A non-gas giant with a mass of 73 times the mass of Earth baffles its explorers
Scientists have been working on models of planet formation since before we knew exoplanets existed. These models were originally driven by the properties of the planets in our solar system, and have proven to be very good at accounting for exoplanets that have no analogues in our solar system, such as the super-Earths and hot Neptunes. If we add to this the ability of planets to move due to gravitational interactions, the properties of exoplanets can be taken into account in general terms.
Today, a large international research team announces that they’ve discovered something our models can’t explain. It’s about the size of Neptune, but it’s about four times larger. Its density – much greater than that of iron – corresponds to the entire planet being almost entirely solid or having an ocean deep enough to submerge entire planets. While the people who discovered it offer two theories for its formation, neither is particularly plausible.
The study of the new planet began, as many now do: it was identified as a region of interest by the Transiting Exoplanet Survey Satellite (TOI, also known as TESS). TOI-1853 is a star slightly smaller than our Sun, about 0.8 times the mass. There were clear signs of a nearby planet, now named TOI-1853 b. The planet orbits close to its host star and completes a full orbit in 1.24 days.
The researchers used this time to determine the planet’s rotational distance. A combination of this distance, the size of the star, and the amount of light the planet blocks can be used to estimate the size of the planet. This is about 3.5 times the radius of Earth, which means it is only slightly smaller than Neptune.
This in itself is not unusual. Many Neptune-sized planets have been discovered. But the combination of size and proximity to the star is unexpected. It places it in the “hot Neptune desert,” where intense star radiates from the planet’s atmosphere. Neptune reaches a hot desert state and eventually strips away its rocky cores and becomes a super-Earth.
So what was TOI-1853 doing in the Ba desert? To find out, the researchers used ground-based observatories to track the motion of its host star, as TOI-1853 b’s gravitational pull changed during its orbit. The acceleration of the star’s motion due to this resistance can be used to estimate the planet’s mass.
It turns out that TOI-1853 has b very from the block. Its mass is estimated at 73 times that of Earth, or more than four times that of Neptune. This obviously means that its composition must be very different from that of Neptune.
Crispy inside and out?
The researchers involved in the discovery spend a significant portion of the text describing how strange TOI-1853 b is. Similar in density, but usually much smaller, are planets that are super-Earths formed by stripping a Neptune-like planet of its atmosphere. There are planets of similar mass, but about twice as massive, that would likely have vast atmospheres and/or oceans. “It occupies a region of the tropics group [distance] The researchers concluded that “the previously body-free hot planetary area corresponds to the drier part of the hot Neptune desert.”
The weirdness doesn’t end there. There are two groups that make sense because of the densities involved here. The first is that the planet is made almost entirely of rocky material like Earth, and has a very thin atmosphere that makes up no more than one percent of its mass. The alternative is that the mass is evenly distributed between the rocky core and a broad layer of water.
Of course, it will not be water as we know it. Because of its proximity to the host star and the enormous pressure from the vast ocean, at least some of the water would be in a supercritical state, and the pressure near the rocky core would force the water to form high-pressure solids. Things will be just as strange in the heart. As the researchers note, “the material properties at such high central pressures remain uncertain.”
Not only do we struggle to understand his present, we are at a loss when it comes to his past. Tiny dust particles from the planet-forming disk will accumulate before TOI-1853 b reaches its current mass, where even a small planet could disrupt the disk. It is unlikely to have formed in its current location, as solids have difficulty condensing there.
Two unlikely possibilities
The researchers suggest two possibilities. One is that a group of minor planets formed outward and then destabilized their orbits as the disk gradually evaporated. Perhaps this led to collisions that shattered many planets and their debris formed a single body. But these processes usually do not result in the creation of a single body, and it would likely take many planets to deliver the 73 planet’s equivalent of material.
The alternative is that several gas giants formed much further away and then destabilized each other’s orbits, leaving one highly eccentric, with part of the orbit extremely close to the host star. This would allow it to collect material from the inner parts of the planet-forming disk, a process that would allow the Jupiter-like planet to nearly double its mass. Its maximum orbit would also allow it to transfer its atmosphere to the star. Once these processes are complete, the tidal interactions between the planet and the star will make its orbit more regular over time.
There is nothing physically impossible in any of these possible formation mechanisms, but both require a series of unexpected events. The universe is big, and these things could be happening somewhere, but it seems unreasonable to expect us to realize the consequences so quickly.
The only thing that can help us understand the origin of TOI-1853 b is the presence of other planets in the system, which can help us understand what is happening in the inner parts of this outer system. TOI-1853 b is so big and so close that it emits a huge signal, and we would have had a hard time spotting any other planet in this system. The researchers estimated that a mass equivalent to 10 Earths could orbit the star, but we missed that. Continuous feedback can be the key to understanding a system.
Nature, 2023. DOI: 10.1038/s41586-023-06499-2 (about digital identifiers).