Astronomers once thought that the planets evolved in their present orbits, which stayed constant over time. Recent observations, theories, and computations, however, have demonstrated that planetary systems are susceptible to alterations and disruptions. Planets are periodically expelled from their star systems to become “rogue planets,” entities that are adrift in the interstellar medium (ISM) and no longer gravitationally connected to any star. Some of these planets might be gas giants with icy moons orbiting them in close proximity that they could introduce into the ISM.
These satellites might have warm water interiors like Jupiter, Saturn, Uranus, and Neptune do, which might be able to support life. Other studies have shown that the combination of geological activity and radioactive decay could enable life on rocky planets with a lot of water on their surfaces. A recent paper by an international team of scientists suggests that our local universe may contain hundreds of rogue planets. They also suggest that far space expeditions may examine these unbound objects more easily than planets still tied to their stars based on their groundbreaking feasibility research.
Manasvi Lingam, an assistant professor at the Florida Institute of Technology’s Department of Aerospace, Physics, and Space Sciences and the Institute for Fusion Studies at the University of Texas at Austin, served as the study’s principal investigator. He was joined by T. Marshall Eubanks, the head scientist of Space Initiatives Inc., and Andreas M. Heinc, a researcher at the Initiative for Interstellar Studies (i4is) and the Interdisciplinary Centre for Security, Reliability, and Trust (SnT) at the University of Luxembourg. Recently, their paper, “Chasing nomadic worlds: A new class of deep space missions,” was published in Acta Astronautica.
Interstellar objects (ISO) are a well-established field of study that was already extremely active by the 1970s, as Lingam and his colleagues highlight in their work. But the discovery of ‘Oumuamua in 2017, the first known encounter with an ISO, and the discovery of 2I/Borisov in 2019 have elevated this field to the fore of academic study. As a result of further studies, their abundances have been restricted because it was discovered that meteors and smaller interstellar objects have visited Earth in the past.
One such object (CNEOS 2014-01-08) was discovered by Harvard astrophysicists Amir Siraj and Professor Avi Loeb (Prof. Lingam’s old advisor) in the meteor catalog of NASA’s Center for Near Earth Object Studies (CNEOS). CNEOS claims that in 2014, this extraterrestrial meteor made landfall in the South Pacific off the coast of Papua New Guinea. Last year, the Galileo Project, under the direction of Prof. Loeb, carried out a sample recovery effort that recovered hundreds of metallic spherules from the meteor’s debris on the ocean floor (360 so far!).
Lingam and his associates look into the prospect of examining far larger things, nevertheless. According to studies conducted in the 1990s, gravitational microlensing tests could help find extrasolar planets, especially ones unattached to any stars. Surveys measuring the distribution of rogue planets have now verified this, demonstrating that they are probably common in our galaxy. This comprises two research that were directed by David Bennett, a Senior Research Scientist with NASA Goddard’s Science Mission Directorate (SMD).
According to the research articles, which are slated for publication in The Astronomical Journal, there may be trillions of rogue planets roving the Milky Way. The profusion of rogue objects and their potential to harbor life gives enormous opportunity for future research, as Prof. Lingam explained to Universe Today via email:
According to estimates, there could be up to 1000 nomadic worlds per star that are at least the size of the Moon. As a result, they would be among the most typical habitats for life, even if only a small portion of them have conditions conducive to life. Because of this, they might be a good target for astrobiology.
Prof. Lingam’s book Life in the Cosmos: From Biosignatures to Technosignatures (co-authored by Prof. Loeb) published in 2021 covered the topic of the potential habitability of rogue planets, satellites, and smaller objects in great detail. For the purpose of this investigation, Prof. Lingam and his colleagues concentrated on objects with diameters between 100 and 1000 meters (330 to 3300 feet), which were far larger than meteorites or the “Oumuamua” and “2I/Borisov.” They also cast a wide net that covered two orders of magnitude, covering everything from planets with radii between Earth and Mars to things similar to Main Belt Asteroids.
“We concentrated on objects with radii between 100 and 10,000 km. These planets may be rocky or ice, and they may be able to sustain liquid water on their surface for up to 100 Myr or longer. As many as 1000 nomadic worlds the size of the moon and larger may exist per star, depending on their size.
They also discovered that smaller things are statistically more likely to be found closer to the inner Solar System and are expected to be significantly more abundant than larger rocky ones. The researchers’ findings also imply that a sphere centered on Earth and extending to the nearest star system (Proxima Centauri) may contain tens of thousands other planet-sized nomadic worlds. These rogue planets are the closest exoplanets outside of our Solar System, with Proxima Centauri having three confirmed exoplanets, one of which is rocky and situated in the star’s habitable zone (Proxima b).
Numerous companies and non-profits want to launch the first interstellar trips to the nearest stars in the near future to study their planetary systems. Examples include Breakthrough Starshot, a mission design that combines directed-energy propulsion (DEP) and gram-scale watercraft to enable interstellar voyages within our lifetimes. Lingam and his team pointed out that by focusing their efforts on potentially habitable rogue planets closer to the Solar System, these missions would save time and money.
In order to achieve this, they looked into a number of propulsion technologies under consideration for interstellar mission architectures. They specifically looked for ideas that would enable missions to investigate planets the size of Earth with a 50-year flight timeframe. Lingam said:
We took into account a variety of propulsion methods, including chemical propulsion, electric and magnetic sails, solar and laser sails, nuclear fusion, laser and nuclear electric propulsion, and sails. We came to the conclusion that, out of all the contenders, laser sails—spacecraft propelled by laser arrays—offer the best chance of arriving at nomadic worlds in a timely manner.
These discoveries offer prospects for both current and future space telescopes. Astronomers intend to expand their search for rogue planets and further limit the number of unbound objects that are currently out there in the coming years. The actual successor to Hubble, the Nancy Grace Roman Space Telescope, will be launched by NASA in 2027 and will bear her name. She was NASA’s first top astronomer and was known as “The Mother of Hubble” since she was instrumental in the development of the telescope.
The same publications, led by David Bennett of NASA Goddard, claim that during its initial mission, Roman could discover as many as 400 Earth-like rogue planets. Gravitational Microlensing is a method essential to this process that is frequently employed to look for exoplanets tied to stars. With this method, light from a faraway star is bent and focused by the gravitational force of large objects, combining features of the Transit Method and Gravitational Lensing. There is a quantifiable brightness dip that can be used to infer the presence of a planet as it transits this star, or passes in front of it relative to the observer (also known as a transit).
Lingam claims that Nancy Grace Roman’s microlensing studies will support their findings and help pinpoint the locations of rogue planets in our solar neighborhood, all of which might be the focus of upcoming exploration missions:
Because gravitational microlensing can identify worlds smaller than the Earth (such moon-sized ones), missions like the Nancy Grace Roman Space Telescope and Euclid are anticipated to objectively constrain the abundances of nomadic worlds.
Without needing to go to far-off stars, scientists can conduct profitable astrobiology missions by investigating renegade objects that have been expelled from their systems. These initiatives are probably going to take place concurrently with missions to the outer Solar System, where robotic explorers will visit icy moons like Europa, Ganymede, Titan, Callisto, etc. and either collect samples from the surface or drill/melt through the surface ice to look for signs of life. The study of rogue bodies will shed significant light on the genesis and evolution of other planetary systems, even when biosignatures are not immediately apparent.
In any case, examining rogue planets and objects will provide us with information about star systems that can only be obtained by traveling to those star systems. This option is quicker, less expensive, and ultimately superior when compared to mounting interstellar missions, which take time, energy, and money. Rogue object exploration might also act as “pathfinder missions,” giving researchers a preview of what to expect elsewhere in the galaxy and directing them toward the most promising areas.
Further Reading: arXiv