First Exoplanet With Exposed Planetary Core Discovered

The discovery provides an unprecedented glimpse into the composition of planet interiors.
Chris Young
Courtesy of University of Warwick

Researchers led by the University of Warwick have discovered the first exposed core of an exoplanet. The discovery provides a glimpse into the interior of planets as never seen before.

The newly discovered exoplanet TOI 849 b, which orbits around a Sun-like star about 730 light-years away, has an exposed core the same size as Neptune. It offers a unique opportunity for the scientific community to learn about the composition of planet interiors. 


An 18-hour year

Led by Christoph Mordasini from the University of Bern, a team of researchers is interpreting the theoretical data being compiled about the exoplanet TOI 849 b. The University of Bern explains that this is the first time in history that an intact exposed core of a gas giant has been discovered orbiting around a star.

TOI 849 b is extremely unusual. Firstly, it is a Neptune-sized planet in a region dubbed as the "Neptune Desert" – a term used by astronomers for a region close to stars where we very rarely see planets of Neptune’s mass or larger.

Secondly, "the planet is strangely close to its star, considering its mass. In other words, we don’t see planets with this mass at these short orbital periods," lead author of the study, Dr. David Armstrong from the University of Warwick explained in a press release.

In fact, TOI 849 b orbits so close to its host star that a year is a mere 18 hours and its surface temperature is around 1,500 °C.

A historic discovery

The researchers believe the planet is a gas giant that was either stripped of its gaseous atmosphere or failed to fully form one during its early life due to special circumstances.

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"We have determined the planet's mass and radius. TOI-849b is about 40 times heavier than the earth, but its radius is just 3.4 earth radii," Christoph Mordasini explains. 

The planet's high density means that it primarily has to consist of iron, rock, and water, but only very little hydrogen and helium.

"Such a small amount of hydrogen and helium is really astonishing for such
a massive planet," Mordasini says. "We would expect a planet this massive to have accreted large quantities of hydrogen and helium when it formed."

David Armstrong adds: "The fact that we don’t see those gases lets us know TOI 849 b is an exposed planetary core."

Two theories for TOI 849 b

Using the "Bern Model of Planet Formation and Evolution," the researchers devised to theories for TOI 849 b's existence and development into an exoplanet with an exposed core.

"The first is that the exoplanet was once similar to Jupiter but lost nearly all of its outer gas through a variety of processes," Christoph Mordasini says.

Tidal disruption, where the planet is ripped apart from orbiting too close to its star, or even a collision with another planet, might have played a role.

Alternatively, TOI 849 b might be a "failed" gas giant: "once the core of the gas giant formed then something very unusual could have happened [meaning] it never formed a massive atmosphere" Christoph Mordasini adds.

"This could have occurred if there was a gap in the disk of dust and gas that the planet formed from due to gravitational interaction with the planet, or if the disk ran out of material right at the very moment when gas accretion normally follows."

The discovery proves that planets with exposed cores exist and that they can be tracked down. Similar findings could provide further insight into the core of exoplanets of different sizes and compositions and provide insights that we couldn't find in our own solar system.

The study by the team led by Dr. David Armstrong from the University of Warwick's Department of Physics is published today in the journal Nature

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