Ellen Phiddian
Cosmos science journalist
Astronomers using the James Webb Space Telescope have discovered a huge exoplanet less than 12 light-years from Earth.
The planet, a gas giant 6 times the size of Jupiter, is the coldest and oldest exoplanet ever directly imaged (that is, not found through indirect means like observing a star).
The international team has published its discovery in Nature.
“We were excited when we realised we had imaged this new planet”, says lead author Dr Elisabeth Matthews, an astrophysicist at the Max Planck Institute for Astronomy in Germany.
“Previous studies had correctly identified a planet in this system but underestimated this super-Jupiter gas giant’s mass and orbital separation.”
The planet is orbiting the orange dwarf star Epsilon Indi A, and it appears to be the only planet in the system, so the researchers have assigned it the name Epsilon Indi Ab.
Epsilon Indi A is 3.6 parsecs, or 11.7 light-years, from Earth, and about 3.5 billion years old.
Its planet appears to be orbiting now at a distance of roughly 15 AU (astronomical unit, the distance between Earth and the Sun). it has an elliptical orbit, and the researchers believe that its furthest distance from its star should be 20-40 AU.
This makes it a very cold world, with a temperature of about 275 Kelvin (less than 2°C).
To date, about 25 exoplanets have been directly imaged. Most exoplanets are found by “classical” means, such as spotting how a star’s light will dim when planets move in front of it.
This means that large, close-orbiting exoplanets are easier to spot than those at move at further distances, because they move between their star and Earth more regularly.
The researchers were using the Mid-Infrared Instrument (MIRI) on the James Webb Space Telescope to study Epsilon Indi A.
MIRI is equipped with a mask called a coronagraph, which can block out light from stars so it can observe objects near them.
“We discovered a signal in our data that did not match the expected exoplanet,” says Matthews.
“But the planet still appeared to be a giant planet.”
The astronomers then confirmed that the light source wasn’t coming from elsewhere, and found some 2019 data from the Very Large Telescope that also indicated a light source at the same point that the planet would be.
The researchers don’t yet know what the surface of Epsilon Indi Ab is like. Some preliminary imaging suggests that it might have a lot of carbon-based molecules, like methane, carbon dioxide, and carbon monoxide in its atmosphere, or it might be a cloudy planet.
“Our next goal is to obtain spectra which provide us a detailed fingerprint of the planet’s climatology and chemical composition,” says Professor Thomas Henning, a director at the Max Planck Institute.
“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.”
