Rogue planets far outnumber planets that orbit stars, study finds

New groundbreaking research conducted jointly by NASA and Japan’s Osaka University has unveiled an astonishing discovery: rogue planets, celestial bodies that roam freely in space without being tethered to any star, vastly outnumber planets that orbit stars. The implications of this study are immense, as NASA’s Nancy Grace Roman Space Telescope, scheduled for launch in the coming years, may potentially spot a staggering 400 Earth-mass rogue planets. Already, the researchers have identified a potential candidate for such a rogue planet.

David Bennett, a senior research scientist at NASA’s Goddard Space Flight Center and co-author of two papers detailing the findings, expressed their estimations, suggesting that our galaxy is home to a mind-boggling 20 times more rogue planets than stars, amounting to trillions of these solitary worlds drifting alone in space. This study is remarkable as it is the first time that the number of rogue planets in our galaxy has been quantified, and it is capable of detecting planets even less massive than Earth.

The team’s extensive nine-year survey, known as MOA (Microlensing Observations in Astrophysics), was conducted at the Mount John University Observatory in New Zealand. Microlensing events occur when an object, such as a planet or star, aligns almost perfectly with a distant background star as observed from our vantage point.

Due to the fact that any object with mass distorts the fabric of space-time, light from the distant star bends around the nearer object as it passes by closely. This natural lensing effect briefly enhances the brightness of the background star’s light, providing astronomers with valuable clues about the intervening object, which cannot be observed through any other means.

Takahiro Sumi, a professor at Osaka University and lead author of one of the papers, emphasized the significance of microlensing, as it enables the discovery of objects like low-mass free-floating planets and even primordial black holes—phenomena that would otherwise remain hidden from direct observation.

The most remarkable revelation from the research is the discovery of a roughly Earth-mass rogue planet, marking only the second finding of its kind. The detailed account of this discovery will be published in an upcoming issue of The Astronomical Journal. Furthermore, the researchers presented a demographic analysis indicating that rogue planets are six times more abundant than planets orbiting stars in our galaxy—a finding set to be published in the same esteemed journal. This newfound understanding of rogue planets is expected to revolutionize our comprehension of the vastness and diversity of celestial objects in the universe.

This animation illustrates the concept of gravitational microlensing with a rogue planet — a planet that does not orbit a star. When the rogue planet appears to pass nearly in front of a background source star, the light rays of the source star bend due to the warped space-time around it. This slightly changes the star’s apparent position on the sky, and can even produce multiple copies of it. Such changes signal the planet’s presence to astronomers. Credit: NASA’s Goddard Space Flight Center/CI Lab

Pint-sized planets

In just a few decades, humanity’s understanding of the cosmos has drastically transformed from pondering the possibility of extraterrestrial worlds in our solar system to the remarkable discovery of over 5,300 planets beyond our own. The vast majority of these newfound planets are either massive or located extremely close to their parent stars, and sometimes both. However, the recent research suggests that rogue planets, untethered to any star, tend to be on the smaller side.

The team of researchers led by Takahiro Sumi revealed that Earth-sized rogue planets are more abundant than their larger counterparts. This finding is crucial in unraveling the mysteries of planetary formation mechanisms. The process of world-building can be turbulent, with celestial bodies interacting gravitationally as they settle into their orbits. Planets with lighter masses experience weaker gravitational ties to their stars, making them susceptible to interactions that propel them into the depths of space. This marks the beginning of their solitary existence, as they wander unnoticed among the darkness between stars.

The concept of such lone planets is not entirely foreign to science fiction enthusiasts, as an episode of the original Star Trek series showcased the discovery of a starless planet, Gothos, hidden amidst a desolate region of stars. Surprisingly, it was found to be habitable. However, the team of researchers emphasizes that the “rogue Earth” they detected probably bears little resemblance to our own planet, besides sharing a similar mass.

This remarkable research has propelled our knowledge of the cosmos to new heights, shedding light on the existence of these intriguing and enigmatic rogue planets. While they may not be teeming with life like Earth, their discovery adds another fascinating layer to our understanding of the vast and diverse universe that surrounds us.

Roman’s hunt for hidden worlds

The detection of solitary planets through microlensing events is an incredibly rare occurrence, prompting the need to expand our search efforts. NASA’s Roman Space Telescope, set to launch by May 2027, aims to achieve precisely that by casting a broader net and observing from space, enhancing its sensitivity to even lower-mass rogue planets.

Naoki Koshimoto, the lead researcher who conducted this study at NASA’s Goddard Space Flight Center and is now an assistant professor at Osaka University, emphasizes that Roman’s wide field of view and sharp vision will allow for more detailed studies of the objects it discovers, surpassing the capabilities of ground-based telescopes.

Initially, estimates based on planets orbiting stars suggested that Roman might detect around 50 terrestrial-mass rogue planets. However, the new research indicates that this number could soar to approximately 400, pending Roman’s actual observations.

To maximize the data obtained, scientists plan to combine Roman’s observations with ground-based data from telescopes like Japan’s PRIME, located at the South African Astronomical Observatory. PRIME, equipped with four detectors from Roman’s development program, will conduct the first extensive microlensing survey in near-infrared light, building upon the previous work of MOA.

Microlensing events are unique and non-repeatable phenomena, but they are not instantaneous. The signal from a rogue planet passing in front of a distant star can last from a few hours to about a day, providing astronomers with an opportunity to conduct simultaneous observations using both Roman and PRIME.

The advantage of observing these events from both Earth and Roman’s position, approximately a million miles away, lies in the enhanced accuracy in measuring the masses of these rogue planets. This unprecedented insight will significantly deepen our understanding of the diverse and enigmatic worlds that inhabit our galaxy. The prospects of the Roman Space Telescope in unraveling the mysteries of the cosmos are truly thrilling, and the collaboration between international space agencies promises to yield exciting discoveries in the near future.

Source: NASA’s Goddard Space Flight Center

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