GJ 523b: The Young Mega-Earth That Challenges Planet Formation Theory

GJ 523b is an unusually dense exoplanet that sits awkwardly between familiar categories. It is 23.5 times as massive as Earth but only 2.55 times Earth’s radius, producing a bulk density of 7.8 ± 1.8 grams per cubic centimeter. That combination points to a world dominated by heavy elements, with no evidence from its measured mass and radius for a substantial hydrogen-helium envelope.

The finding is intriguing rather than settled: the result is based on an arXiv preprint submitted to The Astronomical Journal on 25 March 2026 and has not yet completed peer review.

How astronomers found GJ 523b

The discovery began with the transit method. NASA’s Transiting Exoplanet Survey Satellite, or TESS, monitored the host star GJ 523 and recorded repeated, tiny dips in its brightness. TESS observed the star in five sectors at two-minute cadence and found a candidate signal repeating every 17.74556 days. The candidate was designated TOI-7032.

The depth of those dips, combined with an estimate of the star’s size, indicated a planet with a radius of 2.55 ± 0.15 Earth radii. Transit data reveal a planet’s size, but not its mass, so the candidate required an independent measurement.

That confirmation came from radial-velocity observations with NEID, a high-precision spectrograph on the WIYN telescope. As GJ 523b orbits, its gravity causes the star to move slightly toward and away from Earth. NEID detected that periodic stellar wobble at the same period as the TESS transits, confirming the planetary signal and measuring a mass of 23.5 ± 3.3 Earth masses.

Together, the radius and mass provide the planet’s bulk density. This mass–radius combination is the central reason GJ 523b is considered a likely “mega-Earth”: it is large for a rocky planet, yet far denser than a typical gas-rich sub-Neptune of comparable size.

Why the planet is difficult for standard formation models

The tension comes from what happens when a planetary core becomes very massive while a young system still contains a gas-rich protoplanetary disk.

In conventional core-accretion models, a solid core that reaches roughly 20 Earth masses can begin rapidly capturing surrounding gas. If runaway accretion proceeds, the result is expected to be a gas giant. GJ 523b is more massive than that approximate threshold, but its measured density suggests that it did not acquire a large hydrogen-helium envelope.

Its radius adds to the puzzle. At 2.55 Earth radii, GJ 523b falls within the broad sub-Neptune size range, where planets often retain envelopes of hydrogen and helium. Instead, its density is consistent with a body containing a much larger proportion of rock and other heavy material. This is an inference from the bulk measurements, not a direct atmospheric detection.

The planet also appears to be young. The study estimates an age of 169 million years, with an uncertainty of +100 million and −48 million years. That leaves less time for ordinary, long-term atmospheric escape to remove a once-massive envelope. At the same time, a planet with GJ 523b’s gravity should be comparatively effective at holding on to an atmosphere.

Finally, the orbit may preserve evidence of a complicated dynamical history. The authors infer a stellar spin inclination of 17.6° ± 5.0°. Because the planet transits, that geometry implies a minimum spin-orbit misalignment of 71.4°, consistent with a likely near-polar orbit. The measurement is indirect, but if confirmed it would be difficult to reconcile with a simple, undisturbed, coplanar formation path.

Possible explanations—but no definitive answer yet

The preprint does not establish a single formation mechanism. Instead, it leaves several possibilities open.

Late assembly after the gas disk faded

GJ 523b may have accumulated much of its solid mass after the surrounding gas disk had already dispersed. In that case, a large rocky core could form when there was little nebular hydrogen and helium left to capture.

Gas accretion may have been suppressed

A locally gas-poor disk, rapid dispersal of the disk or heat released during accretion could have prevented the planet from entering runaway gas growth. A hybrid pathway involving both pebbles and planetesimals is one mechanism discussed by the authors: continued solid accretion could heat the growing envelope and delay or inhibit the capture of large amounts of gas.

The planet may have lost an earlier atmosphere

Another possibility is that GJ 523b began with a more substantial envelope and later lost it through intense early irradiation, giant impacts or tidal and dynamical stripping. Simple photoevaporation is less straightforward here than for a close-in stripped core because the planet is relatively massive, orbits every 17.75 days and is still young. Those factors do not rule out atmospheric loss, but they make it harder to treat as the complete explanation.

The system may have been dynamically rearranged

The possible near-polar orbit could reflect migration, a tilted protoplanetary disk or interactions with another body. The authors consider a disk-origin misalignment plausible. A von Zeipel–Lidov–Kozai pathway would require an inclined outer companion that has not yet been detected and is considered less favored in the study.

Why GJ 523b matters

GJ 523b may be a particularly clear example of a proposed class of mega-Earths: planets at least 2.1 Earth radii across with densities of at least 5.5 grams per cubic centimeter. These objects occupy a size range often associated with sub-Neptunes but appear to contain unusually large amounts of heavy material.

That makes GJ 523b relevant to the debate over the exoplanet radius gap, the apparent scarcity of planets between smaller rocky worlds and larger sub-Neptunes. If some planets above that gap can remain extremely dense and gas-poor, then planetary size alone gives an incomplete picture of how planets evolve. Mass measurements are essential for distinguishing a rocky mega-Earth from a lower-density world wrapped in a thick atmosphere.

The discovery also highlights a limit of applying a single threshold too rigidly. A core mass near 20 Earth masses may mark a strong tendency toward gas accretion, but GJ 523b suggests that disk conditions, timing, accretion history and later dynamical evolution can matter just as much.

The sample-of-one problem

GJ 523b is unusual, but one unusual planet cannot establish how common mega-Earths are or prove a universal formation pathway. Its measured mass and radius have uncertainties, and bulk density does not uniquely reveal the planet’s interior structure or atmospheric composition. A dense planet could still possess a secondary or metal-rich atmosphere that is difficult to detect through mass and radius alone.

The inferred obliquity also needs further testing. The current result provides a minimum three-dimensional misalignment based on the star’s inferred spin orientation and the fact that the planet transits; it is not the same as a direct measurement of the sky-projected angle.

For those reasons, “challenges formation theory” is more precise than “breaks the laws of planet formation.” The observation exposes a difficult case for standard expectations, but additional systems are needed to determine whether it represents a rare exception, a stripped planet or a missing but significant class of planetary outcomes.

What observations could resolve the mystery?

Several follow-up measurements could distinguish among the competing explanations:

• More radial-velocity monitoring could improve the mass estimate, constrain orbital eccentricity and search for long-period companions that might have altered the planet’s orbit.
• Additional transit photometry could refine the radius and orbital ephemeris.
• Transit spectroscopy could search directly for a surviving atmosphere and test whether it is hydrogen-helium-poor, metal-rich or effectively absent.
• Rossiter–McLaughlin measurements could determine the sky-projected spin-orbit angle and test whether the inferred near-polar geometry is real.
• A larger sample of young, dense sub-Neptunes could show whether GJ 523b is an isolated anomaly or part of a broader population of massive rocky planets.

For now, GJ 523b is best understood as a well-motivated but provisional case study: TESS found the transit, NEID supplied the mass, and the resulting combination has produced a planet that standard formation models do not explain comfortably.