Mega-Earth GJ 523b is 23 times as massive as our planet—but only 2.5 times as wide

GJ 523b is a dense and massive exoplanet, classified as a "Mega-Earth" due to its size. It is nearly 170 million years old, more than 2.5 times the size of Earth, and the first exoplanet discovered and cataloged by the Wisconsin Center for Origins Research (WiCOR). Max Kroft, a graduate student in the lab of Assistant Professor Thomas Beatty, is the lead author on a paper that characterizes GJ 523b. The paper is under review and available on arXiv. The study highlights the planet's unusual density and size, which makes it an unexpected discovery in the field of exoplanet research.

• GJ 523b is 23 times as massive as Earth.
• It is 2.5 times the size of Earth.
• It is the first exoplanet discovered and cataloged by WiCOR.
• Kroft used a ground-based telescope in Arizona, WIYN, with a high-resolution spectrograph to study the planet.
• The planet's density and atmosphere were determined using data from the James Webb Space Telescope.

The discovery of GJ 523b challenges previous assumptions about how planets form. While dense planets like this are not uncommon, they are usually small rocky planets similar to Earth or Mercury. GJ 523b is two and a half times the size of Earth, which is an unusual development in planetary formation. The study suggests that the planet may have formed in a different way, possibly due to a collision or the loss of its atmosphere. The research team put forward several theories, including the possibility that the planet had its atmosphere ripped off when it orbited too close to its star or that it began as two planets that collided and lost atmosphere in the heat of the crash.

The study is part of WiCOR's efforts to find Hycean exoplanets, which are theorized to have large oceans and temperate atmospheres that could support life. WiCOR launched in 2024 as a collaboration of researchers from seven UW–Madison departments. The research team is hopeful that as planetary researchers analyze more potential planets, more dense and oversized planets like GJ 523b will be cataloged, helping to categorize them. The discovery is expected to be the first of many key findings from WiCOR. The paper is titled "GJ 523b is a Massive, 170 Myr-old Mega-Earth, Likely on a Polar Orbit" and is available on arXiv. The study is expected to contribute to understanding how planets form and evolve.

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