Webb finds an old, cold, and surprising exoplanet

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Using the joint NASA, European Space Agency (ESA), and Canadian Space Agency (CSA) James Webb Space Telescope, astronomers have imaged an exoplanet 12 light-years from Earth. Only about 25 exoplanets have been directly imaged, but this is the coldest and the oldest exoplanet directly observed.

Though the scientists expected to find a planet around the star — named Epsilon Indi A — based on previous studies, Webb’s observation surprised the astronomers. Not only did the exoplanet have properties that were different from what was expected, but it was also found in a location different from what previous models had predicted.

“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,” said lead author Elisabeth Matthews of the Max Planck Institute for Astronomy in Heidelberg, Germany. “It’s about twice as massive, a little farther from its star, and has a different orbit than we expected. The cause of this discrepancy remains an open question.”

The planet is several times the mass of Jupiter and is in an elliptical orbit around a star of a similar age as the Sun. When analyzing the planet’s atmosphere, the astronomers found that it is fainter than expected in shorter wavelengths. This could mean the atmosphere is very cloudy, but it might 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 in different frequencies. The background was obtained from the AllWISE sky survey. (Credit: T. Müller (MPIA/HdA), E. Matthews (MPIA))

The team estimates the planet’s temperature is two degrees Celsius. This is about 100 degrees Celsius warmer than the gas giants in our Solar System, but substantially cooler than any other exoplanet directly observed. This temperature makes older exoplanets harder to observe than younger exoplanets, as the younger exoplanets emit much energy from their formation.

“Cold planets are very faint, and most of their emission is in the mid-infrared,” said Matthews. “Webb is ideally suited to conduct mid-infrared imaging, which is extremely hard to do from the ground. We also needed good spatial resolution to separate the planet and the star in our images, and the large Webb mirror is extremely helpful in this aspect.”

The planet’s star, Epsilon Indi A, is the main star in a star system of three. It’s a K-type red dwarf star, which is smaller and cooler than the Sun. The other two stars are a pair of smaller brown dwarfs that orbit Epsilon Indi A at a large distance.

“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,” said author Caroline Morley of the University of Texas at Austin. “It’s exciting to actually see a planet there ourselves, and begin to measure its properties.”

Earlier studies looked for planets around the star using so-called radial velocity measurements. This method can indirectly detect exoplanets by measuring a slight wobble caused by the planet’s gravitational pull.

“Our prior observations of this system have been more indirect measurements 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 Morley. “That’s why our team chose this system to observe first with Webb.”

While previous studies correctly identified the presence of a planet around Epsilon Indi A, the image returned by Webb was very different from what the astronomers expected. Therefore, 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 exclude the possibility that an object outside the star system happened to be in the background when the image was taken and was now confused for a planet. To do so, the team analyzed earlier observations of Epsilon Indi A and found it unlikely that any background object would have evaded detection in all archived observations. But this didn’t rule out the possibility that it was a temporary burst of light.

Epsilon Indi A as observed by the European Southern Observatory’s Very Large Telescope in 2019. (Credit: Matthews et al.)

The strongest evidence came from the European Southern Observatory’s Very Large Telescope, which observed Epsilon Indi A in 2019, nearly four years earlier than Webb. Because of the large distance between the exoplanet and its star, their relative positions should have changed little in the years between the observations while the star system would have moved substantially compared to any background object.

When the astronomers reanalyzed those observations, they found a faint spot where the planet should have been at the time and concluded this must have been the same object. This confirmed that the light source observed by Webb was indeed a planet orbiting Epsilon Indi A.

Even though the study shows that the previous models were incorrect, it is unclear what caused the error. The team suggests that this discrepancy should 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 run, we hope to also observe other nearby planetary systems to hunt for cold gas giants that may have escaped detection,” said Matthews. “Such a survey would serve as the basis for a better understanding of how gas planets form and evolve.”

Matthews et al.’s results were published in the journal Nature on July 24.

(Lead image: Image of exoplanet Epsilon Indi Ab taken with Webb’s Mid-Infrared Instrument. The star’s light has been blocked by the instrument’s coronagraph, resulting in a dark circle.  Credit: NASA, ESA, CSA, STScI, E. Matthews (MPIA))

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