A supermassive Neptune-sized exoplanet is denser than steel

Astronomers believe the strange composition of the Neptune-sized planet TOI-1853b is the result of a giant planetary collision.

The mass of the Neptune-sized exoplanet TOI-1853b is dominated by heavier elements. Image credit: Luca Naboniello.

TOI-1853Also known as TYC 1468-1031-1 and TIC 73540072, it is a quiet K2 dwarf star with about 85% the mass of our Sun.

The star is located about 167 parsecs (545 light-years) from the Sun in the Boötes constellation.

TOI-1853 presents a giant transiting exoplanet called TOI-1853b in a 1.24-day orbit.

The exoplanet has a radius of 3.5 Earths, a mass of 73 Earths, more than twice that of the Neptune-sized planets known so far, and a density of 9.7 g/cm.3.

The properties of TOI-1853 present a puzzle to conventional theories of planet formation and evolution, and may be the result of multiple protoplanetary collisions or the final state of an early high-eccentricity planet that has migrated closer to its parent star.

„We have strong evidence for high-energy collisions between planetary bodies in our solar system, such as Earth’s moon, and good evidence from a small number of exoplanets,” he said. University of Bristol Astronomer Bill Carter.

„We know there is a huge diversity of planets in exoplanetary systems; many have no analogues in our Solar System, but most have masses and mixtures between rocky planets and Neptune/Uranus.

We found that to form TOI-1853b, the initial planetary body must have been rich in water and experienced an extreme giant impact at speeds greater than 75 km/s.”

„This planet is really amazing,” said Jingyao Du, a postgraduate student at the University of Bristol.

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„In general, we expect planets that form with this much rock to become Jupiter-like gas giants that have densities similar to water.”

„TOI-1853b is the size of Neptune but denser than steel.”

„Our work shows that this can happen if the planet experienced very energetic planet-planet collisions during its formation.”

„These collisions removed some of the lighter atmosphere and water, leaving behind a significantly rock-enriched, high-density planet.”

Zoë Leinhardt, an astronomer at the University of Bristol, said: „We have never studied such extreme giant impacts before.

„There is much work to be done to improve the material models that underlie our simulations and to expand the range of extreme giant impacts.”

This discovery is a Paper In the journal Nature.


L. Naboniello and many others. Largest Neptune-sized planet. Nature, published online August 30, 2023; doi: 10.1038/s41586-023-06499-2

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