Planets Don’t Wait for Their Star to Form First

It looks like we may have to update our theories on how stars and planets form in new solar systems. A team of astronomers has discovered young planets forming in a solar system that’s only about 500,000 years old. Prior to this discovery, astronomers thought that stars are well into their adult life of fusion before planets formed from left over material in the circumstellar disk.

Now, according to a new study, it looks like planets and stars can form and grow up together.

Astronomers have examined plenty of young solar systems. Typically, a young star is surrounded by a disk of dust, and astronomers can see rings being cleared out in the dust by young planets as they form. In those instances, the young star has already gathered its mass. But not in this instance.

“Traditionally it was thought that a star does most of its formation before the planets form, but our observations showed that they form simultaneously.”

Ian Stephens, co-author, CfA

The title of the new research is “Four annular structures in a protostellar disk less than 500,000 years old.” The lead author is Dominique Segura-Cox, a scientist at the Max Planck Institute for Extraterrestrial Physics (MPE) in Germany. The paper is published in the journal Nature.

The study is centered on a young protostar named IRS 63. It’s about 470 light years away from us, in the direction of the constellation Ophiuchus. IRS 63 is deeply embedded in the dense LI709 interstellar cloud.

The dense L1709 region of the Ophiuchus Molecular Cloud mapped by the Herschel Space Telescope, which surrounds and feeds material to the much smaller IRS 63 protostar and planet-forming disk (location marked by the black x).
Credit: MPE/D. Segura-Cox Data credit: ESA/Herschel/SPIRE/PACS/D. Arzoumanian

IRS 63 is different than other young stars. It shows the telltale dust rings that signal the presence of young planets, but it’s only half the age of other young stars that show the same rings. At younger than 500,000 years old, the baby star hasn’t even finished gathering its own mass yet, and is still in the process of forming.

“We observed the young protoplanetary disk called IRS 63 and found gaps and rings within the disk, which is indicative of planet formation,” said study co-author Ian Stephens, an astronomer at the Center for
Astrophysics | Harvard & Smithsonian (CfA). “Traditionally it was thought that a star does most of its formation before the planets form, but our observations showed that they form simultaneously.”

In the same press release, lead author Segura-Cox added, “The rings in the disk around IRS 63 are so young. We used to have this idea that stars entered adulthood first and were the mothers of planets that came afterwards, but now we see that protostars and planets grow and evolve together from early times like siblings.”

In the study, the authors write “The annular substructures we observe towards the disk of the young protostar IRS 63 indicate that the conditions for planetesimal formation probably begin at extremely early times, setting the stage for the first generation of planets to form.”

The team of researchers were able to estimate the masses of the planets that are forming around the young star. The authors write that “…the planetary mass required to open each gap can be estimated from our dust observations.” They added: “Assuming each gap is opened by one planet, for gap G1 the planet mass required is 0.47MJupiter (where MJupiter is the mass of Jupiter), and gap G2 requires a planet mass of 0.31MJupiter.”

The team cautions that these are upper mass limits, and they also mention that these masses are surprising, and larger than expected. “The estimated planet masses in the young disk of IRS 63 are already comparable to Jupiter masses and are surprisingly large at these radii, considering that in the early stages of formation, planets face serious barriers to growing to such large masses on short time scales as well as avoiding runaway accretion at later stages of their formation.”

The rings and gaps in the IRS 63 dust disk are shown next to a sketch of the Solar System orbits drawn at the same size scale and orientation of the IRS 63 disk. The locations of the rings are similar to the locations of objects in our own Solar System, with the inner ring about the size of Neptune’s orbit and the outer ring a little larger than Pluto’s orbit.
Credit: MPE/D. Segura-Cox Data credit: ALMA (ESO/NAOJ/NRAO)

The findings from the IRS 63 system are helping transform our understanding of young solar systems. But they may also be telling us something about our own Solar System, and how it formed.

Gas giants like Jupiter require about 10 Earth masses of solid material to form. That material becomes the core, and a massive amount of gas accretes around it. The team studying IRS 63 measured the amount of material in the young disk and found approximately 150 Earth masses of material, both dust and gas. When the team paired that measurement with measurements of the circumstellar disk and the rings and gaps in it, they learned something.

“These rings and gaps suggest that we are seeing the earliest evidence of planet formation, and that planets certainly start to form within the first half million years, and probably within the first 150,000 years,” said Stephens. “Planets, especially planets like Jupiter, started their own formation at one of the earliest stages of the star formation process.”

Astronomers think that Jupiter formed out beyond Neptune initially, and then migrated inward to eventually take up its present position. Some research shows that it took 700,000 years to migrate. These new observations of the material in IRS 63’s disk seem to back that up. The team thinks that the amount of material in the disk and the young age of the system are very similar to the conditions at a similar age in our own Solar System.

“The size of the disk is very similar to our own solar system,” said Segura-Cox. “Even the mass of the protostar is just a little smaller than the mass of our Sun. Studying such young planet-forming disks around protostars can give us important insights into our own origins.”

This result is surprising, although in a way it shouldn’t be. These young star systems are notoriously difficult to see into. All of the gas and dust makes observing them difficult, meaning there’s bound to be things going on these systems that astronomers haven’t seen yet. There’s been quite a bit of progress in recent years, though. Thankfully, ALMA is able to probe these systems by observing the emissions from dust grains.

An ALMA image of a protoplanetary disk from 2019. This picture of the nearby young star TW Hydrae reveals the classic rings and gaps that signify planets are in formation in this system. Credit: S. Andrews (Harvard-Smithsonian CfA); B. Saxton (NRAO/AUI/NSF); ALMA (ESO/NAOJ/NRAO)

In 2019, astronomers used ALMA (Atacama Large Millimeter-submillimeter Array) to peer into the young solar system forming around TW Hydrae, a young T-Tauri star about 196 light years away. They were able to see the rings and gaps in the disk of material around the star that are the tell-tale signs of planet formation.

In fact, watching as young planets carve gaps in a stellar disk is almost commonplace. Astronomers using ALMA have spotted many of them now.

ALMA’s high-resolution images of nearby protoplanetary disks, which are results of the Disk Substructures at High Angular Resolution Project (DSHARP). Credit: ALMA (ESO/NAOJ/NRAO), S. Andrews et al.; NRAO/AUI/NSF, S. Dagnello

Remarkably, the ESO’s Very Large Telescope was even able to spot two baby planets in one system, in 2018. The images don’t show much detail, and they don’t tell us much, but it’s still a milestone.

This spectacular image from the SPHERE instrument on ESO’s Very Large Telescope is the first clear image of a planet caught in the very act of formation around the dwarf star PDS 70. Credit: ESO/A. Müller et al.

Like a lot of areas of astronomy, the upcoming James Webb Space Telescope will make an important contribution. With its coronagraph and its ability to see in the mid-infrared, it’ll be able to look into these young solar systems to see what’s going on. The Webb’s sensitivity will be far superior to other telescopes. Astronomers will use its power and sensitivity to look into young systems like IRS 63. Who knows what they’ll find.

“Our program is looking at young, newly formed planets and the systems they inhabit, explained co-principal investigator Beth Biller of the University of Edinburgh, in a press release. “Webb is going to allow us to do this in much more detail and at wavelengths we’ve never explored before. So it’s going to be vital for understanding how these objects form, and what these systems are like.”

A Gemini Planet Imager (GPI) image of HR 4796A, the larger star in the binary HR 4796 system. It shows the circumstellar disk around HR 4796A, a ring of dust and planetesimals similar in some ways to a scaled up version of the solar system’s Kuiper Belt. Image credit: Marshall Perrin (Space Telescope Science Institute), Gaspard Duchene (UC Berkeley), Max Millar-Blanchaer (University of Toronto), and the GPI Team.

The James Webb’s Early Release Science program has already chosen a target. HR 4796 is a young binary star systen with planetesimals that astronomers have been watching for 20 years already. Astronomers think that this system, with its planetesimals and its debris ring, is representative of many young solar systems.

Depending on what the JWST finds, we may have to update our theories again. Let’s hope so.

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