About 730 light years away, around a star that closely resembles our Sun, astronomers have found a really strange exoplanet.
It is only slightly smaller than Neptune, which could indicate a gaseous planet … but it is more than twice as massive as Neptune, with a density comparable to Earth and Venus.
This super dense surprise suggests that the exoplanet is rocky, but well above the usual upper size limit for rocky planets. Which, in turn, means that it could be something very rare: what is known as a Chthonian planet, a nucleus of a gas giant that has had its atmosphere removed.
This is a hypothetical class of planets, since the detection of one has never been confirmed before.
The planet in question is called TOI-849b, and it is orbiting a Sun-like star called TOI-849. When we discover exactly what it is, it could help us better understand what is inside the thick atmospheres of gaseous and ice giants, like Neptune, and the processes of formation of these formidable planets.
“TOI 849 b is the most massive terrestrial planet, having a density similar to Earth, discovered. We would expect such a massive planet to have accumulated large amounts of hydrogen and helium when it formed, turning into something similar to Jupiter. The fact that we don’t see those gases let us know that this is an exposed planetary core, “said astronomer David Armstrong of the University of Warwick in the UK.
“This is the first time we have discovered an intact exposed nucleus of a gas giant around a star.”
TOI-849b was located by a survey conducted using NASA’s Transit Exoplanet Survey Satellite (TESS), the exoplanet hunting space telescope. TESS searches for exoplanets looking at the stars, using its sensitive instruments to search for regular, weak drops in its light that indicate that something large, like a planet, is passing in front of the star.
The amount and frequency with which the star’s light is dimmed allows astronomers to calculate things like how big the planet is and how close it is to the star. TOI-849b is very close to its star: it rotates every 18 hours. Such proximity would make it extremely hot, with a surface temperature of around 1,800 Kelvin (1,530 degrees Celsius, or 2,780 degrees Fahrenheit).
This proximity to its star places the exoplanet in a special category: Very few planets the size of Neptune have been found close to its stars, creating what is called a Neptune hot desert.
This would only be remarkable, but then the team made follow-up observations using Doppler spectroscopy.
When a planet orbits a star, it exerts a slight gravitational pull on the star, causing the star to wobble a little on the spot. Doppler spectroscopy measures how the star’s light changes as it wobbles. If the mass of the star is known, astronomers can calculate the mass of the planet based on how much the star wobbles.
This is how the team calculated the mass of the exoplanet: about 39.1 times the mass of Earth and 2.3 times the mass of Neptune. This results in a density of 5.2 grams per centimeter in cubes, very close to the 5.24 g / cm³ for Venus and the 5.51 g / cm³ for Earth.
“While this is an unusually massive planet, it is far from the most massive we know of,” Armstrong explained.
“But it is the most massive we know of for its size, and extremely dense for something the size of Neptune, which tells us that this planet has a very unusual history. The fact that it is in a strange place for its mass also helps. “I don’t see planets with this mass in these short orbital periods.”
Which leads to the conclusion that we are seeing a Chthonian planet. Although how this came about remains a mystery.
It is possible that TOI-849b was formed with a huge gaseous atmosphere similar to that of Jupiter that was later somehow removed.
We know that gas planets next to their stars can have their atmospheres stripped of incredible heat. And one of the only hot Neptunes discovered, Gliese 3470 b, is losing its atmosphere at an incredible rate, evaporated by the heat of its star.
This process would not explain the entirety of the atmospheric loss calculated for TOI-849b, but other events might have played a role, such as collisions with other large objects.
The other option is that TOI-849b began to form as a gas giant, but it did not have enough material, either because it formed late in the evolution of the planetary system, when there was very little material left in the star’s protoplanetary disk, or because it formed in a space on the disc, where there was not enough material to accumulate an atmosphere.
The team plans to continue their investigation with observations to try to determine if TOI-849b has any remaining atmosphere. This could help determine the composition of the core itself.
“In one way or another, TOI 849 b used to be either a gas giant or a ‘failed’ gas giant,” Armstrong said.
“It is the first time he has told us that planets like this exist and can be found. We have an opportunity to see the core of a planet in a way that we cannot do in our own solar system. There are still big open questions.” about the nature of Jupiter’s core, for example, such strange and unusual exoplanets as this give us a window into planet formation that we have no other way to explore. “
The research has been published in Nature.
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