If you are not reading this on an airplane or on the International Space Station, you are currently on the surface of a planet. You are here because the planet is here. But how did the planet get here? Like a rolling snowball that absorbs more snow, planets form of loose dust and gas that surround young stars. As the planets orbit, their gravity pulls in more of the lost material and they grow in mass. We're not sure when the process of planet formation begins in new star orbit, but we have incredible new insights from one of the youngest solar systems ever observed called IRS 63.
The Rho Ophiuchi cloud complex is a nebula of gas and dust that is located in the constellation of Ophiuchus. It is one of the closest star-forming regions to the solar system where the young star system IRS 63 was observed
In the orbit of young stars (or protostars), massive dust and gas disks, so-called circumstellar disks, swirl. These disks are dense enough to be opaque and protect young solar systems from visible light. However, the energy emanating from the protostar heats the dust, which then glows in infrared radiation, penetrating the obstacles more easily 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 enveloped and can only be observed at wavelengths in the sub-millimeter range, which correspond to far-infrared and microwave light. Class I protostars can be observed in the far infrared, class II in the near infrared / red, and finally the surface and solar system of a class III protostar can be observed in visible light as the remaining dust and gas are either blown away by the increase become energy of the star AND / OR has formed into PLANETS! That's where we came from. This leftover material, orbiting newly formed stars, accumulates to form US. The entire process from Class 0 to Class III, in which the solar system leaves its dust cocoon and joins the galaxy, takes about 10 million years. But at what stage does planet formation begin? The youngest circumstellar disks we observed are a million years old and had shown evidence that planet 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.
IRS 63 Circumstellar Disk C. ALMA / Segura-Cox et al. 2020
"Whether or not planets already exist in the disk of IRS 63, it is clear that the planet formation process in the young protostellar phases begins earlier than current theories of planet formation predict."
– Segura-Cox et al. 2020
The picture above shows the protostar IRS 63 as it was imaged by ALMA (Atacama Large Millimeter / Submillimeter Array). ALMA can see through the dusty shroud of the circumstellar disk surrounding the star system. IRS 63 is located 144 parsecs from Earth (approximately 470 light years) with a disk radius of 82AU (astronomical units, or the average distance between Earth and Sun of 150 million km). Although we have identified even younger protostars, their disks are oriented at close-to-edge or close-to-edge angles, making their features difficult to observe. From our point of view, IRS 63 is tilted 45 degrees towards us and offers a view of the early stages of the formation of a solar system. To improve the contrast and details of the image, a computer model was created by IRS 63 that was “smooth”, as if dust and gas had gathered undisturbed around the star – a “perfect” disk. This computer model was then subtracted from the actual image, which increased the differences between the real hard drive and the simulated hard drive.
An international team of scientists led by the astronomer Dominqiue Segura-Cox from the Max Planck Institute observed four key features within the disk – two rings (R1 and R2) and two gaps (G1 and G2). Inner ring R1 is in a radius of 27AU with a width of 6AU, while R2 is in a radius of 51AU with a width of 13AU. G1 has a radius of 19AU with a width of 3.2AU, while G2 has a radius of 37AU with a width of 4.5AU
Left to Right: The original image from the IRS 63, the simulated image and the resulting difference between the two, improving the ring and gap functions. C. ALMA / Segura-Cox et al. 2020Ring and gap functions of IRS 63 c. ALMA / Segura-Cox et al. 2020
Deal with the gap
The gap and ring features may indicate planetary formation or the processes that lead to planetary formation. It is known that gaps observed in more mature circumstellar disks are caused by protoplanets "guarding" dust in clearly observable rings as they cut out a gap in which the planet orbits. Gaps form where disc material has been captured by the gravity of the protoplanet and built into the planet itself. On more mature Class II hard drives, the gaps show almost no infrared dust emission, which means that they are almost dust-free. The gaps of the IRS 63 still show some dust emission, which means that there is still trace dust in the gaps. Then are there planets orbiting IRS 63? 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 roughly 0.47 mass of Jupiter, and G2 could be home to a planet that is 0.31 mass of Jupiter.
While the gaps could be worked out by the accumulation of protoplanets, the rings can also be catalysts of protoplanet formation. There is an external problem in our models of planet formation called the "radial drift problem". The friction between dust in the disk creates a drag effect which causes the dust to lose momentum and drift or "fall" into the star over the radius of the disk. Think less orbit and more orbit around a drain. Sure, we have star systems, so there has to be some natural process that prevents the dust in a system from spiraling in the protostars. The ring structures can save the system. The rings are formed by volatile gases in the circumstellar disk that are pressurized by the star's energy. When dust falls inward, gases in the disk push outward, creating a barrier against which dust accumulates and can aggregate into protoplanets.
Again, we don't know for sure whether planets or protoplanets are present in the swirling gas and dust of IRS 63. When planets exist, the system is too young to be observed directly. However, the research team says, "If planet formation begins on the disk of IRS 63, planets and protostars are likely to grow and coalesce at an early age." Even earlier than expected. The images from IRS 63 also support hypotheses of the formation of gas giants. Closer to the protostar, gases are heated and excited by the energy of the star so that they cannot merge into a protoplanet. Instead, the gases would have to collect outside the radius of the star's “snow line”, where they can be frozen and collect on a planet's surface. Jupiter is currently orbiting at 5.2 AU, but simulations suggest that it formed much further out at almost 30 AU and then moved inward over time. If the gaps in IRS 63 indicate the formation of gas giants, then they agree with models predicting the formation of Jupiter at a more distant radius in our own solar system.
Of all that I have learned about space, this reality of our existence is always the most humble and impressive: the earth, life on earth, you, me – we are literally made of the dust and gases of the stars. Like IRS 63, we all began as a swirling mass that was brought together by fundamental forces of nature to form stones, oceans, clouds, cells, legs, wings, paper, telescopes, computers and spaceships. As Dr. Says Jill Tarter of SETI, "We are all what happens when a primordial mixture of hydrogen and helium evolves for so long that she begins to wonder where it came from."
More to discover:
Four ring-shaped structures in a protostellar disk less than 500,000 years old (original publication) – Segura-Cox et al. 2020
Planet formation in stellar childhood – Smithsonian Astrophysical Observatory
How do planets form? Semarkona meteorite shows some clues – universe today
There is no chemical difference between stars with or without planets – universe today
Was Jupiter born beyond the current orbits of Neptune and Pluto? – PNAS Further mid-infrared study of the young star population of the Rho Ophiuchi cloud: luminosity and mass of the stars before the main sequence – Astrophysical Journal