Knock, Knock. Who’s There? A Free-Floating Planet! Knock, Knock. Who’s There?

An illustration of a free-floating planet
Credit: NASA’s Goddard Space Flight Center Conceptual Image Lab

Title: Formation History of HD106906 and the Vertical Warping of Debris Disks by an External Inclined Companion
Authors: Nathaniel Moore et al.
First Author’s Institution: Georgia Institute of Technology
Status: Accepted to ApJ

When early astronomers theorized how planets formed, they often used the solar system as a model, mainly because that was all we had observationally available at the time. The thing is, the solar system is pretty “well behaved” — the planets are more or less in the same orbital plane, and their orbits are not too eccentric (i.e., they are closer to being circles than ellipses). However, as more exoplanets are found, astronomers begin to question their ideas for how planets formed. The binary system HD 106906, for example, has an asymmetrical debris disk and a planet that is not in the same plane as the disk and is separated from the stars by 730 astronomical units (au; for reference, Earth is 1 au away from the Sun). This system has an unusual architecture, and the authors of today’s article try to theorize how this system formed. Understanding the formation of an usual system like this one allows us to expand our knowledge of planet formation beyond the simplicity of such well-behaved systems such as our own!

There are two main theories for the formation of planets: core accretion and gravitational instability (also called the gas-collapse model). Figure 1 shows the two different scenarios. In both cases, the planets form in a protoplanetary disk, which means that the planets initially start in the same orbital plane. A system like HD 106906 challenges this notion, since it has a very massive planet far away from the disk and in a different orbital plane. The authors of this article explore the idea that the planet HD 106906 b actually formed from the disk, but a recent (about 1–5 million years ago) close encounter with a free-floating planet knocked the planet away from the disk into an eccentric orbit, and the interactions from this close encounter actually caused the disk to become more eccentric as well. The authors explore this idea using N-body simulations (a simulation of how bodies interact over a period of time) of the system combined with simulations of how the observational data would look for this scenario. They then compare the simulations to real observations.

Figure 1. The two possible scenarios for planet formation: accretion model (“bottom-up”) and gravitational instability (“top-down”). Click to enlarge. Credit: NASA and A. Feild (STScI); CC BY 4.0

Companion and Disk Interactions

The authors first try to determine whether the HD 106906 system has been like this for a long time or if its current configuration is the result of a recent event. To do this, they simulate different variations of the planet’s eccentricity, inclination, and semi-major axis. For the simulations, they include the effects of radiation pressure. They also use two different central body configurations: one with a binary star system and another with a single central body and an extra J2 potential term, which emulates the binary system but is more computationally efficient. The main results from these simulations are shown in Figure 2.

Figure 2: The simulations at 1 million years (top left), 5 million years, (bottom left) and 10 million years (bottom right) compared to the Crotts et al. 2021 original observations. After a million years, the simulated disk is very similar to the real image. By 5 and 10 million years, the appearance (size and brightness) of the disk exceeds the observational constraints. Credit: Moore et al. 2023

The simulations lead the authors to conclude that the disk and planet have likely been in this configuration for only 1–5 Myr, which for the system’s age of 13 Myr is quite recent. If it had been there for longer than that, the simulations for 5 and 10 Myr would have been within the observational constraints from the real data.

Figure 3: The free-floating planet, represented in red, can simply “fly by” the system, leaving the original configuration mostly unchanged; be exchanged with the original planet (blue), which then gets ejected; or be captured into the system. Credit: Moore et al. 2023

Knock, Knock. Who’s There?

Next, the authors simulate a close encounter between a 11±1 Jupiter-mass free-floating planet and the native planet of the HD 106906 system to see if this can cause the system’s current arrangement. Figure 3 shows the possible outcomes of simulations of this (un)expected visit. The authors simulate 100,000 initial conditions and see their outcomes. These 100,000 conditions are obtained such that the closest approach distance of the planets is less than 50 au (if it winds up being more than that, the initial condition is rejected).

The team’s final results are shown in Figure 4. From the figure, we can see that a few outcomes in which either the free-floating planet stays in the system or the native planet stays in the system agree with observations. The authors conclude that an encounter with a free-floating planet is a possible explanation for the current architecture of this system. The close encounter only reproduces observational results 0.2% of the time, but this system is quite unusual — so a low probability of a system forming like this is expected!

Figure 4: The final results of the close encounter simulations of the free-floating planet and the HD 106906 system. The dots within the dashed square fall within the expected observational constraints of the companion (i.e., that agree with current estimates for the orbital eccentricity, semi-major axis, and inclination of the companion). The blue dots represent the outcomes that agree with observations where the native planet remains bound to the system. The red dots represent the outcomes where the free-floating planet remains bound to the system. The gray dots represent the other parameters of the 100,000 simulations. Credit: Moore et al. 2022

By Astrobites

Original astrobite edited by H Perry Hatchfield.

Source: American Astronomical Society - AAS Nova

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About the author, Clarissa Do O:

I am a third-year physics graduate student at UC San Diego. I study exoplanet orbital dynamics and also work on exoplanet instrumentation. My current work is on the adaptive optics upgrade of the Gemini Planet Imager 2.0, an instrument that aims to directly image and characterize exoplanets.

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Exiled exoplanet possibly kicked out of star’s local neighborhood

Two direct images of the cometary dust and exoplanet surrounding the young star HD 106906. The wider field in blue shows Hubble Space Telescope data where the star’s blinding light is artificially eclipsed (gray circular mask). The point to the upper right is an 11-Jupiter-mass planet located more than 650 times the distance from the Earth to the sun. A new discovery in these Hubble observations is an extremely asymmetric nebulosity indicating a dynamically disturbed system of comets. Surprisingly, the planet is located 21 degrees above the plane of the nebulosity. The circular orange inset shows a region much closer to the star that can only be detected using advanced adaptive optics from the ground-based Gemini Observatory. Using the Gemini Planet Imager (GPI), researchers found a narrow loop of nebulosity suggesting that a planetary system formed close to the star, but somehow the architecture of the outer regions is severely disrupted. The investigators also find that the planet HD 106906 b may have a dusty ring system of its own, motivating future work with Hubble and ground-based astronomical observatories. Image by Paul Kalas/UC Berkeley. Download Full Image 

ASU scientists zoom in to study the dynamics that drive planet formation.

A planet discovered last year sitting at an unusually large distance from its star – 16 times farther than Pluto is from the sun – may have been kicked out of its birthplace close to the star in a process similar to what may have happened early in our own solar system’s history.

Images from the Gemini Planet Imager (GPI) in the Chilean Andes and the Hubble Space Telescope show that the star has a lopsided comet belt indicative of a very disturbed solar system, and hinting that the planet interactions that roiled the comets closer to the star might have sent the exoplanet into exile as well.

The planet may even have a ring of debris it dragged along with it when it was expelled.

“The measurements we made on the planet suggest it may be dustier than comparison objects, and we are making follow-up observations to check if the planet is really encircled by a disk – an exciting possibility,” said Abhijith Rajan, a graduate student at Arizona State University’s School of Earth and Space Exploration (SESE), who analyzed the planet’s images.

“We think that the planet itself could have captured material from the comet belt, and that the planet is surrounded by a large dust ring or dust shroud,” said Paul Kalas, an adjunct professor of astronomy at UC Berkeley and lead author of a paper about the results, published by the The Astrophysical Journal on November 20, 2015.

Such planets are of interest because in its youth, our own solar system may have had planets that were kicked out of the local neighborhood and are no longer among of the eight planets we see today.

“Is this a picture of our solar system when it was 13 million years old?” asks Kalas. “We know that our own belt of comets, the Kuiper belt, lost a large fraction of its mass as it evolved, but we don’t have a time machine to go back and see how it was decimated. One of the ways, though, is to study these violent episodes of gravitational disturbance around other young stars that kick out many objects, including planets.”

The new observations may have recorded this type of disk disruption around a younger star. “Studying the planets and disks around young stars is a way to investigate the processes that impacted the formation and evolution of our Solar System” says Jennifer Patience, associate professor at Arizona State University’s School of Earth and Space Exploration, who co-lead the development of the target sample for the survey.

The disturbance could have been caused by a passing star that perturbed the inner planets, or a second massive planet in the system. The GPI team also looked for another large planet closer to the star that may have interacted with the exoplanet, but found nothing outside a Uranus-sized orbit.

Kalas and Rajan will discuss the observations during a Google+ Hangout On Air at 7 a.m. Hawaii time (10AM Arizona time) on Dec. 1 during Extreme Solar Systems III, the third in a series of international meetings on exoplanets that this year takes place on the 20 anniversary of the discovery of the first exoplanet in 1995. Viewers without Google+ accounts may participate via YouTube.

Young, 13-million-year-old star

The star, HD 106906, is located 300 light years away in the direction of the constellation Crux and is similar to the sun, but much younger: about 13 million years old, compared to our sun’s 4.5 billion years. Planets are thought to form early in a stars history, however, and in 2014 a team led by Vanessa Bailey at the University of Arizona discovered a planet HD 106906 b around the star weighing a hefty 11 times Jupiter’s mass and located in the star’s distant suburbs, an astounding 650 AU from the star (one AU is the average distance between Earth and the sun, or 93 million miles).

Planets were not thought to form so far from their star and its surrounding protoplanetary disk, so some suggested that the planet formed much like a star, by condensing from its own swirling cloud of gas and dust. The GPI and Hubble discovery of a lopsided comet belt and possible ring around the planet points instead to a normal formation within the debris disk around the star, but a violent episode that forced it into a more distant orbit.

Kalas and a multi-institutional team using GPI first targeted the star in search of other planets in May 2015 and discovered that it was surrounded by a ring of dusty material very close to the size of our own solar system’s Kuiper Belt. The emptiness of the central region – an area about 50 AU in radius, slightly larger than the region occupied by planets in our solar system – indicates that a planetary system has formed there, Kalas said.

He immediately reanalyzed existing images of the star taken earlier by the Hubble Space Telescope and discovered that the ring of dusty material extended much farther away and was extremely lopsided. On the side facing the planet, the dusty material was vertically thin and spanned nearly the huge distance to the known planet, but on the opposite side the dusty material was vertically thick and truncated.

“These discoveries suggest that the entire planetary system has been recently jostled by an unknown perturbation to its current asymmetric state,” he said. The planet is also unusual in that its orbit is possibly tilted 21 degrees away from the plane of the inner planetary system, whereas most planets typically lie close to a common plane.

Kalas and collaborators hypothesized that the planet may have originated from a position closer to the comet belt, and may have captured dusty material that still orbits the planet. To test the hypothesis, they carefully analyzed the GPI and Hubble observations, revealing three properties about the planet consistent with a large dusty ring or shroud surrounding it. However, for each of the three properties, alternate explanations are possible.

The investigators will be pursuing more sensitive observations with the Hubble Space Telescope to determine if HD 106906b is in fact one of the first exoplanets that resembles Saturn and its ring system.

The inner belt of dust around the star has been confirmed by an independent team using the planet-finding instrument SPHERE on the ESO’s Very Large Telescope in Chile. The lopsided nature of the debris disk was not evident, however, until Kalas called up archival images from Hubble’s Advanced Camera for Surveys.

The GPI Exoplanet Survey, operated by a team of astronomers at UC Berkeley and 23 other institutions, is targeting 600 young stars, all less than approximately 100 million years old, to understand how planetary systems evolve over time and what planetary dynamics could shape the final arrangement of planets like we see in our solar system today.

Among Kalas’s coauthors are associate professor Jennifer Patience and graduate student Abhijith Rajan of Arizona State University’s School of Earth and Space Exploration, a unit of ASU’s College of Liberal Arts and Sciences. The research was supported by the NSF and NASA’s Nexus for Exoplanet System Science (NExSS) research coordination network sponsored by NASA’s Science Mission Directorate.

News Link:  https://asunow.asu.edu/20151201-exiled-exoplanet-possibly-kicked-out-local-neigh...

 

 

Source: SESE - School of Earth & Space Exploration/ASU -Arizona State University