Gliese 523b (GJ 523b) is a young, ultra-dense planet that orbits almost perpendicular to its star’s spin, according to a team of U.S. astronomers.
An artist’s impression of a giant exoplanet. Image credit: Sci.News.
“People have been using the phrase ‘mega-Earth’ for more than a decade, but we’ve never had a planet that let us say concretely what one is,” said Dr. Thomas Beatty, an astronomer at the University of Wisconsin-Madison.
“Gliese 523b finally does. What is also striking is that nailing down the definition of a mega-Earth isn’t something us astronomers can really do by ourselves.”
“We need geologists who understand how iron and rock behave at pressures no laboratory on Earth can reach, and atmospheric scientists who can tell us how much of what we measured is rock at all.”
The newfound planet orbits Gliese 523 (GJ 523), a mid-K dwarf star roughly 87 light-years away from Earth, completing one lap every 17.75 days.
The alien world is about 2.55 times Earth’s radius but 23.5 times Earth’s mass, giving it a bulk density of 7.8 grams per cm3 — denser than our home planet itself, despite being large enough to typically host a thick atmosphere.
“This isn’t what we expected at all,” said Max Kroft, a graduate student at the University of Wisconsin-Madison.
“Dense planets like this aren’t uncommon, but they’re usually small rocky planets similar to Earth or Mercury. This planet is two and a half times bigger than the Earth.”
Using radial velocity data from the NEID spectrograph on the WIYN Telescope at Kitt Peak National Observatory, combined with earlier transit detections from NASA’s TESS mission, the astronomers confirmed the planetary nature of Gliese 523b.
“We picked out this planet based on what we thought its size and temperature were,” Kroft said.
“A bigger planet makes a bigger dip, so we get an idea of the size, and based on how often that dip happens, we get the distance of its orbit, and we can use that to estimate the temperature of the exoplanet.”
By analyzing the rotation rates of Gliese 523 and several companion stars moving through space together, the researchers estimated the system is only about 169 million years old.
They also determined that the planet’s orbit is inclined at least 71 degrees relative to its star’s equator, meaning Gliese 523b likely travels over the star’s poles rather than around its equator, similar to how a satellite might orbit Earth pole-to-pole rather than tracing the equator.
The combination of high mass, apparent lack of an atmosphere, youth, and steep orbital tilt is difficult to reconcile with standard theories of how planets form and evolve.