Most planets have a star around which they orbit. But astronomers also know of another class of worlds — rogue planets, which are not bound by gravity to any star. They travel independently through interstellar space, orbiting along with the entire galaxy around its center.
Such planets receive almost no light from stars and therefore remain extremely difficult to observe. Nevertheless, scientists suggest that there could be a vast number of them in the Milky Way — possibly hundreds of billions.
How planets become lonely
Rogue planets are most likely not born "free." Many of them could have formed in ordinary planetary systems and subsequently been ejected from them. A young planetary system is quite a chaotic place. Large planets interact with one another, alter their orbits, and experience mutual gravitational influences. Sometimes this exchange of energy ends with one of the planets acquiring sufficient speed to leave its home system forever.
It is not impossible that something similar occurred in the early Solar System. Some models of its formation work better if we assume the existence of a fifth giant planet, similar to Uranus or Neptune. Later, gravitational interactions could have thrown it out into interstellar space. If this indeed happened, determining the location of such a world today is practically impossible.
How to detect an invisible planet
Conventional methods for searching for exoplanets do not work here. Scientists often detect planets by the way they cause their star to wobble slightly, or by the drop in a star's brightness when a planet passes in front of its disk. A rogue planet simply has no star. Young free-floating planets can be searched for via infrared radiation: after formation, they still retain significant internal heat. But as such a world cools, it becomes practically invisible.
For older and colder objects, another phenomenon helps — gravitational microlensing. If a planet accidentally passes between Earth and a distant star, its gravity warps spacetime and slightly alters the path of the starlight. An observer records a brief change in the star's brightness and, from this signal, can infer the existence of an invisible object.
How many could there be?
Some of the first candidates for freely floating planets were discovered back in the late 1990s. In 2011, a study of microlensing events led to a much larger estimate: hundreds of billions of starless planets could exist in the Milky Way.
This number remains a matter of debate. Some detected objects might turn out to be brown dwarfs or planets that are simply located at extremely large distances from their host stars. Nevertheless, subsequent observations have continued to indicate that rogue planets might indeed be very common.
Can a lonely planet retain heat?
The most surprising aspect of this story is that the absence of a star does not necessarily mean a completely dead and frozen world. A planet continues to receive heat from its own interior. Radioactive decay occurs inside it, and large bodies can retain residual energy left over from their formation. If a planet possesses a sufficiently dense hydrogen atmosphere, it is capable of further trapping this heat.
Therefore, the surface might be covered with ice while liquid water persists deep below. Back in 1999, planetary scientist David Stevenson showed that an ejected Earth-mass planet could, under certain conditions, retain liquid water. The scenario involving a subsurface ocean protected from cosmic cold by a thick layer of ice is particularly interesting.
An ocean without a sun
This possibility forces a reconsideration of the very concept of habitability. When searching for life beyond Earth, astronomers usually focus on planets located within the so-called habitable zone — the region around a star where temperatures allow liquid water to exist on the surface.
For a rogue planet, no such zone exists. But if water lies beneath the surface and derives energy from the planet's interior, it does not need a star at all. On Earth, organisms and ecosystems exist that derive energy from chemical reactions rather than directly from sunlight. Thus, theoretically, a subsurface ocean on a rogue planet cannot be ruled out from the list of potential places for life.
This currently remains a hypothesis: the presence of liquid water does not yet imply the presence of biology. But the mere possibility of warm underground oceans makes these lonely worlds far more intriguing than simple cold rock or gas bodies.
The հidden պopulation of the Milky Way
Rogue planets are gradually changing our understanding of what a planetary world can be. They are capable of existing without a star, traveling through the galaxy, and perhaps retaining heat and water deep beneath their surfaces.
If estimates of their numbers are even approximately correct, there could be hundreds of billions of worlds in the Milky Way that cannot be seen by conventional means. And some of them, despite eternal darkness on the surface, might harbor their own oceans beneath the ice.
Based on materials from National Geographic.
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