Unless you’re reading this in an aircraft or the International Space Station, then you’re currently residing on the surface of a planet. You’re here because the planet is here. But how did the planet get here? Like a rolling snowball picking up more snow, planets form from loose dust and gas surrounding young stars. As the planets orbit, their gravity draws in more of the lose material and they grow in mass. We’re not certain when the process of planet formation begins in orbit of new stars, but we have incredible new insights from one of the youngest solar systems ever observed called IRS 63.
Swirling in orbit of young stars (or protostars) are massive disks of dust and gas called circumstellar disks. These disks are dense enough to be opaque hiding young solar systems from visible light. However, energy emanating from the protostar heats the dust which then glows in infrared radiation which more easily penetrates obstructions than wavelengths of visible light. In fact, the degree to which a newly forming star system is observed in either visible or infrared light determines its classification. Class 0 protostars are completely enshrouded and can only be observed in submillimeter wavelengths corresponding to far-infrared and microwave light. Class I protostars, are observable in the far-infrared, Class II in near-infrared/red, and finally a Class III protostar’s surface and solar system can be observed in visible light as the remaining dust and gas is either blown away by the increasing energy of the star AND/OR has formed into PLANETS! That’s where we came from. That leftover material orbiting newly forming stars is what accumulates to form US. The whole process from Class 0 to Class III, when the solar system leaves its cocoon of dust and joins the galaxy, is about 10 million years. But at what stage does planet formation begin? The youngest circumstellar disks we’d observed are a million years old and had shown evidence that planetary formation had already begun. The recently observed IRS 63 is less than 500,000 years old – Class I – and shows signs of possible planet formation. The excitement? We were surprised to see evidence of planetary formation so early in the life of a solar system.
“Whether planets already exist or not in the disk of IRS 63, it is clear that the planet formation process begins in the young protostellar phases, earlier than predicted by current planet formation theories.”
– Segura-Cox et al. 2020
The picture above is of protostar IRS 63 as imaged by ALMA (Atacama Large Millimeter/Submillimeter Array). ALMA can peer through the dusty shroud of the circumstellar disk surrounding the star system. IRS 63 is located 144 parsecs from Earth (about 470 light years) with a disk radius of 82AU (astronomical units or the average Earth-Sun distance of 150 million km). While we’ve identified even younger protostars, their disks are oriented edge on or at near-edge on angles such that it’s difficult to observe their features. From our vantage point, IRS 63 is tilted toward us at 45 degrees offering a view of the early stages of a solar system’s formation. To enhance the contrast and detail of the image, a computer model of IRS 63 was created that was “smooth” as if dust and gas had accumulated around the star without any disturbances – a “perfect” disk. This computer model was then subtracted from the actual image enhancing the differences between the real disk and simulated disk.
An international team of scientists led by Astronomer Dominqiue Segura-Cox of the Max-Planck Institute observed four key features within the disk – two rings (R1 and R2) and two gaps (G1 and G2). The inner ring, R1 is located at a radius of 27AU with a width of 6AU while R2 is located at a radius of 51AU with a width of 13AU. G1 is at radius 19AU with a width of 3.2AU while G2 is at Radius 37AU with a width of 4.5AU
Mind the Gap
The gap and ring features may be indicative of planet formation or the processes which give rise planet formation. Gaps observed in more mature circumstellar disks are known to be caused by protoplanets that “shepherd” dust into clearly observable rings while carving out a gap where the planet orbits. Gaps form where disk material has been captured by the protoplanet’s gravity and become incorporated into the planet itself. In more mature Class II disks, the gaps show almost no infrared dust emission meaning they are nearly devoid of dust. IRS 63’s gaps still show some dust emission meaning there is still trace dust in the gaps. So, are there planets orbiting IRS 63 then? The team says the answer is “ambiguous.” BUT, if the gaps are created by orbiting protoplanets, their sizes can be estimated. The G1 gap could be home to a planet that is approximately 0.47 Jupiter’s mass and G2 could host a planet 0.31 Jupiter’s mass.
While the gaps could be carved out by accreting protoplanets, the rings may also be catalysts of protoplanet formation. An outlying problem exists in our models of planet formation called the “radial drift problem.” Friction between dust in the disk creates a drag effect that causes the dust to lose momentum and drift or “fall” across the radius of the disk into the star. Think less orbit and more circling a drain. But clearly we have star systems, so there must be a natural process which prevents the dust in a system from spiraling into the protostar. The ring structures may be what saves the system. The rings are formed by volatile gases in the circumstellar disk which are pressurized by the energy of the star. As dust falls inward, gases in the disk push outward creating a barrier where dust piles up and can accrete into protoplanets.
Again, we don’t know for certain if planets or protoplanets exist within the swirling gas and dust of IRS 63. If planets do exist, the system is too young for them to be directly observed. However, the research team says, “if planet formation is already beginning in the disk of IRS 63, then planets and protostars likely grow and evolve together from early times.” Even earlier than anticipated. The images of IRS 63 also support hypotheses of gas giant formation. Closer to the protostar, gases are heated and excited by energy from the star such that they can’t coalesce into a protoplanet. Instead, the gases would have to accrete outside the “snowline” radius from the star where they are frozen and can collect onto a planet surface. Jupiter currently orbits at 5.2 AU but simulations suggest it formed much farther out, at nearly 30AU, and then migrated inward over time. If the gaps in IRS 63 are indicative of gas giant formation, they would be consistent then with models that predict Jupiter’s formation at a more distant radius in our own solar system.
Among all I’ve learned about space, this reality of our existence is always the most humbling and awe inspiring: The Earth, the life on Earth, you, me – we’re literally made of the dust and gases of the stars. We all began, like IRS 63, as a swirling mass collected together by fundamental forces of nature to become rocks, and oceans, and clouds, and cells, and legs, and wings, and paper, and telescopes, and computers, and starships. As Dr. Jill Tarter of SETI says, “We, all of us, are what happens when a primordial mixture of hydrogen and helium evolves for so long that it begins to ask where it came from.”
More to Explore:
Was Jupiter born Beyond the Current Orbits of Neptune and Pluto? – PNASFurther Mid-Infrared Study of the rho Ophiuchi cloud young stellar population: Luminosities and masses of pre-main-sequence stars – Astrophysical Journal