NASA's James Webb Space Telescope has resolved a long-standing mystery by confirming a controversial finding from the Hubble Space Telescope, first observed over 20 years ago. In 2003, Hubble detected evidence of a massive planet orbiting an ancient star, nearly as old as the universe itself. Stars of this age are known to contain very few heavy elements, which are crucial for planet formation. This discovery suggested that planet formation occurred early in the universe's history, with planets growing larger than Jupiter in primordial disks. However, how this could happen remained unclear.
To investigate further, scientists used Webb to study stars in a nearby galaxy that, like the early universe, has a scarcity of heavy elements. Their observations revealed that some stars there have planet-forming disks, and these disks are more enduring than those around younger stars in the Milky Way. "With Webb, we have a really strong confirmation of what we saw with Hubble, and we must rethink how we model planet formation and early evolution in the young universe," said study leader Guido De Marchi of the European Space Research and Technology Centre in Noordwijk, Netherlands.
Protoplanetary Disks in NGC 346 Captured in NIRCam Image.
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| This James Webb Space Telescope image shows NGC 346, a massive star cluster in the Small Magellanic Cloud, providing a nearby proxy for studying early universe stellar environments. Ten yellow circles highlight the positions of the stars surveyed. |
A Different Environment in Early Times: Webb Confirms Longer Lifespan for Planet-Forming Disks.
In the early universe, stars formed mainly from hydrogen and helium, with few heavier elements like carbon and iron. This lack of heavier elements led to the belief that planet-forming disks around stars had short lifespans, preventing planets from growing large. However, Hubble's observations of planets contradicted this, prompting a reevaluation.
To test the idea, scientists focused on the Small Magellanic Cloud, a dwarf galaxy with a similar elemental composition to the early universe, and the NGC 346 star cluster. Despite the conventional belief that planet-forming disks would dissipate after 2-3 million years, Hubble had previously detected stars in this cluster that were 20 to 30 million years old, still surrounded by such disks.
Now, thanks to the Webb Space Telescope, scientists have captured the first-ever spectra of forming, Sun-like stars in this nearby galaxy. Webb's data shows that these stars, even at 20-30 million years old, are still accreting material, indicating that their disks last much longer than expected, allowing more time for planet formation. This challenges existing models and suggests that planet formation can occur more rapidly in some stellar environments.
Protoplanetary Disks in NGC 346 Revealed in NIRSpec Spectra.
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| This graph displays the spectrum of one of the 10 target stars in the study, with yellow highlighting spectral fingerprints of hot atomic helium, cold molecular hydrogen, and hot atomic hydrogen. The magenta spectrum, slightly offset, represents the background environment and lacks the cold molecular hydrogen line. A comparison of the two spectra reveals a large peak of cold molecular hydrogen and atomic hydrogen from the star, indicating the presence of a protoplanetary disk. The data was collected using the NIRSpec instrument on the James Webb Space Telescope. |
A New Way of Thinking.
This new finding challenges earlier theoretical predictions that in environments with few heavier elements, the radiation pressure from stars would quickly blow away planet-forming disks, shortening their lifespan to less than a million years. This raised a major question: if disks don't persist long enough for dust grains to collide and form the cores of planets, how can planets form at all?
The researchers proposed two possible mechanisms, or a combination of both, that could explain the persistence of planet-forming disks in such environments. First, the radiation pressure from the star, which is responsible for dispersing the disk, would be less effective in an environment with fewer heavy elements. NGC 346, for instance, contains only about 10% of the heavier elements found in our Sun, which may mean it takes longer for stars in this cluster to disperse their disks.
The second possibility is that a larger gas cloud is required to form a Sun-like star in an environment rich in hydrogen and helium but poor in heavier elements. A larger gas cloud would result in a bigger, more massive disk, which would take longer to dissipate, even under radiation pressure.
“With more matter around the stars, the accretion lasts for a longer time,” said Sabbi. "The disks take ten times longer to disappear. This has implications for how you form a planet, and the type of system architecture that you can have in these different environments. This is so exciting.”
The study's findings are detailed in the December 16 issue of The Astrophysical Journal.
NGC 346: A Comparison of Hubble and Webb Observations.
The James Webb Space Telescope (JWST) is the world’s leading space science observatory, playing a pivotal role in unraveling mysteries within our solar system and beyond. By studying distant worlds orbiting other stars and exploring the origins and structures of the universe, Webb is transforming our understanding of the cosmos. This groundbreaking mission is led by NASA, with strong international collaboration from the European Space Agency (ESA) and the Canadian Space Agency (CSA).
Meanwhile, the Hubble Space Telescope, operational for over three decades, continues to push the boundaries of space exploration. Its ongoing discoveries are fundamental to shaping our understanding of the universe. Hubble is a joint endeavor between NASA and ESA, with NASA’s Goddard Space Flight Center in Greenbelt overseeing its management and mission operations. Lockheed Martin Space, based in Denver, supports operations at Goddard, while the Space Telescope Science Institute in Baltimore, managed by the Association of Universities for Research in Astronomy, conducts Hubble’s science operations for NASA.
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