Last update: November 5, 2025

Nemesis: The Theory of the Sun's Companion Star

Illustration of the Nemesis theory, hypothetical companion star of the Sun

Origin of the Hypothesis: Nemesis, the Missing Sister Star

The idea of a companion star to the Sun, named Nemesis, originated from the observation of mass extinction cycles that seem to occur every 26 to 30 million years. In the 1980s, researchers David Raup (1933-2015) and Jack Sepkoski (1948-1999) highlighted these cycles based on fossil records.

To explain these periodic extinctions, astronomers Richard A. Muller (1944-), Marc Davis (1947-), and Piet Hut (1952-) proposed in 1984 the existence of a faint companion star in a highly elliptical orbit around the Sun. This star, named Nemesis, could, during each close pass, disrupt the trajectories of comets in the Oort Cloud and cause a shower of projectiles toward the inner Solar System.

N.B.:
In galactic dynamics, an isolated star can result from violent gravitational interactions within its natal cluster. About 70% of massive stars are now observed in multiple systems, suggesting that the Sun's solitude is likely a secondary situation.

Research and Controversies

Despite decades of research, no direct evidence of Nemesis's existence has been found. Modern astronomical surveys like WISE (Wide-field Infrared Survey Explorer) have mapped the sky in infrared and detected no brown dwarf or faint star at the proposed distances for Nemesis. Many astronomers consider the Nemesis hypothesis unlikely for the following reasons:

Lack of detection: In sensitive infrared surveys, no object compatible with Nemesis has been observed in the data. This rules out hypotheses of relatively warm and massive brown or red dwarfs within ~1.5 light-years of the Sun.
Difficulty maintaining a stable orbit: Over billions of years in a disturbed galactic environment, encounters with other stars, especially in the galactic arm or the Sun's natal cluster, cause significant gravitational perturbations, altering the eccentricity or inclination of the companion's orbit.
Periodicity of extinctions: Major extinctions vary widely (from 51 Ma to 135 Ma), which obscures the idea of a strict cycle, and minor extinctions are irregular and random.
Periodic comet showers: Perturbations of the Oort Cloud, which send comets toward the inner Solar System, do not necessarily require a companion star to occur. The Milky Way's tidal force on the Oort Cloud or the passage of nearby stars is sufficient to cause disturbances.

N.B.:
The Oort Cloud is a "theoretical" region located between 2,000 and 100,000 astronomical units from the Sun, containing billions of extremely small and dark cometary nuclei. Its existence is inferred from the orbits of a few long-period comets that arrive from all possible directions, forming a sphere around the Solar System.

The Idea of a Binary Past for the Sun Retains Astrophysical Interest

Recent studies on stellar formation show that more than 60% of Sun-like stars are born in binary or multiple systems. The work of Steven Stahler (University of California, 2017) suggests that the Sun very likely had a twin star at its birth, separated by gravitational interactions within its original cluster.

This companion "Nemesis," if it existed, could have been ejected from the early Solar System. Indeed, stars rarely form in isolation but within dense star clusters. Close gravitational interactions between young stars in these chaotic environments are frequent and can easily separate newly formed binary pairs.

Thus, the Sun very likely had one or more companion stars in its youth, but these would have been torn away by gravitational perturbations. The hypothetical Nemesis, if it existed, would today orbit independently around the galactic center, light-years away from us, or have been captured by another stellar system. This possibility makes it even more difficult to search for such an object today.