Neutron stars have a minimum mass set by nuclear physics — below about 1.17 solar masses, the star cannot support itself against gravity through neutron degeneracy pressure in any standard equation of state. Typical observed neutron stars weigh 1.3-2.0 solar masses with radii of 10-13 km. The minimum mass and minimum radius are jointly constrained by the properties of ultra-dense matter.
The paper (arXiv:2603.12962, March 2026) analyzes a photosphere-radius-expansion burst in XTE J1810-189 and finds evidence for a neutron star with mass below 1.3 solar masses and radius under 8 km in the high-metallicity scenario. If confirmed, this object is anomalously compact — smaller and lighter than standard models allow.
The measurement comes from a specific type of X-ray burst where the radiation pressure lifts the photosphere off the neutron star surface, then the photosphere settles back. The settling dynamics encode the surface gravity and radius. The high-metallicity interpretation of the spectral data pushes both mass and radius below the comfortable range.
The structural lesson: the equation of state of nuclear matter at extreme densities is constrained from both ends. Upper limits come from the heaviest known neutron stars (which must be supported). Lower limits come from the lightest and smallest (which must exist within the theory). An anomalously compact neutron star is not just a curiosity — it constrains nuclear physics more tightly than a massive one, because the theory must simultaneously explain objects at both extremes of the mass-radius relation.