Scholz's Star: When a Red Dwarf Grazed the Sun 70,000 Years Ago

A Discovery Born from a Motion Anomaly

A Dwarf Spotted in WISE Infrared Data

WISE 0720-0846 was spotted in 2013 by Ralf-Dieter Scholz, who identified it in data from the WISE (wide-field Infrared Survey Explorer) infrared satellite as a candidate late-type dwarf at a short distance from the Sun. The system had escaped previous surveys due to its low galactic latitude and its modest proper motion for such a nearby star. Complementary observations, notably those of Adam Burgasser and his collaborators published in 2015, confirmed its binary nature: a red dwarf of spectral type M9.5 and a brown dwarf of type T5, separated by about 0.8 astronomical units, with an orbital period of two to four years.

A Velocity that Intrigued Eric Mamajek

It was this slowness of apparent motion, combined with a high and outward-directed radial velocity, that intrigued Eric Mamajek. Two hypotheses were open to the team: either the star was heading for a future close passage to the Solar System, or it had just left us after a previous passage. Radial velocity measurements decided in favor of the second option: the system is moving away from the Sun, which made it possible to reconstruct its past trajectory.

A Passage Within One Light-Year of the Sun

A Trajectory Reconstructed by Simulation

In their study published in 2015 in The Astrophysical Journal Letters, Mamajek and his collaborators integrated the galactic orbit of the binary system (about 0.15 solar masses in total) and that of the Sun. The result: 70,000 years ago, with an uncertainty margin of plus or minus 15,000 years, Scholz's star would have passed 52,000 astronomical units from the Sun, about 0.25 parsecs or 0.8 light-years. Out of 10,000 trajectory simulations accounting for measurement uncertainties, 98% had the system passing within the outer Oort cloud.

A Dynamically Weak Encounter

At its closest point, the duo would have displayed an apparent magnitude of about 11.4, too faint to be seen with the naked eye, but sufficient to be detected by modern instruments. The authors point out, however, that due to its low mass and relatively high speed, the encounter was dynamically weak: it probably did not trigger a massive comet shower towards the inner Solar System.

Estimates Revised with Gaia Data

Towards a Slightly Older Date

Position and velocity measurements have been refined since 2015, notably thanks to the Gaia satellite. A subsequent reassessment of the trajectory, based on astrometric data and an updated Solar System model, slightly shifted the date and distance of the passage: the median is now around 80,000 years, for a minimum distance of about 0.33 parsecs, or about 1.1 light-years. These revisions remain compatible with the initial estimate within the uncertainty margins and confirm that the system indeed crossed the fringes of the Oort cloud.

This type of recalculation is common in astrometry: each new generation of data reduces the error bars without undermining the central conclusion. Scholz's star remains, to this day, the best-constrained stellar flyby ever measured, in both distance and speed.

A Trace in the Orbits of Comets?

A Study on 340 Hyperbolic Orbits

In 2018, Spanish astronomers Carlos and Raúl de la Fuente Marcos, with researcher Sverre Aarseth, examined the hyperbolic orbits of nearly 340 small bodies in the Solar System, i.e., objects following trajectories so open that they will permanently leave the solar neighborhood. By integrating these orbits into the past over 100,000 years, the team identified concentrations of radiant points in the sky compatible with a gravitational perturbation caused by the passage of Scholz's star.

A Real Effect, But No Immediate Consequence

This study, published in Monthly Notices of the Royal Astronomical Society Letters, suggests that the impact of the flyby was more significant than initially estimated. Comets perturbed in this way from the Oort cloud would, however, take about two million years to reach the inner Solar System, which rules out any link with recent geological or climatic events on Earth.

Table: Characteristics of the System and the Stellar Encounter

A Rare but Not Exceptional Phenomenon

A Frequency Estimated by Galactic Models

A stellar flyby as close as the Oort cloud is not unique on a cosmic scale. Models indicate that a star passes through the Oort cloud on average about every 100,000 years, while an approach as close as that of Scholz's star (52,000 astronomical units or less) would only occur about every nine million years. These events are part of the normal dynamics of the solar neighborhood within the Milky Way.

Unprecedented Trajectory Precision

A well-documented precedent already existed before Scholz: the idea that stellar passages could disturb the Oort cloud dates back to the work of Jan Oort himself on the structure of the cloud that bears his name. What distinguishes Scholz's star is the unprecedented precision with which its past trajectory was reconstructed, thanks to exceptionally well-constrained radial and transverse velocity measurements for such a faint star.