Scientists have been working on models of planet formation since before we knew exoplanets existed. These models were originally guided by the properties of the 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 the 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 that our models cannot explain. It is about the size of Neptune, but four times larger. Its density – much higher than that of iron – is consistent with 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 signs of the existence of a planet close to the star, 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 at which the planet rotates. Based on a combination of that distance, the size of the star, and the amount of light blocked by the planet, it is possible to estimate the size of the planet. This turns out to be about 3.5 times the radius of Earth, which means it is 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. And 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 stripped of their rocky core, making them a super-Earth.
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 due to this drag 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 the mass of Earth, or more than four times the mass of Neptune. Obviously, this means that its composition must be completely different from that of Neptune.
Crunchy inside and out?
The researchers involved in its discovery spend a fair portion of the text describing how weird TOI-1853 b is. There are planets with similar densities, but usually much smaller, which are super-Earths formed by stripping a Neptune-like planet of its atmosphere. There are planets with similar masses, but about twice as large and likely to have extensive atmospheres and/or oceans. “It occupies a region of the orbital cluster [distance] The researchers concluded that “the area of the hot planets that was previously devoid of objects corresponds to the drier region of Neptune’s hot desert.”
The oddities do not end there. There are two combinations that make sense given the densities at play here. One is that the planet is made up 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. Given its proximity to its host star and the enormous pressures from that 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 just as strange inside the heart. As the researchers note, “the properties of the material at such high central pressures remain uncertain.”
Not only do we struggle to understand its present, we are at a loss when it comes to its past. Small dust particles would stop accumulating from the planet-forming disk before TOI-1853 b reached its current mass, as even a smaller planet might disrupt the disk. It is unlikely to have formed in its current location, since solids have difficulty condensing there.
Two possibilities, not likely
The researchers suggest two possibilities. One is that a group of planetesimals formed farther away and then their orbits became destabilized as the disk gradually evaporated. This could have led to collisions that shattered multiple planets, which then saw their debris form a single body. But these processes tend not to form single objects, and it would likely take a lot of planets to carry 73% of the Earth’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 one part of the orbit extremely close to the host star. This would allow it to collect material from the interior of the planet-forming disk, a process that could allow a Jupiter-like planet to nearly double its mass. Its maximum orbit will also allow it to transfer its atmosphere to the star. After these processes are complete, 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 these things are likely to happen somewhere, but it seems unreasonable to expect that we would find their results so quickly.
One 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 happening in the inner parts of this outer system. TOI-1853 b is so large and so close that it emits a massive signal, and we would have had trouble detecting any other planets in this system. Researchers estimate that something as massive as 10 terrestrials could also be orbiting close to the star, and we would have missed that. Continuous feedback may be the key to understanding the system.
Nature, 2023. DOI: 10.1038/s41586-023-06499-2 (About digital IDs).
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