Astronomers from the University of Wisconsin–Madison have reported the discovery of an unusual exoplanet, GJ 523b. It is the first planet discovered and cataloged as part of the Wisconsin Center for Origins Research project.
The planet is located approximately 87 light-years from Earth. Its radius is about 2,55 Earth radii, and its mass is approximately 23,5 Earth masses. With these parameters, its density turns out to be very high—about 7,8 g/cm³.
The high density indicates that GJ 523b is predominantly rocky and contains little gas in its atmosphere. Such worlds are informally called "mega-Earths."
The planet was first spotted using the TESS space telescope by periodic dimming of the star's light. It was then confirmed by observations at the 3,5-meter WIYN telescope at Kitt Peak Observatory using the NEID spectrograph.
The orbital period is 17,75 days. The age of the system is estimated at approximately 170 million years—a very young object by cosmic standards.
According to standard planet formation models, a core of such mass should have attracted a thick gas envelope and turned into a gas giant. However, GJ 523b has remained dense and rocky.
"This is not at all what we expected," noted lead author of the study Max Croft.
Possible explanations include the loss of its atmosphere due to proximity to the star or a collision between two protoplanets. There is not yet enough data for a definitive conclusion.
The discovery helps refine the concept of a "mega-Earth": a planet with a radius of at least 2,1 Earth radii and a density of at least 5,5 g/cm³. How exactly such objects form remains to be determined.
