Introduction

The undiscovered stellar companion of the sun Nemesis moves along a highly elliptical orbit, which would periodically disturb the Oort cloud and is thought to be responsible of the periodicity of about 26 million years in extinction rates on the Earth over the last 250 million years.^[1] It was named after the ancient Greek goddess of revenge. Astronomers Whitmire, Jackson, and Davis et al. first proposed Nemesis being a binary stellar companion of the sun in 1984, which moves with it around a common centre of mass. The existence of a Jupiter-sized brown dwarf, however, is lacking for the completeness of evidence in the solar system on the existence of Nemesis, and existing literature have still been regarding Pluto as a planet.^[2]

Scientific Evidence

The preliminary search, initiated by Percival Lowell on the Planet X, led to the discovery of Pluto on February 18, 1930 at the Lowell Observatory in Flagstaff, Arizona after his death.^[3][4] The discovery by astronomer Clyde W. Tombaugh, with contributions from William H. Pickering, was categorized according to Lowell's theoretical input, but only to be recognized for being a dwarf in 2006.^[5]

While Lowell's paradigm originated from celestial mechanics, later theorizarion formalizing the Nemesis hypothesis combined archeoogy with cosmology.^[6] Marc Davis, Piet Hut, and Richard A. Muller weighed the archeological evidence of species extinction on earth with a 26 million year periodicity, and proposed an unseen solar companion traveling in a moderately eccentric orbit, which at its closest approach (perihelion) passes through the "Oort cloud" of comets which is believed to surround the sun.^[7]

Correlative evidence were summarized by J. P. Philippov and M. I. Chobanu:^[2]

1. Approximately 50% of all main sequence stars are part of double or multiple stars systems, and current observational evidence cannot rule out the Nemesis hypothesis, with a possibility of a brown dwarf companion to the Sun with a mass of 0.0002 solar masses;
2. Palaeontologists J. Sepkoski and D. M. Raup found 12 events corresponding to sharp increases in mortality of living organisms in the last 250 million years of Earth's history, including the alleged disappearance of the dinosaurs 65 million years ago;
3. The periodicity of comets coming from the Oort cloud appears to be directly related to the periodicity in movement of an unknown massive celestial body;^[8]
4. It can only be easily explained by the existence of a massive celestial body located at the Solar System periphery that Pluto, Eris, Sedna, and other trans-Neptunian and extreme trans-Neptunian objects have very elongated orbits and large values of orbit inclinations to the plane of the ecliptic;
5. Evidence from studies on circumstellar discs of stars exhibited the possibility for a stellar protoplanetary disc occupying exceptionally wide orbit;
6. A number of massive planets may be located beyond the orbit of Pluto at distances of about 205-264, 346-416, 610-692, and 972-1,001 AU from the Sun;
7. Phenomena similar or comparable to Nemesis in other galaxies such as alpha Centauri.

The estimation on the mass of Nemesis M_N, in terms of the sun's mass M_⊙, is evaluated to be M_N ≤ 0.042M_⊙ by Bhalerao and Vahia, 2.00×10⁻⁴ ≤ M_N ≤ 7.00×10⁻²M_⊙ by Whitmire and Jackson, and 1.241×10⁻² ≤ M_N ≤ 7.00×10⁻² M_⊙ by Philippov and Chobanu, with a radius of approximately 71,492 kilometers and effective temperature between 300 and 3,000 Kelvins.^[2][9][10] Its orbital period is estimated to be between 10⁴ and 6.2×10⁷ years with a semi-major axis of 8.8×10⁴ AU and 0.9 eccentricity.^[9][10] It is also estimated that the orbital period should be exactly 2.6×10⁷ years to a heliocentric distance between 8.8×10³ and 1.672×10⁵ AU.^[2]

The Mass Extinction Scenario

With the continued explorations for Nemesis and other proposed planetary or stellar and substellar bodies, Nemesis' correlations to the mass extinction events on Earth have been challenged. Reexaminations on the fossil evidence by Adrian L. Melott and Richard K. Bambach found strong evidence for the periodicity in biodiversity to be 62 million years, with the cross-spectrum peak of extinction intensity in 99.9 million years.^[11] The work of Melott and Bambach is rigorious with geological time as compared to time proper and astronomical time, and they stated that a recent sharp peak at 26.8 million years in mass extinction expressed in terms of time proper does not justify a linear retrogression analysis of the time intervals used in the Nemesis hypothesis. Radom distribution was used in their statistical tests, indicating to the subjective bias in the original Nemesis hypothesis, and the recent maximum they have tested is at 11 million years ago.^[12]