We tend to define planets as objects that need a star around which to revolve. Yet, as the history of astronomical research has shown time and again, the Universe doesn’t seem to care much for such clear-cut definitions. Scattered across the vast distances of interstellar space, there are alien worlds which freely roam in the perpetual darkness that reigns between the stars. These free-floating, or “rogue” planets as they are better known, have been an active topic of research in recent years, offering astronomers the chance to further study the processes that drive exoplanetary formation and evolution, while helping them to better constrain the line that separates massive planets in general from the “failed stars” known as brown dwarfs. A new study by an international team of astronomers has for the first time managed to detect what appears to be possible weather patterns on such a rogue extrasolar planet, indicating that far from being dead and frozen celestial husks, these alien worlds are as equally fascinating as their more “proper” planetary siblings which bask in the warmth and light of other stars.
The prospect of rogue planets that freely wonder the galaxy in the vast distances between the stars had been hypothesized for decades, yet its confirmation would have to wait for the advent of exoplanet discoveries that have taken place during the last 20 years. The bulk of the extrasolar worlds detected to date have been found with the help of mainly two planet-hunting techniques: the transit method, which looks for the dimming of a star’s light that is caused by the passage of a planet across the star’s face, and the radial velocity method, which looks for the gentle “wobble” in a star’s motion that is caused by the gravitational tug of the planets orbiting around it. Evidently, these methods work for planets that revolve around stars, but what about the free-floating ones? In order to uncover the latter, astronomers either rely on direct imaging or utilise the effects of gravitational microlensing. More specifically, if a free-floating planet happens to lie sufficiently close to Earth its thermal emission can be directly observed by telescopes in infrared wavelengths, a task which is much easier than imaging orbiting exoplanets whose faint light is lost in the overwhelmingly bright glare of their host stars. Several exoplanets that are much more distant for their infrared glow to be visible at all have been found with the help of gravity: When a distant rogue planet happens to cross in front of a background star, its gravity will bend and brighten the star’s light in a minuscule but nevertheless discernible way which can be detected by astronomers on Earth. By using these two techniques, astronomers have managed to discover dozens of rogue planet candidates in recent years with masses that range from several to dozens of times that of Jupiter.
One such free-floating planet, named PSO J318.5-22, was discovered back in 2013 with the Panoramic Survey Telescope & Rapid Response System, or Pan-STARRS, PS1 wide-field telescope on Haleakala, Maui, in Hawaii. Located approximately 80 light-years away in the constellation of Capricornus, PSO J318.5-22 was readily visible in infrared wavelengths by its faint red glow and was found through spectroscopy to exhibit properties that were different from those of brown dwarfs and was much more like the gas giants that have been detected by the hundreds around other stars. With a mass not greater than eight times that of Jupiter and an age of approximately 12 million years, PSO J318.5-22 was found to be similar to the gas giants in the HR 8799 planetary system, but instead of being tied to a star, it roamed the galaxy alone. “We have never before seen an object free-floating in space that looks like this,” would say at the time Dr. Michael Liu, an astronomer at the University of Hawaii and leader of the team that had made the discovery. “It has all the characteristics of young planets found around other stars, but it is drifting out there all alone.”
An international team of astronomers, led by Dr Beth Biller, a researcher at the University of Edinburgh in the UK, re-visited PSO J318.5-22 one year later in October and November 2014 while utilising the European Southern Observatory’s New Technology Telescope in Chile, as part of a wider imaging survey of a total of 22 rogue planetary-mass objects, in order to study the brightness variability of free-floating planets in near-infrared wavelengths. The analysis of PSO J318.5-22’s light-curve showed that the latter exhibited a rotational period between five and 10 hours, which is similar to Jupiter’s 10-hour rotation in our own Solar System, over which time the astronomers detected a 10 percent variability on the planet’s overall brightness. Taking into account its relatively low measured temperature of 1,160 Kelvin, Biller’s team concluded that PSO J318.5-22 was too cold for this variability to be caused from starspots or other stellar-like process and was most probably due to the presence of various layers of thick and thin cloud covers in its atmosphere. “PSO J318.5-22 is too cool to have starspots and likely too old for ongoing accretion,” write the researchers in their study, which was accepted for publication at The Astrophysical Journal Letters. “From its red colors, PSO J318.5-22 must be entirely cloudy. Thus the likely mechanism producing
the observed variability is inhomogeneous cloud cover, as has been found previously to drive variability in higher-mass brown dwarfs with similar temperatures.”
At PSO J318.5-22’s surface temperature, the planet’s weather patterns wouldn’t be anything we’d like to experience. At the conditions that are prevalent on PSO J318.5-22’s atmosphere, elements like hydrogen and helium would be in a liquid metallic state, whereas the cloud covers would be loaded with molten metals that would create a constant downpour. “These are likely hot silicates and iron droplet clouds,” Biller told New Scientist in regards to her team’s discovery. “This makes Venus look like a nice place.”
PSO J318.5-22’s properties also make it an important link between planetary-mass objects and brown dwarfs, offering astronomers the chance to better study the physical processes that are both common and different between these two classes of objects. “Only one exoplanet rotation period has been measured to date (7-9 hours for Beta Pictoris b). PSO J318.5-22 is only the second young planetary mass object with constraints placed on its rotational period and is likely also a fast rotator like B Pic b, with possible rotation periods from 5-20 hours. PSO J318.5-22 is thus an important link between the rotational properties of exoplanet companions and those of old, isolated Y dwarfs with similar masses,” conclude the researchers in their study.
The study of free-floating planets like PSO J318.5-22 is of great importance to many different areas of planetary science and astrophysics for various reasons. For instance, rogue planets are now considered to be greatly abundant in the galaxy, with several different studies in recent years suggesting that they could well outnumber the “usual” ones which are tied around stars. Furthermore, the mechanisms by which rogue planets are thought to have been expelled from their solar systems are thought to have played a crucial role in the evolution of our own Solar System as well. Thus the study of rogue planets could provide important clues to our understanding of how our Solar System and others have evolved over time.
Yet, more importantly, rogue planets could turn to be of interest from an astrobiological perspective as well. While at face value, far from the warmth and light of a star, one could think of such planets as being the least conductive to life, a growing body of research has suggested that this may not be the case at all. Computer simulations have shown that a rocky free-floating exoplanet with a size three times that of Earth could retain enough internal heat to maintain a liquid water ocean underneath a surface ice cover, similar to the one that is believed to exist on the interior of Jupiter’s moon Europa. In addition, a free-floating planet that would posses a thick hydrogen atmosphere would be able to maintain the temperature on its surface above the freezing point of water. Hydrogen doesn’t freeze over even at very low temperatures and can act like a thermal blanket, keeping a free-floating planet’s internal heat from totally escaping into space, thus creating the conditions for water to remain liquid on its surface.
In the search for answers regarding the evolution of solar systems and the prevalence of life in the Universe, it may be that the inconspicuous planetary wanderers of the galactic deep could be some of the best places to closely look at.