The newly-discovered planetary core, named TOI-849b, is a gas giant that was either stripped of its gaseous atmosphere or that failed to form one in its early life.
An artist’s impression of a Neptune-sized exoplanet. Image credit: Mark Garlick / University of Warwick.
TOI-849 is a G-dwarf star located 741 light-years away in the constellation of Sculptor.
Also catalogued as TIC33595516 and TYC 6431-1158-1, the star is slightly smaller and less massive than the Sun and is 6.7 billion years old.
TOI-849b has a radius smaller than Neptune’s but an anomalously large mass of 39.1 times that of Earth, nearly half the mass of Saturn.
The object is 3.4 times bigger than our home planet and its mean density is 5.2 g/cm3, similar to Earth’s, making it the densest Neptune-sized planet discovered so far.
TOI-849b has an equilibrium temperature of 1,527 degrees Celsius (2,781 degrees Fahrenheit) and an orbital period of only 18 hours, making it an ‘ultra-short-period’ object.
The planetary core is also in the so-called Neptunian desert, a region close to stars where astronomers rarely see planets of Neptune’s mass or larger.
“While this is an unusually massive planet, it’s a long way from the most massive we know,” said Dr. David Armstrong, a researcher in the Centre for Exoplanets and Habitability and the Department of Physics at the University of Warwick.
“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.”
“TOI-849b has a high density and therefore has to primarily consist of iron, rock and water, but only very little hydrogen and helium,” said Dr. Christoph Mordasini, an astronomer in the Physics Institute at the University of Bern.
“Such a small amount of hydrogen and helium is really astonishing for such a massive planet.”
“We would expect a planet this massive to have accreted large quantities of hydrogen and helium when it formed.”
TOI-849b was discovered by NASA’s Transiting Exoplanet Survey Satellite (TESS) and then confirmed using the High Accuracy Radial velocity Planet Searcher (HARPS) spectrograph mounted on the 3.6-m telescope at ESO’s La Silla Observatory.
“There are two theories as to why we are seeing the planet’s core, rather than a typical gas giant,” the astronomers said.
“The first is that it was once similar to Jupiter but lost nearly all of its outer gas through a variety of 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.”
“Large-scale photoevaporation of the atmosphere could also play a role, but can’t account for all the gas that has been lost.”
“Alternatively, it could be a failed gas giant,” they said.
“We believe that once the core of the gas giant formed then something could have gone wrong and it never formed an atmosphere.”
“This could have occurred if there was a gap in the disk of dust that the planet formed from, or if it formed late and the disk ran out of material.”
“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,” Dr. Armstrong said.
“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.”
“Although we don’t have any information on its chemical composition yet, we can follow it up with other telescopes.”
“Because TOI-849b 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.”
The discovery is reported in a paper published in the July 1, 2020 issue of the journal Nature.
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D.J. Armstrong et al. A remnant planetary core in the hot-Neptune desert. Nature 583, 39-42; doi: 10.1038/s41586-020-2421-7
