Comets are delicate and transient visitors from our Solar System’s frigid, dark twilight regions beyond the orbit of the outermost major planet, Neptune. These dazzling objects come screaming into the warm and well-lit inner Solar System, close to the brilliant fires and melting heat of our Sun, with their sparkling thrashing tails flashing as they streak across the sky. Alas, when alien stars pass too close to our Solar System, they can push these frozen objects out of their original home in the remote Oort cloud into the inner regions around our Star, and thus stellar close encounters are an important factor in determining the risk of dangerously large cosmic impactors striking our Earth–with catastrophic results. In August 2017, Dr. Coryn Bailer-Jones from the Max Planck Institute for Astronomy in Germany announced that he has used information derived from the European Space Agency’s (ESA’s) Gaia satellite to give the first systematic estimate of the rate of such close stellar encounters of the worst kind. According to the new research, every million years, up to two dozen alien stars float within a few light-years of our Sun, making for an almost-constant state of tragic perturbation–and just such a dangerous star will invade our Solar System in 1.3 million years, potentially sending comets screeching towards Earth.
The Oort cloud, named in honor of the Dutch astronomer Jan Oort (1900-1992), is a still-hypothetical cloud composed primarily of icy comet nuclei that is believed to surround our Sun at a distance of about 50,000 to 200,000 astronomical units (AU). One AU is the average Earth-Sun separation, which is 93,000,000 miles. This very distant cloud, composed of frozen comets, is thought to form an enormous shell around our entire Solar System.
Comets crashing down on our planet are among the most destructive of cosmic catastrophes. Probably the best-known disaster of this sort was the mass extinction event, that occurred approximately 66 million years ago, that hastened the demise of the dinosaurs. This mass extinction paved the way for mammals to emerge, evolve, and survive on Earth. However, it has not been definitely determined if the impactor, in this case, was a comet or an asteroid.
Catastrophic impact events that cause regional or global destruction on Earth are rare, and occur at the rate of only one every million years. In addition, monitoring systems provide a reasonably complete inventory of larger comets and asteroids–and not one is on a collision course with Earth.
Nevertheless, the threat of such a catastrophe is serious enough to require investigation. The outer limits of our Solar System, where the Oort cloud is located, is believed to host a multitude of frozen, icy comet nuclei. The gravitational nudges of wandering stars can push these comets inward towards our Sun–and some of the icy objects will migrate into the inner Solar System where Earth is located. For this reason, these objects can potentially enter a collision course with our planet. This is the reason why a scientific understanding of these stellar encounters and their properties is necessary.
Frozen Visitors From Afar
Comets are really icy planetesimals. This means that they are the ancient leftover building blocks of the quartet of giant gaseous planets inhabiting the outer Solar System: Jupiter, Saturn, Uranus, and Neptune. Many scientists think that comets hold in their frozen hearts the most pristine of primordial elements that went into the construction of our Solar System about 4.56 billion years ago. These very ancient ingredients have been preserved in a kind of “deep freeze” at the outermost fringe of our Solar System where it is both frigid and dark.
In contrast, the primordial rocky planetesimals were similar to the asteroids that dwell in our Solar System today. Asteroids, that are primarily found in the Main Asteroid Belt between Mars and Jupiter, are the relic ancient building blocks of the four inner, solid, and relatively small planets: Mercury, Venus, Earth, and Mars. Both icy and rocky planetesimals collided with one another and merged, thus creating increasingly larger and larger bodies when our Sun and its family were first forming billions of years ago.
The frozen, dusty comets wander into Earth’s warm inner kingdom from the remote Oort cloud, as well as from the Kuiper Belt and Scattered disk. The Kuiper Belt and Scattered Disk revolve around our Star beyond the orbit of Neptune, and they are the source of short-period comets, which are comets that invade the inner Solar System more frequently than every two hundred years. The much more remote Oort cloud is the distant domain of long-period comets which take at least two hundred years to fly into our Solar System’s inner kingdom. Because the Kuiper Belt is so much closer to us than the Oort cloud, short-period comets have played a more important role in our planet’s history than the long-period comets. Nevertheless, Kuiper Belt Objects (KBOs) are sufficiently distant, dim, and small to have been beyond the reach of our technology until 1992. Astronomers have not been able to observe the very remote Oort cloud that is thought to reach at least 10% of the way to the nearest star beyond our own Sun.
Every time a comet comes squealing into the inner Solar System, it loses a small amount of its mass due to sublimation of its surface ices to gas. For example, the well-known Halley’s Comet, is thought to have a lifetime of less than 100,000 years. The comets that we can see today, as they brightly streak into the sky above us, will eventually disappear as a result of their sublimation of ices to gas, only to be replaced by a fresh, new collection of comets, that will come brilliantly soaring inwards toward our Star from their frozen homes in the Oort cloud, Scattered Disk, and Kuiper Belt.
The frozen heart–or core–of a comet is called its nucleus, and it is primarily made up of ice and dust that is coated by a blanket of dark organic material. The ice itself is composed of frozen water, but there are other frozen ingredients existing as well–such as carbon dioxide, carbon monoxide, ammonia, and methane. The nucleus might also contain a small rocky core.
As the comet migrates in the direction of our Sun, the ice on the surface of its nucleus turns to gas, and forms a cloud called a coma. Radiation from the Sun pushes the particles of dust away from the coma, and this creates a flashing, thrashing, dusty tail. Charged particles from our Star change the comet’s gases into ions, thus forming an ion tail. Because the tails of comets are shaped by our Sun’s glare and the solar wind, they invariably point away from our Star.
Most comets have nuclei no larger than 10 miles–or even less. However, some comets possess comas that can be almost 1 million miles wide. Some outstanding comets have tails that are 100 million miles long.
Comets leave behind a trail of debris that can cause meteor showers on Earth. The Perseid meteor shower occurs every year between August 9 and 13 when our planet travels through the orbit of the Swift-Tuttle Comet.
We can observe some comets with the unaided eye when they come screaming inward towards our Sun. This is because their comas and tails reflect sunlight, and sometimes they are bright because of the energy they absorb from our Star. However, most comets are too dim or small to be seen without a telescope.
The Oort Cloud
In 1932 the Estonian astronomer Ernst Opik (1893-1985) proposed that long-period comets came from an orbiting cloud at the outer limits of our Solar System. The Dutch astronomer Jan Oort independently gave new life to this theory in 1950 in an effort to resolve a paradox.
Over the course of our Solar System’s history the orbits of comets have become unstable and ultimately dynamics dictate that a comet must either crash into the Sun or a planet or, alternatively, be rudely evicted from our Solar System altogether by planetary gravitational perturbations. Furthermore, their volatile composition means that, as they repeatedly migrate towards our Sun, radiation eventually boils the volatiles away until the comet either fragments or forms an insulating crust that shields it from additional outgassing.
Taking everything into account, Oort reasoned that a comet could not have formed while in its current orbit. Instead, it must have inhabited a frigid outer reservoir of comet nuclei for almost its entire existence.
Estimates have placed the outermost edge of the Oort cloud between 100,000 and 200,000 AU. The region itself can be subdivided into a spherical outer Oort cloud of 20,000 to 50,000 AU, and a torus-shaped inner Oort cloud at 2,000 to 20,000 AU. The outermost region of this vast cloud is only weakly bound gravitationally to our Star and it is the original home of the long-period comets that invade the inner Solar System. The inner Oort cloud, known as the Hills cloud, is named in honor of Dr. Jack G. Hills, a retired Laboratory Fellow of the Los Alamos National Lab (New Mexico), who proposed its existence in 1981. Models predict that the inner cloud should host tens or hundreds of times more cometary nuclei than the outer halo–and it is a possible source of new comets that resupply the thin and delicate outer cloud, as the latter’s numbers gradually diminish. The Hills cloud does, indeed, explain the continued existence of the Oort cloud over a time span of billions of years.
The Oort cloud itself is believed to be a lingering relic of the original protoplanetary accretion disc that formed around our newborn Sun. The most widely accepted theory suggests that the Oort cloud’s numerous icy inhabitants first coalesced closer to our brilliant, hot, and fiery baby Sun as part of the same process that created both the eight major planets, as well as the minor planets. However, a gravitational dance with youthful gas-giants like Jupiter hurled these objects into extremely long elliptical or parabolic orbits. Indeed, recent research conducted by NASA scientists indicates that our Sun’s sibling stars (stars that were born in the same stellar cluster as our Sun) eventually drifted apart, and went their separate ways, when they were still young. In addition, many–possibly even the majority– of icy Oort cloud denizens did not form close to our Star. Supercomputer simulations of the evolution of the Oort cloud from the birth of our Solar System to the present indicate that the cloud’s mass peaked approximately 800 million years after its formation, as the rate of accretion and collision slowed down, and depletion started to overtake supply.
Wandering Stars And Cosmic Disasters
Dr. Bailer-Jones has now published the first systematic estimate of the rate of such stellar encounters of the worst kind. The new result uses data collected from the first data release (DR 1) of the Gaia mission that combines new Gaia measurements with older measurements that were made by ESA’s Hipparcos satellite. Dr. Bailer-Jones modeled each candidate for a catastrophic stellar encounter as a swarm of virtual stars. In this way, he demonstrated how uncertainties in the orbital data will influence the derived rate of encounters.
Dr. Bailer-Jones found that over the span of a typical million years, 490 to 600 stars float past our Sun, within a distance of 16.3 light-years–or less. All of these stars would be close enough to wreak havoc, hurling comets out of the Oort cloud, and into the golden light and warmth that exists closer to our Star. Between 19 and 24 stars will pass at 3.26 light-years, or less. All of these hundreds of stars would be close enough to nudge cometary nuclei out of the Oort cloud, and into the inner Solar System. The new results add strength to theories that postulate an earlier, less systematic estimate. Indeed, according to the new research, when it comes to stellar encounters, traffic in our Solar System is rather heavy.
Looking into the distant future, Dr. Bailer-Jones found that in about 1.3 million years, a star named Gliese 710 will wander within 1.4 million miles of our Sun, and it may well send a screaming host of rampaging comets towards the inner Solar System–wreaking havoc.
The current results of this study address a period of time that extends about 5 million years into our Solar System’s past–as well as into the future. With Gaia’s next data release– DR2 scheduled for April 2018–this could be extended 25 million years in both directions. Astronomers are now planning to hunt for the stellar culprits that could have been responsible for hurling the deadly comet that contributed to the demise of the dinosaurs 65 million years ago. In order to do this, astronomers will need to know our Milky Way Galaxy and its mass distribution in greater detail than they do now–a long-term plan of the researchers involved in Gaia and related projects.