Scientists have been working on models of planet formation since before exoplanets were known to exist. Although these models were originally derived from the properties of the planets in our solar system, they have proven to be very good at accounting for exoplanets that have no equivalents in our solar system, such as super-Earths and hot Neptune. It turns out. Add in the planet’s ability to move around thanks to gravitational interactions, and the properties of exoplanets can usually be explained.
Today, a large international research team announced findings that our models cannot explain. It is about the same size as Neptune, but has four times the mass. Its density is much higher than that of iron, comparable to either the entire planet being almost completely solid, or having oceans deep enough to drown the entire planet. Those who discovered it have put forward several theories about its formation, neither of which is particularly likely.
unusual outlier
The study of this new planet began as many people do today. The planet was identified as an object of interest by the Transiting Exoplanet Survey Satellite (TOI, TESS Object of Interest). TOI-1853 is a slightly smaller star than the Sun, with a mass approximately 0.8 times that of the Sun. And near this star there were clear signs of a planet, now called TOI-1853 b. The planet orbits very close to its host star, completing a complete orbit in 1.24 days.
Researchers used that time to determine the distance the planet orbited. It is possible to estimate the size of a planet based on a combination of its distance, the size of the star, and the amount of light blocked by the planet. It turns out to be about 3.5 times the radius of the Earth, meaning it’s a little smaller than Neptune.
That in itself is not unusual. Many Neptune-sized planets have been discovered. But the combination of its size and proximity to the star was unexpected. It places it in what is called the “hot Neptune desert”, where intense radiation from the stars expels the planet’s atmosphere. Neptune, which has reached a hot desert state, will eventually be stripped down to its rocky core, leaving behind a super-Earth.
So what was TOI-1853 b doing in the desert? To find out, researchers used ground-based observatories to observe how gravity changes as TOI-1853 b moves through its orbit. The movement of the host star was tracked at that time. The acceleration of the star’s motion due to this gravitational force can be used to estimate the mass of the planet.
TOI-1853 b has many of mass. Its mass is estimated to be 73 times that of Earth, or more than four times the mass of Neptune. Quite obviously, that means its composition must be very different from Neptune.
Crispy inside and outside?
The researchers involved in its discovery spend a considerable amount of text explaining how much of an outlier this makes TOI-1853 b. There are some planets with similar densities, but they are usually much smaller and are super-Earths formed by stripping away the atmosphere of planets similar to Neptune. Although there are several planets of similar mass, they are all nearly twice as large and may have extensive atmospheres and oceans. “It occupies the region of the mass trajectory [distance] “This is a previously unoccupied region of hot planetary space, and corresponds to the driest regions of hot Neptune’s deserts,” the researchers conclude.
That’s not the only strange thing. Considering the density here, there are two configurations that make sense. One is that the planet is composed almost entirely of rocky material similar to Earth, with a very thin atmosphere that accounts for no more than 1 percent of its mass. Another way is that the clumps are evenly distributed between the rock core and a huge film of water.
Of course, it’s not water as we know it. Given its proximity to the star and the enormous pressure exerted by its oceans, at least some of that water is in a supercritical state, meaning that the pressure near the rock’s core causes it to form a high-pressure solid. Things will get weird inside the core as well. “The properties of matter at such high central pressures are still uncertain,” the researchers said.
Not only do we have a hard time understanding its present, but we also feel a little lost when it comes to its past. Accumulation of small dust particles from the planet-forming disk will cease before TOI-1853 b reaches its current mass. This is because even smaller planets will have their disks destroyed. And it is unlikely that it formed in its current location, given that solids are less likely to condense there.
Two possibilities, both unlikely
The researchers suggest two possibilities. One is that a group of smaller planets formed further out, and then their orbits became unstable as the disk gradually evaporated. This may have caused a collision, shattering several planets, and the fragments forming a single celestial body. However, these processes do not tend to form a single object, and many planets would probably be needed to transport 73 Earths’ worth of material.
Alternatively, several gas giants formed far outside and then destabilized each other’s orbits, one of them becoming highly eccentric and part of its orbit very close to its star. That is what it is. This would allow material to be collected from within the planet-forming disk, a process that could roughly double the mass of Jupiter-like planets. Its extreme orbit would also allow it to transfer its atmosphere to the star. Once these processes are complete, tidal interactions between planets and stars will eventually make their orbits more regular.
There is nothing physically impossible about either of these potential formation mechanisms, but both require a series of unlikely events. The universe is large, so those things will probably happen somewhere, but it seems unreasonable to expect us to encounter the consequences this quickly.
One thing that might help us understand the origin of TOI-1853 b is the presence of other planets in this system. This may help us understand what was happening inside this exosolar system. TOI-1853 b is so large and so close that it produced a huge signal that would have made it difficult to detect other planets in this system. Researchers estimate that something as large as 10 Earths could be orbiting near the star, and we would have missed it. Continuous observation may be the key to understanding the system.
Nature, 2023. DOIs: 10.1038/s41586-023-06499-2 (About DOI).