What are the rogue planets? Thinking about the planets we usually imagine globes orbiting other “suns”, or just other stars located somewhere in our galaxy. At this time (10 March 2014) we know 1771 extrasolar planets owned by about 1100 planetary systems, therefore exoplanets are not uncommon and their existence is obvious.
What is more, with every year scientists find dozens of them. The primary goal of the study and exploration of other planetary systems is finding planets similar to Earth – and capable of sustaining life in one form or another. We already have some candidates for so-called habitable worlds. But this is also accompanied by numerous discoveries of other interesting globes, including the planet made of diamond, the planet with extremely hot and melting surface, or devastated planets orbiting cold, dead stars known as neutron stars, or pulsars.
All these worlds have something in common: they circle their host stars on more or less regular, elliptic orbits. It also happens that the planet orbits two stars, belonging to a binary system, at the same time: it is permitted by the rules of celestial mechanics. Orbits of these planets are highly eccentric and may take the form of a figure-8 shaped curve or, for example, the planet can be strongly associated with only one of the stars. System like this may be also unstable and over time the planet is not attracted by both stars anymore, soon becoming a free traveler in space. The planet may also be ejected from the system due to many different events, including interaction with the gravity of another, nearby mass, or collision with a comet or asteroid which may totally change its orbit, so it is no longer associated with a parent star.
In fact, scientists know such planets. They are called rouge or interstellar planets, or (less often) nomad planet. The class includes all the celestial bodies with typical planetary masses, orbiting the Galaxy directly. While a “normal” planet orbits the star, and this star simultaneously orbits the center of mass of the galaxy itself, every rouge planet does exactly the same thing, but without a star.
Interstellar planet does not have to be formed in a binary or multiple star system. Planets may as well be ejected from the typical planetary system in which they were born. Researchers believe that there are even planets which were never gravitationally bound to any star. Moreover, some claim (example: F. Dyson) that formerly solar and now interstellar planets can exist, citing the presence of large and hard to classify objects orbiting the Sun beyond Pluto and located within the Kuiper belt. The gravitational pull is very weak at this distance, so small planets, comets or asteroids may have already left our system.
What is the most spectacular implication of the existence of nomad planets? Most of all, they are good candidates for so-called MACHO (Massive astrophysical compact halo) object. MACHO is a general name for the entire class of bodies that may explain the apparent presence of dark matter in the galaxies. They should emit little or no radiation. Dark and difficult to detect planets drifting freely in space might populate most galaxies. If so, their numbers have to be very large because dark matter seems to provide the most of all the matter in the Universe. Or there are also another numerous MACHO objects – for example black holes, neutron stars or brown dwarfs – relatively cold stars which were too tiny to initiate nuclear fusion in their cores and start to shine like the Sun.
Speaking of brown dwarfs, they are most common explanation of the presence of these rouge planets that were born alone, or without the star system. To understand why we first have to learn how such might-have-been stars form. The standard scenario of star birth involves gravitational collapse of an interstellar cloud of gas and dust. This mass contracts and heats up due to the release of its gravitational energy. After some time the core of a protostar becomes sufficiently dense to keep the thermal radiation inside the object, which results in the further increase of the central temperature and density. The pressure of released radiation slows down the contraction. If the center of the star is hot enough, thermonuclear reactions occur and the star starts to radiate and becomes a normal star of the main-sequence. If, however, the initial mass of the cloud is less than about 0.08 solar mass, its contraction is too weak to efficiently heat it, so the density and temperature in the core of the protostar are too low to ignite nuclear fusion. Protostar soon turns into a dim, cool brown dwarf which radiates mainly in infrared.
But the distinction between a big, gaseous planet and small brown dwarf is not obvious. In fact the biggest planet of the Solar System, Jupiter, is also an example of an object which contracted from a cloud of gas with the release of thermal radiation. It had too low a mass to start the fusion, but it still radiates more heat than it receives from the Sun due to its former contraction, and its radius shrinks by about 2 centimeters per year. For this reason astronomers introduced the new class of so-called sub-brown dwarfs – objects which form through the collapse of a gas cloud but with initial masses lower than the limiting mass for thermonuclear fusion of deuterium. Of course if such object exists freely in space, it is indistinguishable from a rogue planet that once formed around a star (or stars), and was later ejected from the orbit.
It is not easy to detect a rouge planet. Because of the lack of a host star scientists cannot use popular indirect methods of detecting exoplanets, such as measuring the small deviations in star’s motions, or the observation of planetary transits in front of their parent stars, visible as periodic drop of star’s total brightness. We have only two reliable methods of detecting rogue planets: gravitational microlensing and direct imaging. The second method allows us to detect young and bright planets only, while the first is more complicated but very good at finding also dimmer and smaller bodies. It is based on the phenomenon of gravitational lenses in which the visible picture of a small object passing in front of a massive object (i.e. star or galaxy) in the background is increased dramatically by the presence of gravitational field of a more massive body.
Author: Elzbieta Kuligowska
References & further reading:
11 awesome Exoplanet facts (Stellar Planet)
Sub-brown dwarf (Wikipedia)