April 13, 2024

Where do all these rebel planets come from?

There is a population of planets that roam the space free from any star. They are called rogue planets or floating planets (FFPs). Some FFPs graduate as loners, having never enjoyed the company of a star. But most are ejected from solar systems in some way, and this can happen in different ways.

A researcher decided to try to understand the FFP population and how it came to be.

FFPs are also called isolated planetary-mass objects (iPMOs) in scientific literature, but regardless of the name used, they are the same thing. These planets roam alone in interstellar space, divorced from any relationship with stars or other planets.

FFPs are mysterious because they are extremely difficult to detect. But astronomers are getting better at it and getting better tools for the task. In 2021, astronomers made a determined effort to detect them in Upper Scorpius and Ophiuchus and detected 70 of them, possibly many more.

This image shows the location of 115 potential rogue planets, highlighted with red circles, recently discovered in 2021 by a team of astronomers in a region of the sky occupied by Upper Scorpius and Ophiucus. The exact number of rogue planets found by the team is between 70 and 170, depending on the age assumed for the study region. This image was created assuming an intermediate age, resulting in a number of planet candidates between the two extremes of the study. Image credit: ESO/N. Risinger (skysurvey.org)

Broadly speaking, there are two ways FFPs can form. They can form like most planets, in protoplanetary disks around young stars. These planets form by the accumulation of dust and gas. Or they may form like stars, collapsing into a cloud of gas and dust unrelated to a star.

For planets that form around stars and are eventually expelled, there are different ejection mechanisms. They may be ejected by interactions with their stars in a binary star system, they may be ejected by a stellar flyby, or they may be ejected by planet-planet scattering.

In an effort to better understand the FFP population, one researcher examined ejected FFP. He simulated rogue planets that result from planet-planet interactions and those that come from binary star systems, where interactions with their binary stars eject them. Could there be a way to differentiate them and better understand how these objects arise?

A new paper titled “On the Properties of Floating Planets Originating in Circumbinary Planetary Systems” has addressed the problem. The author is Gavin Coleman, from the Department of Physics and Astronomy at Queen Mary University of London. The article will be published in the Monthly Notices of the Royal Astronomical Society.

In his article, Coleman points out that researchers have explored how FFPs form, but there is more to do. “Numerous work has explored mechanisms for forming such objects, but has not yet provided predictions about their distributions that could differentiate the formation mechanisms,” he writes.

Coleman focuses on ejected stars rather than stars that formed as rebels. It avoids rogue planets that result from interactions with other planets because planet-to-planet scattering is not as significant as other types of ejections. “It is important to note that planet-to-planet scattering around individual stars cannot explain the large number of FFPs seen in observations,” explains Coleman.

This artist's impression shows an example of a rogue planet with the Rho Ophiuchi cloud complex visible in the background.  The rogue planets have masses comparable to the planets in our Solar System, but they do not orbit a star, roaming freely on their own.  Image credit: ESO/M.  Kornmesser/S.  Guisard
This artist’s impression shows an example of a rogue planet with the Rho Ophiuchi cloud complex visible in the background. The rogue planets have masses comparable to the planets in our Solar System, but they do not orbit a star, roaming freely on their own. Image credit: ESO/M. Kornmesser/S. Guisard

Coleman highlights binary star systems and their circumbinary planets in his work. Previous research shows that planets are naturally ejected from circumbinary systems. In his research, Coleman simulated binary star systems and how planets ejected from those systems behave. “We found significant differences between planets ejected through planet-planet interactions and those by binary stars,” he writes.

Coleman based his simulations on a binary star system called TOI 1338. TOI 1338 has a known circumbinary planet called BEBOP-1. Using a known binary system with a confirmed circumbinary planet provides a solid basis for simulations of it. It also allowed him to compare his results with other simulations based on BEBOP-1.

The simulation varied several parameters: the initial mass of the disk, the binary separation, the resistance of the external environment and the level of turbulence in the disk. These parameters strongly govern which planets form. Other parameters used only a single value: the combined stellar mass, mass ratio, and binary eccentricity. The combined stellar mass of TOI 1338 is about 1.3 solar masses, in line with the average in binary systems of about 1.5 solar masses.

Each simulation lasted 10 million years, long enough for the solar system to take shape.

Coleman found that circumbinary systems produce FFPs efficiently. In simulations, each binary system ejects on average between two and seven planets with more than one Earth mass. For giant planets with more than 100 Earth masses, the number of ejected planets drops to 0.6 ejected planets per system.

This figure from the article shows the masses of the ejected planets.  The blue line represents all planets, the red line represents planets with less than one Earth mass, and the yellow line represents huge planets with more than 100 Earth masses.  Image credit: Coleman 2024.
This figure from the article shows the masses of the ejected planets. The blue line represents all planets, the red line represents planets with less than one Earth mass, and the yellow line represents huge planets with more than 100 Earth masses. Image credit: Coleman 2024.

The simulations also showed that most planets are ejected from their circumbinary disks between 0.4 and 4 million years after the start of the simulation. At this age, the circumbinary disc has not dissipated and exploded.

This figure shows the ejection time for planets of different masses.  Most planets that become FFPs are ejected within the first million years.  Image credit: Coleman 2024.
This figure shows the ejection time for planets of different masses. Most planets that become FFPs are ejected within the first million years. Image credit: Coleman 2024.

The most important result may concern the speed dispersions of the FFPs. “As planets are ejected from systems, they retain significant excess velocities, between 8–16 km?1. This is much larger than the observed velocity dispersions of stars in local star-forming regions,” explains Coleman. Therefore, this means that the velocity dispersions of FFPs can be used to differentiate ejecta from those that formed as loners.

Velocity dispersions provide another window into the FFP population. Coleman’s simulations show that the velocity dispersion of FFPs ejected through interactions with binary stars is about three times greater than the dispersion of planets ejected by planet-planet dispersion.

This figure shows the excess speed of the FPP population ejected in the simulations.  The color-coded bar on the right shows the amount of speeding.  The x-axis shows the distance from the pericenter because
This figure shows the excess speed of the FPP population ejected in the simulations. The color-coded bar on the right shows the amount of speeding. The x-axis shows the distance from the pericenter because “it gives an approximate location for the final interaction that led to the ejection of the planet,” according to the author. Image credit: Coleman 2024.

Coleman also found that the level of turbulence in the disk affects the planet’s ejection. The weaker the turbulence, the more planets will be ejected. Turbulence also affects the mass of ejected planets: weaker turbulence ejects less massive planets, where about 96% of ejected planets are less than 100 Earth masses.

This research issue shows how the number of ejected planets depends on the turbulence in the system.  Lower turbulence (blue) ejects more planets than intermediate (red) or strong (yellow) turbulence.  The x-axis shows the number of planets ejected per system and the y-axis shows the cumulative distribution function.  Image credit: Coleman, 2024.
This research issue shows how the number of ejected planets depends on the turbulence in the system. Lower turbulence (blue) ejects more planets than intermediate (red) or strong (yellow) turbulence. The x-axis shows the number of planets ejected per system and the y-axis shows the cumulative distribution function. Image credit: Coleman, 2024.

Taken together, the simulations provide a way to observe the FFP population and determine its origins. “Differences in FFP mass distributions, frequencies, and excess velocities may indicate whether individual stars or circumbinary systems are the fundamental birthplace of FFPs,” Coleman writes in his conclusion.

But the author also recognizes the disadvantages of his simulations and clarifies what the sims don’t tell us.

“However, although this work contains numerous simulations and explores a broad parameter space, it does not constitute a complete population of circumbinary systems in formation,” Coleman writes in his conclusion. According to Coleman, with current technology it is not feasible to obtain a complete population of these systems.

“If such a population is carried out in future work, then comparisons between this population and observed populations would give even more valuable insight into the formation of these intriguing objects,” he explains.

There’s still a lot astronomers don’t know about binary systems and how they form and eject planets. On the one hand, planetary formation models are constantly being revised and updated with new information.

We also don’t have a clear idea of ​​how many FFPs there are. Some researchers think there could be trillions of them. The upcoming Nancy Grace Roman Space Telescope will use gravitational lensing to take a census of exoplanets, including a sample of FFPs with masses as small as Mars.

In future work, Coleman aims to determine whether there are differences in the chemical composition between FFPs. This would constrain the types of stars around which they form and where they form in their protoplanetary disks. This would require spectroscopic studies of FFPs.

But, at least for now, Coleman has developed an increasingly better way of understanding FFPs. Using this data, astronomers can begin to discern where individual FFPs came from and better understand the population at large.

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