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About 5 million years ago, the exoplanet PDS 70b was born. This planet, 370 light years from Earth, is a cosmic newborn that formed well after the extinction of the dinosaurs. It’s part of a pair of planetary siblings thought to be in their final stages of growth. Their characteristics offer scientists unparalleled insight into planetary development. Now, a new measurement of PDS 70b’s atmosphere could help build better models of planetary evolution (Astrophys. J., Lett. 2024, DOI: 10.3847/2041-8213/ad95e8).
“For the first time, we measured the composition of a baby planet’s atmosphere directly,” says the study’s lead author, Chih-Chun (Dino) Hsu of Northwestern University. The measurement was possible only with a specialized, high-resolution spectrometer at the W. M. Keck Observatory, he says. There, the research team collected the infrared light emitted by the hot gases in PDS 70b’s atmosphere by carefully aligning the telescope to the planet’s location in the sky.
Hsu used the signals in the spectrum from carbon monoxide and water to determine the atomic ratio of carbon to oxygen. He then compared the planetary ratio to that of the protoplanetary disk and the system’s star.
“We found a mismatch with its natal environment,” Hsu says. The simplest models of planetary birth predict that the chemistry of a planet should mirror that of the protoplanetary disk from which it was born, he explains. Instead, PDS 70b’s carbon to oxygen ratio more closely matches that of the star it orbits. With this new measurement, Hsu says, “we validated the suspicion that this picture might be too simplified.”
Yifan Zhou, an astronomer studying PDS 70b at the University of Virginia who was not involved in the new work, says that “the fact that we can measure the composition, the carbon to oxygen ratio, of this planet is groundbreaking.” Carbon-to-oxygen ratios have long been used in models as a tracer for planet formation, but “this is the very first time that we have an observational constraint from a planet that is still forming,” Zhou says. Such a measurement provides modelers an anchor point to test the accuracy of their models on planetary evolution.
Hsu plans to collect new infrared data from the system’s second gas giant, PDS 70c, in the spring of 2025. But he’s also interested in other protoplanets of various ages. “By looking at the whole population,” he says, “we should be able to tell how they form and how they evolve in a more statistically robust way.”
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