Circular orbits are common in binary star systems with periods shorter than three days

Star system: EBLM J0040+01 A | Distance from the Sun: 307 light years

An eclipsing binary star system consists of two stars orbiting each other so close that they periodically block each other's light as seen from Earth. Over millions to billions of years, gravity between these stars reshapes their orbits and spinning rates in a process called tidal evolution, which helps us understand how both close star pairs and exoplanet systems change over time.

Researchers examined 68 binary pairs (among them EBLM J0040+01) from the EBLM survey using NASA's TESS space telescope and over a decade of radial velocity measurements — a technique that detects tiny stellar movements caused by gravitational pull. Each system contains a sun-like star paired with either a red dwarf or orange dwarf. The team measured how fast each primary star rotates and how circular or stretched each orbit is.

The study found that about three-quarters of these systems have evolved nearly circular orbits, with the remaining stretched orbits appearing only in systems with orbital periods longer than three days. About 78 percent show synchronized rotation, meaning the primary star completes exactly one rotation during each complete orbit. Nearly all systems with orbital periods under three days fall within a well-defined synchronization zone. However, some systems remain stubbornly asynchronous in ways that current tidal models struggle to explain.

Key findings
- About 75 percent of the binary systems have circularized to nearly perfect circular orbits
- Stretched orbits appear only in systems with orbital periods longer than three days, with minor eccentricity less than 0.25
- About 78 percent of systems show synchronized rotation matching their orbital period
- Systems with orbital periods under three days almost always synchronize within a measurable zone
- Some systems remain unexpectedly asynchronous, defying current theoretical predictions

Scientific study

EBLM XVII - Tidal Synchronization and Circularization in Tight Stellar Binaries (external link)