Home Science & Technology The gasless giant has a mass 73 times that of the Earth, which amazes its discoverers – Ars Technica

The gasless giant has a mass 73 times that of the Earth, which amazes its discoverers – Ars Technica

The gasless giant has a mass 73 times that of the Earth, which amazes its discoverers – Ars Technica

Artist's concept of a massive planet near a star.

Scientists were working on models of planet formation before we knew exoplanets existed. Originally based on the properties of the solar system’s planets, these models have proven to be extremely good at also accounting for exoplanets that have no counterpart in the solar system, such as super-Earths and hot Neptunes. Add to this the ability of planets to move due to gravitational interactions, and the properties of exoplanets could usually be explained.

Today, a large international team of researchers announces the discovery of something our models cannot explain. It is about the size of Neptune, but four times more massive. Its density – much greater than that of iron – can be reconciled with either the entire planet being almost entirely solid or having an ocean deep enough to submerge entire planets. Although the people who discovered it put forward several theories about its origin, none of them is particularly likely.

Bizarre exception

The study of the new planet began as many do now: 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 slightly smaller than our Sun, with about 0.8 times its mass. 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 complete revolution in 1.24 days.

Scientists used this time to determine the distance at which the planet orbits. Based on a combination of this 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. It turned out to be about 3.5 times the radius of Earth, making it slightly smaller than Neptune.

This in itself is not unusual; many planets the size of Neptune have been discovered. But the combination of the star’s size and proximity is unexpected. He places it in the so-called “hot desert of Neptune”, where the star’s intense radiation displaces the planet’s atmosphere. When Neptune reaches a hot desert state, it is stripped down to a rocky core, making it 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 movement of its parent star as TOI-1853 b’s gravitational pull changes as it moves around its orbit. The acceleration of the star’s motion due to this attraction can be used to estimate the mass of the planet.

It turned out that TOI-1853 b has plot weight. It is estimated to be 73 times the mass of Earth or more than four times the mass of Neptune. This quite clearly means that its composition must be very different from that of Neptune.

Crispy inside and out?

The researchers involved in its discovery devoted a lot of text describing how outlier the object makes TOI-1853 b. There are planets with similar densities, but they are usually much smaller – super-Earths formed by the removal of the atmosphere from a Neptune-like planet. There are planets with similar masses, but almost all are twice as large and probably have extensive atmospheres and/or oceans. “It occupies an area of ​​mass orbit [distance] hot planet space, which was previously devoid of objects, corresponding to the driest region of the desert of hot Neptune” – the scientists conclude.

The weirdness doesn’t end there. There are two compositions that make sense given the density here. First, the planet is made up almost entirely of rocky material like Earth, with an extremely thin atmosphere that is at most one percent of its mass. The alternative is to evenly distribute the mass between the rocky core and the huge shell of water.

Of course, it wouldn’t be water as we know it. Given the proximity of the host star and the enormous pressure of such a large ocean, at least some of this water would be in a supercritical state, and the pressure near the rocky core would force the water to form high-pressure solids. It would be just as weird in the core. As the researchers note, “The properties of matter at such high central pressures are still uncertain.”

Not only do we have a hard time understanding his present, but we’re a little lost when it comes to his past. The accumulation of small dust particles from the planet-forming disk would be stopped before TOI-1853 b reached its current mass, as even a smaller planet could tear the disk apart. It is unlikely to form at its present location, given that solids have difficulty condensing there.

Two possibilities, neither unlikely

The researchers suggest two possibilities. First, a group of smaller planets formed further on, and their orbits became destabilized as the disk gradually evaporated. This could result in collisions that broke up several planets, and then their debris formed a single body. However, these processes do not produce single bodies, and it is likely that multiple planets are needed to accommodate the 73 terrestrial materials.

The alternative is that several gas giants formed much farther apart and then destabilized their orbits, leaving one very eccentric with one portion of its orbit very close to its host star. This would collect material from the inner parts of the planet-forming disc, a process that could roughly double the mass of a Jupiter-like planet. Its extreme orbit would also allow the transfer of the atmosphere to the star. Once these processes are complete, tidal interactions between the planet and the star will eventually make its orbit much 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, and probably things like this are happening somewhere, but it seems unreasonable to expect that we will run into their results so quickly.

The thing that might help us understand the origin of TOI-1853 b is the presence of other planets in this system, which could help us understand what was going on in the inner parts of this exosolar system. TOI-1853 b is so large and so close that it produces a huge signal, and we would have trouble detecting any other planets in this system. Scientists estimate that an object as massive as 10th Earth could also be orbiting close to the star, and we would have missed it. Continuous observations may be the key to understanding the system.

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


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