How did we get here? Few topics in astronomy intrigue us more than this simple five-word query. Yet the simplicity hides multiple layers of complexity. How did the universe come into existence? How did stars form out of an initial mix of mostly hydrogen and helium gas? How did planets grow in the dusty disks surrounding these stars? And even the answers to these questions can’t explain how life and intelligence appeared.
When the James Webb Space Telescope (JWST) began its mission, scientists hoped it would shed light on the key step of planet formation. Mission accomplished. A team of scientists has found convincing evidence that planets in the early days of the cosmos developed over surprisingly long periods.
Before JWST, astronomers mostly studied Milky Way star-forming regions with compositions similar to the Sun. These observations showed that planet-forming disks don’t last much longer than 2 million years. That’s not much time for dust grains to stick together, coalesce into small rocks, and create massive cores that could grow into planet-sized bodies.
And these findings couldn’t account for the majority of the universe’s stars, which formed billions of years ago. Dust was scarce because few elements heavier than helium, what astronomers call metals, existed back then. Theorists thought the lack of dust would allow a star’s X-ray emission to blow away any disk within a million years or so.
A Hubble shock
But the Hubble Space Telescope raised serious questions about this scenario. In 2003, it discovered a planet with 2.5 times the mass of Jupiter circling a neutron star-white dwarf binary in the Milky Way globular cluster M4. These clusters have few metals and yet, somehow, one of them managed to forge a massive planet.
Later that decade, Hubble studied the young star cluster NGC 346 in the Small Magellanic Cloud (SMC), a companion galaxy to the Milky Way located some 200,000 light-years away. Although nearby, the SMC boasts a metallicity only about one-eighth that of the Sun, mimicking what scientists find at much earlier cosmic times. Hubble found some 200 cluster stars up to 30 million years old that seemed to possess planet-forming disks.
JWST proves the point
“The Hubble findings were controversial, going against not only empirical evidence in our galaxy but also against the current models,” said team leader Guido De Marchi of the European Space Research and Technology Centre in Noordwijk, Netherlands, in a press release. Yet JWST confirmed the Hubble results. The scientists used JWST’s Near-Infrared Spectrograph to study 10 Sun-like stars between 20 million and 30 million years old in NGC 346. The spectra — the first of any Sun-like stars in an external galaxy — revealed molecular hydrogen, confirming the existence of circumstellar disks.
“With Webb, we have a really strong confirmation of what we saw with Hubble,” said De Marchi. “We must rethink how we model planet formation and early evolution in the young universe.” The team reported its results in the Dec. 20, 2024, issue of The Astrophysical Journal.
Astronomers now can breathe easier knowing that planets have more time to develop around low-metallicity stars than they do in nearby regions of the Milky Way Galaxy. One possibility the researchers suggest is that stars in metal-poor regions begin life in larger gas clouds that produce bigger disks.
