Non-gas giant has 73 times Earth’s mass, bewildering its discoverers

Scientists have been working on models of planet formation since before we knew exoplanets existed. Originally guided by the properties of the planets in our Solar System, these models turned out to be remarkably good at also accounting for exoplanets without an equivalent in our Solar System, like super Earths and hot Neptunes. Add in the ability of planets to move around thanks to gravitational interactions, and the properties of exoplanets could usually be accounted for.

Today, a large international team of researchers is announcing the discovery of something our models can't explain. It's roughly Neptune's size but four times more massive. Its density—well above that of iron—is compatible with either the entire planet being almost entirely solid or it having an ocean deep enough to drown entire planets. While the people who discovered it offer a couple of theories for its formation, neither is especially likely.

A freakish outlier

The study of the new planet started as many now do: It was identified as an object of interest by the Transiting Exoplanet Survey Satellite (TOI, for TESS Object of Interest). TOI-1853 is a star somewhat smaller than our Sun, with about 0.8 times its mass. And there were clear indications of a planet near the star, now called TOI-1853 b. The planet orbits quite 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 orbits. Based on a combination of that distance, the size of the star, and the amount of light blocked by the planet, it's possible to estimate the planet's size. That turned out to be about 3.5 times Earth's radius, meaning it's a bit smaller than Neptune.

On its own, that's 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 what's called the "hot Neptune desert," where intense radiation from the star drives off a planet's atmosphere. Neptunes that reach the hot desert state end up stripped down to their rocky cores, which leaves them as super-Earths.

So what was TOI-1853 b doing in the desert? To find out, the researchers used ground-based observatories to track the movement of its host star as the gravitational pull of TOI-1853 b shifted as it moved through its orbit. The acceleration in the star's motion due to this pull can be used to estimate the planet's mass.

It turned out that TOI-1853 b has a lot of mass. It's estimated to be 73 times the mass of Earth or over four times the mass of Neptune. Pretty clearly, that means its composition must be very different from Neptune's.

Crunchy on the inside and outside?

The researchers involved in its discovery spend a fair bit of text describing just how much of an outlier this makes TOI-1853 b. There are planets with similar densities, but they're typically significantly smaller—the super-Earths formed by stripping away the atmosphere on a Neptune-like planet. There are planets with similar masses, but they're almost all twice as large and are likely to have extensive atmospheres and/or oceans. "It occupies a region of the mass–orbital [distance] space of hot planets that was previously devoid of objects, corresponding to the driest area of the hot-Neptune desert," the researchers conclude.

The oddities don't end there. There are two compositions that make sense given the densities at play here. One has the planet being almost entirely composed of rocky material like Earth's, with an extremely thin atmosphere that accounts for one percent of its mass at most. The alternative is that the mass is evenly distributed between a rocky core and an immense coating of water.

Of course, that wouldn't be water as we know it. Given its proximity to its host star and the massive pressures from that much ocean, at least some of that water would be in a supercritical state, and the pressure near the rocky core would force water to form high pressure solids. Things would be equally weird inside the core. As the researchers note, "The properties of matter at such high central pressures are still uncertain."

Not only do we struggle to understand its present, but we're at a bit of a loss when it comes to its past. Accumulation of small dust particles from the planet-forming disk would shut off before TOI-1853 b reached its present mass, as even a smaller planet would disrupt the disk. And it is unlikely to have formed in its current location, given that solids have difficulty condensing there.

Two possibilities, neither likely

The researchers suggest two possibilities. One is that a group of smaller planets formed further out and then had their orbits destabilized as the disk gradually evaporated. This could have resulted in collisions that shattered several planets, which then saw their debris form a single body. But these processes don't tend to form single bodies, and it would probably take a lot of planets to carry 73 Earth's worth of materials.

The alternative is that several gas giants formed much further out and then destabilized each other's orbits, leaving one highly eccentric, with one portion of the orbit extremely close to the host star. This would allow it to gather material from the inner portions of the planet-forming disk, a process that could allow a Jupiter-like planet to roughly double in mass. Its extreme orbit would also allow it to transfer its atmosphere to the star. After these processes were complete, tidal interactions between the planet and the star would eventually make its orbit far more regular.

There's nothing physically impossible about either of those potential formation mechanisms, but both require a series of unlikely events. The Universe is big, and those things probably happen somewhere, but it seems unreasonable to expect we'll stumble across their results this quickly.

One thing that might help us understand TOI-1853 b's origin is the presence of other planets in the system, which might help us understand what was happening in the inner portions of this exosolar system. TOI-1853 b is so big and so close that it creates an enormous 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 close to the star, and we would have missed it. Continuing observations may be key to understanding the system.

Nature, 2023. DOI: 10.1038/s41586-023-06499-2  (About DOIs).