Universe Today
Free-Floating Planets (FFP), also known as "Rogue Planets," were first discovered in 2000 by astronomers searching the Orion Nebula. Since then, hundreds of FFP candidates have been detected, and scientists predict that there may be trillions of them throughout the Milky Way. This would mean that they outnumber stars by 20 to 1, and potentially habitable planets by a factor of 25 or more. Research has also shown that many of these planets are likely to bring some of their moons along with them.
This was determined by LMU physicist Dr. Giulia Roccetti in a previous study, where she found that gas giants ejected from their star systems could retain some of their "exomoons." In a new study, Roccetti and researchers from the Ludwig-Maximilians-University Munich (LMU) and the Max Planck Institute for Extraterrestrial Physics (MPG) investigated hydrogen as a potential heat trap that could ensure habitability on some of these exomoons. Their findings show that with a sufficient amount of atmospheric hydrogen, these moons could remain habitable for billions of years!
The notion of exomoon habitability comes directly from what we know about moons in our own Solar System. Around Jupiter, Saturn, Uranus, and Neptune, dozens of moons are believed to have interior oceans, which are kept warm through tidal heating. Basically, the powerful gravitational interaction between these moons and their gas giants caused their rocky-metallic interiors to flex, releasing heat and energy.
This heat is released, along with minerals essential for life, into the moons' interior oceans through hydrothermal vents located at the core-mantle boundary. For decades, astronomers have been hoping to get a closer look at these "Ocean Worlds," which are so-named because they possess more water in their interiors than all the oceans on Earth combined. However, things would work differently for exomoons orbiting FFPs, since the ejection process would alter their orbits.
The resulting tidal forces would then deform the lunar body by compressing its interior and generating heat through friction. This heating would be enough to maintain oceans on the surface of these moons, despite them having no star to draw heat and energy from. However, retaining this heat on the moon's surface depends on the presence of an atmosphere with sufficient greenhouse gases.
On Earth, carbon dioxide (CO2) is the predominant greenhouse gas, which helps our atmosphere retain heat, and is also driving anthropogenic Climate Change. According to the research team, earlier studies have found that a CO2-rich atmosphere could support habitable conditions for up to 1.6 billion years. However, for exomoons orbiting FFPs, the extreme cold could cause this CO2 to condense, thereby allowing heat to escape.
The team considered an alternative heat trap in the form of an atmosphere rich in molecular hydrogen. While this element is largely transparent to heat, under high pressure, collisions between atoms allow for heat absorption. In addition, hydrogen remains stable at very low temperatures, including those present in the interstellar medium (ISM). In essence, "Ocean Worlds" in the ISM could harbor life in surface oceans rather than in interior oceans, as observed here at home.
*This is an artist's illustration of a potentially habitable exomoon orbiting a giant planet. Credit: NASA GSFC/Jay Friedlander and Britt Griswold*
The findings could also provide insight into the origins of life. For instance, the tidal disruptions that deform the moons' interiors would also give rise to a "water cycle," in which water evaporates and condenses again. These cycles are considered an important mechanism for the formation of complex molecules that would eventually give rise to life. In this respect, tidal forces would not only supply heat but could also drive chemical evolution on bodies orbiting Rogue Planets.
David Dahlbüdding, doctoral researcher at LMU and lead author of the study, explained:
Our collaboration with the team of Prof. Braun helped us recognize that the cradle of life does not necessarily require a Sun. We discovered a clear connection between these distant moons and the early Earth, where high concentrations of hydrogen through asteroid impacts could have created the conditions for life.
These findings bode well for scientists engaged in the search for life in the Universe (aka abiogenesis). Given how common FFPs are in our galaxy and the fact that these moons could provide stable habitats for billions of years, interstellar space may be teeming with life. The findings may also offer clues as to how life has been distributed throughout the cosmos (panspermia).
But most of all, they reveal that life could exist in the darkest regions of the Universe, challenging a long-held assumption that habitable worlds exist only around stars.
Further Reading: IDW
