Oort cloud

The Oort cloud marks the outer boundary of the solar system, where the Sun’s gravitational influence ends, and it answers the question: where do the long-period comets come from?

Overview

Comets are believed to be dirty snowballs. “Snow” in this context means mostly water snow but also frozen carbon dioxide, frozen carbon monoxide, frozen methane, frozen methanol, and perhaps others. “Dirty” refers to bits of carbon, silicate pebbles, and other materials mixing with these ices. As a comet approaches the Sun, the ices sublime to vapor. The swiftly flowing vapors carry dust aloft, and the dust and vapor form a cloud called a coma around the cometary nucleus. The solar wind passing through the coma drags the vapors and dust out into a tail. If the comet comes close to Earth and if the tail is spectacular, it can become visible to those on the surface of Earth.

Based on their orbital periods, comets are placed in one of two classes. Long-period comets have orbits that take more than two hundred years to complete, while short-period comets have periods of two hundred years or less. The orbits of short-period comets lie more or less in the ecliptic plane, which is the plane of Earth’s orbit around the Sun. It is called the ecliptic because when the Moon is also in this plane, there might be an eclipse. The orbits of the other planets are also close to the ecliptic. Long-period comets, on the other hand, come in from all angles with respect to the ecliptic, and from this, astronomers have inferred that the source of long-period comets must be a spherical halo surrounding the solar system.

In 1950, a Dutch astronomer named Jan Hendrik Oort pondered the source of long-period comets. In examining the orbits of first-time, long-period comets, Oort found that, on average, they came in from 44,000 astronomical units (AU) from the Sun. (One AU is the distance from the Sun to Earth, about 150 million kilometers.) Comets coming into the inner solar system for the first time have more volatile material on, and just below the surface than do repeat comets, so first-time comets develop a coma farther away from the Sun and brighten more than do repeat comets. Oort proposed that there is a vast spherical cloud of comets extending 100,000 AU outward from the Sun, with a concentration at 44,000 AU. This comet cloud has since been named the Oort cloud.

Another peak in the aphelia (orbital points farthest from the Sun) of first-time comets occurs at 10,000 AU. This suggests that there is an inner Oort cloud, also called the Hills cloud (for astronomer Jack G. Hills). This cloud is a wedge-shaped doughnut centered on the ecliptic plane. It begins about 1,000–2,000 AU out, far beyond the outer edges of the Kuiper belt, and extends outward to 20,000 AU, where it shades into the spherical outer Oort cloud.

The outer limits of the Oort cloud may extend outward as far as 100,000–150,000 AU (1.6–2.4 light-years), potentially more than halfway to the nearest star (Proxima Centauri, at 4.2 light-years). The Sun’s gravity is so weak at these distances that comets there are easily affected by the gravitational pull of passing stars or by the tidal forces exerted by the Milky Way galaxy itself. The Sun orbits the center of the Milky Way, passing through the galaxy’s spiral arms perhaps four times in seven hundred million years, and may come close to massive gas clouds called nebulae. The gravitational pull from these clouds may stir up the Oort cloud of comets. Although they are generally too far apart, occasionally, the comets themselves may come close enough to interact gravitationally. The results of any stirring of the Oort cloud may be to randomize comet orbits so that they occupy a sphere, to strip some comets from the cloud and send them out into the space between stars, or to send them inward toward the Sun. Since comets from the Sun’s comet cloud can escape into interstellar space, it may be that some of the comets in the Oort cloud have drifted in from other stars. Such alien comets might be recognized by having different isotopic abundances of various elements than have home-grown comets. So far, no comets from other stars have been definitively identified.

Another possibility for stirring the Oort cloud is that there might be a large planet or a small star far from the Sun but gravitationally bound to it. Research indicates that the majority of stars are part of a binary star system, so the Sun would be in the minority if it had no companion. If such a solar companion existed and if it had a thirty-million-year orbit, it would stir up the Oort cloud every thirty million years as it passed through. As a result of such stirring, for two to three million years, the number of comets striking Earth would increase a few hundredfold. If large comets struck Earth, a series of mass extinctions would result.

Periodic comet showers might also result from the Sun sinking below and then rising above the galactic disk, which contains most of the stars, gas, and dust in the region of the Sun’s orbit about the galactic center. As the planets revolve around the Sun, they move along with the Sun as it revolves about the center of the Milky Way. When the Sun is "above" the disk, the mass of the disk pulls the Sun downward; once the Sun passes through and goes "below" the disk, gravity pulls the Sun back up toward and through it again. This cycle repeats again and again, much like the motion of a carousel horse moving up and down while the carousel carries the horse around and around. As the intensity and direction of the disk’s gravitational pull on the Oort cloud changes, the cloud will be stirred up. This oscillation of the Sun through the galactic disk may have a period of thirty million years. If the Sun lacks a companion star, and so far, there is no evidence to suggest otherwise, this will be the dominant external effect on the Oort cloud.

If either the mass extinctions on Earth or the crater age throughout geologic history were periodic, a nonterrestrial cause would be the most likely since there are no known cycles on Earth with periods of tens of millions of years. Neither crater ages nor mass extinctions display a strongly periodic timing, however. A 30- to 35-million-year cycle would match some crater ages: the Chesapeake Bay crater, the nearby Toms Canyon crater, and the Popigai crater in Siberia all date to between 35 and 36 million years ago, and the Mistastin crater in Labrador, Canada, dates to about 36.4 million years ago. It is plausible that all four of the impacting bodies that created these craters resulted from the same stirring of the Oort cloud. The Chicxulub crater in Mexico's Yucatán Peninsula is about 66 million years old and coincides with the Cretaceous–Paleogene extinction event that marked the demise of three-quarters of all species on Earth, including the dinosaurs. Going back another 30 million years, however, there is no good candidate crater dating to around 95 million years ago.

Thus, while Earth may be pummeled with comet showers from time to time, there is no conclusive evidence of a regular comet cycle as represented by impact craters on Earth. A study by Michael Rampino and Ken Caldeira, published in the Monthly Notices of the Royal Astronomical Society in late 2015, reported having found new evidence of an approximately 26-million-year cycle. However, a follow-up study by Matthias Meier and Sanna Holm-Alwmark, published in a June 2017 issue of the same journal, reported that new radiometric argon–argon (⁴⁰Ar–³⁹Ar) dating analysis—more accurate than the potassium–argon (K–Ar) dating technique that it superseded—had revised the ages of many of the impacts cited in the previous study, negating the evidence of periodicity.

Knowledge Gained

One possible Oort cloud object is 90377 Sedna, a trans-Neptunian object (TNO) discovered in 2003 and named for Sedna, the Inuit goddess of the sea. Its perihelion (orbital point closest to the Sun) is about 76.07 AU, and its aphelion (orbital point farthest from the Sun) is about 890.50 AU, near the beginning of the inner Oort cloud. It has an estimated diameter of approximately 1,800 kilometers, an orbital period of 10,624.6 years, a rotational period of 10.273 hours, and a maximum temperature of about 33 kelvins (–240 degrees Celsius, or –400 degrees Fahrenheit). Other potential inner Oort cloud objects include the TNOs 2000 CR₁₀₅, with a perihelion of 44.25 AU and an aphelion of 398.08 AU, and 2012 VP₁₁₃, with a perihelion of 80.47 AU and an aphelion of 431.17 AU.

Since it is not possible to observe any confirmed Oort cloud object directly, scientists must learn about them by studying comets, Oort cloud objects that nature has delivered to Earth's doorstep. Comets seem to be largely water ice along with carbon dioxide ice, carbon monoxide ice, methane ice, methanol ice, ammonia ice, amorphous (noncrystalline) carbon, silicates and other stony materials, sodium, carbonates, simple hydrocarbons, and clays. National Aeronautics and Space Administration (NASA)'s Deep Impact mission found the comet 9P/Tempel, also called Tempel 1, to be covered by a dust layer tens of meters deep and to have a nucleus that was 75 percent empty space, making it structurally weak. The minor constituents especially, which differ from one comet to another, imply that comets form at various distances from the Sun.

Astronomers have observed extended dust disks around many stars, including Vega (alpha Lyrae), Fomalhaut (alpha Piscis Austrini), beta Pictoris, and Ran (epsilon Eridani). They correspond with what would be expected during the formation of planets and perhaps even during the initial ejection of icy asteroids into the outer reaches of their systems. The large number of objects in the Sun’s early Kuiper Belt would have caused them to smash into each other, making the larger ones larger and smashing smaller ones to dust. Radiation pressure would then drive the dust from the system, or gravity would pull it into the star, depending on the size of the dust particles. In any case, the system will be relatively free from dust once collisions cease.

No space probes have yet reached even the inner Oort cloud. Of those spacecraft currently in transit, NASA's Voyager 1 probe, launched in 1977, will be the first, having been projected to reach the Oort cloud in about three hundred years; however, power to its instruments will be shut off in 2025, after which engineering data will only be retrievable for another ten years. NASA’s Voyager 2, New Horizons, Pioneer 10, and Pioneer 11 are all projected to eventually reach the Oort cloud, though their power sources will all be dead before they reach its outer edge. The Whipple mission, a proposal for a space-based telescope to search for objects in the Kuiper Belt and the Oort cloud by conducting an occultation survey, was selected for technological development in 2011 as part of NASA's Discovery Program and was submitted again for the next round of proposals in 2014, but it was ultimately not chosen for implementation. Although no space probes have yet to reach the Oort cloud, advances in observational technology have allowed scientists to study Oort Cloud comets from greater distances. Further, scientists continue to make discoveries regarding the Oort cloud into the twenty-first century. For example, studies in the 2020s suggest there may be a new planet hiding in the Oort Cloud, beyond the known boundaries of the solar system.

Context

When astronomers first began finding planets orbiting other stars by analyzing those stars’ light for minute Doppler shifts, they did not expect early results, but that is what they got. They found something no one imagined existed, namely “hot Jupiters,” or Jupiter-sized planets very close to their parent stars. Hot Jupiters produced a large signal that was easy to detect. The immediate problem was that there should not have been enough material available to make a Jupiter that close to the parent star. Astronomers recognized that the current understanding of planetary formation might be insufficient. An alternative was that perhaps planets migrate away from the location where they were formed. Theoreticians worked on the problem and decided that they should have included planetary migration all along. While the disk of gas, dust, and asteroids not yet incorporated into a planet is still present, planets can migrate.

Whether the planets migrate inward or outward depends on the stage of planet formation, the state of the disk near the planet, and the location of the planet in the disk. Jupiter migrated inward by an estimated 2 percent, while Saturn, Uranus, and Neptune migrated outward by an estimated 10 percent, 15 percent, and 30 percent, respectively. During the tens of millions of years of migration, these planets all flung icy asteroids inward and outward. Jupiter’s strong gravity tossed them out into the Oort cloud, while Saturn, Uranus, and especially Neptune launched them into the Kuiper Belt and perhaps also into the inner Oort cloud. This explains conditions in Earth's solar system and is consistent with what is known of extrasolar planetary systems and with observed dust disks around other stars.

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