Giant planets and comets battle in planet-forming disk

A study of dust in the disk around the star HD 163296 suggests we’re glimpsing a gravitational interaction between giant planets and much-smaller objects, the future asteroids and comets of this newly forming solar system.

A yellow star in the midst of an oval-shaped, orange disk, with 3 concentric rings.

Image from the ALMA telescope in Chile – not an artist’s concept – of the young star HD 163296. The star is surrounded by a disk of gas and dust, where at least 3 giant planets are thought to be forming … duking it out with this system’s future comets and asteroids. Image via ALMA/S. Dagnello/Istituto Nazionale di Astrofisica.

The Istituto Nazionale di Astrofisica (INAF) – headquartered in Rome, Italy – announced a new study on May 23, 2019, that provides a key glimpse into the process by which solar systems build their planets. The study is based on observations with the ALMA telescope in Chile. It explored whether the anomalous features in the dust and gas distributions in the planet-forming disk of a distant star – called HD 163296 – could arise from an interaction of the system’s giant planets with its planetesimals, or planet-building blocks. Leftover planetesimals, those that don’t go into forming planets, will one day become this system’s asteroids and comets.

The new study is published in the peer-reviewed Astrophysical Journal.

For centuries, astronomers have theorized that planets form in a flattened disk of gas and dust encircling a newly born star. In 2014, ALMA became the first to capture detailed images of these circumstellar disks, specifically a first image of bright concentric rings in a disk, around the star HL Tau. Thus the process by which solar systems are born is being revealed. Since then, ALMA has been capturing even smaller-scale structures in circumstellar disks – gaps, rings and spiral arms – most of them believed to be linked to the presence of young planets and to arise from the interplay of the new planets’ gravity with their surroundings. A statement from INAF explained:

Among the best-studied disks observed by ALMA is that surrounding HD 163296, a 5 million-year-old star about twice the mass of our sun. HD 163296’s disk is both massive (a bit less than one tenth of the sun’s mass) and wide (about 500 au, twice the outer boundary of the Kuiper Belt in the solar system) and has been proposed to be home to at least three planets with masses comprised between twice that of Uranus and the one of Jupiter. ALMA’s most recent observations allowed to spatially and compositionally characterize the structure of HD 163296’s disk to a level previously undreamed of and showed how dust is still quite abundant (more than 300 times the mass of the Earth) in this disk notwithstanding its age and having produced at least three giant planets. The same observations also revealed some strange behaviors of the dust spatial distribution that could not easily be explained only as the result of its interplay with the gas and the newly formed giant planets.

As planets form in a disk, the dust in the disk is thought to be swept up, so that it decreases over time. Astronomers expected dust to disappear over time from the region immediately inside HD 163296’s innermost planet. At the same time, they thought, dust coming from the outer regions of the disk should pile up outside the orbits of the second and third planets. ALMA’s observations revealed instead that the regions inside the first planet and between the first and second planets have some of the highest concentrations of dust of the whole disk. The new study explored whether these anomalous dust features could arise from the interaction of the giant planets with a component of the disk previously unaccounted for: the planetesimals.

Diego Turrini of INAF – lead author of the study – said:

From the study of the solar system we know that mature circumstellar disks like HD 163296 are not composed only by gas and dust, but also contain an invisible population of small planetary objects similar to our asteroids and comets.

Headshot of a handsome, bearded young man standing in a desert landscape.

Astronomer Diego Turrini of the Istituto Nazionale di Astrofisica, headquartered in Rome. He led a team that explored small-scale structures in the dusk in the disk around the star HD 163296. Image via INAF.

Turrini and his colleagues performed computer simulations showing how, during the growth of HD 163296’s three giant planets, a larger and larger fraction of the surrounding population of planetesimals is injected on very eccentric and very inclined orbits similar to those of the comets in our solar system. Francesco Marzari of the University of Padova, co-author of the study, commented:

The main outcome of this dynamical excitation is a higher rate of violent collisions among the planetesimals.

The team found that the collisions among planetesimals remain quite gentle until the giant planets approach their final masses but then they rapidly grow a hundredfold in violence and start grinding down the planetesimals. Marzari said:

These violent collisions replenish the dust population in the disk. The new dust produced by this process, however, has a different orbital distribution than the original one and mainly concentrates in two places: the orbital region within the first giant planet and the ring between the first and the second giant planets.

Read more details about the outcome of the study here

Leonardo Testi, also co-author of the study and head of the ALMA Support Center of the European Southern Observatory, said:

This study was started as a pathfinder project to explore whether the dynamical excitation caused by newly formed giant planets could actually produce observable effects. As such, we just scratched the surface of this process and its implications. Nevertheless, its physical recipe is quite simple: massive planets forming in a disk of planetesimals. Given the widespread signatures of possible young giant planets we are discovering with ALMA and the extended duration of the dynamical effects caused by their appearance, we might be looking to a process that is quite common among circumstellar disks.

And so the question of how our Earth and solar system were made is being answered!

The concentric rings around the star HD 163296, viewed from the top down and edgewise.

Graphic showing the disk of icy planetesimals hidden in HD 163296’s circumstellar disk seen from above and the side. The young giant planets rapidly create a large population of exocomets acting as high-speed projectiles for the other bodies. Image via D. Turrini/INAF-IAPS.

Bottom line: New telescopic observations of young star HD 163296 show rings of dust in its surrounding dust cloud indicating that giant planets are interacting with small bodies that will become asteroids and comets.

Source: Dust-to-gas Ratio Resurgence in Circumstellar Disks Due to the Formation of Giant Planets: The Case of HD 163296

Read more via INAF

Deborah Byrd