Scientists have been working on models of planet formation since before we knew exoplanets existed. These models were originally guided by the properties of planets in our solar system, and have been shown to be remarkably good at accounting for exoplanets that have no equivalent in our solar system, such as super-Earths and hot Neptunes. Add to this the planets’ ability to move thanks to gravitational interactions, and the properties of exoplanets can usually be taken into account.
Today, a large international team of researchers announces the discovery of something our models can’t explain. It’s about the size of Neptune, but it’s about four times larger. Its density – much higher 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 likely.
The study of the new planet began as many do now: it was identified as an object of interest by the Transiting Exoplanet Survey (TOI, for TESS Object of Interest) satellite. TOI-1853 is a star somewhat smaller than our Sun, with a mass of about 0.8 times. There were clear indications of a nearby planet, now called TOI-1853 b. The planet orbits close to its host star, completing a full orbit in 1.24 days.
The researchers used that time to determine the distance the planet orbited. Based on a combination of that distance, the size of the star, and the amount of light the planet blocks, it is possible to estimate the planet’s size. It turns out that this is about 3.5 times the radius of Earth, which means it’s 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 so-called “hot Neptune desert,” where intense radiation from the star shoots from the planet’s atmosphere. Neptunes that reach the hot desert state end up stripping them of their rocky core, making them super-Earths.
So what was TOI-1853 b doing in the 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 as it moved through its orbit. The acceleration of the star’s motion caused by this cloud can be used to estimate the planet’s mass.
It turns out that TOI-1853 b has a lot 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.
Crunchy inside and out?
The researchers involved in its discovery spend a fair bit of text describing how strange what makes TOI-1853 b. There are planets with similar densities, but usually much smaller, which are super-Earths formed by stripping a Neptune-like planet from its atmosphere. There are planets with similar masses, but about twice as massive that would likely have extensive atmospheres and/or oceans. “It occupies a region of the orbital cluster [distance] The researchers concluded that “the area of hot planets that was previously devoid of bodies corresponds to the drier region of the hot Neptune desert.”
The oddities don’t end there. Two combinations make sense given the densities at play here. One is that the planet is made almost entirely of rocky material like Earth, with a very thin atmosphere accounting for one percent of its mass at most. The alternative is that the mass is evenly distributed between the rocky core and an enormous layer of water.
Of course, this will not be water as we know it. Due to its proximity to its host star and the enormous pressures from this large ocean, at least some of that 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 equally strange inside the heart. As the researchers note, “the properties of matter at such high central pressures remain uncertain.”
Not only do we struggle to understand its present, we are also at a loss when it comes to its past. Small dust particles from the planet-forming disk will stop accumulating before TOI-1853 b reaches its current mass, as even a smaller planet could disrupt the disk. It is unlikely that it would have formed in its current location, given that solids have difficulty condensing there.
Two possibilities, not likely
The researchers suggest two possibilities. One is that a group of minor planets formed further out and then destabilized their orbits as the disk gradually evaporated. This could have led to collisions that shattered many planets, which then saw their debris form a single body. But these processes tend not to form single bodies, and it would probably take many planets to carry the equivalent of 73 Earths worth 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 one 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 could allow a Jupiter-like planet to nearly double its mass. Its maximum orbit would also allow it to transfer its atmosphere to the star. After these processes are complete, the tidal interactions between the planet and the star will eventually make its orbit more regular.
There is nothing physically impossible about any of these possible formation mechanisms, but both require a series of unexpected events. The universe is big, and it’s possible that these things are happening somewhere, but it seems unreasonable to expect that we’d find their consequences so quickly.
The only thing that might help us understand the origin of TOI-1853 b is the presence of other planets in the system, which might help us understand what was going on in the inner parts of this outer system. TOI-1853 b is so big and so close that it gives off a massive signal, and we would have had trouble detecting any other planets in this system. The researchers estimate that something as massive as 10 Earths could also be orbiting near the star, and we’d missed that. Continuous feedback may be the key to understanding the system.
Nature, 2023. DOI: 10.1038/s41586-023-06499-2 (about digital identifiers).
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