Supermassive Neptune-Sized Exoplanet Has Density Higher than Steel

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

Heavy elements dominate the mass of the Neptune-sized exoplanet TOI-1853b. Image credit: Luca Naponiello.

Heavy elements dominate the mass of the Neptune-sized exoplanet TOI-1853b. Image credit: Luca Naponiello.

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

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

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

The alien world has a radius of 3.5 Earth radii, a mass of 73 Earth masses, almost twice that of any other Neptune-sized planet known so far, and a density of 9.7 grams per cm3.

TOI-1853’s properties present a puzzle for conventional theories of planetary formation and evolution, and could be the result of several protoplanet collisions or the final state of an initially high-eccentricity planet that migrated closer to its parent star.

“We have strong evidence for highly energetic collisions between planetary bodies in our Solar System, such as the existence of Earth’s Moon, and good evidence from a small number of exoplanets,” said University of Bristol astronomer Phil Carter.

“We know that there is a huge diversity of planets in exoplanetary systems; many have no analog in our Solar System but often have masses and compositions between that of the rocky planets and Neptune/Uranus.”

“We found that the initial planetary body would likely have needed to be water-rich and suffer an extreme giant impact at a speed of greater than 75 km/s in order to produce TOI-1853b as it is observed.”

“This planet is very surprising,” said University of Bristol postgraduate student Jingyao Dou.

“Normally we expect planets forming with this much rock to become gas giants like Jupiter which have densities similar to water.”

“TOI-1853b is the size of Neptune but has a density higher than steel.”

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

“These collisions stripped away some of the lighter atmosphere and water leaving a substantially rock-enriched, high-density planet.”

“We had not previously investigated such extreme giant impacts as they are not something we had expected,” said University of Bristol astronomer Zoë Leinhardt.

“There is much work to be done to improve the material models that underlie our simulations, and to extend the range of extreme giant impacts modeled.”

The discovery is reported in a paper in the journal Nature.


L. Naponiello et al. A super-massive Neptune-sized planet. Nature, published online August 30, 2023; doi: 10.1038/s41586-023-06499-2

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