Astronomers have discovered a massive, dense exoplanet named GJ 523b that challenges conventional ideas about how rocky planets form. The planet is roughly 2.5 times wider than Earth while packing approximately 23 times our planet’s mass into that compact size. Worlds of this scale and density are informally known as “mega-Earths.”
Discovery of GJ 523b by the University of Wisconsin–Madison
According to a statement from the University of Wisconsin–Madison, GJ 523b features an unusually high density that suggests it contains relatively little atmosphere, despite being large enough that astronomers would typically expect it to have accumulated a substantial gaseous envelope. This isn’t what we expected at all,
said Max Kroft, lead author of the study.
Observations by NASA’s Transiting Exoplanet Survey Satellite
The exoplanet was initially identified as a candidate by NASA’s Transiting Exoplanet Survey Satellite (TESS), which detects periodic dips in starlight as planets cross in front of their host stars. Researchers subsequently used the ground-based WIYN 3.5-meter Telescope at Kitt Peak National Observatory in Arizona with a spectrograph to measure the planet’s gravitational pull on its star.
Measurements from the WIYN 3.5-meter Telescope
Calculations show that GJ 523b has a mass about 23.5 times that of Earth, a radius 2.55 times larger, and a density of approximately 126.82 grams per cubic inch. It circles its star every 17.75 days within a system estimated to be nearly 170 million years old.
Planet Formation Models and Theories
This combination of characteristics creates a puzzle for planet formation models, which suggest that planets build cores of rock and metal that can pull in hydrogen and helium from surrounding gas and dust. While solar system gas giants like Jupiter and Saturn began rapidly accumulating gaseous envelopes once their growing cores reached roughly 20 times Earth’s mass, GJ 523b exceeds that threshold yet remains predominantly rocky. Researchers are considering whether the planet formed with a thick atmosphere that was later stripped away, or if it formed through a collision between two planets that blasted much of their gaseous envelopes into space.
