Astrophysicists have discovered a planet that has only its core remaining.

Among the over five thousand exoplanets already discovered in the cosmos, most are located extremely close to their stars, and they are significantly more massive than Earth. The reason is that such worlds are the easiest to detect: the primary method of their search is the so-called transit method. When a planet, especially a very large and close one, passes in front of its star, the star's light dims slightly. It is easy to guess that this occurs at regular intervals. This is how astronomers understand that the star is not alone.

They also find smaller worlds that are much further from their suns, but particular questions arise concerning those that are perilously close to their stars. These are referred to as ultra-short-period planets. This means they have ultra-short orbital periods — less than one Earth day. Scientists are trying to understand how they end up there.

According to calculations, planets cannot form at such minuscule distances from stars. Therefore, they must be brought there through gravitational interactions with other planets. This implies that such planets must exist within the system, meaning they need to be detected to understand the situation.

Thus, the K2-360 star system (EPIC 201595106), located 764 light-years from Earth, serves as an important illustrative example. Its mass is nearly that of the Sun. Two planets are observed near it: both are many times closer to it than Mercury is to the Sun (the distance between our star and its first planet is about 58 million kilometers).

The planet K2-360 a is situated 13.5 million kilometers from its sun, while K2-360 b is located just three million kilometers away. The more distant one completes an orbit around the star in less than 10 Earth days.

The "year" for the closer planet lasts only 21 hours. It is also notable for its size and mass ratio: with a radius one and a half times that of Earth, it "weighs" nearly as much as eight Earths. This results in a density of about 11 grams per cubic centimeter, comparable to the density of lead. According to planetary scientists, K2-360 b may be composed half of iron and half of other elements.

Recently, the K2-360 system was studied by an international team of researchers led by specialists from the Astrobiology Center of the National Institute of Natural Sciences NINS (Japan). As they write in their article for Scientific Reports, the dense structure of K2-360 b raises suspicions that it is not a complete planet but rather what remains of it — a "bare" solid inner core devoid of all external layers. It is estimated that the entire planet was originally 9-11 times heavier than Earth, thus classifying it as a sub-Neptune: Neptune is 17 times more massive than our planet.

This scenario is supported by the presence of a second planet, which "weighs" approximately 15 Earths. Its size, however, has not been established since this planet does not transit across the star's disk. It was calculated based on the "behavior" of the star (which wobbles slightly as it moves around the common center of mass). In any case, this second planet, as the researchers suggested, might have once "pushed" the former sub-Neptune toward its inevitable doom — directly into the star's hell.