Sub-Neptunes, January 30

Sub-Neptunes are a type of planet that is smaller than Neptune (as the name suggests).

In our solar system, we have “terrestrial planets” (Mercury, Venus, Earth, and Mars), which are small and rocky planets, and then “gas giants” (Jupiter and Saturn), massive, gassy hydrogen/helium bodies, and then “ice giants” (Uranus and Neptune), slightly less massive and made up more of ice than the gas giants.

It turns out, when we observe other systems, the majority of planets that we observe fall into the size zone between Neptune and Earth, a place that is completely empty in our solar system. This was a surprise: we assumed that our solar system was average, and that we would find bodies similarly distributed to our own. That means terrestrial planets like Mercury, Venus, Earth and Mars; gas giants like Jupiter and Saturn; and ice giants like Uranus and Neptune. Instead, sub-Neptunes are neither of these three categories.

The idea that our solar system is atypical runs counter to a concept astronomers hold dear: the Principle of Mediocrity, or the idea that Earth is completely average. In our history of understanding the solar system, we first thought the Sun orbited around us (disproven by Copernicus); then that we were the only planet with a moon (disproven by Galileo); and then that there was only one galaxy (Hubble discovered otherwise). Since the 1920s, astronomers have carefully tried to say that Earth and the Sun are completely average.

In the age of exoplanets, we have been able to examine this assumption of our average-ness more closely. It turns out, Earth is special in a lot of ways; it seems like we have been in a special place in our solar system to not get bombarded too frequently by meteors; to not get blasted by the Sun’s radiation; it seems tectonic plates are rare; etc. And even our type of planet is uncommon, as sub-Neptunes prove.

Although we have managed to figure out all these facts about other planets, of the 6000+ we have observed so far, we are still limited in the data we can get from them.

The primary way exoplanets are observed these days is with what’s called the transit method, where a planet passes in front of another star. From Earth, we can see the star’s dimming and deduce that a planet exists in that system. With this method, we can figure out a planet’s radius, and with another detection method, we can figure out a planet’s mass. This is still only two pieces of observational data. This means we can know a planet’s density, but not what it is made of.

This is an issue in the case of sub-Neptunes. If we see Jupiter-like planets, or rocky, Earth-sized planets, we can make an educated guess about the planet’s composition. We do this by coupling our understanding of planetary formation processes from our solar system with our knowledge of what primeval blocks make up a new system. We know how to create a Jupiter and an Earth with specific starting ingredients, a specific cooking (formation) process, and at a specific distance (the distance from the planet’s star).

However, while we know the starting ingredients for sub-Neptunes, we don’t know the formation process that results in that type of planet. Many different formation pathways could result in the mass/radius combination that we observe.

This is a hot topic of debate in my field: some people think that variations of this planet could host life, while others imagine them as fiery oceans made of magma.

The following diagram shows three possible solutions to our observations, that all have the same density, but very different formation pathways and different chemical make ups.

All three of these possibly explain our observations.

The first world is a world larger than Earth that held on to its hydrogen atmosphere. Earth once had a hydrogen atmosphere, too, but we lost it as we weren’t heavy enough to hold onto the weight.

The second world is a “Hycean” world, coined by Prof. Madhusadhan (future topic). Earth was formed closer to the Sun, in a region where water would have existed as a gas, and not as a solid, and so therefore ended up with less initial water. This world would have been formed closer to where Uranus and Neptune were formed, and therefore gained water that existed as ice, and therefore had much more ice than Earth.

The final world is like Neptune, but smaller.

One way that we can help determine what sub-Neptunes are made of is with observations of its atmosphere. The transit method makes it possible to “see” different molecules in a planet’s atmosphere, as the signal of a planet passing in front of a star changes depending on what molecules are present there.

The Hycean world is not proven to exist, but the concept of a world that is all ocean, with a base of ice, is appealing to fans of science fiction. Many scientists believe that the atmospheric detections of a Hycean planet - a lot of hydrogen, water, and CO₂ in the atmosphere - matches just as well with a roiling, boiling magma planet. This magma planet would be completely unfriendly to life.

Prof. Madhusadhan goes further than claim Hyceans exist - he has claimed that he has potentially detected signs of DMS on the planet K218-b in its atmosphere. DMS is a type of sulfur which, on Earth, is produced almost exclusively by ocean phytoplankton. Prof. Madhusadhan is implying that ocean phytoplankton exist in the sea of Hycean world.

However, others have said his detection is not strong enough to prove that he has found DMS; there is a signal, but it’s a different molecule; or that Hyceans don’t exist at all.

Many astrophysicists are working on the problem of determining the difference between a rocky world/Hycean world/mini-Neptune—to break the degeneracy, as we call it.

But as I sit there, looking at small, tiny pieces of data, it’s amazing to realize I’m attempting to understand worlds tens of light-years away, planets circling unfamiliar suns, and figure out what exactly they are made of, and who, if anything, could live there.