This detection is significant for several reasons. It marks the first time an exoplanet has been imaged with JWST that had not been previously imaged from the ground and is much colder than the gas planets JWST has studied so far. An 'image' here means that the planet appears as a bright dot on the images, representing direct evidence. In contrast, the transit and radial velocity methods provide indirect evidence, as the planet only reveals itself through its mediated effect.

JWST Observations Update Previous Measurements
The planet orbits the main component of the nearby triple star system Epsilon Indi, or Eps Ind for short. Astronomical conventions assign the label Eps Ind A to that primary star, a red dwarf slightly smaller and cooler than the sun. To name the planet, a "b" is appended, resulting in the designation Eps Ind Ab.

The new JWST data indicate that this super-Jupiter has a mass six times that of Jupiter in our Solar System. Eps Ind Ab follows an eccentric, elliptical orbit with a separation ranging from 20 to 40 astronomical units from Eps Ind A. One astronomical unit is the average distance between Earth and the Sun, approximately 150 million kilometres. These new values differ significantly from earlier studies, leading the team to label this a "new" planet.

Cool Planets, Hot Science
Only a few cold gas-giant planets orbiting solar-age stars are known, and these have all been inferred indirectly from radial velocity measurements. By imaging and analyzing the spectra of these planets, astronomers can study their atmospheres and track the evolution of planetary systems compared to computational models. Investigating planets in settled planetary systems helps refine our understanding of the late stages of planetary evolution and general planet formation and evolution processes.

These observations pave the way for discovering more cold gas-giant planets. This will enable astronomers to study a new class of exoplanets and compare them to the gas giants in our solar system.

How to Detect Cold Gas Planets
These planets are difficult to find using traditional detection methods. Planets far from their host stars are typically very cold, unlike the hot Jupiters that orbit close to their stars. Wide orbits are unlikely to align with our line of sight to produce a transit signal. Additionally, measuring their signals with the radial-velocity method is challenging when only a small section of the orbit can be monitored.

Previous studies tried to investigate a giant planet orbiting Eps Ind A using radial velocity measurements, but extrapolating from a small part of the orbit led to incorrect conclusions about the planet's properties. Eps Ind Ab takes around 200 years to orbit its star, so observations over a few years are insufficient to determine the orbit precisely.

Therefore, Matthews and her team employed a different approach: direct imaging of the known planet. Exoplanet host stars are so bright they outshine nearby objects, making regular cameras ineffective due to the blinding starlight.

To address this, the team used JWST's MIRI (Mid-Infrared Instrument) camera with a coronagraph. This light-blocking mask acts like an artificial eclipse, covering the star. Eps Ind's proximity to Earth, only 12 light-years away, also helped. The closer the star, the larger the separation between two objects in an image, reducing the interference from the host star. MIRI was ideal because it observes in the thermal or mid-infrared, where cold objects shine brightly.

What Do We Know About Eps Ind Ab?
"We discovered a signal in our data that did not match the expected exoplanet," says Matthews. The point of light in the image was not in the predicted location. "But the planet still appeared to be a giant planet," adds Matthews. However, before confirming this, the astronomers had to rule out that the signal was from a background source unrelated to Eps Ind A.

"It is always hard to be certain, but from the data, it seemed quite unlikely the signal was coming from an extragalactic background source," explains Leindert Boogaard, another MPIA scientist and a co-author of the research article. While examining astronomical databases for other observations of Eps Ind, the team found imaging data from 2019 obtained with the VISIR infrared camera on the European Southern Observatory's (ESO) Very Large Telescope (VLT). Re-analyzing these images, they found a faint object precisely where it should be if the source imaged with JWST was indeed associated with Eps Ind A.

The scientists also attempted to understand the exoplanet's atmosphere based on images of the planet in three colors: two from JWST/MIRI and one from VLT/VISIR. Eps Ind Ab is fainter than expected at short wavelengths, which could indicate substantial amounts of heavy elements like carbon, forming molecules such as methane, carbon dioxide, and carbon monoxide, commonly found in gas-giant planets. Alternatively, it might indicate that the planet has a cloudy atmosphere. More research is needed to draw a definitive conclusion.

Plans and Prospects
This work is only the first step toward characterizing Eps Ind Ab. "Our next goal is to obtain spectra that provide a detailed fingerprint of the planet's climatology and chemical composition," says Thomas Henning, Emeritus Director at MPIA, co-PI of the MIRI instrument, and a co-author of the underlying article.

"In the long run, we hope to also observe other nearby planetary systems to hunt for cold gas giants that may have escaped detection," says Matthews. "Such a survey would serve as the basis for a better understanding of how gas planets form and evolve."

Full Caption: The inserts show cropped versions of the MIRI images obtained at mid-infrared wavelengths 10.65 (left) and 15.55 micrometres (right), which depict the area around the star Eps Ind A, whose position is indicated by star symbols. A coronagraph blocks the light from the star that would outshine both images. Instead, a new object becomes visible to the top left. This source is the exoplanet Eps Ind Ab. The background was obtained from the AllWISE sky survey. Research Report:A temperate super-Jupiter imaged with JWST mid-infrared imaging

Related Links
Max Planck Institute for Astronomy
Lands Beyond Beyond - extra solar planets - news and science
Life Beyond Earth