Free Astronomy Magazine September-October 2024

“Our prior observations of this system have been more indirect measure- ments of the star, which actually allowed us to see ahead of time that there was likely a giant planet in this system tugging on the star,” said team mem- ber Caroline Morley of the University of Texas at Austin. “That’s why our team chose this system to observe first with Webb.” “This discovery is exciting because the planet is quite similar to Jupiter — it is a little warmer and is more massive, but is more similar to Jupiter than any other planet that has been imaged so far,” added lead author Elisabeth Matthews of the Max Planck Insti- tute for Astronomy in Germany. Previously imaged exoplanets tend 47 SEPTEMBER-OCTOBER 2024 to be the youngest, hottest exo- planets that are still radiating much of the energy from when they first formed. As planets cool and con- tract over their lifetime, they be- come significantly fainter and therefore harder to image. “Cold planets are very faint, and most of their emission is in the mid- infrared,” explained Matthews. “Webb is ideally suited to conduct mid-infrared imaging, which is ex- tremely hard to do from the ground. We also needed good spa- tial resolution to separate the planet and the star in our images, and the large Webb mirror is ex- tremely helpful in this aspect.” Epsilon Indi Ab is one of the coldest exoplanets to be directly detected, with an estimated temperature of 35 degrees Fahrenheit (2 degrees Celsius) — colder than any other im- aged planet beyond our solar sys- tem, and colder than all but one free-floating brown dwarf. The planet is only around 180 degrees Fahrenheit (100 degrees Celsius) warmer than gas giants in our solar system. This provides a rare oppor- tunity for astronomers to study the atmospheric composition of true solar system analogs. “Astronomers have been imagining planets in this system for decades; fictional planets orbiting Epsilon Indi have been the sites of Star Trek episodes, novels, and video games like Halo,” added Morley. “It’s excit- ing to actually see a planet there ourselves, and begin to measure its properties.” Epsilon Indi Ab is the twelfth closest exoplanet to Earth known to date and the closest planet more massive than Jupiter. The science team chose to study Eps Ind A because the sys- tem showed hints of a possible plan- etary body using a technique called radial velocity, which measures the back-and-forth wobbles of the host star along our line of sight. “While we expected to image a planet in this system, because there were radial velocity indications of its presence, the planet we found isn’t what we had predicted,” shared Matthews. “It’s about twice as massive, a little farther from its star, and has a different orbit than we expected. The cause of this dis- crepancy remains an open ques- tion. The atmosphere of the planet also appears to be a little different than the model predictions. So far we only have a few photometric measurements of the atmosphere, meaning that it is hard to draw con- clusions, but the planet is fainter than expected at shorter wave- lengths.” The team believes this may mean there is significant methane, carbon monoxide, and carbon dioxide in the planet’s atmosphere that are absorbing the shorter wavelengths of light. It might also suggest a very cloudy atmosphere. The direct imaging of exoplanets is particularly valuable for characteri- zation. Scientists can directly collect light from the observed planet and compare its brightness at different wavelengths. So far, the science team has only detected Epsilon Indi Ab at a few wavelengths, but they hope to revisit the planet with Webb to conduct both photometric and spectroscopic observations in the future. They also hope to detect other similar planets with Webb to find possible trends about their at- mospheres and how these objects form. NASA’s upcoming Nancy Grace Roman Space Telescope will use a coronagraph to demonstrate direct imaging technology by photograph- ing Jupiter-like worlds orbiting Sun- like stars – something that has never been done before. These results will pave the way for future missions to study worlds that are even more Earth-like. P revious page: This image of the gas- giant exoplanet Epsilon Indi Ab was taken with the coronagraph on NASA’s James Webb Space Telescope’s MIRI (Mid-Infrared Instru- ment). A star symbol marks the location of the host star Epsilon Indi A, whose light has been blocked by the corona- graph, resulting in the dark circle marked with a dashed white line. Ep- silon Indi Ab is one of the coldest exoplanets ever directly imaged. Light at 10.6 microns was assigned the color blue, while light at 15.5 microns was assigned the color orange. MIRI did not resolve the planet, which is a point source. [NASA, ESA, CSA, STScI, Elisabeth Matthews (MPIA)] !