A newly discovered exoplanet has such peculiar characteristics that astronomers believe it must have suffered a giant collision in its past.
TOI-1853b is an exoplanet only slightly smaller than Neptune, but nearly twice as dense as Earth, suggesting a rock-rich composition that is difficult to explain through normal channels of planet formation and evolution.
Instead, a team – led by physicist Luca Naponiello from the University of Rome Tor Vergata in Italy and the University of Bristol in the UK – believe it was once the heart of a much larger world vast and more gaseous which has lost its atmosphere due to extreme violence.
“This planet is very surprising! Normally, we would expect planets formed with so much rock to become gas giants like Jupiter, which have densities similar to water,” says physicist Jingyao Dou of the University of Bristol.
“TOI-1853b is the size of Neptune but has a greater density than steel. Our work shows that this can occur if the planet experienced extremely energetic planet-planet collisions during its formation. These collisions have destroys some of the lighter atmosphere and the water coming out of it, a planet that is high in density and significantly enriched in rocks.
TOI-1853b is a rarity among exoplanets. It is firmly planted in a gap known as the Neptunian Desert – a world the size of Neptune, in a close orbit to its star.
Only a small handful of worlds matching this description have been discovered, out of the more than 5,500 exoplanets confirmed to date. Understanding why there are so few exoplanets in the Neptunian desert would help us better understand how planets form and evolve.
TOI-1853b is 3.46 times the radius of the Earth; Neptune measures 3.88 Earth radii. But the similarities end there. The exoplanet orbits its host star, an orange dwarf about 80% the size of the Sun, once every 1.24 days. Even if its radius doesn’t stretch the imagination too far, its mass is truly puzzling: 73.2 times the mass of Earth. Neptune is only 17.15 Earth masses.
At this size and mass, the team calculates, TOI-1853b has a density of 9.7 grams per cubic centimeter. It’s wild. Neptune has an average density of 1.64 grams per cubic centimeter. The Earth average is 5.15 grams. Iron has a density of 7.87 grams per cubic centimeter and the density of steel is about the same.
Neptune’s density is so low because it has a thick and extended atmosphere. What the density of TOI-1853b tells us is that its composition must contain a lot of denser material and little atmosphere. (Also, the density of the Earth’s core can reach 13 grams; matter inside a massive body is compressed by all the mass above it, so its density increases.)
Naponiello and his team conducted simulations to determine how a planet might become like this in the galaxy. They found that the most likely explanation is a high-speed impact between two massive, still-forming exoplanets that crushed them and ejected the atmosphere.
“Our contribution to the study was to model extreme giant impacts that could potentially remove the lighter atmosphere and water/ice from the larger home planet to produce the extreme density measured,” says the physicist. Phil Carter of the University of Bristol.
“We found that the initial planetary body would likely have had to be water-rich and undergo an extreme giant impact at a velocity greater than 75 kilometers per second in order to produce TOI-1853b as observed.”
The team plans to conduct follow-up observations to look for traces of the atmosphere around TOI-1853b and analyze its composition to determine if their collision scenario is likely.
Interestingly, another similar exoplanet has just been discovered by another group of scientists. TOI-332b measures 3.2 Earth radii, 57.2 Earth masses, in an 18.72 hour orbit around an orange dwarf and has a density of 9.6 grams per cubic centimeter. Perhaps the two teams could combine their efforts.
“We hadn’t studied such extreme giant impacts before, because they’re not something we expected,” says physicist Zoë Leinhardt from the University of Bristol.
“There is a lot of work to be done to improve the material models that underpin our simulations and to expand the range of extreme giant impacts modeled.”
The research has been published in Nature.
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