An astronomy team has discovered a rocky, super-dense exoplanet with more than 23 times the mass of Earth. Despite its diameter being only about two and a half times that of Earth’s, this discovery poses a new puzzle for prevailing theories on planet formation and evolution.

The planet is named GJ 523b and researchers classify it as one of the so-called “mega-Earths”, which are large worlds composed mainly of rock and metal. Unlike larger planets, which usually consist of thick layers of hydrogen and helium.

According to measurements, the planet’s mass is about 23.5 times that of Earth, its diameter is 2.55 times that of Earth’s, and its estimated density is about 7.8 grams per cubic centimeter, compared to 5.5 grams for Earth. These characteristics make it one of the largest rocky planets and most dense among known exoplanets.

Maxwell Croft, lead researcher at the University of Wisconsin–Madison, said that the result was completely contrary to the team’s expectations: high-density planets are well known to astronomers, but they usually consist of small rocky worlds similar to Earth or Mercury, not planets of this size and mass.

GJ 523b was first observed as a candidate planet by NASA's TESS satellite. The observatory monitors the slight periodic drops in star brightness that occur when a planet passes in front of its star and blocks part of its light.

The team then conducted observations from Earth using the WIYN 3.5-meter telescope at the National Kitt Peak Observatory in Arizona. The researchers used a high-precision spectrograph to measure the tiny wobble of the star caused by the planet's gravitational pull.

Combining transit and radial velocity measurements allowed them to determine the planet’s size, mass, and density. The results showed that it completes one orbit around its star every 17.75 days, with an estimated equilibrium temperature of about 538 Kelvin (265 degrees Celsius), assuming it does not reflect any of the radiation falling on it.

The planet orbits a medium-sized orange dwarf located about 87 light years from Earth. The system is estimated to be around 169 million years old, with a wide margin of uncertainty, making it a young system by cosmic standards.

The significance of the discovery lies in the fact that the planet’s mass exceeds the threshold at which a rocky core is thought to begin attracting large amounts of surrounding hydrogen and helium around a young star.

Common models suggest that massive planetary cores, when reaching a mass of about twenty times Earth's mass, become capable of rapidly accumulating a dense gaseous envelope, gradually transforming into Neptune-like planets or gas giants in our solar system.

But GJ 523b has exceeded this mass, and yet its high density suggests that it remained a rocky and metallic world with only a limited amount of gases. Here researchers pose the question: How did the planet avoid its expected fate and become a “super-Earth” instead of becoming a gas giant?

Missing envelope

The team proposes several possible scenarios to explain this unusual structure. The planet may have originally formed with a dense atmosphere, before losing most of it due to extreme heat or interactions with its star.

Another possibility is that GJ 523b was the result of a massive collision between two planets; such an impact could merge their rocky and metallic material into one extremely large body while ejecting most of their gaseous envelopes into space.

The study gives additional weight to the collision scenario, not only because of the planet's density alone, but also because its orbit appears highly tilted relative to its star's rotation axis, possibly close to the polar orbit. This unusual orientation may be a result of a disturbance or violent impact during the early stages of the system's life.

However, the youthfulness of the system makes the explanation more difficult; it has not had much time by astronomical standards, raising questions about how the planet was able to lose its significant gaseous envelope in such a short period.

New Class

Astronomers have been using the term “super-Earth” for more than a decade, but it has not yet become an official agreed-upon classification. The study proposes an observational definition of these worlds that includes planets with a radius at least 2.1 times Earth’s and a density of 5.5 grams per cubic centimeter or greater.

However, the researchers caution against building a complete theory on just one example. Finding twenty or thirty similar planets could reveal common patterns in their masses, orbits, or ages, helping to determine whether “super-Earths” are rare cases or an independent planetary class.

The study has been submitted for publication in The Astronomical Journal but is currently available as a preprint on arXiv. At the time of writing this article, it had not yet undergone peer review. Therefore, details about the planet’s internal composition remain speculative, based on measurements of mass and radius and computational models rather than direct observations of its interior.