This Strange Alien World May Be the Core of a Stripped-Down Gas Giant

Its mysterious evolutionary path is challenging current ideas on how planetary formation works.

A bizarre new exoplanet discovered orbing a Sun-like star some 730 light-years away could turn out to be the stripped-down core of an ancient Gas Giant similar to Jupiter. According to astronomers, the alien planet is approximately 40 times more massive than our planet. Given its peculiar characteristics, astronomers say the world is different from all other planets discovered to date, and chances are we are looking at the core of a giant planet or a giant planet in the making whose growth stopped abruptly.


In 2018, astronomers using the NASA TESS telescope spotted a strange world designated TOI-849b. The existence of the exoplanet was confirmed by astronomers using the La Silla Observatory in Chile shortly after. Now, follow-up studies have revealed peculiar characteristics of the distant exoplanet.

Led by David Armstrong of the University of Warwick, a team of astronomers recently published a paper in the preprint server arXiv, detailing what they believe could be the origins of the distant world. In the new study—still to be peer-reviewed—the researchers explain that TOI-849b has a mass 40 times that of our planet and diameter 3.45 times greater. Altogether, this information hints the exoplanet has a density comparable to Earth’s, making it the densest Neptune-size planet identified to date.

Researchers believe this exoplanet could be the solid core of an ancient and powerful gas giant, now devoid of the thick blanket of gas that once surrounded it. A long time ago, the researchers theorize, its appearance may have been similar to that of our Jupiter, Saturn, Uranus or Neptune.

The interior of Gas Giants remains little known even to the worlds of the Solar System, as difficulties in observation lead to great uncertainties in the properties of those planetary nuclei. So, planets that have undergone strange evolutionary pathways can provide us with a new route to understand those planetary interiors, the scientists revealed in their study.

“We have never before seen planets of this density that are this size,” Armstrong explained.

Indeed, astronomers would expect, based on our current knowledge of exoplanets and our own solar system, that an exoplanet like TOI-849b would have built up a thick and dense atmosphere around it, resembling gas giant planets like Jupiter in our own solar system. However, current knowledge of TOI-849b tells us that this was not the case, and scientists really don’t know why.

One possible theory proposes that TOI-849b might have run out of gas as it formed, so it managed to build a core but not an atmosphere. However, it is not clear how this process could develop. According to the authors, the planet would be much more likely to form as a “normal” gas giant, and then somehow lose its gas, either because it got too close to its star and its atmosphere was swept away by it, or due to a cosmic collision with another giant planet.

Neptunian desert

TOI-849b’s location in its host star system is also weird. In fact, it orbits its star once every 18.4 hours, so close to it that its surface temperature is approximately 1,500 degrees Celsius. Generally, worlds the size of Neptune so close to their stars are torn apart or partially vaporized by powerful solar radiation, leading astronomers to name that particular area a so-called hot Neptunian desert; This area receives strong irradiation from the star, meaning the planets do not retain their gaseous atmosphere as they evaporate leaving just a rocky core.

Whatever the case may be with TOI-849b, one thing is certain; this distant alien world did not follow normal patterns of planetary evolution. In fact, it is strange not only in comparison to the planets in our Solar System but also with respect to the more than 4,000 known exoplanets discovered to date. It is, for now, a unique planet and its mysterious evolutionary path is challenging current ideas on how planetary formation works.

If TOI-849b really formed like a normal gas giant and its atmosphere was later destroyed by some catastrophic event, it could help astronomers learn many new things about exoplanets and the history of planetary formation.

Another theory put forth by experts explaining the existence of the strange world is that maybe TOI-840b just formed pretty late in the history of its solar system, at a time when much of the system’s protoplanetary disk was already gone.

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