It helps us figure out whether GJ 504 b really is a planet or not

–   by Briley Lewis    –

An image of the 'pink planet', GJ 504b
GJ 504 b. Image Credit: NASA/Goddard Space Flight Center/S. Wiessinger

Ammonia (NH3) is a common chemical here on Earth. It’s in fertiliser, refrigerator coolant and even some biological waste – if you’ve ever cleaned a fish tank, you’re surely familiar with the molecule’s scent. It’s also common beyond our planet, as its component elements of nitrogen and hydrogen are both readily produced in stars. Astronomers first spotted ammonia floating in the spaces between stars around 1970.

Now Nasa’s James Webb Space Telescope (JWST) appears to have made the first detection of ammonia on an exoplanet (a world orbiting a star other than the Sun). Around 57 light years away and with a temperature of about 500 degrees Kelvin (equivalent to about 227 degrees Celsius), GJ 504 b is one of the coldest exoplanets ever to be photographed, as they usually need to be young and warm for us to image them.

However, we’re still not certain whether GJ 504 b really is a planet or not. Astronomers think it may be a large gas giant planet about four times Jupiter’s mass, or it could be a brown dwarf, which is a bit too big to be a planet but not large enough to sustain fusion like a star. That’s where ammonia comes in: this key molecule helps narrow down GJ 504 b’s size, and therefore its mass. The JWST data suggest that GJ 504 b is, in fact, small enough to be called “planet-sized”, but more observations are needed to be sure.

Only JWST can provide those additional observations, and only JWST could possibly have made this confident detection of ammonia in the first place. The space telescope’s mid-infrared MIRI instrument was designed to find the NH3 predicted to linger in planetary atmospheres, and its capabilities open a door to imaging planets at longer wavelengths than was previously possible. This provides a treasure trove for scientists looking to understand the atmospheres of giant exoplanets, which in turn helps narrow down how these planets could have formed in the first place.

JWST has already made a number of other spectacular observations, including sandy silicate clouds on the exoplanet VHS 1256 b and an atmosphere quite similar to Jupiter’s on another, Epsilon Indi Ab. And with 20 or more years left in its scientific lifetime, there are sure to be more discoveries to come.

This article is from New Humanist’s Spring 2025 issue. Subscribe now.