Nemesis is a possible red dwarf or brown dwarf star that was first suggested in 1984. It was thought to orbit the Sun about 95,000 AU (1.5 light-years) away, beyond the Oort cloud. This idea was proposed to explain a pattern of mass extinctions in Earth's history, which seem to happen more often every 26 million years. In 2017, scientists Sarah Sadavoy and Steven Stahler suggested that the Sun might have been part of a binary star system when it formed. They believed this could mean a star like Nemesis once existed but separated from the system about four billion years ago. This would mean Nemesis could not be responsible for more recent extinction patterns.
Later theories suggest other causes for changes in the orbits of distant Solar System objects. These include the gravitational pull of nearby stars or the way the galaxy's gravity affects the outer parts of the Solar System (called the Shiva hypothesis). In 2010, scientists found evidence in fossils that supported the 26-million-year extinction cycle, but over a longer time period. However, in 2011, researchers studied the ages of Earth's craters and found no strong evidence of regular impacts, suggesting earlier results were due to small sample sizes.
The Infrared Astronomical Satellite (IRAS) and the 2MASS survey, which mapped the sky from 1997 to 2001, did not find Nemesis. More recent surveys, like the Wide-field Infrared Survey Explorer (WISE), used advanced infrared technology to search for brown dwarfs as cold as 150 K up to 10 light-years away. These surveys also failed to detect Nemesis. In 2011, David Morrison, a NASA scientist, stated that there is no strong evidence for Nemesis, as it should have been discovered in infrared sky surveys.
Claimed periodicity of mass extinctions
In 1977, Alfred G. Fischer and Michael Arthur first proposed that life in open ocean areas repeated in cycles, based on fossil evidence. They believed these cycles were caused by changes in climate and ocean currents on Earth, not by events from space.
In 1984, paleontologists David Raup and Jack Sepkoski studied fossil records of marine animals and found patterns in extinction rates over 250 million years. They identified 12 major extinction events, with an average of 26 million years between them. Two of these events matched known asteroid impacts. Although they did not explain the cause of the pattern, they suggested it might be linked to something outside Earth. Astronomers later tried to find a possible explanation.
In 2010, Melott and Bambach reviewed fossil data with more accurate dating and used a second database. They found a repeating pattern of increased extinction rates every 27 million years, dating back 500 million years, with stronger statistical support than earlier studies.
Today, scientists still debate whether these extinction cycles are real. Some recent research argues that the patterns may not be statistically significant.
Development of the Nemesis hypotheses
Two groups of scientists, Daniel P. Whitmire and Albert A. Jackson IV, and Marc Davis, Piet Hut, and Richard A. Muller, published similar ideas in the same issue of the journal Nature to explain the pattern of mass extinctions discovered by Raup and Sepkoski. Their theory suggests that the Sun might have an unseen companion star in an oval-shaped orbit that sometimes disturbs comets in the Oort cloud. This causes more comets to enter the inner Solar System, increasing the chances of comet impacts on Earth. This idea became known as the "Nemesis" or "Death Star" hypothesis.
If Nemesis exists, its exact nature is unknown. Richard A. Muller believes it is most likely a red dwarf star with a brightness level between 7 and 12. Daniel P. Whitmire and Albert A. Jackson think it might be a brown dwarf instead. If it is a red dwarf, it could already be in star catalogs, but its distance from the Sun would make it hard to detect using older methods that look for how stars move across the sky. These methods might not find it because Nemesis would move slowly compared to other stars, like Barnard's Star, which was discovered in 1916. Muller believes Nemesis will be found when surveys that measure star distances reach a brightness level of 10.
As of 2012, scientists had identified more than 1,800 brown dwarfs. Research suggests there are fewer brown dwarfs near our Solar System than previously thought. For every brown dwarf, there may be as many as six regular stars. Most stars similar to the Sun are single stars, not part of multiple-star systems. Earlier ideas suggested many stars were in groups, but this is less common than once believed.
Richard A. Muller, using the timing of a mass extinction that happened 11 million years ago, proposed that Nemesis has an orbit with a longest point about 1.5 light-years (95,000 AU) from the Sun. He suggests Nemesis is near the constellation Hydra, based on the paths of some long-period comets that match his theory. Muller's most recent paper on this topic was published in 2002. In that paper, he suggested Nemesis was moved into an oval-shaped orbit 400 million years ago by a passing star.
In 2010 and again in 2013, Melott and Bambach found evidence of a pattern showing more extinctions every 27 million years. However, because Nemesis is far from the Sun, it is likely affected by other stars passing nearby. This would change its orbit by 15–30%, making it unlikely that a clear 27-million-year pattern in extinction events is caused by Nemesis.
Orbit of Sedna
The trans-Neptunian object Sedna follows a very long and unusual oval-shaped path around the Sun, moving between 76 and 937 astronomical units (AU) from the Sun. Sedna takes about 11,400 years to complete one full orbit. Michael Brown, who discovered Sedna, wrote in an article that Sedna’s position in space was confusing. He said, “Sedna shouldn’t be there. There’s no way to put Sedna where it is. It never comes close enough to be influenced by the Sun, but it never moves far enough away to be affected by other stars.” Brown suggested that one or more stars not part of our solar system, which passed near the Sun billions of years ago, might have pulled Sedna into its current orbit. In 2004, Kenyon supported this idea after studying Sedna’s orbit and using computer simulations to test how ancient star passes might have affected it.
An alternative idea was proposed by John Matese and Daniel Whitmire. They suggested that a large gas giant planet, named Tyche (the sister of the mythological star Nemesis), might explain Sedna’s orbit. Tyche would need to be 3 to 4 times the mass of Jupiter. In 2016, Michael Brown and Konstantin Batygin studied Sedna’s orbit and the orbits of 14 other extreme trans-Neptunian objects. They proposed that a massive super-Earth, called Planet Nine, could explain their unusual paths. However, this idea is not directly connected to the Nemesis star hypothesis.
Past, current, and pending searches for Nemesis
Searching for Nemesis using infrared light is important because cooler stars appear brighter in this type of light. The University of California's Leuschner Observatory did not find Nemesis by 1986. The Infrared Astronomical Satellite (IRAS) also did not find Nemesis during the 1980s. The 2MASS survey, which operated from 1997 to 2001, did not detect a star or brown dwarf in the Solar System. If Nemesis exists, it may be found by the Pan-STARRS or LSST surveys in the future.
If Nemesis is a red dwarf or brown dwarf, the WISE mission (an infrared sky survey that mapped most of the solar neighborhood using parallax measurements) was expected to find it. WISE can detect brown dwarfs as cold as 150 kelvin up to 10 light-years away. The closer a brown dwarf is, the easier it is to detect. Preliminary results from the WISE survey were shared on April 14, 2011. The full WISE catalog was released on March 14, 2012. In 2014, WISE data showed no evidence of a Saturn-sized or larger object in the Oort cloud beyond 10,000 AU.
Studies from the 1980s suggested that a Nemesis object would have an irregular orbit due to disturbances from the galaxy and passing stars. Research by Melott and Bambach found a very regular signal, which does not match the expected irregular orbit of Nemesis. This supports the idea of regular extinction patterns but does not support the Nemesis hypothesis. It is still possible that other types of substellar objects could explain these patterns. According to a 2011 NASA news release, "recent scientific analysis no longer supports the idea that extinctions on Earth happen at regular, repeating intervals, and thus, the Nemesis hypothesis is no longer needed."