Not All Planets Have a Star
When you think of a planet, you probably think of one of the planets in our own solar system, like Earth, Jupiter, or Mars.
Most of the exoplanets we’ve discovered have been like those in our solar system. They orbit their parent stars like clockwork.
However, not all planets exist in such orderly systems. Some planets are floating through the galaxy alone, not bound to any star in particular. These loners are called “rogue planets” or sometimes “free-floating planets.”
They’re not easy to study, since they don’t orbit any star, and can’t be spotted reliably or repeatedly. Scientists basically need to get lucky and be looking as a rogue planet passes in front of a random star to detect one at all.
What Are Rogue Planets Like?
Rogue planets, much like the planets you find in solar systems, come in every size.
Some are enormous and gaseous, much like Jupiter (and sometimes even larger), while others are more like the ice giants Neptune or Uranus, which are smaller.
And some of them are even like our own planet—small and rocky.
The major feature that ties them all together is temperature.
Because they’re not warmed by a parent star, the surfaces of rogue planets are all quite cold. What heat they have comes from within.
Some of that inner heat is mechanical. Just like when you bench a piece of plastic repeatedly, the mechanical forces at work during a planet’s formation generate heat, which is thought to be enough to make the surface of rocky planets completely molten. The heat gradually fades as the planet ages.
Another source of heat is radioactive decay. Heavy elements (including radioactive ones) wind up trapped in the centers of planets. As those radioactive elements undergo decay, the core is continuously heated.
Collectively, those two sources could keep the inside of a planet extremely toasty for a very long time. Take Earth, for example. Earth is some 4.5 billion years old, and it is believed that the core of the Earth is somewhere between 5,000 and 6,000 degrees Celsius—aboutthe same temperature as the surface of the Sun. The rhythmic gravitational pull of the Moon also adds a bit too.
As the heat from their formation and the radioactive materials in their core winds down over billions of years, rogue planets will wind up completely frozen.
How Do Rogue Planets Form?
Some rogue planets form “normally,” like the ones in our solar system. As the soup of dust and gas (properly called the protoplanetary disk) whirls around a newly-forming star, particles gradually bump into each other and begin to clump up. Over millions of years, some of these clumps grow large enough to start vacuuming up everything in their orbits, forming planets as we know them.
However, these early years in a solar system aren’t tranquil times. They’re marked by rapid changes and instability. As multiple planets form, instability in the system results in planets migrating into different orbits. Sometimes, if a planet is particularly unlucky, it is completely ejected from the solar system, resulting in a rogue planet. Another star (or black hole or neutron star) passing nearby can also destabilize an existing solar system, resulting in a planet being ejected.
Alternatively, scientists believe that it is probably possible for rogue planets to form directly out of clouds of material floating around in space, much like stars themselves do. There is some debate about just how big such planets would be, however.
Either way, once a planet begins meandering through the galaxy on its own, it’ll likely stay that way forever.
Can Life Exist on a Rogue Planet?
Probably, but it would likely be much more limited than what you might find on planets orbiting stars.
There are multiple places in our solar system that can provide some clues: the Earth and the moons of gas giants.
Life on Earth-Like Rogue Planets
On Earth, hydrothermal vents, where the normally frigid ocean water is heated by volcanic processes to hundreds of degrees, are hotbeds of life.
The water is warm and has an abundance of dissolved minerals, which encourages the growth of bacteria and other simple organisms. Once those are present, larger, more complex organisms quickly follow. There is a decent amount of evidence that suggests the very first life on Earth arose in such areas.
If Earth were to suddenly become a rogue planet, the planet would cool, and over the course of months or a year, the entire ocean would become covered in ice. That ice would act as an insulator, slowing the loss of energy to space.
It is possible that deep parts of the ocean would take millions of years to fully freeze, and that hydrothermal vents could create oases for simple lifeforms to thrive so long as the planet remained geologically active, which will probably be the case for at least another billion years.
Life on Rogue Gas Giants and Their Moons
Besides Earth, the moons of gas giants like Jupiter are thought to be the best locations for life in our solar system.
Though clad with icy exteriors, the tidal stresses due to the gravitational interactions with Jupiter and the other moons heat the interiors of the moons. This heating can get pretty intense—it has resulted in Io, Jupiter’s 4th largest moon, becoming the most volcanically active body in the solar system.
Io itself is not thought to be hiding an ocean, but salty oceans could exist on several moons in the solar system, including Jupiter’s moons Europa, Callisto, Ganymede, and Saturn’s moons Titan and Enceladus.
If a gas giant and its moons were ejected from a solar system, it is entirely possible the tidal heating could maintain the conditions required for life on the moons for billions of years.
While the life you find on rogue, Earth-like planets or the moons of rogue gas giants isn’t going to be the sort of life Captain Kirk would be excited to talk to, it certainly could exist. So, while the company might not be much for conversation, at least there is a possibility these planets havesomefriends as they roam the cosmos.
