1st detection of a giant planet remnant core: unprecedented glimpse inside other worlds

Centro de Astrobiología (CAB, CSIC-INTA)
  • An exoplanet with anomalous characteristics orbiting Sun-Like Star TOI-849 is discovered by a University of Warwick-led team with participation of the Centro de Astrobiología
  • The first exposed planetary core was found in the "Neptunian desert", where objects of such size and density are rarely seen
  • The core could be part of a former gas giant similar to Jupiter that lost nearly all of its outer gas, or a ‘failed’ gas giant that failed to form an atmosphere in its early life
  • The finding represents a unique opportunity to better understand the internal structure and formation of gas giant


At the Neptunian Desert and the insides of giant planets

The interiors of giant planets remain poorly understood. Even for the planets in the Solar System, the study of their cores is extremely complicated due to observational difficulties. In this context, exoplanets that appear to have undergone rare evolutionary processes provide scientists with a new way to understand planetary interiors. Some of them are found in the so-called "Neptunian desert", a term used by astronomers to refer to a region close to stars where we rarely see planets of Neptune’s mass or larger.

Among the few cases detected to date, it has been found that such planets are unusually dense. This suggests that they have undergone processes of atmospheric erosion due to high radiation from their host star, partially ripping away their outer atmosphere. These planets are believed to be so rare because most of them end up evaporating in just a few billion years.

A scientific team led by David Armstrong, from the University of Warwick, with researchers from the Centro de Astrobiología (CAB, CSIC-INTA), reported in the journal Nature the discovery of one of these anomalous exoplanets, called TOI-849b. This is thought to be the first time the exposed core of a planet has ever been observed. The discovery stems from a survey of stars by NASA’s Transiting Exoplanet Survey Satellite (TESS) using the so-called transit method. This procedure involves the observation of stars to detect the decrease in brightness that indicates that a planet has passed in front of them.

The transit method detects the passage of planets in front of stars owing to the difference of light recorded during the event.

The telescopes used were specifically designed to detect the very shallow dips in brightness from exoplanets transits, in this case, as little as only one-tenth of one percent of the star’s brightness. The planet was then confirmed by the European Southern Observatory’s La Silla Observatory (Chile). The exoplanet orbits a solar-type star called TOI-849, which is about 730 light years from Earth and its surface temperature is around 1800K. It orbits so close to its host star that a year lasts only 18 hours.


TOI-849b, an exceptional case

Studies of TOI-849b have confirmed that its characteristics are quite abnormal. Its radius, about 3.5 times greater than that of Earth, first suggested that it was a gaseous planet similar to Neptune and Uranus. However, when measuring its mass using the High Accuracy Radial velocity Planet Searcher (HARPS) spectrograph, mounted on the 3.6-m telescope at ESO’s La Silla Observatory, something different was found: an unusually large mass was obtained, some 40 times greater than that of Earth (Neptune's mass is approximately 16 times greater than that of the Earth).

TOI-849b radius and mass values indicate that its density is similar to that of planet Earth, making it a gigantic solid planet, the largest one found to date, containing, at most, 3% of its mass as a gaseous atmosphere. Taking into account these peculiar characteristics, the researchers hypothesize that we are facing the bare core of what was once a gas giant like Jupiter, but that has lost nearly all of its gas envelope.

To explain this immense loss of mass, the researchers propose several scenarios. In one of them, the planet initially formed as a gas giant away from its star, then migrated inland and got so close to its star that tidal effects ripped away the atmosphere. Another possibility is a large planetary collision with another planet in the system. Alternatively, TOI-849b could be a “failed” gas giant: after the formation of its core, something could happen in the system that prevented the planet from accumulating the necessary gas to form an atmosphere. But are these the only possibilities?

Artist's impression showing a planet the size of Neptune in the so-called "Neptunian desert". By Mark Garlick; Uni. Warwick.

One of the tasks of the CAB researchers who participated in the study was precisely to rule out other feasible scenarios. In particular, in the context of the TROY Project, led by CAB researcher Jorge Lillo-Box, co-author of this study, another possibility was evaluated: the possibility that the wobble of the star – used to measure the planet’s mass - was not being caused by only one, but two planets located in the same orbit. Planets in such a situation are known as co-orbital planets.

As Lillo-Box pointed out, “the analysis carried out allowed ruling out the presence of planets co-orbiting TOI-849b with a mass greater than 8 Earth masses. These results have allowed us to definitively rule out the co-orbital scenario as the source of the planet's great mass, thus confirming the hypothesis that TOI-849b is definitely a bare rocky core, probably a gigantic sphere composed mainly of iron and silicates", he concludes. TOI-849b represents, therefore, a unique case where the material of the primordial core of the formation of a gaseous planet can be studied. The origin of this strange planet has yet to be determined and future observations will be necessary to clarify its origin.


Further plans together with the European Space Agency

The European Space Agency plans to launch a mission similar to TESS, albeit a much more ambitious one, where the National Institute of Aerospace Technology (INTA) and the Centro de Astrobiología (CAB), along with other Spanish institutions such as the Institute of Astrophysics of Andalusia (IAA-CSIC), the University of Granada (UGR) and the Instituto de Astrofísica de Canarias (IAC), have a very relevant role.

Currently under development, the PLATO satellite, whose mission is the detection and characterization of Earth-like planets in orbits around Sun-like stars, is at a crucial time in its design and implementation and requires strong support from the Spanish State Research Agency (AEI) and the Spanish Centre for the Development of Industrial Technology (CDTI).

As David Barrado, a CAB researcher involved in this study, concludes “The results of the TESS satellite, as illustrated by the case of TOI-849b, show that a determined and consistent investment over time leads to remarkable advances in our knowledge of the Universe and our role in it.”


Image credits:

Transit method illustration was downloaded from Wikimedia Commons and licensed via a Creative Commons Attribution-ShareAlike 4.0 International (CC BY-SA 4.0) license.

Neptune-sized planet at the Neptunian Desert artist illustration by Mark Garlick, University of Warwick, was kindly provided by kindly CAB, CSIC-INTA.

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