‘Alien weather forecast’ by Leicester scientist provides insight into distant world

A science team has gained new insight into the atmosphere of a ‘sub-Neptune,’ a class of planet common in the galaxy but about which little is known, with the help of a ‘weather forecast’ by a University of Leicester scientist.

NASA’s JWST has observed a distant planet outside our solar system – and unlike anything in it – to reveal what is likely a highly reflective world with a steamy atmosphere. It’s the closest look yet at the mysterious world, which was largely opaque to previous observations.

The observations utilised JWST’s Mid-Infrared Instrument (MIRI), which Leicester scientists were involved in the design, manufacture and testing of, as well as modelling work by Dr Michael Roman from the University’s School of Physics and Astronomy. 

Dr Michael Roman from the University of Leicester School of Physics and Astronomy said: “A decade ago, astronomers tried to determine GJ 1214b’s atmospheric composition using the Hubble Space Telescope, but when they examined the data, they found that the planet was veiled in a thick layer of cloud or haze. This thick, blanketing haze layer prevented them from seeing the composition of the atmosphere. What we needed were observations that could peer through this layer.”

Thermal infrared observations had the potential to do just that, and so it was the perfect target for the new mid-infrared instrument (MIRI) on JWST.

GJ 1214 b is bigger than Earth but smaller than Neptune, and it orbits its host star so closely that it circles it every 38 hours.  However, astronomers are finding that planets like this are actually quite common across the galaxy--perhaps even the most common--and so scientists are eager to better understand them.  Everyone wants to know what they are made of and how they formed.  Theory suggests these ‘sub-Neptunes’ have thick hydrogen atmospheres (perhaps similar to a warmer version of Neptune), but observations are needed to confirm the theories.

“The planet’s atmosphere is totally blanketed by some sort of haze or cloud layer,” said Eliza Kempton, a researcher at the University of Maryland and lead author of a new paper on the planet. “The atmosphere just remained totally hidden from us until this observation.” 

To penetrate such a thick barrier, the research team took a chance on a novel approach: In addition to making the standard observation – capturing light from the host star that’s filtered through the planet’s atmosphere – they tracked GJ 1214 b through its entire orbit around the star, demonstrating the power of Webb’s Mid-Infrared Instrument (MIRI).

Infrared radiation lies outside the part of the light spectrum that human eyes can see. But using MIRI, the research team was able to create a kind of ‘heat map’ of the planet as it orbited the star.

“The MIRI instrument was indeed able to see through some of this thick haze, revealing, for the first time, hints of GJ 1214b’s atmospheric composition and temperature,” said Dr Roman. “We detected a feature that appears consistent with the presence of water vapour and possibly methane.  We also found that this thick haze was quite reflective—enough so to reflect about half of the light receives from its host star, causing the planet to be cooler than we expected, although it is still over 550 K (280˚ C) and the daylit side of the planet, while about 100 K less on the nightside.”

The heat map revealed – just before the planet’s orbit carried it behind the star, and as it emerged on the other side – both its day and night sides, unveiling for the first time details of the atmosphere’s composition.

“The ability to get a full orbit was really critical to understand how the planet distributes heat from the day side to the night side,” Kempton said. “There’s a lot of contrast between day and night. The night side is colder than the day side.” In fact, the temperatures shifted from 535 degrees to 326 degrees Fahrenheit (279 Celsius to 165 Celsius).

Such a big shift is only possible in an atmosphere made up of heavier molecules, such as water or methane, which appear similar when observed by the Webb telescope’s instruments. That means the atmosphere of GJ 1214 b is not composed mainly of lighter hydrogen molecules, Kempton said, a potentially important clue to the planet’s history and formation – and perhaps its watery start. She noted that, if indeed water-rich, the planet could be a ‘water world,’ with large amounts of watery and icy material at the time of its formation.  And while GJ 1214 b is too hot to harbour liquid oceans, water in vaporized form still could be a major part of its makeup.

Dr Roman assisted with the interpretation of the results and, in collaboration with colleagues, performed some of the many 3-D simulations of the atmospheric winds and temperatures.  “We essentially employed global weather forecast models tailored to the alien conditions of GJ 1214b.  The goal was to try and generate a model of the atmosphere that could help us to interpret what we were seeing in the observations.  Several of us produced models for this purpose. It turns out none of them were perfectly consistent with the actual observations for reasons we are still investigating, but they did help reveal that the clouds and/or hazes must be surprisingly bright and reflective.

“We now have a better understanding of the atmosphere, but the precise composition is still very uncertain. We are hoping to use JWST-MIRI for follow up observations soon.  The hope is that we can get even more precise measurements of the planet’s atmosphere and better determine the composition of this mysterious, cloud-veiled world.”

The new observations could open the door to deeper knowledge of a planet type so far shrouded in uncertainty. Sub-Neptunes are the most common type of planet in the galaxy, but mysterious to us because they don’t occur in our solar system. Measurements so far show they are broadly similar to, say, a downsized version of our own Neptune. Beyond that, little is known.

The University of Leicester provided the Mechanical Engineering Lead for the Mid-Infrared Instrument (MIRI) on JWST, has been responsible for design and provision of the MIRI Primary Structure (in collaboration with the Danish National Space Centre), and provided support for MIRI test and calibration activities. As well as heavy involvement in the MIRI instrument development, University scientists are strongly participating in the JWST science programme, in a wide range of projects covering many aspects of astrophysics and planetary science.

Dr Roman is working with Professor Leigh Fletcher at the University to use MIRI to analyse the giant planets of our own Solar System.  He adds: “The amazing sensitivity and precision of the MIRI instrument is allowing for exciting new discoveries about the atmospheres of other planets, both within our Solar System and far beyond.”

• A reflective, metal-rich atmosphere for GJ 1214b from its JWST phase curve is published in Nature, DOI: 10.1038/s41586-023-06159-5