Max Planck researchers refute the claim that Earth is periodically hit by asteroids or comets. Nemesis is a hypothetical hard-to-detect red dwarf star, white dwarf star or brown dwarf, orbiting the Sun at a distance of about 50,000 to 100,000 AU (about 0.8-1.5 light-years), somewhat beyond the Oort cloud. Nemesis is theorized to disturb the Oort cloud and periodically cast dangerous comets towards the inner solar system and Earth.
Danger looms from out of space: asteroids and comets are a threat to our planet. The history of Earth has always been punctuated by cosmic catastrophes. Several studies have claimed to have found periodic variations, with the probability of giant impacts increasing and decreasing in a regular pattern.
Image: Wikipedia
This Nemesis star was originally postulated to exist as part of a hypothesis to explain a perceived cycle of mass extinctions in the geological record, which seem to occur once every 26 million years or so. In addition, observations by astronomers of the sharp edges of Oort clouds, similar to that of the Solar System, around various binary (double) star systems, in contrast to the diffuse edges of the Oort clouds around single-star systems, has prompted some scientists to postulate that a dwarf star may be co-orbiting the Sun.
Counter-theories also exist that other forces (like the angular effect of the galactic gravity plane) may be the cause of the sharp-edged Oort cloud pattern around the Sun. To date the issue remains unsettled in the scientific community.
Now a new analysis by Coryn Bailer-Jones from the Max Planck Institute for Astronomy (MPIA) shows those simple periodic patterns to be statistical artifacts. His results indicate either that the Earth is as likely to suffer a major impact now as it was in the past, or that there has been a slight increase impact rate events over the past 250 million years
Barringer Crater, also known as Meteor Crater, in Arizona. This crater was formed around 50,000 years ago by the impact of a nickel-iron meteorite. Near the top of the image, the visitors center, complete with tour buses on the parking lot, provides a sense of scale.
© National Map Seamless Viewer/US Geological Service
Giant impacts by comets or asteroids have been linked to several mass extinction events on Earth, most famously to the demise of the dinosaurs 65 million years ago. Nearly 200 identifiable craters on the Earth's surface, some of them hundreds of kilometers in diameter, bear witness to these catastrophic collisions.
Understanding the way impact rates might have varied over time is not just an academic question. It is an important ingredient when scientists estimate the risk Earth currently faces from catastrophic cosmic impacts.
Since the mid-1980s, a number of authors have claimed to have identified periodic variations in the impact rate. Using crater data, notably the age estimates for the different craters, they derive a regular pattern where, every so-and-so-many million years (values vary between 13 and 50 million years), an era with fewer impacts is followed by an era with increased impact activity, and so on.
Giant impacts by comets or asteroids have been linked to several mass extinction events on Earth, most famously to the demise of the dinosaurs 65 million years ago. Nearly 200 identifiable craters on the Earth's surface, some of them hundreds of kilometers in diameter, bear witness to these catastrophic collisions.
Understanding the way impact rates might have varied over time is not just an academic question. It is an important ingredient when scientists estimate the risk Earth currently faces from catastrophic cosmic impacts.
Since the mid-1980s, a number of authors have claimed to have identified periodic variations in the impact rate. Using crater data, notably the age estimates for the different craters, they derive a regular pattern where, every so-and-so-many million years (values vary between 13 and 50 million years), an era with fewer impacts is followed by an era with increased impact activity, and so on.
The Nördlinger Ries, or Ries, was formed when a meteor hit the area 15 million years ago. The resulting crater, roughly 20 km in diameter, has since been filled in and eroded. In this natural-colour satellite image, it can just be made out as a circular structure, much less clearly defined than the Barringer Crater, which is significantly younger.Artist's concept of Nemesis Star, a red dwarf
This Nemesis star was originally postulated to exist as part of a hypothesis to explain a perceived cycle of mass extinctions in the geological record, which seem to occur once every 26 million years or so. In addition, observations by astronomers of the sharp edges of Oort clouds, similar to that of the Solar System, around various binary (double) star systems, in contrast to the diffuse edges of the Oort clouds around single-star systems, has prompted some scientists to postulate that a dwarf star may be co-orbiting the Sun.
Counter-theories also exist that other forces (like the angular effect of the galactic gravity plane) may be the cause of the sharp-edged Oort cloud pattern around the Sun. To date the issue remains unsettled in the scientific community.
Now a new analysis by Coryn Bailer-Jones from the Max Planck Institute for Astronomy (MPIA) shows those simple periodic patterns to be statistical artifacts. His results indicate either that the Earth is as likely to suffer a major impact now as it was in the past, or that there has been a slight increase impact rate events over the past 250 million years
Barringer Crater, also known as Meteor Crater, in Arizona. This crater was formed around 50,000 years ago by the impact of a nickel-iron meteorite. Near the top of the image, the visitors center, complete with tour buses on the parking lot, provides a sense of scale.
Giant impacts by comets or asteroids have been linked to several mass extinction events on Earth, most famously to the demise of the dinosaurs 65 million years ago. Nearly 200 identifiable craters on the Earth's surface, some of them hundreds of kilometers in diameter, bear witness to these catastrophic collisions.
Understanding the way impact rates might have varied over time is not just an academic question. It is an important ingredient when scientists estimate the risk Earth currently faces from catastrophic cosmic impacts.
Since the mid-1980s, a number of authors have claimed to have identified periodic variations in the impact rate. Using crater data, notably the age estimates for the different craters, they derive a regular pattern where, every so-and-so-many million years (values vary between 13 and 50 million years), an era with fewer impacts is followed by an era with increased impact activity, and so on.
Giant impacts by comets or asteroids have been linked to several mass extinction events on Earth, most famously to the demise of the dinosaurs 65 million years ago. Nearly 200 identifiable craters on the Earth's surface, some of them hundreds of kilometers in diameter, bear witness to these catastrophic collisions.
Understanding the way impact rates might have varied over time is not just an academic question. It is an important ingredient when scientists estimate the risk Earth currently faces from catastrophic cosmic impacts.
Since the mid-1980s, a number of authors have claimed to have identified periodic variations in the impact rate. Using crater data, notably the age estimates for the different craters, they derive a regular pattern where, every so-and-so-many million years (values vary between 13 and 50 million years), an era with fewer impacts is followed by an era with increased impact activity, and so on.
One proposed mechanism for these variations is the periodic motion of our Solar System relative to the main plane of the Milky Way Galaxy. This could lead to differences in the way that the minute gravitational influence of nearby stars tugs on the objects in the Oort cloud, a giant repository of comets that forms a shell around the outer Solar System, nearly a light-year away from the Sun, leading to episodes in which more comets than usual leave the Oort cloud to make their way into the inner Solar System – and, potentially, towards a collision with the Earth.
A more spectacular proposal posits the existence of an as-yet undetected companion star to the Sun, dubbed “Nemesis”. Its highly elongated orbit, the reasoning goes, would periodically bring Nemesis closer to the Oort cloud, again triggering an increase in the number of comets setting course for Earth.
In 1984, paleontologists David Raup and Jack Sepkoski published a paper claiming that they had identified a statistical periodicity in extinction rates over the last 250 million years using various forms of time series analysis. They focused on the extinction intensity of fossil families of marine vertebrates, invertebrates, and protozoans, identifying 12 extinction events over the time period in question. The average time interval between extinction events was determined as 26 million years. At the time, two of the identified extinction events (Cretaceous-Tertiary and Late Eocene) could be shown to coincide with large impact events. Although Raup and Sepkoski could not identify the cause of their supposed periodicity, they suggested that there might be a non-terrestrial connection. The challenge to propose a mechanism was quickly addressed by several teams of astronomers.
Two teams of astronomers, Whitmire and Jackson, and Davis, Hut, and Muller, independently published similar hypotheses to explain Raup and Sepkoski's extinction periodicity in the same issue of the journal Nature. This hypothesis proposes that the Sun may have an as yet undetected companion star in a highly elliptical orbit that periodically disturbs comets in the Oort cloud, causing a large increase in the number of comets visiting the inner solar system with a consequential increase in impact events on Earth. This became known as the Nemesis (or, more colorfully, Death Star) hypothesis.
If it does exist, the exact nature of Nemesis is uncertain. Richard A. Muller suggests that the most likely object is a red dwarf with magnitude between 7 and 12,while Daniel P. Whitmire and Albert A. Jackson argue for a brown dwarf. If a red dwarf, it would undoubtedly already exist in star catalogs, but its true nature would only be detectable by measuring its parallax; due to orbiting the Sun it would have a very low proper motion and would escape detection by proper motion surveys that have found stars like the 9th magnitude Barnard's star.
The last major extinction event was about 5 million years ago, so Muller posits that Nemesis is likely 1.0 to 1.5 light-years (63,000–95,000 AU) away at present, and even has ideas of what area of the sky it might be in (supported by Yarris, 1987), near Hydra, based on a hypothetical orbit derived from original apogees of a number of atypical long-period comets that describe an orbital arc meeting the specifications of Muller's hypothesis.
If Nemesis exists, then it may be detected by the planned Pan-STARRS or LSST astronomical surveys.
In particular, if Nemesis is a red dwarf star or a brown dwarf, then the WISE mission (an infrared sky survey that covered most of our solar neighborhood in movement-verifying parallax measurements) is expected to be able to find it, if it exists.[9] Preliminary results of the WISE survey were released on 14 April 2011. The final release of analyzed results is scheduled to be released in March 2012.
Contacts and sources:
Dr. Coryn Bailer-Jones
Max Planck Institute for Astronomy
Max Planck Institute
Citation: C. A. L. Bailer-Jones Bayesian time series analysis of terrestrial impact cratering
Monthly Notices of the Royal Astronomical Society
Citation: C. A. L. Bailer-Jones Bayesian time series analysis of terrestrial impact cratering
Monthly Notices of the Royal Astronomical Society
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