James Webb Spots A Cold Gas Giant Planet Six Times Larger Than Jupiter

The James Webb Space Telescope (JWST) shoots an exoplanet six times larger than Jupiter. It’s the oldest and coldest planet outside the Solar System observed directly, and it wasn’t where astronomers expected.

A distant and cold neighbour

‘It is about double the mass, a little further away from its star, and has a different orbit than expected,’ reported Elisabeth Matthews of the Max Planck Institute for Astronomy (MPIA) in Germany, one of the authors of the Webb Cycle 1 GO #2243 study. The study’s results were published 24 July in the journal Nature.

Astronomers previously knew about this planet. Its name sounds like Epsilon Indi Ab. Epsilon (or ‘Eps’) is the fifth letter of the Greek alphabet or – in this case – the fifth brightest star in the constellation Indus, where ‘Ind’ comes from. The ‘A’ is the star, and the ‘b’ denotes the planet.

Scientists’ initial calculations showed the planet’s mass to be 3.25 times that of Jupiter. Using data from the new images, they found that its mass is six times that of Jupiter. They also learnt that the planet orbits the star every 200 years rather than every 45.2 years. 

The planet is bright enough in the infrared to correspond to a temperature of about 275 K (1.85 °C), which means it is one of the coldest exoplanets observed.

It is the same distance from its star as our Neptune is from the Sun. It forms a triple system with two ‘brown dwarfs’ – objects that never got big enough to cause hydrogen fusion in their cores – that orbit around it more than 1,000 times further away than Eps Ind Ab.

The First Exoplanet Ever Revealed By Direct Imaging With The JWST

However, James Webb’s high sensitivity made it possible to overcome these limits: the new image showed an exoplanet about as old as those in the Solar System.

‘Cold planets are very faint, and most of their radiation occurs in the mid-infrared,’ Mathews 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 Webb’s large mirror is extremely useful in this aspect.’

To find the planet, the international team of scientists aimed the telescope so that the star was at the centre of its field of view. Using the MIRI (Mid-Infrared Instrument) instrument, they then utilised the coronagraph capability.

The resulting image showed the planet, a ‘gas giant’ composed mainly of hydrogen gas. The researchers believe its atmosphere has significant amounts of methane, carbon monoxide and carbon dioxide, which absorb shorter wavelengths of light, which could mean a very cloudy atmosphere.

Elizabeth Matthews said scientists will continue to study Eps Ind Ab to determine its atmosphere.

Conclusion

Although not involved in the study, Paul Delaney, an astronomer and emeritus in the Department of Physics and Astronomy at York University in Toronto, said it powerfully illustrated JWST’s capabilities.

‘It’s a finely tuned telescope, and it’s producing data of such high quality that it’s hitting the headlines,’ Delaney said. In just two years, the amount of data collected by the telescope has already surpassed that of its predecessor, the Hubble Space Telescope, launched in 1990.

While discovering new planets is not JWST’s primary mission, it will focus on studying those already found to learn more about these distant worlds and help astronomers better understand how planets like ours form.