A super-tight collision product?

A super-tight collision product?

Artist’s impression of the enigmatic exoplanet TOI-1853b. © Luca Naponiello

Denser than steel: Astronomers report a Neptune-sized exoplanet with a gigantic mass. It is about twice as large as any other known planet in this category. Apparently TOI-1853b consists of a surprisingly high proportion of rock. Modeling suggests that the exoplanet may be the result of massive collisions between protoplanets.

When it comes to the dimensions in our solar system, a celestial body occupies an intermediate position between the rocky planets and the gas giants: Neptune. It has around 17 times the mass of the Earth. Exoplanet research has already shown that planets in this category also exist in many other star systems. They have different characteristics that have to do with their distance from the star and the history of their formation. Some, like our Neptune, are icy worlds with thick atmospheres of hydrogen and helium. However, denser specimens are also known, which consist largely of water or rock and only have thin atmospheres. The newly discovered Neptune-sized planet is now the most extreme example known to date.

Density record for the exo-Neptunes

It was discovered by an international team of astronomers in the star system TOI-1853, approximately 535 light-years away from us. The scientists tracked down the planet using data from the Transiting Exoplanet Survey Satellite (TESS) space telescope using the transit method. It became apparent that TOI 1853b has around 3.5 times the radius of Earth and is therefore about the size of our Neptune. In contrast to this, however, it orbits extremely close to its star: TOI 1853b only needs 1.24 days to complete one orbit.

The big surprise, however, was the mass of the planet, which could be determined based on its gravitational influence on the star. Accordingly, TOI-1853b is around 75 times heavier than Earth. The mass of TOI-1853b is therefore about twice as large as that of all other known examples from the category of Neptune-sized exoplanets. In combination with the information about its dimensions, this results in a surprisingly high density. This means that the planet consists of a particularly high proportion of heavy material such as rock.

Probably a “bombastic” origin story

“This planet has very surprising features: it is the size of Neptune but has a density higher than steel,” says co-author Jingyao Dou from the University of Bristol. “It would actually be assumed that such a planet would have developed into a gas giant whose density would ultimately be similar to that of water,” explains Dou. In order to investigate the question of how the amazing features of TOI-1853b could have arisen, he and his colleagues developed model simulations on the computer.

As they report, it became apparent that the planet could have been formed through collisions between protoplanets. The simulations show that the bombastic events would have removed part of the lighter atmosphere and water and left behind large amounts of rock. “It turned out that the original planetary body could have been rich in water. “To create the features of TOI-1853b would have required massive impacts at high speeds,” says co-author Philip Carter from the University of Bristol.

The newly discovered planet now contributes to previous evidence that massive collisions can play an important role in the formation of planetary systems, say the scientists. “We have strong evidence of high-energy collisions between planetary bodies in our solar system, such as the existence of our Earth’s moon. “There are also already some indications for a small number of exoplanets,” says Carter. TOI-1853b now joins this list. That’s why the unusual celestial body will continue to be in the sights of astronomers.

Source: University of Bristol, specialist article: Nature, doi: 10.1038/s41586-023-06499-2

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