Discovering Tyche

Is there an undiscovered massive object orbiting the Sun in the Oort Cloud, elusively hidden in the perpetual frigid darkness? The Oort Cloud occupies an immense region of space surrounding the Sun; from a couple of thousand AU to as far as 50000 AU from the Sun! The term AU is the acronym for Astronomical Unit, where one AU is the mean distance of the Earth from the Sun and it has a value of 149.6 million kilometers.

The Oort Cloud is estimated to contain several trillion objects larger than 1 kilometer in diameter, with each object spaced tens of millions of kilometers away from its closest neighbor! To put the size of the Oort Cloud into perspective, even the distance of Pluto from the Sun is less than 0.1 percent the distance to the edge of the Oort Cloud!

A paper by John J. Matese and Daniel P. Whitmire (2010) entitled “Persistent Evidence of a Jovian Mass Solar Companion in the Oort Cloud” describes the possibilities of a Jupiter-mass object orbiting the Sun at a distance large enough to place it within the Oort Cloud. Tyche is the name that has been suggested for this hypothetical object. The name Tyche, which means “luck” in Greek, is also the good sister of Nemesis in Greek mythology.

In this paper, the possible existence of Tyche was inferred from dynamical and statistical analysis of the orbits of comets entering the Solar System from the Oort Cloud. The gravitational perturbations from a distant Jupiter-mass object like Tyche could also explain the peculiar orbits of extended scattered disc objects such as Sedna. These objects orbit the Sun on highly elliptical orbits that take them out to hundreds of AU from the Sun. Sedna for example, has a very elongated orbit which takes it from a minimum of 76 AU from the Sun out to an incredible 976 AU from the Sun and it takes over 12 thousand years to orbit the Sun once.

Being located at such a huge distance from the Sun, the amount of insolation that Tyche gets from the Sun will be negligible. Tyche will be a gas giant world like Jupiter and it is expected to glow feebly at a temperature of about 200 Kelvin from heat emanating from its warm interior. Therefore, Tyche can only be detected in the infrared band since such a cool object is expected to emit almost no visible light. Interestingly, NASA’s recently launched Wide-field Infrared Survey Explorer (WISE) will be able to easily detect the presence of such an object in the Oort Cloud! Visit http://wise.ssl.berkeley.edu/ to find out more about WISE.

Although a positive detection of Tyche might not be much of a surprise, the discovery of such a world will be extremely fascinating. Jupiter is currently by far the most massive known object in orbit around the Sun and the discovery of something more massive than Jupiter will have interesting implication regarding our perspectives of things in orbit around the Sun. What kind of moons will orbit this object and might some of these moons be similar to the ones in orbit around Jupiter? What kind of exploratory robotic spacecraft might possibly be sent there? Additionally, since the formation mechanisms for such an object are probably be very different compared to the formation mechanisms for the planets in our Solar System, should Tyche be classified as a planet or a brown dwarf?

Quaoar-Weywot

Quaoar is the name of a Kuiper Belt object which orbits the Sun at a mean distance of 43.6 times the Earth-Sun distance. At that orbital distance, Quaoar takes 288 Earth years to go around the Sun once. Travelling at a speed of 20 kilometres per second, it will take roughly a decade to travel from the Earth to Quaoar. Additionally, Quaoar also has a moon named Weywot which orbits it with period of close to twelve and a half days. Weywot orbits Quaoar at a mean orbital distance of approximately 14500 kilometres from Quaoar.

A paper by W. C. Fraser and M. E. Brown (2010) entitled “Quaoar: a Rock in the Kuiper Belt” describes the unique properties of the Quaoar-Weywot system and some new observations of this fascinating far-flung system. Quaoar is about 900 kilometres in diameter and in comparison; Pluto has a diameter of 2300 kilometres. The orbit of Weywot around Quaoar reveals that Quaoar has a mass that is approximately 12 percent of Pluto’s. This gives Quaoar an estimated mean density of 4.2 grams per cubic centimetre which makes Quaoar one of the densest known objects in the Kuiper Belt. Additionally, Quaoar’s moon Weywot is estimated to have a diameter of 74 kilometres. A human being with a weight of 70 kilograms on the Earth’s surface will weigh less than 4 kilograms on the surface of Quaoar!

Quaoar’s unusually high density implies that it contains proportionally less icy materials than other Kuiper Belt objects and its high density is also reminiscent of objects in the main asteroid belt which are located much closer to the Sun than Quaoar. Therefore, a substantial bulk of Quaoar is probably made up of much denser rocky material instead of the less dense icy materials.

One theory which explains Quaoar’s unusually high density states that Quaoar collided with another object which stripped away most of Quaoar’s less dense icy mantle and left behind the denser rocky core. This collision event increased the mean density of Quaoar to the current observed value as a larger proportion of Quaoar’s mass is now comprised of denser rocky material.

Another theory which might explain Quaoar unusually high density states that Quaoar formed much closer to the Sun in the main asteroid belt where objects formed there typically have densities similar to the current density observed for Quaoar. Subsequently, gravitational interaction with the planets scattered Quaoar further from the Sun and into the frigid realm of the distant Kuiper Belt objects where Quaoar has been residing ever since.

Regarding the exploration of objects in the Kuiper Belt, NASA’s New Horizons spacecraft is currently on its way to Pluto and it is scheduled to make closest approach to Pluto on 14 July 2015. This will be the first ever flyby of a Kuiper Belt object. In fact on 17 October 2010, New Horizons would have travelled half the flight time to reach Pluto since its launch on 19 January 2006. After making its flyby of Pluto and its moons, Charon, Nix and Hydra, New Horizons is also scheduled to flyby one or more Kuiper Belt objects. Visit http://pluto.jhuapl.edu/ to obtain all the latest news about this mission.

True Heavyweight

In the previous two posts, I wrote about some fascinating stuff involving the application of micro black holes and I also explored an interesting alternative to conventional black holes. In this post, I am going to write about the most massive black hole currently known in the universe, even though there are probably a lot more yet-to-be-discovered black holes which can be more massive than this current record holder.

OJ 287 is a pair of supermassive black holes residing in the heart of a distant galaxy located 3.5 billion light years away, where one light year is the distance light travels in one year. The primary black hole of OJ 287 contains an incredible 18 billion times the mass of the Sun while the secondary black hole contains 150 million times the mass of the Sun. This makes the primary black hole of OJ 287 one of the most massive known black holes in the universe. To put things into perspective, the supermassive black hole in the core of our Milky Way Galaxy is a mere 4 million times the mass of our Sun and the Sun alone is already 333 thousand times more massive than the Earth.

With 18 billion times the mass of the Sun, the event horizon of the monstrous primary black hole of OJ 287 will span an astonishing 110 billion kilometres in diameter. This means that about 80 thousand Suns or 9 million Earths placed end-to-end are required to span the diameter of the black hole’s event horizon. Note that the event horizon of a black hole is a region surrounding it where gravity becomes so strong that it does not let even light to escape.

The much less massive secondary black hole of OJ 287 orbits the primary black hole with a period of 11 to 12 years. Two outbursts are observed from OJ 287 every 11 to 12 years as the secondary black hole intersects the accretion disk of the much more massive primary black hole with a frequency of twice per orbit. The orbit of the secondary black hole around the primary black hole is gradually decaying via the emission of gravitational radiation and the secondary black hole is expected to merge with the primary black hole within 10 thousand years.

Eternally Collapsing Object

Last week, I wrote about some fascinating applications of micro black holes and in my first sentence, I defined a black hole as an object that is so dense and compact that within a certain distance from it, its gravitational pull becomes so strong that it does not let even light to escape. This boundary is known as the black hole’s event horizon and anything that happens to enter it will never escape. By this definition, a black hole does not have a true physical surface and its event horizon is basically a region of space within which nothing, including light, can escape.

In this article, I investigate a kind of black hole that is very different from what was defined in the above paragraph and I will be using the phase “progenitor star” to define an object that is currently in the process of collapsing under its own immense gravity towards forming a black hole. As the progenitor star collapses, it never reaches a true black hole state, but instead becomes a General Relativistic Radiation Pressure Supported Star (GRRPSS). What a mouthful! Although this scenario is purely theoretical, its exciting properties are surely worth considering.

The name “General Relativistic Radiation Pressure Supported Star” somewhat speaks for itself. So, how does radiation pressure works? Take for example our Sun - a ferocious hot ball of hydrogen and helium with 333 thousand times the mass of the Earth, sitting in the middle of our Solar System. The enormous amount of radiation produced via nuclear fusion within the Sun’s core produces an incredible amount of radiation pressure which tries to blow the Sun apart, while gravity tries to crush the Sun inwards. It is this perfect balance between radiation pressure and gravity which gives our Sun the size that we constantly observe it to be.

As the progenitor star collapses, the gravitational field in the vicinity of the star becomes increasingly stronger as the star becomes ever more dense and compact. As the physical diameter of the progenitor star collapses and approaches the diameter of the event horizon for a black hole of its mass, radiation emitted from the surface of the progenitor star will become increasingly red-shifted. This means that the radiation emitted from the progenitor star’s surface gets stretch into ever longer wavelengths.

When the progenitor star collapses below one and a half times the diameter of the event horizon for a black hole of its mass, light emitted at or near the tangent to the star’s surface will not be able to escape into space and will eventually fall back to the surface. As the progenitor star collapses until its physical size approaches the diameter of the event horizon for a black hole of its mass, only light that is being emitted vertically upwards from the star’s surface will escape into space instead of falling back somewhere else on the star’s surface. Therefore, as the progenitor star collapses, only light that is emitted at an ever decreasing angle from the local vertical will escape into space and the self-gravitational trapping of radiation by the progenitor star becomes more and more effective.

The radiation pressure created from the self-gravitational trapping of radiation by the progenitor star prevents it from collapsing to a true black hole state. Instead, the progenitor star will continue collapsing as an incredibly hot ball of quark gluon plasma which asymptotically tends towards a true black hole state but never reaches it. In fact, such an object will appear totally dark since almost no radiation will be able to escape the super strong gravity of the star and it will appear very much like a true black hole.

It might also be true that as the collapsing General Relativistic Radiation Pressure Supported Star tends to become a true black hole, its lifetime in this phase becomes infinite. Such as object can be called an Eternally Collapsing Object (ECO) and this class of objects represents an alternative to conventional black holes.

Sources:

1. Abhas Mitra and Norman K. Glendenning (2010), “Likely Formation of General Relativistic Radiation Pressure Supported Stars or Eternally Collapsing Objects”, arXiv:1003.3518v2

2. Abhas Mitra (2006), “Radiation Pressure Supported Stars in Einstein Gravity - Eternally Collapsing Objects”, arXiv:gr-qc/0603055v3

3. Abhas Mitra (2006), “Sources of Stellar Energy, Einstein- Eddington Timescale of Gravitational Contraction and Eternally Collapsing Objects”, arXiv:astro-ph/0608178v3