JWST Observes Protoplanetary Disks that Challenge Models of Planet Formation

JWST (The James Webb Space Telescope) was specifically meant to address some of the greatest unsettled questions in cosmology. These include all of the key questions experts have been pondering since HST (the Hubble Space Telescope) took its deepest views of the Cosmos: the Hubble Tension, how planetary systems formed, how the first stars and galaxies came together, and when the first black holes appeared. Particularly, Hubble spotted something very fascinating in 2003 when observing a star nearly as old as the Cosmos itself. Orbiting this ancient star was a gigantic planet whose very existence challenged accepted models of planet formation since stars in the early Cosmos did not have enough time to produce sufficient heavy elements for planets to form.

Thanks to recent observations by the Webb, an international team of experts announced that they may have solved this mystery. By observing stars in the Small Magellanic Cloud (LMC), which lack huge amounts of heavy elements, they discovered stars with planet-forming disks that are far longer-lived compared to those seen around young stars in our galaxy Milky Way. The research was led by Guido De Marchi (an astronomer at the European Space Research and Technology Centre) in Noordwijk, Netherlands. He was joined by scientists from the INAF Osservatorio Astronomico di Roma, Gemini Observatory/NSF NOIRLab, the Space Telescope Science Institute (STScI), the UK Astronomy Technology Centre (UK ATC), the Leiden Observatory, the Institute for Astronomy at the University of Edinburgh, the European Space Agency (ESA), NASA’s Ames Research Center, and NASA’s Jet Propulsion Laboratory. The paper detailing their findings appeared on 16^th December in The Astrophysical Journal.

According to recognized cosmological models, the first stars in the Cosmos (Population III stars) formed 13.7 billion years back, just a few hundred million years post the Big Bang. These stars were extremely hot, bright, colossal, short-lived, and composed of helium and hydrogen, with very little in the way of heavy elements. These elements were slowly forged in the interiors of Population III stars, which distributed them all over the Cosmos once they exploded in a supernova and blew off their outer layers to create star-forming nebulae.

These nebulae and their traces of weightier elements would form the next generation of stars (Population II). After these stars formed from dust and gas in the nebula that suffered gravitational collapse, the residual material fell around the new stars to form protoplanetary disks. As a consequence, subsequent populations of stars contained far higher concentrations of metals (aka. metallicity). The presence of these heavy elements, ranging from oxygen and carbon to iron and silica, led to the formation of the first planets.

As such, HST‘s discovery of a colossal planet (2.5 times the mass of Jupiter) around a star that existed just 1 billion years post the Big Bang puzzled scientists since early stars contained only miniscule amounts of heavier elements. This implied that planet formation started when the Cosmos was very young, and some planets had time to become particularly immense. Elena Sabbi (the chief scientist for the Gemini Observatory at the National Science Foundation’s NOIRLab) explained in a NASA press release:

“Current models predict that with so few heavier elements, the disks around stars have a short lifetime, so short in fact that planets cannot grow big. But Hubble did see those planets, so what if the models were not correct and disks could live longer?”

To examine this theory, the team used JWST to observe the colossal, star-forming cluster NGC 346 in the Small Magellanic Cloud (a dwarf galaxy and one of the Milky Way’s closest neighbors). This star cluster is also known to have comparatively low amounts of heavier elements and served as a nearby proxy for stellar environments during the early Cosmos. Earlier observations of NGC 346 by HST revealed that many young stars in the cluster (~20 to 30 million years old) seemed to still have protoplanetary disks around them. This was also astonishing since such disks were thought to dissipate after 2 to 3 million years.

Thanks to JWST’s high-resolution and chic spectrometers, researchers now have the first-ever spectra of young Sun-like stars and their environments in a proximate galaxy. As study leader Guido De Marchi (the European Space Research and Technology Centre in Noordwijk) put it:

“The Hubble findings were controversial, going against not only empirical evidence in our galaxy but also against the current models. This was intriguing, but without a way to obtain spectra of those stars, we could not really establish whether we were witnessing genuine accretion and the presence of disks, or just some artificial effects.”

“We see that these stars are indeed surrounded by disks and are still in the process of gobbling material, even at the relatively old age of 20 or 30 million years. This also implies that planets have more time to form and grow around these stars than in nearby star-forming regions in our own galaxy.”