How planets without stars can possibly host Life on them

A look at ways in which starless planets can retain heat

An artist’s impression of the free-floating planet CFBDSIR J214947.2–040308.9 showing an infrared view of the object. The object appears blueish in this near-infrared view because most of the longer infrared wavelengths is absorbed by methane and other molecules in the planet’s atmosphere. In visible light the object is so cool that it would only shine dimly with a deep red color when seen close-up. Source: ESO

Rogue planets are planets without stars and thus orbit the galaxy directly. This makes them incredibly difficult to find because they don’t reflect any starlight or are undetectable by common exoplanet detection methods. Yet science paved the way to find just one such planet lurking in the dark about 80 light years away. The faint red glow of the rogue planet PSO J318.5–22 was directly imaged by the Pan-STARRS PS1 telescope. Seeing that such rogue planets are hard to find, it makes sense that one is 6 times more massive than Jupiter, making it relatively easy to spot.

A direct image of the first rogue planet to be discovered - PSO J318.5–22. Source: Wikipedia

Rogue planets are everywhere

Gravitational microlensing observations of 50 million stars in the Milky Way found 10 Jupiter-sized rogue planets. This helped scientists estimate that there are at least 2 rogue planets for every star in the galaxy (Paper). This means that there are potentially billions of rogue planets out there, lurking silently in the dark. Many of them are likely Earth-sized too.

A dead and desolate planet with no heat and light from a sun seems like the last place to look for Life. However, there are ways rogue planets can retain heat and have suitable conditions for Life:

#1: Rogue planets with hydrogen dominated atmosphere can retain heat

Not having a sun has its advantages too, one being that exposure of the planet’s atmosphere to ultra-violet (UV) light is minimal. This helps even an Earth-sized rogue planet retain a hydrogen dominated atmosphere. The paper Life-sustaining planets in interstellar space? suggests that such an atmosphere can trap the planet’s heat through pressure induced far-infrared radiation. Under the right conditions (like a fully convective atmosphere), liquid water oceans are also possible.

#2: Rogue planets can be geologically active

A large natural satellite like our Moon generates tidal forces on Earth that cause significant heating due to friction. This heating increases geological activity of the planet.

The tidal force of the Moon on the Earth (shown by the bulges) is a result of the centripetal force caused by the rotation around a common center of mass (yellow arrows) and the gravitational force caused by the Moon (red arrows). Source: Wikipedia

Early on after planetary formation, terrestrial planets with a large satellite (like the Earth-Moon system) can interact with Jupiter-sized bodies and get kicked out of the star system. Simulations of the same suggest that about 5% of such terrestrial planets retain their satellite even after being pushed out of the system. The tidal heating caused by such a moon provides a longer span for geological activity for such rogue planets, giving a chance for Life to arise.

#3: Moons of rogue planets can potentially harbour Life

Free floating planetary-mass objects can eventually form a mini planetary system around them, much like the Jupiter mini-system. The sub-brown dwarf OTS-44 is known to be surrounded by a disk of at least 10 Earth masses. OTS-44 can thus have a set of moons going around it in the future.

An artist’s impression of the sub-brown dwarf OTS 44 showing the disk of at least 10 Earth masses, which might evolve into a set of moons around the rogue planet. Source: Wikipedia

Just like how the tidal forces of Jupiter are responsible for its moon Europa’s subsurface ocean, moons around such rogue planets can have subsurface liquid water oceans too by the very same mechanism.

An artist’s illustration showing the internal structure of Jupiter’s moon Europa. Beneath the thick ice covering, lies an ocean of liquid water caused by the tidal heating effects of Jupiter’s gravity. Source: Source: NASA

Coupled this with possible sea floor volcanism and hydrothermal vents (that are considered to have been the birthplace of Life on Earth), such exomoons of these rogue exoplanets stand a chance at producing Life, even if bacterial.

In 2013, a possible discovery of an exomoon around a rogue planet was announced in this paper.

The Future

The possibilities of Life existing on rogue planets is thus not as tiny as we think at first go. Our current capabilities of finding rogue planets is not so good but NASA’s upcoming Wide Field InfraRed Survey Telescope (WFIRST) is expected to expand our search significantly. Only then with significantly more data, can we even begin to contemplate the real possibilities of Life on such dark worlds.