Thanks to the James Webb Space Telescope, a joint effort by NASA, the European Space Agency (ESA) and the Canadian Space Agency (CSA), astronomers have been able to photograph an exoplanet located 12 light-years from Earth. Only about 25 exoplanets have been directly photographed, but this one is the coldest and oldest exoplanet observed directly.
Scientists had expected to find a planet around the star, dubbed Epsilon Indi A, based on previous studies. But Webb’s observation surprised astronomers. Not only did the exoplanet have different properties than expected, it was also found in a different location than previous models predicted.
“We expected to image a planet in this system because there were radial velocity indications of its presence, but the planet we found is not the one we predicted,” said Elisabeth Matthews, lead author of the study and a member of the Max Planck Institute for Astronomy in Heidelberg, Germany. “It is about twice as massive, a little further from its star, and has a different orbit than we expected. The cause of this difference remains an open question.”
The planet has a mass several times that of Jupiter and is in an elliptical orbit around a star the same age as the Sun. When astronomers analyzed the planet’s atmosphere, they found that it was fainter than expected at shorter wavelengths. This could mean that the atmosphere is very cloudy, but it could also indicate the presence of carbon-based molecules such as methane, carbon monoxide, and carbon dioxide.
Two observations of Epsilon Indi Ab by Webb’s mid-infrared instrument at different frequencies. Background was obtained from the AllWISE sky survey. (Credit: T. Müller (MPIA/HdA), E. Matthews (MPIA))
The team estimates the planet’s temperature to be two degrees Celsius. That’s about 100 degrees Celsius warmer than the gas giants in our solar system, but considerably colder than any other directly observed exoplanet. This temperature makes older exoplanets harder to observe than younger ones, because the latter emit a lot of energy during their formation.
“Cool planets are very faint and most of their emissions are in the mid-infrared,” Matthews explained. “Webb is ideally suited to imaging in the mid-infrared, which is extremely difficult to do from the ground. We also needed good spatial resolution to separate the planet and star in our images, and the large Webb mirror is extremely useful for this.”
The planet’s star, Epsilon Indi A, is the primary star in a three-star system. It is a K-type red dwarf, smaller and cooler than the Sun. The other two stars are a pair of smaller brown dwarfs that orbit Epsilon Indi A at a great distance.
“Astronomers have been imagining planets in this system for decades; fictional planets orbiting Epsilon Indi have been featured in Star Trek episodes, novels and video games like Halo,” said Caroline Morley, an author at the University of Texas at Austin. “It’s exciting to see a planet there and start measuring its properties.”
Need to beat the heat?
Webb has taken a direct image of Epsilon Indi Ab: the coldest, most Jupiter-like exoplanet ever photographed. To make this discovery, Webb masked the starlight (represented by the star symbol) with a mask called a coronagraph: https://t.co/C8HE4Zsz13 pic.twitter.com/oL8rye3pzH
— NASA Webb Telescope (@NASAWebb) July 24, 2024
Previous studies have searched for planets around the star using radial velocity measurements. This method indirectly detects exoplanets by measuring a slight wobble caused by the planet’s gravitational pull.
“Our previous observations of this system were indirect measurements of the star, which allowed us to see in advance that there was likely a giant planet in this system pulling on the star,” Morley said. “That’s why our team chose this system to observe first with Webb.”
While previous studies had correctly identified the presence of a planet around Epsilon Indi A, the image Webb returned was very different from what astronomers expected. So the scientist first had to confirm that the bright spot in the image was indeed the planet they were looking for.
Most importantly, they had to rule out the possibility that an object from outside the star system was in the background when the image was taken and had been mistaken for a planet. To do this, the team analyzed previous observations of Epsilon Indi A and found that it was unlikely that a background object had escaped detection in all the archived observations. But that didn’t rule out the possibility that it was just a temporary burst of light.
Epsilon Indi A observed by the European Southern Observatory’s Very Large Telescope in 2019. (Credit: Matthews et al.)
The most compelling evidence came from the European Southern Observatory’s Very Large Telescope, which observed Epsilon Indi A in 2019, nearly four years before Webb. Because of the exoplanet’s great distance from its star, their relative positions would have changed little in the years between observations, while the star system would have moved considerably relative to any background objects.
When astronomers reanalyzed these observations, they found a faint spot where the planet should have been at that time and concluded that it must be the same object. This confirmed that the bright source Webb had observed was indeed a planet orbiting Epsilon Indi A.
Although the study shows that previous models were wrong, it is unclear what caused this error. The team suggests that this discrepancy be explored in future studies. Additionally, future observations could study the planet’s atmosphere and composition in more detail and help astronomers better understand similar cold gas giants.
“In the long term, we also hope to be able to observe other nearby planetary systems to hunt for cold gas giants that have escaped detection,” Matthews said. “Such a study would provide a foundation for a better understanding of the formation and evolution of gas planets.”
Matthews et al.’s results were published in the journal Nature on July 24.
(Main image: Image of the exoplanet Epsilon Indi Ab taken with Webb’s mid-infrared instrument. The star’s light was blocked by the instrument’s coronagraph, resulting in a dark circle. Credits: NASA, ESA, CSA, STScI, E. Matthews (MPIA))