon monoxide should not freeze out lower in the atmosphere, but only higher since it gets warmer as the pressure increases in a thick atmosphere like on Venus. The main idea here that is not supported by principles is the magma ocean. A deep atmospheric temperature of 530 C is only a little more than Venus atmosphere at 464 C, much too low for there to be a magma ocean. What were are seeing here is a “pressure cooked planet,” as has been mentioned for Hycean worlds which are not in the life belt, but on the hot side too close for liquid water. It might have an water ocean if it was in the colder side in the life belt or perhaps even past the snow line since a large atmosphere allows a higher boiling point of water also already mentioned and also a great greenhouse effect.
What were are seeing here is something similar to Venus, but since there is so much water all of it is in the atmosphere greatly increasing the pressure and greenhouse effects. It also has a larger gravity and escape velocity than Venus and therefore TOI-270d can hold water vapor longer and therefore has more water vapor due to it being more abundant than Venus which due to its lower gravity and solar wind stripping, etc and H20 photo lysis or photo dissociation has lost most of its water leaving the heavy hydrogen behind, the DH20. The age of the system maters here. This system is less than a billion years old so it has not had enough time too loose all of its water vapor due to the lack of a magnetic field to protect it from the solar wind.
The NIR leaks through the upper atmosphere of Venus and TOI-270d therefore allows us to see the higher temperatures of the lower atmosphere, the hotter thermal infra red in a thick atmosphere. The spectral bands are broadened in the NIR which happens because the molecules are more energetic with more collisions between them. The MIR lets us see the top of the atmosphere. Carbon Dioxide absorbs strongly in the MIR which is why our atmosphere is opaque to it and the JWST has to be above the atmosphere to see in the MIR. Since CO2 is evenly mixed through an atmosphere, we can see the spectral lines of it in TOI-270d.
I believe, and may be completely confused, that by frozen they mean the ratio of CO2 to other elements becomes ‘frozen’ or confined at lower altitudes resulting in the upper atmosphere being depleted of CO2. The use of the term is clunky.
As to a molten surface; could a hot dense atmosphere reduce heat flux to the point where the surface remains molten?
Harold, you’ve got this right. ‘Frozen’ means the ratio of the gases becomes fixed by chemical equilibration. The paper goes into this in considerable depth.
If the atmosphere were a perfect insulator, the surface temperature would rise to that of the core. I can imagine that with volcanism, magma reaching the surface would have difficulty cooling, remaining in a hot, liquid state, slowly increasing in depth. Add in the stellar radiation that gets trapped. The Venusian surface is hotter than Mercury’s, and there is no active volcanism.
To suggest magma “oceans”, someone has run the numbers to reach that conclusion, whether from trapped stellar radiation, radiogenic heating, and/or active volcanism.
Quote by Alex Tolley: “To suggest magma “oceans”, someone has run the numbers to reach that conclusion, whether from trapped stellar radiation, radiogenic heating, and/or active volcanism.” I agree with this. The distance from the star gives us the intensity and flux of black body radiation across the electromagnetic spectrum. If we move TOI-270 d much closer to the star there is a much higher intensity and larger radiation cross section. We could move it close enough to get the necessary surface temperature for the to be molten and we still should be able to see that temperature which should be higher than 530 C. I assume the atmospheric temperature of TOI-270 d is 530 C by its brightness in the infra red thermal black body radiation through the near infra red which gives us the temperature in the lower atmosphere which is indicative of the surface temperature. The same techniques in spectroscopy work on Venus.
Volcanism is interesting since the generally accepted idea in astrophysics is that super Earths and Sub Neptune’s don’t have any plate tectonics and carbon cycle. Oceans could still take up some of the carbon dioxide from volcanism, but TOI-270 d does not have any oceans and without any carbon cycle, the carbon dioxide level in the atmosphere would be much higher which helped a runaway greenhouse effect.
It would be nice to see the atmospheric spectra of sub Neptune and super Earth in the life belt and past the snow line.
Just a note generally about magma oceans. When we think of magma oceans, we think of bubbling, red-hot rock flowing, but those features come from the release of pressure, gases escaping and water expanding into steam as the driving force.
The pressure at the bottom of TOI-270d’s atmosphere is something like 1000 miles down on Earth, and the rock there is essentially solid with slight plasticity.
At the bottom of the atmosphere, heat from the planet’s core will escape through conduction until it reaches the bottom of the atmosphere at which point convection will take over. The atmosphere at this point will have approximately the density of water. As it rises to the top of the atmosphere, the atmosphere expands and cools, and as the atmosphere descends again, it will compress back to a point of red heat. So, there will be a considerable temperature gradient from the top the bottom of the atmosphere just from the effects of compression.
@Dave
What would such a magma ocean look like at these immense depths? Relatively smooth, glowing, molten rock overlaid with convecting water?
I’ve had a few more thoughts on this since my last post. Firstly, there is probably some sort of process going on at the boundary, much like there is between earth’s core and mantle.
Secondly, let us consider convection: convection occurs when a fluid that is heated changes density. Becoming less dense than the surrounding fluid, it floats upward.
Now let’s consider the superfluid mixture of water, H & He under 500 Kbar sitting on the planet’s mantle. It has about the density of water, but if it’s heated 10°C, under 500kbar, how much will it expand? If this expansion is infinitesimal, its density will remain the same and there won’t be any circulation. Also, consider its opacity. Water is pretty good at transmitting light (compared to rock) so radiation may be the most effective way the red heat is transmit upwards. Radiation’s ability transmit energy goes up by the 4th power of temperature, so transmission of heat out of the core may be far more efficiently done by radiation, leaving the bottom part of the atmosphere stagnant. At some point as you rise toward the surface and the pressure goes down, heating will expand the superfluid sufficiently to start convective circulation. In a way the planetary atmosphere would in someways mimic the interior of a G-type star where you have a radiative zone overlaid by a convective zone.
We could all agree that this is a very complicated world. And placing it in context is complicated too, in as much as there has not been very much written or published about the TOI-270 system thus far – save connecting it to the “hycean proposition”.
Trying to get a grasp, I searched for system descriptions. A rule of thumb I have used for habitable worlds ( whether this one was examined as such or not) is that the stellar surface flux temperature extended out to an equivalent radius where it would reduce to around 400 K, like in the vicinity of Earth is HZ figure of merit. It’s hotter. TOI 270d is 6 million kilometers from a M1 red dwarf.
My calculation of the corresponding “flux temperature” at that distance is 825 K.
While it is the farthest out known exoplanet in the TOI 270 system, it is also in a 2:1 resonant orbit with its inner next door neighbor. These are big worlds, not Galilean moons like Io, Europa, Ganymede and Callisto. The spread is wider, but periods are similar. … Whether the TOI-270 exoplanets are stellar synchronous in their rotation or not, I don’t know how we would be able to judge, considering how fluid their compositions are presumed to be. In the case of Neptune and Triton, for example, Neptune’s equatorial wind streams run to the west perhaps synchronously ( or attempting to be so) with Triton’s retrograde orbit.
Also, I could not find an estimate of how old this stellar system is. Heat of formation would tend to leak out of a planet’s interior, disregarding contributions of nuclear reactions for relatively short lived isotopes. If TOI 270 is two, five or ten billion years old, the remaining interior heat in TOI 270 d could be significantly different or released by differing internal circulatory processes.
Comparison to K2-18b is, of course, is warranted. Similar masses, similar stars, but there could be differences in ages, how tightly they are bound and how they are perturbed by other objects.
It would appear, after all, that research into hycean worlds is only beginning.
I stand corrected. I should have read the paper more carefully. I was thinking about the possibility false positives since carbon monoxide won’t be frozen due to photo lysis of carbon dioxide CO2 into oxygen and carbon monoxide happening mostly in the upper atmosphere on Venus. CO still could be in chemical equilibrium though. It is interesting that there is no Ammonia NH3. There are other possibilities: Carbon dioxide can combine with Ammonia to make ammonium carbamate. Also Ammonia is easier broken broken down by ultra violet photodissociation and methane due to its strong carbon bonds. Low Nitrogen exoplanet is another possibility besides a molten surface. Google AI
Molten surfaces can only happen in extreme environments events like the Wasp system, a giant meteor or the giant impact hypothesis. Ibid. Google AI agrees with the idea that there are molten oceans on sub Neptunes do to their thick atmosphere. I respectfully disagree as the melting temperature of rock is just to high. I don’t think that idea sticks to the priniciples of physics, chemistry and geology.
There’s a lot in this paper. It’s not just a result, but it contains numerous formulae that describe the relations between a planet’s spectra and its composition.
A few points: the papers talks about the mass fraction ratio between H/He and the heavier volatiles such as O, C, N, and S and been around 50%. This is not the mole ratio. That would be something like 16:1. If we do some rough calculations and assume the bulk of the heavier volatiles is water, then the mole ratio of H2 gas to Oxygen is 8:1. One H2 is require to add to an Oxygen atom to make H2O, which brings it down to 7:1. If we assume that a quarter of the H/He is Helium, then this brings the ratio down to 5:1, so for every molecule of water, there are 5 of H2 and about 2 of He. This is still a Hydrogen dominated atmosphere.
The proportions of CH4 and CO2 are calculated to be 2.3% and 2.1% by mass, which means that they are minor constituents of an atmosphere dominated by H, He and H20.
The mass of this atmosphere is phenomenal, 0.29 Earth’s mass, which is equivalent to 25 lunar masses, or 1244 times the mass of Earth’s oceans. Assuming Earth’s oceans, vaporized produce an atmosphere of 350 bar, then the pressure at the bottom of this atmosphere is about 500 kilobar, which is close to the pressure at which high pressure ice forms at a temperature of 1000°C.
This means that even minor constituents in this atmosphere have an enormous mass.
A note about Nitrogen. The authors think there is little in the atmosphere as it forms Ammonia and is dissolved in the magma mantle. I’m a little dubious of this. I would point out that they can’t detect N2 spectroscopically (stated in the paper) and even a tiny fraction of the atmosphere being N2 would be an enormous mass of Nitrogen.
A final point about the cloud deck. These spectra from TOI-270d is excellent, indicating a lack of haze or clouds, but these spectra are taken at the 1 millibar level which is about at 175,000 feet on Earth and the only clouds at that level are noctilucent clouds. There maybe other clouds in the planet’s atmosphere at lower levels; although, the authors think that the temperature profile is such that water vapor doesn’t condense out all the way to the top of the atmosphere.
Lots of fascinating stuff in this paper.
“Quenching” should always excite us a little. At high temperatures, entropy prevails, and chemicals break themselves to bits; at low temperatures, enthalpy rules, and they recombine. At high pressures, gasses are forced to ally into larger compounds to reduce their pressure. Whenever we say the proportions of something are “frozen”, in disequilibrium, it means there is a negative free energy change to be had: the release of energy. This implies that a chemoautotrophic organism, producing the right enzymes, could generate metabolic energy from that environment, powered ultimately by the convection of the planet itself.
I love the idea that an organism could use convection and pressure changes to support its metabolic processes. So different from methanogens at the ocean vents using the serpentinization reaction to create free hydrogen for metabolism, which reduces CO2 to CH4.
The organism might use high pressure to ease the reaction, but then migrate up in the rising plume to a lower pressure to gain some advantage there, before cycling back down again. This might allow more primitive enzymes to effectively drive the reactions to capture energy to grow and replicate.
For the a less dramatic case than TOI-270 d:
There is plenty of convection afloat in the atmosphere of Jupiter. And plenty of hydrocarbons. Choose a level, band or belt.