Could Homeless Planets Be a Home?
January 10th, 2025
Learning about astronomy is always better when you encounter outliers, strange behaviors, and weird astronomical bodies that all seem like coding errors in the Universe’s framework. One of them is rogue planets. This article will discuss what rogue planets are, how they can be detected, and the criteria necessary for them to host life.
Rogue planets are planets which are not bound to any star, roaming free in space until an astronomical body (such as a star) gravitationally attracts it. Due to an absence of a host star, these planets are difficult to track and study closely, as evidenced by the lack of a portfolio for ‘cool-looking rogue planets’ or rogue planets with ‘unique features.’
A typical planet forms when dust particles begin to clump together–becoming larger and larger–following the formation of a star. In order to be a planet (sorry Pluto), a body must be orbiting a star, resemble a spherical shape (often achieved by having a certain amount of gravitational force, which translated to its mass as well), and have sufficient gravitational force to attract or repel other mass around it (with cleared orbital path). Rogue planets, on the other hand, are an interesting conundrum.
There are many theories as to how rogue planets are formed. One explanation is stated by Forbes, where a rogue planet manifests itself in space, similar to a star. However, the most common origin stories of these planets are through planetary birth or ejection. In planetary birth, rogue planets could be pushed out of their star system during their formation, or fail to be strongly attracted by the central star. With ejection theories, these planets could have been born normally, yet been kicked out of their system by some event or force–one example being a collision between two star systems, where a planet’s orbit is disturbed. These rogue planets aren’t just mysterious strangers; one could have existed in our own star system. Researchers via Scientific American noted that “there's about a 0.5% chance that one of these wayward planets might have formed in our own system and ended up in the Oort cloud as it drifted away from the sun.” Yet, the opposite could also be the case: our Sun pulled a rogue planet into the edge of our star system.
So how do we detect rogue planets, especially if they are lacking a bright host star. The most common method is gravitational microlensing (can also be used to identify exoplanets).
Here’s a simple breakdown of how gravitational microlensing would work, and can be used to find exoplanets and rogue planets:
Image Courtesy of The European Space Agency (ESA) via EarthSky
If a closer star and a distant star align, greater brightness is achieved since the distant star’s light is essentially magnified. When scientists observe a brightness/magnification vs. time graph, they will observe a singular peak (hill-like shape) because the closer star’s gravity bends the light and magnifies the distant star’s brightness. This can be shown by the graph on the left side of the image
When an exoplanet is involved (orbiting the closer star), the graph will depict two bumps–one central peak, followed by a slightly smaller peak. Here, the closer star’s gravity bends the light of the distant star, but the exoplanet’s gravity also bends the light. You can see this with the graph on the right side of the image
Now let’s say the rogue planet becomes the closer star (remember, no host star accompanies the rogue planet) and aligns itself with a distant star. Because the rogue planet’s gravity bends the light, there will be a peak in brightness. This shouldn’t be the case if there’s only one star, indicating that there’s been interference. Hence, a rogue star is detected. This would replicate a normal peak (graph on the left side of the image); a normal star with no interference would not have this peak
One of the most frequently-asked questions when scientists find these new astronomical events or bodies is do aliens live there? Was it caused by aliens? Is that an alien? While people might initially believe these rogue planets are incapable of sustaining life due to their lack of a host star, that might not be completely true.
According to a research paper published by Dirk Schulze-Makuch and Alberto G. Fairen, “Evaluating the Microbial Habitability of Rogue Planets and Proposing Speculative Scenarios on How They Might Act as Vectors for Panspermia,” rogue planets shouldn’t be overlooked despite their extremely low chance of having life or traces of past life. The paper details how rogue planets’ atmospheres collapse and their surfaces freeze due to the absence of a host star; however, their inner cores could still be able to sustain subsurface liquid water through radioactive decay. The duo note how “water is such a common compound in the universe (given also that hydrogen is the most common element and oxygen the third-most common element) [7], it seems reasonable that many of these icy worlds may have ice caps consisting dominantly of water ice with liquid water beneath.” They additionally propose another possibility to support the presence of water on a rogue planet: a thick-enough hydrogen atmosphere that creates adequate pressure and insulation.
Their research further delves into the growth of life and the essential component of photosynthesis. According to the team, without a star, rogue planets could still undergo a similar process to capture energy through hydrothermal vents (which has been observed with Earth’s marine organisms too deep to have sunlight).
For the possibilities listed above, the researchers are referring to rocky rogue planets in particular. Rocky rogue planets have the possibility of having liquid water/ice. Other rogue planets are similar to gas giants, where they lack solid surfaces other than the planet’s core, making chances of life improbable
However, we might not need an investigation of a rogue planet to see how plausible these possibilities are. Moons like Europa and Titan have similar conditions to a rogue planet, and while these moons have a host star, they still remain distant from it. These moons are some of the best locations for life to exist in our solar system, and if life does exist there, it might give scientists some clues or further knowledge to understand how habitable rogue planets can be.