James Webb telescope discovers Zhúlóng, the Milky Way's long-lost 'twin' near the dawn of time
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When astronomers used the James Webb Space Telescope (JWST) to peer deep into the early universe, they made a serendipitous discovery: a galaxy that appears to be the Milky Way's ancient twin sibling waving its spiral arms back at us.
In new images that capture light emitted just 1 billion years after the Big Bang, when the universe was roughly one-fourteenth its current age, the newly discovered galaxy appears fully formed, with a central bulge of old stars, a vibrant disk of stellar newborns, and two distinct spiral arms. Given its recognizable features and impressive size, the researchers have dubbed this galaxy the most distant Milky Way "twin" ever observed.
The detection of such a fully formed spiral galaxy so early in cosmic time adds to many recent JWST discoveries that challenge our best theories of cosmology, which predict that large galaxies like this should take several billion years to form through an arduous series of smaller galaxy mergers.
A full description of the galaxy — nicknamed Zhúlóng, after a mythical Chinese sun dragon whose eyes controlled the day and night cycle — was published April 16 in the journal Astronomy & Astrophysics.
"What makes Zhúlóng stand out is just how much it resembles the Milky Way — both in shape, size, and stellar mass," first study author Mengyuan Xiao, a postdoctoral researcher at the University of Geneva, said in a statement.
While Zhúlóng is unquestionably older than the Milky Way, it could be mistaken for our galaxy's "little" sibling. The new research estimates that Zhúlóng's star-forming disk measures about 60,000 light-years across, compared with our galaxy's 100,000 light-year breadth, and it contains about 100 billion solar masses, compared with the Milky Way's 1.5 trillion.
Still, Zhúlóng is by far the largest Milky Way look-alike spotted at such an early time in the universe's history. It formed more than a billion years earlier than the similarly shaped spiral galaxy Ceers-2112, which JWST discovered in 2023 at about 11.7 billion light-years from Earth.
Related: 42 jaw-dropping James Webb Space Telescope images
No genealogical tests were needed to find this long-lost sibling; the researchers discovered it by accident in PANORAMIC, a wide-field survey of billions of distant objects. The survey was taken in JWST's unique "pure parallel" mode — essentially, a way for the telescope to use two of its infrared instruments simultaneously to observe two different regions of space at once.
"This allows JWST to map large areas of the sky, which is essential for discovering massive galaxies, as they are incredibly rare," study co-author Christina Williams, an assistant astronomer at the National Science Foundation's NOIRLab and principal investigator of the PANORAMIC survey, said in the statement.
The serendipitous discovery of Zhúlóng adds fuel to an ongoing cosmological fire started by JWST several years ago. The telescope's observations of the early universe consistently show that objects there, including gargantuan galaxies and supermassive black holes, seem to have grown too big, too fast for our current best theories to explain.
It's thought that the Milky Way took several billion years to form, while Zhúlóng seems to have achieved a similar shape and size in less than 1 billion years. How this is possible remains "an open question," the team wrote in the study.
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"This discovery shows how JWST is fundamentally changing our view of the early universe," study co-author Pascal Oesch, an associate professor of astronomy at the University of Geneva and co-principal investigator of the PANORAMIC program, concluded in the statement.
The researchers are proposing follow-up observations with JWST, as well as with the ground-based Atacama Large Millimeter/submillimeter Array in the Chilean desert, to get to know our galaxy's long-lost twin even better.
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Sign up for CNN's Wonder Theory science newsletter. Explore the universe with news on fascinating discoveries, scientific advancements and more. A tiny sign revealed in April seemed like it might change the universe as we know it. Astronomers had detected just a hint, a glimmer of two molecules swirling in the atmosphere of a distant planet called K2-18b — molecules that on Earth are produced only by living things. It was a tantalizing prospect: the most promising evidence yet of an extraterrestrial biosignature, or traces of life linked to biological activity. But only weeks later, new findings suggest the search must continue. 'It was exciting, but it immediately raised several red flags because that claim of a potential biosignature would be historic, but also the significance or the strength of the statistical evidence seemed to be too high for the data,' said Dr. Luis Welbanks, a postdoctoral research scholar at Arizona State University's School of Earth and Space Exploration. While the molecules identified on K2-18b by the April study — dimethyl sulfide, or DMS, and dimethyl disulfide, or DMDS — are associated largely with microbial organisms on our planet, scientists point out that the compounds can also form without the presence of life. Now, three teams of astronomers not involved with the research, including Welbanks, have assessed the models and data used in the original biosignature discovery and got very different results, which they have submitted for peer review. Meanwhile, the lead author of the April study, Nikku Madhusudhan, and his colleagues have conducted additional research that they say reinforces their previous finding about the planet. And it's likely that additional observations and research from multiple groups of scientists are on the horizon. The succession of research papers revolving around K2-18b offers a glimpse of the scientific process unfolding in real time. It's a window into the complexities and nuances of how researchers search for evidence of life beyond Earth — and shows why the burden of proof is so high and difficult to reach. Located 124 light-years from Earth, K2-18b is generally considered a worthy target to scour for signs of life. It is thought to be a Hycean world, a planet entirely covered in liquid water with a hydrogen-rich atmosphere, according to previous research led by Madhusudhan, a professor of astrophysics and exoplanetary science at the University of Cambridge's Institute of Astronomy. And as such, K2-18b has rapidly attracted attention as a potentially habitable place beyond our solar system. Convinced of K2-18b's promise, Madhusudhan and his Cambridge colleagues used observations of the planet by the largest space telescope in operation, the James Webb Space Telescope, to study the planet further. But two scientists at the University of Chicago — Dr. Rafael Luque, a postdoctoral scholar in the university's department of astronomy and astrophysics, and Michael Zhang, a 51 Pegasi b / Burbidge postdoctoral fellow — spotted some problems with what they found. After reviewing Madhusudhan and his team's April paper, which followed up on their 2023 research, Luque and Zhang noticed that the Webb data looked 'noisy,' Luque said. Noise, caused by imperfections in the telescope and the rate at which different particles of light reach the telescope, is just one challenge astronomers face when they study distant exoplanets. Noise can distort observations and introduce uncertainties into the data, Zhang said. Trying to detect specific gases in distant exoplanet atmospheres introduces even more uncertainty. 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Such harsh temperatures could change the way astronomers think about the planet's potential habitability, Zhang said, especially because cooler temperatures persist in the top of the atmosphere — the area that Webb can detect — and the surface or ocean below would likely have even higher temperatures. 'This is just an inference only from the atmosphere, but it would certainly affect how we think about the planet in general,' Luque said. Part of the issue, he said, is that the April analysis didn't include data collected from all three Webb instruments Madhusudhan's team used over the past few years. So Luque, Zhang and their colleagues conducted a study combining all the available data to see whether they could achieve the same results, or even find a higher amount of dimethyl sulfide. They found 'insufficient evidence' of both molecules in the planet's atmosphere. Instead, Luque and Zhang's team spotted other molecules, like ethane, that could fit the same profile. 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'So the main thing we wanted to do was assess whether other chemical species could provide an adequate fit to the data.' When the model was expanded, the evidence for dimethyl sulfide or dimethyl disulfide 'just disappears,' Welbanks said. Madhusudhan believes the studies that have come out after his April paper are 'very encouraging' and 'enabling a healthy discussion on the interpretation of our data on K2-18b.' He reviewed Luque and Zhang's work and agreed that their findings don't show a 'strong detection for DMS or DMDS.' When Madhusudhan's team published the paper in April, he said the observations reached the three-sigma level of significance, or a 0.3% probability that the detections occurred by chance. For a scientific discovery that is highly unlikely to have occurred by chance, the observations must meet a five-sigma threshold, or below a 0.00006% probability that the observations occurred by chance. 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