A rogue planet, also known as a free-floating or interstellar planet, is a type of planet that drifts through space without being bound to a star or any solar system, sometimes also referred to as “nomad planets”. They are intriguing objects in astronomy due to their unique nature and the mysteries surrounding their origin, characteristics, and potential abundance.

1. 1. Origin and Formation Theories

• • • Ejection from a Planetary System: Rogue planets may form like normal planets within a protoplanetary disk around a star. Through interactions with other massive bodies—like larger planets, stars, or binary companions—they can be gravitationally slingshot out of their system. This is one of the leading theories for their existence.
    • Starless Formation: Some rogue planets might form in the same way stars do, by the collapse of a dense cloud of gas and dust, but without gaining enough mass to become a star. These planets would essentially be “failed stars” with insufficient mass to initiate nuclear fusion.
      
      

1. 2. Detection Challenges

Since rogue planets do not orbit a star, they do not reflect starlight like regular planets, making them exceptionally difficult to detect.

• • • • Gravitational Microlensing: This occurs when a rogue planet passes in front of a distant star. The planet’s gravity bends and magnifies the star’s light, creating a temporary brightening effect. This is currently the most effective method of identifying rogue planets.
      • Infrared Emissions: Rogue planets might still emit heat from their formation or from internal sources like residual radioactive decay, which can be detected in the infrared spectrum, especially if they are large or young.
        
        

1. 3. Prevalence and Abundance

Studies suggest that rogue planets could be incredibly common. Some estimates propose that rogue planets might outnumber stars in our galaxy by a wide margin. In 2021, astronomers announced the discovery of at least 70 free-floating planets in our galaxy using data from the European Southern Observatory’s Very Large Telescope. Some studies even speculate that there could be billions of rogue planets drifting through the Milky Way alone.

1. 4. Composition and Characteristics

• • • Size: Rogue planets could range from small, Earth-sized planets to gas giants many times the size of Jupiter. In fact, some massive rogue planets might resemble brown dwarfs, objects larger than planets but not massive enough to sustain nuclear fusion like stars.
    • Temperature: Rogue planets are typically cold due to the lack of a star to provide heat. However, some could have residual heat from their formation or internal radioactive decay, keeping their interiors relatively warm.
    • Atmosphere: A thick atmosphere might help some rogue planets retain heat. For example, gas giants could have dense, hydrogen-rich atmospheres that insulate the planet, allowing them to maintain higher temperatures than might be expected in the cold of interstellar space.
      
      

1. 5. Potential for Life

While rogue planets seem inhospitable at first glance due to their cold, dark environments, there are speculative possibilities for life.

• • • • Subsurface Oceans: If a rogue planet retains enough internal heat, it could sustain liquid water beneath its surface, insulated by an icy crust. Similar to moons like Europa and Enceladus in our Solar System, these subsurface oceans could potentially harbour microbial life.
      • Geothermal Energy: Even without sunlight, rogue planets could have geothermal activity (from tidal forces or radioactive decay), providing an energy source for life.
        
        

1. 6. Role in Galactic Dynamics

• • • Star-Forming Regions: Some rogue planets could form in star-forming regions and be ejected early in the system’s development. Their presence could influence the formation of other planets and the architecture of planetary systems.
    • Potential for Capture: Some rogue planets might get recaptured into other star systems if they pass close enough to a star, becoming part of a new solar system.
      
      

1. 7. Famous Discoveries

• • • PSO J318.5-22: One of the most famous rogue planets, discovered in 2013, this object is about six times the mass of Jupiter and was found about 80 light-years away. It’s particularly notable because it has atmospheric characteristics like gas giants in other star systems, but it freely floats through space.
    • OGLE-2016-BLG-1928: This planet was detected in 2020 through gravitational microlensing and is one of the smallest rogue planets discovered, with a mass similar to Earth’s.