Rogue planets
A rogue planet — also known as an interstellar planet, nomad planet, free-floating planet or orphan planet — is a planetary-mass object which has either been ejected from its system or was never gravitationally bound to any star, brown dwarf or other such object, and that therefore orbits the galaxy directly.Astronomers believe that either way, the definition of planet should depend on current observable state and not origin.
Larger planetary-mass objects which were not ejected, but have always been free-floating, are thought to have formed in a similar way to stars, and the IAU has proposed that those objects be called sub-brown dwarfs(an example of this is Cha 110913-773444, which may be an ejected rogue planet, or it may have formed on its own and be a sub-brown dwarf).The closest to earth yet discovered is around 100 light years away.
Observation
When a planetary-sized object passes in front of a background star, its gravitational field causes a momentary increase in the visible brightness of the background star. This is known as microlensing. Astrophysicist Takahiro Sumi of Osaka University in Japan and colleagues, who form the Microlensing Observations in Astrophysics (MOA) and the Optical Gravitational Lensing Experiment (OGLE) collaborations, carried out a study of microlensing which they published in 2011. They observed 50 million stars in our galaxy using the 1.8 meter MOA-II telescope at New Zealand's Mount John Observatory and the 1.3 meter University of Warsaw telescope at Chile's Las Campanas Observatory. They found 474 incidents of microlensing, just 10 of which were brief enough to be planets of around Jupiter's size with no associated star in the immediate vicinity. The researchers estimated from their observations that there are nearly two free-floaters for every star in our galaxy. Other estimations suggest a much larger number, up to 100,000 times more free-floating planets than stars in our Milky Way.
Retention of heat in interstellar space
In 1998, David J. Stevenson theorizedthat some planet-sized objects drift in the vast expanses of cold interstellar space and could possibly sustain a thick atmosphere which would not freeze out due to radiative heat loss. He proposes that atmospheres are preserved by the pressure-induced far-infrared radiation opacity of a thick hydrogen-containing atmosphere.
It is thought that during planetary-system formation, several small protoplanetary bodies may be ejected from the forming system. With the reduced ultraviolet light associated with its increasing distance from the parent star, the planet's predominantly hydrogen- and helium-containing atmosphere would be easily confined even by an Earth-sized body's gravity.
It is calculated that for an Earth-sized object at a kilobar hydrogen atmospheric pressures in which a convective gas adiabat has formed, geothermal energy from residual core radioisotope decay will be sufficient to heat the surface to temperatures above the melting point of water.Thus, it is proposed that interstellar planetary bodies with extensive liquid-water oceans may exist. It is further suggested that these planets are likely to remain geologically active for long periods, providing a geodynamo-created protective magnetosphere and possible sea floor volcanism which could provide an energy source for life.The author admits these bodies will be difficult to detect due to the intrinsically weak thermal microwave radiation emissions emanating from the lower reaches of the atmosphere, although later research suggests that reflected solar radiation and far-IR thermal emissions may be detected if one were to pass within 1000 AU of Earth.
A study of simulated planet ejection scenarios has suggested that around five percent of Earth-sized planets with Moon-sized natural satellites would retain their satellites after ejection. A large satellite would be a source of significant geological tidal heating.[14]
Proplyds of planetars
Recently, it has been discovered that some extrasolar planets such as the planemo 2M1207b, orbiting the brown dwarf 2M1207, have debris discs. If some large interstellar objects are considered stars (sub-brown dwarfs), then the debris could coalesce into planets, meaning the disks are proplyds. If these are considered planets, then the debris would coalesce as satellites. The term planetar exists for those accretion masses that seem to fall between stars and planets.
Known or possible rogue planets
There is no current way of telling whether these are planets that have been ejected from orbiting a star or were originally formed on their own as sub-brown dwarfs.
Sub-Brown Dwarf
A sub-brown dwarf is an astronomical object formed in the same manner as stars and brown dwarfs (i.e. through the collapse of a gas cloud) but that has a mass below the limiting mass for thermonuclear fusion of deuterium (about 13 Jupiter masses) and that hence is not a brown dwarf.
Failed brown dwarfs
Sub-brown dwarfs are formed in the manner of stars, through the collapse of a gas cloud (perhaps with the help of photo-erosion) but there is no consensus amongst astronomers on whether the formation process should be taken into account when classifying an object as a planet.Free-floating sub-brown dwarfs can be observationally indistinguishable from rogue planets that originally formed around a star and were ejected from orbit, and on the other hand a sub-brown dwarf formed free-floating in a star cluster may get captured into orbit around a star.
Lower mass limit
The smallest mass of gas cloud that could collapse to form a sub-brown dwarf is about 1 MJ.This is because to collapse by gravitational contraction requires radiating away energy as heat and this is limited by the opacity of the gas. A 3 MJ candidate is described in the paper Dusty Disks at Bottom of IMF.
List of possible sub-brown dwarfs
Orbiting a brown-dwarf
There is no consensus whether these companions of brown dwarfs should be considered sub-brown dwarfs or planets.
- The 5–10MJ companion of 2MASS J04414489+2301513
- 2M1207b
Free-floating
Ocean Planet
An ocean planet (also termed a waterworld) is a type of planet whose surface is completely covered with an ocean of water.
Planetary objects that form in the outer Solar System begin as a comet-like mixture of roughly half water and half rock by mass. Simulations of Solar System formation have shown that planets are likely to migrate inward or outward as they form, presenting the possibility that icy planets could move to orbits where their ice melts into liquid form, turning them into ocean planets. This possibility was first discussed in the professional astronomical literature by Marc Kuchner and Alain Légerin 2003. Such planets could therefore theoretically support life that would be aquatic.
The oceans on such planets would be hundreds of kilometers deep, much deeper than the oceans of Earth. The immense pressures in the lower regions of these oceans could lead to the formation of a mantle of exotic forms of ice. This ice would not necessarily be as cold as conventional ice. If the planet is close enough to its sun that the water's temperature reaches the boiling point, the water will become supercritical and lack a well-defined surface. Even on cooler water-dominated planets, the atmosphere can be much thicker than that of Earth, and composed largely of water vapor, producing a very strong greenhouse effect.
The extrasolar
planet GJ
1214 b is the most likely known candidate for an ocean planet. Many
more such objects are expected to be discovered by the ongoing Kepler
spacecraft mission, such as the recently discovered ocean planet
candidate Kepler-22b.
Smaller ocean planets would have less dense atmospheres and lower gravity; thus, liquid could evaporate much more easily than on more massive ocean planets. Theoretically, such planets could have higher waves than their more massive counterparts due to their lower gravity.
Other types of ocean
Oceans, seas, lakes, etc., can be composed of liquids other than water: e.g. the hydrocarbon lakes on Titan. The possibility of seas of nitrogen on Triton was also considered but ruled out. Underneath the thick atmospheres of Uranus and Neptune it is expected that these planets are composed of oceans of hot high-density fluid mixtures of water, ammonia and other volatiles. The gaseous outer layers of Jupiter and Saturn transition smoothly into oceans of liquid hydrogen. There is evidence that the icy surfaces of the moons Ganymede, Callisto, Europa, Titan and Enceladus are shells floating on oceans of very dense liquid water or water-ammonia. Our own planet Earth is often called the ocean planet since it is 70% covered in water. The atmosphere of Venus is 96.5% carbon dioxide and at the surface the pressure makes the CO2 a supercritical fluid. Extrasolar terrestrial planets that are extremely close to their parent star will be tidally locked and so one half of the planet will be a magma ocean. It is also possible that terrestrial planets had magma oceans at some point during their formation as a result of giant impacts. Where there are suitable temperatures and pressures, volatile chemicals which might exist as liquids in abundant quantities on planets include ammonia, argon, carbon disulfide, ethane, hydrazine, hydrogen, hydrogen cyanide, hydrogen sulfide, methane, neon, nitrogen, nitric oxide, phosphine, silane, sulfuric acid, and water. Hot Neptunes close to their star could lose their atmospheres via hydrodynamic escape, leaving behind their cores with various liquids on the surface.
Terrestrial planets will acquire water during their accretion, some of which will be buried in the magma ocean but most of it will go into a steam atmosphere, and when the atmosphere cools it will collapse on to the surface forming an ocean. There will also be outgassing of water from the mantle as the magma solidifies - this will happen even for planets with a low percentage of their mass composed of water, so "super-Earth exoplanets may be expected to commonly produce water oceans within tens to hundreds of millions of years of their last major accretionary impact."