James Webb Space Telescope Directly Images Coldest Planet Yet

Astronomers have identified thousands of exoplanets among the stars, but very few have been directly imaged. We can add another to that exclusive list thanks to the James Webb Space Telescope. A team from the Max Planck Institute for Astronomy (MPIA) has used NASA’s flagship telescope to take a look at a nearby star, and they found a surprise. The star, called Epsilon Indi A, is home to the coldest and oldest exoplanet ever imaged.

Most confirmed exoplanets have never been seen—we can only infer their presence from the gravitational wobbles (radial velocity) of stars or dips in luminance as these elusive worlds transit in front of the star. Planets are so much dimmer than the stars they orbit that it’s usually impossible to see them with current optical instruments. Even the Webb telescope is ill-equipped for this particular task. However, some worlds with particularly long orbits have been photographed. That’s the situation in the Epsilon Indi system, which is a mere 12 light-years away.

Epsilon Indi is a triple star system, and the newly discovered planet orbits the largest of the three, known as Epsilon Indi A. The discovery has been dubbed planet “b” as its orbit is much different than what the team expected from previous radial-velocity measurements of the star. That makes the world “Epsilon Indi Ab.” It’s the first exoplanet to be imaged by Webb that was not previously imaged by ground-based telescopes.

The team, led by Elisabeth Matthews of the MPIA, probed Epsilon Indi A using Webb’s MIRI (Mid-Infrared Instrument) camera, which includes a coronagraph. This component can block the light from a star, making orbiting exoplanets easier to spot, provided they follow a long path around the star, and that definitely describes Epsilon Indi Ab. A year on this exoplanet is about 200 Earth years.

“Previous studies had correctly identified a planet in this system but underestimated this super-Jupiter gas giant’s mass and orbital separation,” said Matthews. Only a handful of cold gas giants have ever been detected around solar-age stars, and this is the first one ever imaged directly, the team explains.

Epsilon Indi Ab is a frigid gas giant, with an estimated temperature of 0 degrees Celsius. The team’s analysis suggests Epsilon Indi Ab is about six times the mass of Jupiter, but even a planet this size wouldn’t be visible to other observatories. MIRI was the ideal instrument to go planet hunting in Epsilon Indi because it can detect faint infrared emissions from cooler objects like this one.

The team’s next step is to obtain spectra from the exoplanet, which could reveal details of its atmosphere. Scientists have never had the opportunity to directly study an exoplanet quite like this one—the team hopes that Webb could reveal more cold gas giants orbiting nearby stars, helping us better understand how large planets form.