Friday, July 08, 2011

The Reference: Tyche

Tyche,” the title of a song appearing on Dronolith, a surprisingly progressive and dynamic drone album from Blackwolfgoat, is an astronomical reference, as well as being a minor goddess in the Greek pantheon representing luck and fortune. In 1999, planetary astronomers John Matese and Daniel Whitmire proposed the existence of a large planet in the far reaches of the Kuiper Belt, or perhaps the inner region of the Oort Cloud, in order to explain the orbits of some long period comets. The astronomers have named this hypothetical planet Tyche, which may be as much as four times the mass of Jupiter.

After the Sun, the Solar System can be thought of as being divided into distinct regions. The inner planets from Mercury to the asteroid belt (composed of metals and silicates), the hydrogen gas giants Jupiter and Saturn, the “ice giants” (colder gases, such as methane and ammonia) Uranus and Neptune, and then the icy debris left over from the formation of the Solar System. The icy debris is separated into two regions, the Kuiper Belt and the Oort Cloud.

The Kuiper Belt is named after planetary scientist Gerard Kuiper, who proposed the existence of the region in the 1950s. The Kuiper Belt is a disk shaped field of debris within the planetary plane of the Solar System (the ecliptic) extending from about the orbit of Neptune, about 30 AU from the Sun, out to a distance of about 55 AU or so (an AU, or Astronomical Unit, is defined as the mean distance from the Earth to the Sun).

The Kuiper Belt’s existence was originally inferred by examining the orbits of short period comets. Short period comets are comets characterized by very eccentric, elliptical orbits that occur within, or close to, the plane of the ecliptic. The orbital periods of such comets are 200 years or less. Because all of these comets reside in the same orbital plane as the planets, and have similar, relatively short periods, they come from the same region; hence, the inferred existence of a disk of debris relatively close to the Sun’s family of planets. The most famous short period comet is Comet Halley.

The most famous member of the Kuiper Belt is Pluto (actually, a double planetoid consisting of Pluto and a companion, Charon, which is about one third the mass of Pluto). Objects of similar size in the Kuiper Belt are referred to as “dwarf planets,” and the smaller pieces of debris are sometimes called iceteroids.

Many sizable Kuiper Belt Objects (KBOs) have been discovered within the last 20 years or so with deep telescope surveys. These dwarf planets are named after denizens of the underworld in various mythologies, such as Quaoar and Eris (which is actually bigger than Pluto).

The principal observational astronomer responsible for these discoveries is David Jewitt. Most of his research has focused on one area of the Kuiper Belt, so a complete survey of this vast region is nowhere near finished, even though more than 1000 KBOs have been discovered. Calculations estimate that there may be as many as 70,000 KBOs with diameters exceeding 100 km. Here is an artist's rendition of the largest discovered thus far…

As large as the Kuiper Belt may be, the spherically shaped Oort Cloud is much larger, extending out to a distance of about 50,000 AU from the Sun, which is nearly a light year. The Oort Cloud, named after Dutch planetary scientist Jan Oort, is a much larger field of debris that gradually extends from the far reaches of the Kuiper Belt into a sphere centered on the Sun.

The existence of the Oort Cloud is inferred from a number of different observations. First, the Cloud is the source of long period comets. Long period comets differ from their short period brethren in a number of ways. These objects come into the inner Solar System on very nearly parabolic orbits. This means that the orbital period of such objects is measured in terms of thousands of years, at least. Two famous recent examples are Comets Hale- Bopp (perihelion, 1997), and McNaught (perihelion, 2006). Comet Hale- Bopp has an orbital period of a few thousand years, where McNaught is truly an Oort Cloud object, with a period of nearly 100,000 years.

Comet Hale- Bopp

Comet McNaught

Second, these objects are quite large. For example, the nucleus of Comet Hale-Bopp is in the neighborhood of 60 km across. That is gigantic compared to the nuclei of short period comets. The reason for this discrepancy is that, simply, short period comets have been eroded by repeated passes around the Sun. Some long period comets are probably in the inner system for the first time or pass very infrequently; hence, they’ve been essentially untouched since the Solar System’s condensation from the Solar Nebula 4.6 billion years ago.

Third, long period comets enter the inner Solar System from no preferential direction; therefore, the Oort Cloud is spherically shaped, rather than a disk lining up with the plane of the ecliptic like the Kuiper Belt. The fact that the long period comets have such long periods indicates that the Oort Cloud is truly vast.

Detailed observations with today’s very powerful ground-based and orbital-based telescopes provide photographic proof of the existence of Kuiper Belts and Oort Clouds around mature stars, and, in particular, newly formed stars with forming planetary systems.

Here are a few famous examples. First, a composite of a number of forming stars deep within the Orion Nebula, photographed by the Hubble Space Telescope. The nascent stars are surrounded by protoplanetary disks. After planet formation has been completed, some of the leftover, distant debris will become Kuiper Belts and Oort Clouds.

This is a masked HST photo of the nearby mature star Fomalhaut. Clearly visible is a huge dust ring, a Kuiper Belt analog along with one of the first photographs of a planet orbiting a star other than the Sun.

So far, only four objects in the Sun’s Oort Cloud have been directly observed. The most famous of which is called Sedna, which is currently, by chance, close to perihelion. The orbital period of Sedna is about 10,000 years, and Sedna is probably an inner Oort Cloud object. The estimated number of objects in the Oort Cloud could be upwards of a trillion, covering an extremely large volume.

The proposed existence of Tyche comes from a complicated interplay of orbital dynamics. Data suggests that the number of long period comets coming in from the Oort Cloud slightly favors one direction over another, suggesting gravitational perturbations from a previously unknown massive object; hence, Tyche. From what I’ve read, however, the sample size of the cometary orbital data is small and is, therefore, statistically suspect.

Gravitational perturbation theory
, a complicated effect of gravitational dynamics, however, does explain why objects from both the Kuiper Belt and the Oort Cloud are kicked into the inner Solar System, so the idea is not without precedent.

A similar idea was proposed in the 1980s, called Nemesis, which sought to explain perceived regularities in mass extinctions on Earth. The Nemesis hypothesis proposed the existence of a red dwarf companion to the Sun on a very long period orbit to explain the mass extinction frequency in the fossil record. Hypothetically, when Nemesis is close to the Sun, the star would gravitationally perturb iceteroids in the Oort Cloud, sending them into the inner system, heightening the chances of impacts on Earth. That idea has been ruled out for a number of reasons, not the least of which is the lack of a discovery of such a companion. Isaac Asimov wrote a novel about the idea that he loosely connected to his famous Foundation sequence. Incidentally, Tyche is also sort of a tongue-in-cheek humorous name given for the proposed object, referring to the mythology of the Greek goddess Nemesis, who is the balance to Tyche’s fortune.

If Tyche exists, the recently completed survey mission WISE, a space-based infrared telescope, is perfect for detecting such an object.

The WISE data should yield a final answer to the question when data analysis has been completed within stages over the next two years.

Kuiper Belt/ Oort Cloud references in metal (at least, that I could quickly find)…

Aurora Borealis Timeline: The Beginning and End of Everything (song title)



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