In 2003, Hubble provided evidence of giant exoplanets around very old stars. Such stars have only small amounts of the heavy elements that make up planets. This suggests that some planetary formation occurred when our universe was very young, and that those planets had time to form and grow large within the primordial disk, becoming even larger than Jupiter. I am. But how? To answer this question, astronomers used the NASA/ESA/CSA James Webb Space Telescope to study stars in the nearby Small Magellanic Cloud, which, like the early Universe, lacks large amounts of heavy elements. They discovered that not only do some stars there have planet-forming disks, but that those disks are longer-lived than the disks found around young stars in our Milky Way galaxy.
This web image shows NGC 346, a massive star cluster in the Small Magellanic Cloud. Yellow circles superimposed on the image indicate the positions of the 10 stars investigated in the study. Image credits: NASA/ESA/CSA/STScI/Olivia C. Jones, UK ATC/Guido De Marchi, ESTEC/Margaret Meixner, USRA.
“With Webb, we have strong confirmation of what we saw with Hubble, and we need to rethink how we model planet formation and early evolution in the young Universe.” European Space Research Agency said Dr. Guido de Marchi, a researcher at Technology Center.
“In the early universe, stars formed primarily from hydrogen and helium, with few heavier elements such as carbon or iron, and were later born from supernova explosions.”
“Current models predict that because heavy elements are so scarce, the lifetime of the disk around the star is short, so short that in fact planets cannot grow,” said a researcher at NSF's NOIRLab's Gemini Observatory. said lead scientist Dr. Elena Sabbi.
“But Hubble actually observed those planets. So what happens if the model is incorrect and the disks have a longer lifespan?”
To test this idea, the astronomers trained Webb in the Small Magellanic Cloud, a dwarf galaxy that is one of the closest galaxies to the Milky Way.
In particular, they examined the massive star-forming cluster NGC 346, which also has a relative lack of heavy elements.
This cluster served as a nearby proxy for studying stellar environments with similar conditions in the distant early universe.
Hubble observations of NGC 346 since the mid-2000s have revealed that there are many stars around 20 to 30 million years old that are thought to still have planet-forming disks around them.
This was contrary to the conventional idea that such disks would disappear after two or three million years.
“Hubble's discovery was controversial and went against not only the empirical evidence for the galaxy, but also current models,” Dr. De Marchi said.
“This was interesting, but without a way to obtain the spectra of these stars, we will not know whether what we are witnessing is genuine accretion and the presence of a disk, or just an artificial effect. I couldn't actually confirm it.”
Now, thanks to Webb's sensitivity and resolution, scientists have, for the first time, spectra of the formation of Sun-like stars and their surrounding environments in nearby galaxies.
“We can see that these stars are actually surrounded by a disk and are still in the process of engulfing material even though they are relatively old, 20 or 30 million years old,” De Marchi said. Ta.
“This also means that planets have more time to form and grow around these stars than in nearby star-forming regions in our galaxy.”
This discovery contradicts previous theoretical predictions that if there were very few heavy elements in the gas around the disk, the star would quickly blow away the disk.
Therefore, the lifespan of the disk is very short, probably less than 1 million years.
But how can planets form if dust grains stick together to form pebbles and the disk doesn't stay around the star long enough to become the planet's core?
The researchers explained that two different mechanisms, or a combination of them, may exist for planet-forming disks to persist in environments low in heavy elements.
First, the star applies radiation pressure to blow the disk away.
For this pressure to be effective, an element heavier than hydrogen or helium must be present in the gas.
However, the massive star cluster NGC 346 contains only about 10 percent of the heavy elements present in the Sun's chemical composition.
Perhaps the stars in this cluster just need time to disperse their disks.
A second possibility is that for a Sun-like star to form when there are few heavier elements, it would need to start with a larger cloud of gas.
As the gas cloud grows larger, it produces larger disks. Therefore, because there is more mass in the disk, it will take longer to blow it away, even if the radiation pressure is acting the same.
“The more material around the star, the longer the accretion will last,” Sabbi says.
“It takes 10 times longer for the disk to disappear. This has implications for how planets form and the types of system architectures that can be used in different environments. This is very exciting.”
of study Published today on astrophysical journal.
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Guido de Marchi others. 2024. Protoplanetary disks around Sun-like stars appear to live longer when they are less metallic. APJ 977,214;Doi: 10.3847/1538-4357/ad7a63
This article is adapted from an original release by the Webb Mission Team at NASA's Goddard Space Flight Center.
