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Exposed 'remnant' core of gas giant 730 light years away is helping scientists study the planet's interiors

Located approximately 730 light years away, TOI 849 b orbits so close to its host star that a year is a mere 18 hours and its surface temperature is around 1,800K
(University of Warwick/Mark Garlick)
(University of Warwick/Mark Garlick)

The interiors of giant planets remain poorly understood. Even for the planets in our solar system, difficulties in observation lead to large uncertainties in the properties of planetary cores, say scientists. This could change as astronomers have discovered the surviving core of a gas giant orbiting a distant star, offering an unprecedented glimpse into the interior of a planet and the unique opportunity to learn about its composition.

Scientists believe that this is the first time the exposed core of a planet has been observed. Located around a star much like our own, approximately 730 light years away, the core named TOI 849 b orbits so close to its host star that a year is a mere 18 hours and its surface temperature is around 1,800K. The core of the exoplanet (a planet outside the Solar System) is the same size as Neptune, and is believed to be a gas giant that was either stripped of its gaseous atmosphere or that failed to form one in its early life, says the research team from the University of Warwick, UK. 

Determining the planet's mass and radius, the team says TOI 849 b is about 40 times heavier than the earth, but its radius is just 3.4 earth radii.

According to lead author Dr David Armstrong from the Department of Physics at the University of Warwick, TOI 849 b is the most massive terrestrial planet discovered that has an Earth-like density. "The discovery of TOI-849b demonstrates that we can find exposed planetary cores by looking for planets very close to their host stars. TOI 849 b itself is much more massive than we expect even gas giant planetary cores to be, and this might imply a new planet formation or evolution pathway which we don’t yet understand," Armstrong tells MEA WorldWide (MEAWW).

"We would expect a planet this massive to have accreted large quantities of hydrogen and helium when it formed, growing into something similar to Jupiter. The fact that we don't see those gases lets us know this is an exposed planetary core. This is the first time that we've discovered an intact exposed core of a gas giant around a star," he explains in the analysis published in Nature. Dr Armstrong says with more work, scientists can build up a population of cores. "This will be a huge step in our understanding of the interiors of planets. Investigating planetary cores is very challenging, with large uncertainties still present even for the planets in our own solar system. The core is the first part of a planet to form, so if we want to understand planet formation in general, we need to understand the core," he tells MEAWW.

Some of the other institutes involved in the research work are Aix-Marseille Université, France, Universidade do Porto, Portugal, University of Cambridge, UK, Harvard-Smithsonian Center for Astrophysics, US, University of Arizona, US, University of Bern, Switzerland, and Massachusetts Institute of Technology, US, among others.

The core is the same size as Neptune, says the study (Getty Images)

TOI 849 b was found in a survey of stars by NASA's Transiting Exoplanet Survey Satellite (TESS). When a planet crosses in front of its star from our perspective, an event called a transit, its passage causes a distinct dip in the star's brightness. Using this method, TESS can help detect planets by recording the tell-tale dip in a star's brightness, which indicates that a planet has passed in front of them. The object was then analyzed using the HARPS instrument, on a program led by the University of Warwick at the European Southern Observatory's La Silla Observatory in Chile.

According to the analysis, TOI 849 b was located in the "Neptunian desert" — a term used by astronomers for a region close to stars where one rarely sees planets of Neptune's mass or larger. The host star TOI-849 is a late G dwarf, implying stars that have a similar mass to the Sun.

The team determined that the object's mass is two to three times higher than Neptune but it is also incredibly dense with all the material that makes up that mass squashed into an object the same size. "While this is an unusually massive planet, it's a long way from the most massive we know. But it is the most massive we know for its size, and extremely dense for something the size of Neptune, which tells us this planet has a very unusual history. The fact that it's in a strange location for its mass also helps. We don't see planets with this mass at these short orbital periods," says Dr Armstrong.

Scientists say that there are two theories as to why they saw the planet's core, rather than a typical gas giant. The first is that it was once similar to Jupiter but lost nearly all of its outer gas through multiple methods. These could include tidal disruption, where the planet is ripped apart from orbiting too close to its star, or even a collision with another planet.

Largescale photoevaporation of the atmosphere could also play a role, but cannot account for all the gas that has been lost, says the study. Another explanation is that it could be a "failed" gas giant. The team believes it is possible that once the core of the gas giant formed, something could have gone wrong and it never formed an atmosphere. This could have occurred if there was a gap in the disc of dust that the planet formed from, or if it formed late and the disc ran out of material. 

TOI 849 b was found in a survey of stars by NASA's Transiting Exoplanet Survey Satellite (TESS).

"The planet could have been a gas giant before undergoing extreme mass loss via thermal self-disruption or giant planet collisions, or it could have avoided substantial gas accretion, perhaps through gap opening or late formation. Although photoevaporation rates cannot account for the mass loss required to reduce a Jupiter-like gas giant, they can remove a small (a few Earth masses) hydrogen and helium envelope on timescales of several billion years, implying that any remaining atmosphere on TOI-849b is likely to be enriched by water or other volatiles from the planetary interior. We conclude that TOI-849b is the remnant core of a giant planet," the team concludes. 

According to Dr Armstrong, the findings indicate that planets like this exist and can be found. “We have the opportunity to look at the core of a planet in a way that we can't do in our own solar system. There are still big open questions about the nature of Jupiter's core, for example, so strange and unusual exoplanets like this give us a window into planet formation that we have no other way to explore," he explains. 

While the research team does not have any information yet on its chemical composition, they say they can follow it up with other telescopes. "Because TOI 849 b is so close to the star, any remaining atmosphere around the planet has to be constantly replenished from the core. So if we can measure that atmosphere then we can get an insight into the composition of the core itself,” says Dr Armstrong.