Scientists have studied the fate of rogue planets and their moons that have been ejected from their original star systems. Research shows that the moons of such planets may retain conditions favorable for the emergence of life for billions of years.

Rogue Planets: What Are They?

Currently, hundreds of planets are known to roam freely through the Galaxy, not orbiting any star but moving independently. Most often, these are gas giants several times more massive than Jupiter, discovered in regions where stars are born. It is believed that most of these objects originally form alone: they could have become stars but did not accumulate enough mass.

There have also been discoveries of rogue planets only slightly larger than Earth. According to planetary scientists, such bodies can be ejected from their systems due to dynamic processes in young star systems or if they form near a massive star, such as Bellatrix, which has a mass about eight and a half times that of the Sun.

How Planets Become Rogues

Massive stars have relatively short lifespans—just 10 to 20 million years. Once they exhaust their fuel, they explode as supernovae, throwing off their outer layers. Planets nearby can be propelled away by the shockwave, turning into rogue planets.

The Role of Moons in Supporting Life

Having a moon can help such a planet maintain conditions necessary for life. Astrophysicists from Hungary modeled over four thousand scenarios for the evolution of such planets and their moons.

In all cases studied, the moons remained with their planets, and their distances from the planet changed very little. Only a slight increase in orbital eccentricity was observed, which could even be a beneficial factor.

The largest moons of Jupiter also do not have perfectly circular orbits. This causes the moon to periodically move closer to and farther from the planet, generating tidal forces that create internal friction and heat within the moon.

Thanks to this process, moons like Europa, Ganymede, Callisto, and others in the Solar System may have subsurface oceans of liquid water, increasing the likelihood of life.

The Potential for Life on Moons of Rogue Planets

Calculations suggest that if a hypothetical Jupiter-like planet with a moon similar to Europa were ejected from its system by a supernova explosion, the moon could receive even more internal heat from tidal forces than it would with a star present. In 12–15% of the modeled cases, the moon became warm enough to support the emergence of life, and these conditions could persist for billions of years.

For this to happen, several conditions must be met: the moon should be no farther than 15 planetary radii from its planet, and the planet itself should be at least 383 astronomical units away from any star to prevent the moon’s water from evaporating.

Rarity and Prospects

Planets on such distant orbits are rare, but there is a possibility of their formation around less massive companion stars, since massive stars often prevent planets from forming due to their intense radiation.