Monthly Roundup: Exo-Neptunes and Sub-Neptunes

With nearly 6,000 exoplanets discovered to date, it’s clear that not all of the planets in our galaxy resemble the planets in our solar system. Today’s Monthly Roundup explores three types of planets with no analogs in our solar system: a warm Neptune orbiting its tiny host star in less than 4 days; a polar Neptune orbiting perpendicular to its host star; and a famous sub-Neptune whose structure is a matter of debate.

Close-In Neptune, Breaker of Chains

LHS 3154b is a typical Neptune-mass exoplanet in an atypical place. This planet has a mass of at least 13.2 Earth masses, but it orbits an M-dwarf star just 11% the mass of the Sun, swinging around its tiny host star every 3.7 days. It’s not clear how such a setup came to be, as conventional theories of planet formation provide few avenues for such massive planets to form around low-mass stars.

Donald Liveoak and Sarah Millholland (Massachusetts Institute of Technology) have proposed an explanation for LHS 3154b: it’s several planets in a trench coat. In other words, rather than having formed as the single planet observed today, LHS 3154b is the product of a series of collisions between multiple planets that once existed in the system.

To test this theory, Liveoak and Millholland simulated the evolution of a system containing 11 small planets with an average mass of 2 Earth masses. The planets in this system are arrayed in what’s known as a resonant chain, in which the orbital periods of the planets are integer multiples of one another. This configuration is thought to arise naturally when young planets migrate within a protoplanetary disk. Liveoak and Millholland nudged the planets out of this stable configuration by having the planets begin to lose their atmospheres. With their orbital stability disrupted, the planets abandoned their carefully balanced orbits and collided.

masses and orbital periods of planets in systems with a close-in Neptune-mass planet

Masses and orbital periods of the planets remaining in the six simulated systems that generated a close-in Neptune-mass planet. The planetary system architectures are remarkably similar. [Liveoak & Millholland 2024]

Planets akin to LHS 3154b — defined here as planets with masses of 12–20 Earth masses and orbital periods less than seven days — arise in just 1.2% of simulations. This result shows that while it’s possible to create a planet like LHS 3154b this way, it’s not surprising that more planets like LHS 3154b have yet to turn up. The simulations did provide a clue to find evidence of this process in other systems, though: the systems yielding close-in Neptunes always had one or two companions. The Neptune-mass planet is always the closest planet to the star, and the remaining planets have orbital periods around 30 days. By searching for systems containing a short-period Neptune with an outer companion, researchers can study the chain-breaking process that may be responsible for LHS 3154b’s existence.

Stability of Polar Neptunes

The planets in our solar system orbit in nearly the same plane as the one defined by the Sun’s spin, but it’s clear that not all planetary systems in our galaxy are so orderly. A sizeable chunk of the exoplanet population orbits nearly perpendicular to their stars’ spins. Curiously, roughly half of this population are planets with masses similar to that of Neptune. These highly inclined exo-Neptunes share several characteristics, including slightly elongated orbits, puffy atmospheres with the possibility of past mass loss, and, in some cases, a massive planet in the same system, orbiting farther from the host star.

Emma Louden (Yale University) and Sarah Millholland (Massachusetts Institute of Technology) investigated whether disk-driven resonance could torque exo-Neptunes into their perpendicular orbits while also explaining the other properties of this exoplanet population. In this framework, a young exo-Neptune’s orbit would evolve under the gravitational influence of an outer giant planet and a dissipating planetary disk. Over time, the dissipation of the disk combines with a nodal resonance between the inner Neptune and the outer giant planet, eventually launching the inner Neptune into a highly inclined orbit. This mechanism clearly links polar-orbiting Neptunes with outer giant planets.

diagram showing the process of disk-driven resonance

A diagram that describes the disk-driven resonance for creating polar-orbiting planets. [Louden & Millholland 2024]

This process is thought to take place in the first 10 million years of a planetary system, and it’s not yet clear whether this setup is stable over billions of years. Louden and Millholland simulated the evolution of this setup under the influence of tides. Remarkably, the authors found that exo-Neptunes in polar orbits are stable over long stretches of time, though some planets with smaller orbital inclinations, in the 45–80-degree range, are not stable.

Louden and Millholland then considered two known polar Neptunes, WASP-107b and HAT-P-11b. Both of these planets have an outer giant planet in their system, and both planets show signs of mass loss due to tidal heating. Louden and Millholland showed that these planets’ orbital configurations are incredibly stable, on par with the orbital stability of Uranus and Neptune in our own solar system. While this study doesn’t provide proof of the disk-driven resonance hypothesis, it does demonstrate the feasibility of the concept, and further observational evidence can strengthen the hypothesis.

Is K2-18b Covered in a Supercritical Ocean?

Of the many sub-Neptune exoplanets — those with masses between the mass of Earth and the mass of Neptune — K2-18b is perhaps the most famous. Discovered in 2015 in data from the Kepler Space Telescope, K2-18b has a mass of 8.63 Earth masses and a radius of 2.6 Earth radii. The structure of planets of this size is a matter of debate: do sub-Neptunes have solid surfaces, like Earth, or are they primarily gaseous, like Neptune? Or are they somewhere in between, unlike any of the planets in our solar system?

Possible structures for K2-18b

Possible structures for K2-18b that have been explored in previous work and will be tested in this work (far right). Click to enlarge. [Luu et al. 2024]

K2-18b’s structure has been difficult to pin down, even with the help of JWST. Atmospheric spectroscopy enabled the detection of methane (CH4) and carbon dioxide (CO2) in K2-18b’s atmosphere. Water, carbon monoxide, and ammonia were not detected. Through modeling, some researchers have found these data to be consistent with the planet having a thin atmosphere with a habitable — even speculated to be inhabited — surface, while others have concluded that K2-18b is gas rich, with no solid surface.

Now, Cindy Luu (University of Texas at San Antonio) and collaborators have explored yet another possibility: that K2-18b is covered with an ocean of hot, supercritical water. A supercritical fluid is hot enough to become gaseous but is under enough pressure that it is left in a strange in-between state that shares properties with both its liquid and gaseous forms. Pure water goes supercritical at 647.15K (705℉/374℃).

Luu’s team used geochemical modeling to test the hypothesis that a global supercritical water ocean could explain the observed chemical abundances of K2-18b’s atmosphere. In this scenario, the boundary between the ocean and the atmosphere would be fuzzy, with a greater degree of mixing than would be found at a typical ocean–atmosphere boundary. The team found that an ocean with a temperature between 710K and 1050K could reproduce the observed chemical abundance ratios in K2-18b’s atmosphere. What’s more, this scenario naturally reproduces the non-detection of carbon monoxide.

While the current investigation shows that a supercritical water ocean is consistent with the existing atmospheric abundances measured with JWST, more work is needed to explore this exotic possibility further. More data on carbon monoxide and ammonia, in particular, will be critical to future research.

Citation

“Formation of Close-In Neptunes around Low-Mass Stars through Breaking Resonant Chains,” Donald Liveoak and Sarah C. Millholland 2024 ApJ 974 207. doi:10.3847/1538-4357/ad7383

“Polar Neptunes Are Stable to Tides,” Emma M. Louden and Sarah C. Millholland 2024 ApJ 974 304. doi:10.3847/1538-4357/ad74ff

“Volatile-Rich Sub-Neptunes as Hydrothermal Worlds: The Case of K2-18b,” Cindy N. Luu et al 2024 ApJL 977 L51. doi:10.3847/2041-8213/ad9eb1