Water Ice Cirrus Clouds May Envelop Jupiter-Exoplanet

First Demonstration: The detection of clouds around exoplanets is an important step in the search for a second Earth

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To the point

• New observations: Astronomers have used the James Webb Space Telescope to study the atmosphere of a massive Jupiter-analogue.
• Evidence for clouds: Surprisingly, the observations indicate the presence of water-ice clouds, in alignment with theoretical models on the computer - previous exoplanet-models had been too simple.
• Part of a larger search: Observations and analyses serve as a kind of test run for the challenges that also arise in the search for a "second Earth."

A team of astronomers led by Elisabeth Matthews at the Max Planck Institute for Astronomy (MPIA) has made a discovery that highlights the limits of most current models of exoplanet atmospheres: water-ice clouds on a distant Jupiter-like exoplanet called Epsilon Indi Ab. The way the observations were made has broader implications for exoplanet research: as an interesting immediate step on the path towards eventually finding and characterizing an Earth-analogue exoplanet.

Step by step towards a second Earth

Exoplanet research has an ambitious long-term goal: at some time within the next few decades, astronomers hope to be able to detect traces of life on an exoplanet. On the path towards that goal, exoplanet research has gone through several stages. In the first stage of research, from 1995 to about 2022, the main focus of exoplanet researchers was on detecting more and more exoplanets, using indirect methods that gave them information about the masses of some exoplanets, the diameters of others, and in some cases both mass and diameter.

When the James Webb Space Telescope (JWST) began operating in earnest in 2022, exoplanet research entered a second stage: High-quality, detailed information about the atmospheres of many exoplanets became available for a considerable number of planets, and researchers began to reconstruct the properties of such atmospheres in some detail. This is still at least one stage removed from realistic searches for life on exoplanets, which are expected to require the next generation of space telescopes.

With the new study, the astronomers are exploring some aspects of these next-level methods - although not yet for a planet like Earth. Elisabeth Matthews (Max Planck Institute for Astronomy), the study's lead author, says: "JWST is finally allowing us to study solar-system analogue planets in detail. If we were aliens, several light years away, and looking back at the Sun, JWST is the first telescope that would allow us to study Jupiter in detail. For studying Earth in detail, we would need much more advanced telescopes, though."

Elusive exo-Jupiters

But as amazing as results from JWST about exoplanet atmospheres are, studying the analogues of our Solar System's Jupiter has proven surprisingly difficult. Almost all gas giants studied with JWST so far differ from Jupiter in that they are much, much hotter - for the most common method of studying exoplanet atmospheres to work, the planet needs to pass in front of its host star from the perspective of an observer on Earth, and the probability for that configuration is much higher when the planet is closer to its star, which in turns makes the planet comparatively hot. The new study by Elisabeth Matthews and her colleagues uses a different technique. This is the closest observers have come to studying a Jupiter-analogue - and it has provided at least one surprise!

Matthews and her colleagues used JWST's mid-infrared instrument MIRI to obtain direct images of the planet Epsilon Indi Ab. Naming conventions for exoplanets are such that this designation indicates the first planet discovered to orbit the star Epsilon Indi A in the constellation Indus (in the southern sky). Bhavesh Rajpoot, a PhD student at the Max Planck Institute for Astronomy who contributed to the study, says: "This planet has a considerably greater mass than Jupiter - the new study fixes its mass at 7.6 Jupiter masses - but the diameter is about the same as for its solar-system cousin."

A more massive, slightly warmer Jupiter

Epsilon Indi Ab is about four times as distant from its central star as Jupiter is from the Sun. The star Epsilon Indi A itself is a bit less massive and a bit less hot than our Sun. This makes the surface temperature of Epsilon Indi Ab very low, at about 200 to 300 Kelvin (between -70 and +20 degrees Celsius). The reason the planet is slightly warmer than Jupiter (140 K) is that there is still a lot of heat remaining from the planet formation phase. Over the next billions of years, Epsilon Indi Ab will steadily cool down, eventually becoming colder than Jupiter.

The astronomers used the coronagraph of the MIRI instrument to block out the central star's light, which would otherwise outshine the planet's much dimmer light. They then took an image through a very particular filter: 11.3 μm, which is just outside the wavelength region close to 10.6 μm that is characteristic for ammonia molecules NH₃. The comparison with images at 10.6 μm that Matthews and her team had already taken in 2024 enabled the astronomers to estimate the amount of ammonia present. (Incidentally, both the mechanical filter wheels placing the coronagraph and the filter in front of the MIRI camera were constructed at MPIA, one of the German contributions to the JWST.)

Surprising evidence for clouds

For Jupiter, both ammonia gas and ammonia clouds dominate the upper layers of the atmosphere that are visible in observations. Given its properties, Epsilon Indi Ab was thought to have massive amounts of ammonia gas as well, although not ammonia clouds. Surprisingly, the photometric comparison showed somewhat less ammonia than expected. The best explanation Matthews and her colleagues found for this deficit was the presence of thick but patchy water-ice clouds, similar to the high-altitude cirrus clouds in Earth's atmosphere - an unexpected complication!

In interpreting observations of this kind, astronomers compare their data to simulations of planetary atmospheres. But most of the published models neglect to include clouds, as the presence of clouds makes the computation that much more complicated - clearly something theorists will need to fix! James Mang (University of Texas at Austin), a co-author of the study, says: "It's a great problem to have, and it speaks to the immense progress we're making thanks to JWST. What once seemed impossible to detect is now within reach, allowing us to probe the structure of these atmospheres, including the presence of clouds. This reveals new layers of complexity that our models are now beginning to capture, and opens the door to even more detailed characterization of these cold, distant worlds."

An opportunity for the Roman Space Telescope

On the upside, there is an upcoming opportunity for observing the water-ice clouds, which are very reflective directly: NASA's Nancy Grace Roman Space Telescope, where MPIA is a partner, is slated for launch in 2026-2027, and should be suitable for exactly that kind of observation. In the meantime, Matthews and her colleagues are applying for JWST observation time to target additional cold Jupiter-analogues. And at the same time that Matthews and other astronomers are learning more about cold exo-Jupiters, their observational techniques are laying the groundwork that, if all goes well, will help future observers target earthlike planets, in search of life.

Background information

The results described here have been published as E. C. Matthews et al., "A second visit to Eps Ind Ab with JWST: new photometry confirms ammonia and suggests thick clouds in the exoplanet atmosphere of the closest super-Jupiter" in the Astrophysical Journal Letters.

The MPIA researchers involved are Elisabeth Matthews and Bhavesh Rajpoot, in collaboration with James Mang and Caroline Morley (University of Texas at Austin), Aarynn Carter and Mathilde Mâlin (Space Telescope Science Institute), and others.