Bizarre Neptune-sized planet ‘denser than steel’ discovered

“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 Phil Carter, a new study’s co-author from the University of Bristol, in an official release. 

Giant planetary collision led to this density  

TOI-1853b is a planet that circles the orange dwarf star TOI-1853. 

This exoplanetary system was identified by NASA's Transiting Exoplanet Survey Satellite (TESS) and is located around 544 light-years away from Earth in the direction of the Boötes constellation.

TOI-1853b's density is composed of a greater proportion of rock than would be expected for a Neptune-sized planet. The planet’s mass is double that of any other known similar-sized planet, implying that its density is significantly high. 

It is still unknown how the planet came to be. However, the researchers created a computer model that indicates it was formed by a catastrophic planetary collision. 

This powerful cosmic impact left massive pieces of rock behind, and its violent intensity might have stripped off the lighter atmosphere and water that existed on the planet prior to this collision. The team speculates that this planet was initially a water-rich world. 

As a result, the giant impact transformed the world into a high-density exoplanet, as it is observed. 

“This planet is very surprising! 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,” said Jingyao Dou, a postgraduate student and the co-author of this study. 

Why does this study matter?

Exoplanetary science is a relatively new field in astronomy. The first confirmed exoplanet discovery was made in the 1990s, and since then, scientists have been discovering and learning about a diverse array of planets orbiting other stars. 

Earlier scientific studies have shed light on the existence of a wide range of Neptune-size planets, some being icy worlds hosting dense atmospheres of hydrogen and helium. 

This new study provides an incredible revelation about how such violent impacts may result in the formation of Neptune-sized exoplanets in our Milky Way and in other galaxies. 

The new study also adds to our understanding of the origin and evolution of planetary systems outside our solar system. The statement adds: “This discovery helps to connect theories for planet formation based on the solar system to the formation of exoplanets.” 

Following this study, the team intends to conduct extensive observations of TOI-1853b using powerful telescopes in order to determine the presence and composition of the residual atmosphere.

“We had not previously investigated such extreme giant impacts as they are not something we had expected. 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,” concluded Zoë Leinhardt, one of the study co-authors. 

The detailed observations have been published in the journal Nature. 

Study abstract:

Neptune-sized planets exhibit a wide range of compositions and densities, depending on factors related to their formation and evolution history, such as the distance from their host stars and atmospheric escape processes. They can vary from relatively low-density planets with thick hydrogen–helium atmospheres to higher-density planets with a substantial amount of water or a rocky interior with a thinner atmosphere, such as HD 95338 b, TOI-849 b and TOI-2196 b. The discovery of exoplanets in the hot-Neptune desert, a region close to the host stars with a deficit of Neptune-sized planets, provides insights into the formation and evolution of planetary systems, including the existence of this region itself. Here we show observations of the transiting planet TOI-1853 b, which has a radius of 3.46 ± 0.08 Earth radii and orbits a dwarf star every 1.24 days. This planet has a mass of 73.2 ± 2.7 Earth masses, almost twice that of any other Neptune-sized planet known so far, and a density of 9.7 ± 0.8 grams per cubic centimetre.