“I like to joke that for this paper we ‘used the coronagraphs wrong,” joked William Balmer, Ph.D. student at Johns Hopkins University. But the backhanded compliment conceals the creative scientific vision behind the James Webb Space Telescope’s (JWST) latest accomplishment in taking pictures of exoplanets. By deliberately tweaking the coronagraphs a instrument designed to block starlight the scientists have uncovered unprecedented views of distant worlds, lighting up their atmospheres, chemistry, and history.
The JWST coronagraphs, originally planned to look at the corona of the Sun, were then redirected to take images of exoplanets. The blinding starlight glare is blocked by the instruments so astronomers are able to observe distant planets orbiting near-by stars. Balmer’s group did it differently: they adapted the coronagraph masks to permit a narrow beam of starlight through. This seemingly counterintuitive strategy allowed them to more efficiently eliminate planetary signals. “Because [the JWST] is so stable, the differences between the reference and target images are smaller than the light from the planets around our targets [allowing us to detect them more clearly],” Balmer described.
One of the targets in the sky was HR 8799, a star system 130 light-years distant in the constellation Pegasus. This new system, a paltry 30 million years old, possesses four giant gas planets HR 8799 b, c, d, and e emitting infrared radiation from their newly hot origin. The JWST imaged the first-ever picture of HR 8799 e at 4.6 microns, a wavelength within the mid-infrared part of the spectrum which is absorbed by Earth’s atmosphere and ground observations virtually impossible. “Earth’s atmosphere has only a brief window of transparency at 4.6 microns,” Balmer said in describing the importance of the telescope being in orbit.
The readings showed the HR 8799 planets to be carbon dioxide-rich, a finding with deep implications for where they originated. Carbon dioxide in a planet’s atmosphere can signal heavy elements such as oxygen, magnesium, and iron elements employed in the core accretion process. It is the same mechanism that formed Jupiter and Saturn, where rock and ice cores accumulate sufficient mass to tap into dense hydrogen and other gas atmospheres.”Since the strength of the carbon dioxide feature in the HR 8799 planets’ atmospheres is so strong, we are fairly confident that they have a larger fraction of heavy elements compared to its host star,” Balmer said.
The surprises did not end there, however. The JWST observations also revealed hidden diversity in the atmospheres of the inner planets of HR 8799. Although the planets had been assumed to be similar to one another previously, mid-infrared data revealed diversity in their vertical mixing process. HR 8799 e, for example, showed giant abundances of carbon monoxide to methane in its upper atmosphere a result that was not expected under its temperature. “Vertical mixing has moved warm, CO-rich gas from the deeper layers of the atmosphere up into the outer layers, where it has ‘out competed’ the methane that should be there,” Balmer said. It’s one example of how active and turbulent planetary atmospheres are.
The telescope also looked at 51 Eridani b, a 97-light-year-distant young exoplanet. This world, darker than anticipated, showed clear signs of carbon dioxide and carbon monoxide in its upper atmosphere, which indicates that it is metal-rich. The light-gathering power of the JWST at 4.3 microns a wavelength fully absorbed by Earth’s atmosphere were critical to these discoveries. “The most exciting wavelength we had access to with the JWST is at 4.3 microns, where none of these planets had been observed before,” Balmer said.
Not only do these findings add to the discovery of a single exoplanet, but to the potential understanding of planet formation in general. Comparing the HR 8799 system with our own as a model system, scientists are seeking to gain a greater understanding of the similarity or lack thereof of Earth and its cousins.”Our hope with this kind of research is to understand our own Solar System, life, and ourselves in the comparison to other exoplanetary systems,” Balmer said.
The team was awarded further JWST observing time in the future to track four more planetary systems. The goal of these tests is to see whether gas giants throughout the universe also formed through core accretion, and what that says about how much they affected small, potentially Earth-sized planets. As described by astronomer Laurent Pueyo, “How common is this for planets we can directly image? We don’t know yet, but we’re proposing more Webb observations to answer that question.”
The James Webb Space Telescope keeps pushing the frontiers of exoplanetary science with its constancy and cutting-edge equipment, revealing the intricate secrets of distant worlds on the edge of vision. With every observation, it puts us in a better position to know the solutions to the major questions of the universe and our place within it.
