The James Webb Space Telescope (JWST) was specifically designed to answer some of the biggest unsolved questions in cosmology. These include all the important questions scientists have been pondering since the Hubble Space Telescope (HST) gained its deepest insights into the universe: the Hubble voltage, how the first stars and galaxies came together, how planetary systems formed, and when the first black appeared Holes appeared. In particular, Hubble discovered something very interesting in 2003 when he observed a star almost as old as the universe itself.
This ancient star was orbited by a massive planet, whose very existence contradicted accepted models of planet formation because stars in the early universe did not have time to produce enough heavy elements for planet formation. Thanks to recent observations from JWST, an international team of scientists announced that they may have solved this mystery. By observing stars in the Small Magellanic Cloud (LMC), where there are no large amounts of heavy elements, they found stars with planet-forming disks that are longer-lived than those seen around young stars in our Milky Way Galaxy.
The study was led by Guido De Marchi, an astronomer at the European Space Research and Technology Center (ESTEC) in Noordwijk, Netherlands. He was joined by researchers from the INAF Osservatorio Astronomico di Roma, the Space Telescope Science Institute (STScI), the Gemini Observatory/NSF NOIRLab, the UK Astronomy Technology Center (UK ATC), the Institute for Astronomy at the University of Edinburgh and the University Leiden Observatory, the European Space Agency (ESA), NASA's Ames Research Center and NASA's Jet Propulsion Laboratory. The paper detailing their findings appeared Dec. 16 in the Astrophysical Journal.
James Webb Space Telescope image of NGC 346, a massive star cluster in the Small Magellanic Cloud. Image credit: NASA/ESA/CSA/STScI/Olivia C. Jones (UK ATC)/Guido De Marchi (ESTEC)/Margaret Meixner (USRA)
According to accepted cosmological models, the first stars in the universe (Population III stars) formed 13.7 billion years ago, just a few hundred million years after the Big Bang. These stars were very hot, bright, massive, short-lived, and consisted of hydrogen and helium, with hardly any heavy elements. These elements were gradually forged inside Population III stars, distributing them throughout the universe as they exploded in a supernova, blowing off their outer layers to form star-forming nebulae.
These nebulae and their traces of heavier elements would form the next generation of stars (Population II). After these stars formed from gas and dust in the nebula, which underwent gravitational collapse, the remaining material fell around the new stars, forming protoplanetary disks. As a result, subsequent stellar populations contained higher concentrations of metals (also known as metallicity). The presence of these heavy elements, from carbon and oxygen to silicon and iron, led to the formation of the first planets.
Hubble's discovery of a massive planet (2.5 times Jupiter's mass) around a star that existed just a billion years after the Big Bang stunned scientists because early stars contained only tiny amounts of heavier elements. This suggested that planet formation began when the universe was very young and some planets had time to become particularly massive. Elena Sabbi, the chief scientist of 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 short lifespans, so short that planets cannot grow large. But Hubble has seen these planets. So what if the models weren’t accurate and discs could last longer?”
James Webb Space Telescope image of NGC 346, a massive star cluster in the Small Magellanic Cloud. Image credit: NASA/ESA/CSA/STScI/Olivia C. Jones (UK ATC)/Guido De Marchi (ESTEC)/Margaret Meixner (USRA)
To test this theory, the team used Webb to observe the massive star-forming star 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 contain relatively small amounts of heavier elements and served as a close proxy for stellar environments in the early Universe. Previous observations of NGC 346 by Hubble showed that many young stars in the cluster (around 20 to 30 million years old) apparently still had protoplanetary disks around them. This was also surprising since such disks were thought to disintegrate after 2 to 3 million years.
Thanks to Webb's high-resolution and sophisticated spectrometers, scientists now have the first spectra of young Sun-like stars and their surroundings in a nearby galaxy. As study leader Guido De Marchi from the European Center for Space Research and Technology in Noordwijk put it:
“The Hubble results were controversial and contradicted not only empirical evidence in our galaxy but also current models. This was fascinating, but without the ability to obtain spectra of these stars, we couldn't really determine whether this was real accretion and the presence of disks or just artificial effects.”
“We see that these stars are actually surrounded by disks and are still devouring material even at a relatively old age of 20 or 30 million years. This also means that planets around these stars have more time to form and grow than in nearby star-forming regions in our own galaxy.”
The direct comparison shows a Hubble image of the massive star cluster NGC 346 (left) with a Webb image of the same star cluster (right). Image credit: NASA/ESA/CSA/STScI/Olivia C. Jones (UK ATC)/Guido De Marchi (ESTEC)/Margaret Meixner (USRA)/Antonella Nota (ESA)
These findings naturally raise the question of how disks with few heavy elements (the actual building blocks of planets) can survive for so long. The researchers proposed two different mechanisms that could explain these observations alone or in combination. One possibility is that a star's radiation pressure is effective only when elements heavier than hydrogen and helium are present in sufficient quantities in the disk. However, the star cluster NGC 346 only contains about ten percent of the heavier elements of our sun, so it can take longer for a star in this cluster to dissolve its disk.
The second possibility is that where heavier elements are scarce, a Sun-like star would have to form from a larger cloud of gas. This would also create a larger and more massive protoplanetary disk, which would take longer to destroy the stellar radiation. Sabbi said:
“The more matter there is around the stars, the longer the accretion takes. It takes ten times longer for the slices to disappear. This impacts how you design a planet and what kind of system architecture you can have in these different environments. This is so exciting.”
“With Webb we have a really strong confirmation of what we saw with Hubble, and we need to rethink how we model planet formation and early evolution in the early universe,” Marchi added.
Like many of Webb's observations, these results are a fitting reminder of what the next-generation space telescope was designed to do. The JWST not only confirmed the Hubble tension, but also observed more galaxies (and larger ones!) in the early universe than the models predicted. It was also observed that the seeds of supermassive black holes (SMBH) were more massive than expected. In this regard, the JWST does its job by making astronomers rethink theories that have been accepted for decades. This will result in new theories and discoveries that could upend what we think we know about the cosmos.
Further reading: NASA, The Astrophysical Journal
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