Nemesis (star)

from Wikipedia, the free encyclopedia

Nemesis is the name of a hypothetical companion of the sun who, as a brown dwarf or dwarf star, is said to orbit the sun at a distance of about one to three light years . The hypothesis is intended to provide an explanation for a possible periodicity of comet impacts and species extinctions on Earth.

With the data of sky surveys available in the 21st century , the existence of such a celestial body within the observation horizon can be largely excluded. However, recent findings suggest that sun-like stars usually form in pairs. Single stars like the sun would be the result of broken binary systems . A possible companion star like Nemesis could then have left the system so far that it could no longer be assigned to the original system.

The name goes back to Nemesis , the goddess of righteous anger and retribution in Greek mythology .

Origin of the hypothesis

Nemesis was postulated , among others, by the physicist Richard A. Muller, who is particularly known for his popular science books . This was suggested by Walter Alvarez , the founder of the hypothesis that the dinosaurs became extinct as a result of a comet impact on earth . Since the meteorite craters on earth show a possibly matching age classification, Alvarez at least includes this hypothesis in his considerations.

In 1984 David M. Raup and J. John Sepkoski investigated the earlier extinctions of species on Earth and arranged them in time. In doing so, they came to the conclusion that there were regular mass extinctions with intervals of between 26 and 33 million years. A period of around 27 million years serves as the starting point for most theoretical calculations. More comet impacts occurred at approximately the same time interval, so that a connection between these two events was suspected. In the period that followed, a cause for the periodically frequent cometary impacts was sought. An explanation for this is provided by a possibly existing companion of the sun, which crosses the Oort cloud at regular intervals and changes the orbits of the comets there with its gravitational field. Such comets then move into the inner areas of the solar system, where, due to the increased number of comets, there are statistically more frequent impacts on planets . This hypothetical companion of the sun was called Nemesis.

However, these results are in principle questioned, since with a measurement error of ten percent the period length in question is within the measurement error after 320 million years and, on the other hand, there is not the slightest evidence of this star that would be supported by verifiable direct observations.

Nemesis as a brown dwarf or dwarf star

If a celestial body is responsible for the periodically increased number of comets, this has physical parameters. Since the appearance of the theory, attempts have been made to narrow it down as much as possible with the help of known physical laws and existing observations .

Davis, Hut and Muller describe the hypothetical companion of the sun as a brown dwarf in an orbit with average eccentricity and its perihelion in the Oort cloud. This is disturbed by Nemesis each time it passes through, as a result of which more than 10 9 comets are diverted into orbits that lead them into the inner solar system , where about ten to 200 of them hit the earth .

The assumption that the increased impacts on earth were caused by comets captured by the sun was rejected because it is extremely difficult to arrive at stable periods with this model. In addition, the analysis of impact craters suggests objects originating in the solar system.

The Jackson and Whitmire model is essentially the same as the above. However, they receive an eccentricity e ≥ 0.9, which was even more critical than in the case of Davis, Hut and Muller. With increasing eccentricity, elliptical orbits require better and better coordination of the angular momentum with the orbit, since otherwise they are not stable.

According to the numerical investigations carried out by Weinberg, Shapiro and Wasserman , such weakly coupled binary star systems with the required properties ( semi-major axis ~ 10 5 astronomical units and 27 million years orbital period) have a low but not negligible probability . These simulations take into account the disturbances caused by passing stars and interstellar gas clouds as well as the structure of the gas clouds and the galactic tidal forces .

Varum Bhalerao and MN Vahia have addressed the question of the greatest possible mass of Nemesis. Based on a connection between the frequent comet impacts on earth and an object that causes a disturbance in the Oort cloud, this object was assigned an orbital period of 27 million years.

Lagrange points

The cause of the disturbances that occur, which lead to an increased number of comets, is assumed to be that the orbits of these small bodies are disturbed when they come near the first Lagrange point , which lies between the sun and the nemesis. Based on this, the luminosity of the star should be determined. In addition, it is assumed that Nemesis is about as old as the sun, which means that much heavier stars can be excluded, as these have a significantly shorter lifespan and would have been discovered due to their higher luminosity. Neutron stars and black holes can also be excluded because they have a relatively high luminosity due to accretion .

On the basis of these assumptions, numerical simulations were carried out in order to further limit the parameters of Nemesis. The resulting curves of luminosity as a function of mass were compared with previous observations. Since the Tycho-2 catalog up to m = 11.0 and the Guide Star Catalog II up to J = 19.5 are complete, it can be assumed that Nemesis has a lower luminosity, as it has not yet been observed. This leads to an upper limit mass, which is around 44 Jupiter masses. The influence of the assumed period on this value is very small. However, a strongly elliptical orbit, the perihelion of which is close to the Sun, could still cause a deviation of a maximum of 47 Jupiter masses, since the luminosity would decrease again by 0.045 L ☉ if the star is in aphelion.

ER Harrison studied pulsars and their period of rotation. This decreases with increasing age of the pulsar. In doing so, he came across a group whose period is decreasing unusually slowly. One explanation for this would be a companion of the sun, which accelerates the center of gravity of the solar system. Harrison suspected it could be a white , red, or even a "black dwarf," now known as a brown dwarf .

Counter arguments

Henrichs and Staller looked at Harrison's hypothesis and found various arguments that refuted it. For one thing, such an object would disrupt the movement of the planets. Longitudinal deviations in the orbit of Neptune, for example, cannot be explained with this theory and even speak against it. On the other hand, a brown dwarf can also be excluded because, if it exists, it would have about the luminosity of Betelgeuse in the infrared range and would therefore have already been discovered (especially when the entire sky was surveyed by the WISE mission launched by NASA in 2009 with their satellite-based IR telescope ). The same applies to a red and a white dwarf.

Nemesis might have escaped these observations if she were a black hole or a neutron star . However, all three scientists consider this to be very unlikely.

Recent publications question the explanation of a 27 million year periodicity in extinction rates by a nemesis-like celestial body, among other things because the orbit of a celestial body this far from the Sun is too subject to disturbance from neighboring stars and galactic tidal effects is to have a sufficiently constant orbital period over 500 million years. The actual existence of the extinction data periodicity itself is also called into question, which is merely a statistical artifact. Both of these arguments separate the nemesis hypothesis from its foundation.

Alternative theory

According to Victor Clube ( University of Oxford ) and William Napier ( University of Cardiff ), Davis, Hut and Muller's model is unstable in environments dominated by interstellar gas clouds. If one takes into account the gravitational disturbances of the highly elliptical Nemesis orbit caused by the gas clouds, the major semiaxis shortens to 100 astronomical units after just a few orbits . Furthermore, the model is out of phase and an additional periodicity of around 250 million years is known.

Clube and Napier therefore start from the assumption that the solar system moves periodically through a layer of extrasolar objects. This layer of debris lies in the plane of the Milky Way . This assumption gives a period on the right scale, about 35 million years.

Lisa Randall argues in a book that such a layered mass accumulation on the galactic plane is caused by collapsed dark matter .

Davis, Hut and Muller criticize a phase shift in this hypothesis : the sun is currently close to the galactic plane, but the last extinction was about eleven million years, i.e. only half a period.

See also

Individual evidence

  1. ^ Robert Sanders: New evidence that all stars are born in pairs. In: Berkeley News. University of California Berkeley, June 13, 2017, accessed on October 27, 2017 (English, "sunlike stars are not primordial [...] They are the result of the breakup of binaries [...] We are saying, yes, there probably was a Nemesis, a long time ago " ).
  2. ↑ Walter Alvarez, Richard A. Muller, Nature April 1984, pp. 718 ff
  3. ↑ David M. Raup, J. John Sepkoski Jr .: Periodicity of extinctions in the geologic past. (PDF) In: Proc. Natl. Acad. Sci., USA, vol. 81, pp. 801-805, Feb. 1984. October 11, 1983, accessed April 7, 2016 (English).
  4. ^ Marc Davis, Piet Hut, Richard A. Muller, Nature, April 1984, pp. 715 ff.
  5. ^ Marc Davis, Piet Hut, Richard A. Muller, Nature February 1985, p. 503
  6. ^ Daniel P. Whitmire, Albert A. Jackson IV, Nature, Apr. 1984, p. 713
  7. ↑ Martin D. Weinberg, Stuart L. Shapiro, Ira Wasserman, 1986 Icarus 65, page 27 ff.
  8. ↑ Martin D. Weinberg, Stuart L. Shapiro, Ira Wasserman, The Astrophysical Journal 312, January 1987, p. 367 ff.
  9. ↑ Varum Bhalerao, MN Vahia, 2005 Bull. Astr. Soc. India 33, page 27 ff.
  10. ^ ER Harrison, Nature, November 1977, page 324 ff.
  11. ↑ HF Henrichs, RFA Staller, Nature, May 1978, page 132 ff.
  12. ↑ Adrian L. Melott, Richard K. Bambach: Nemesis Reconsidered . In: Monthly Notices of the Royal Astronomical Society Letters . 407, 2010, pp. L99-L102. arxiv : 1007.0437 . bibcode : 2010MNRAS.407L..99M . doi : 10.1111 / j.1745-3933.2010.00913.x .
  13. ↑ Nemesis is a myth. Max Planck Society, August 1, 2011, accessed April 5, 2016 .
  14. ↑ CAL Bailer-Jones, F. Feng: Evidence for periodicities in the extinction record? Response to Melott & Bambach . July 7, 2013, arxiv : 1307.4266 .
  15. ↑ Did the solar system 'bounce' finish the dinosaurs? Cardiff University, May 2, 2008, accessed November 2, 2017 (press release). Original article : JT Wickramasinghe and WM Napier: Impact cratering and the Oort Cloud . In: Monthly Notices of the Royal Astronomical Society . tape 387 , no. 1 , June 11, 2008, doi : 10.1111 / j.1365-2966.2008.13098.x (Open Access).
  16. ↑ Lisa Randall, Dark Matter and the Dinosaurs: The Astounding Interconnectedness of the Universe, Ecco 2015
  17. ↑ Nicola Davis, Dark matter and dinosaurs: meet Lisa Randall, America's superstar scientist , The Guardian, Jan. 12, 2016