Well, this is a big deal: Astronomers have discovered a very nearby gas giant exoplanet through direct imaging with JWST! A few exoplanets have been seen by JWST before, but this is the first one discovered in JWST data. On top of that, it’s the second closest Jovian (Jupiter-like) exoplanet known!

Epsilon Indi is visible to the naked eye as a faint orange star in the southern constellation of Indus. It’s actually a triple system: The primary (called Epsilon Indi A, or Eps Ind A for short) is a K-type star, meaning it’s slightly less massive, cooler, and smaller than the Sun. It’s orbited by a binary pair of brown dwarfs. Such a system consisting of a single star orbited by a binary pair is called a hierarchical triple, just so’s you know. The brown dwarfs are very far from the primary, about a fifth of a light-year, and take millennia to orbit. The system as a whole is about 11.9 light-years from Earth, making it one of the closest to us (the 20^th closest known in fact). 

Over the years, careful observations of Eps Ind A showed some peculiarities. Its motion through space has a slight wobble to it, hinting at a big planet orbiting it, tugging it gravitationally as it moves. There’s also a slight Doppler shift in its spectrum (called the radial or reflex velocity) that indicated a planet as well. The data aren’t clear, but the best guess was that the planet was about three times the mass of Jupiter on an orbit roughly 1.6 billion kilometers out from the star, a bit farther out than Saturn is from the Sun.

A planet like that should be relatively cool in temperature, which means it emits infrared light. JWST is designed to look at those wavelengths, so astronomers pointed the observatory at Eps Ind A to see what they could see [link to journal paper].

And what they got was a surprise. They put the primary star behind an occulting mask, a piece of metal that blocks the glare from bright objects so that fainter nearby objects can be seen. And sure enough, right next to the star is a fainter point of light. Could it be the planet? 

They were able to statistically rule out the object being a distant background galaxy — such a galaxy would be extended in the image, and the object is clearly unresolved, a dot. Also, the Eps Ind system moves pretty rapidly through space, and over the years would have moved significantly relative to any background source. Older infrared data shows no such object at the position seen, when it should be pretty obvious. That means the object is moving along with Eps Ind A.

Also, the brightness and color (the difference in brightness between two images taken in different filters) are consistent with it being a gas giant exoplanet. The formal designation for it is Epsilon Indi Ab. 

But this is where things get weird. For one, it’s not at all in the position predicted using the previous data. It’s also far too bright to be a planet with three times Jupiter’s mass. What went wrong? The astronomers aren’t sure, but it’s likely the predictions made were inaccurate due to the uncertainties in the original pre-JWST data. It’s possible to get multiple solutions for a planet’s characteristics when the data aren’t great, so the old estimates probably just converged on the wrong solution. 

Combining the old observations with the new ones from JWST, the astronomers find that Epsilon Indi Ab has a mass of a little over 6 times that of Jupiter on an elliptical orbit well over 4 billion kilometers out from the star. That’s much more massive and farther out than the previous estimates!

Does that mean there may yet be another planet orbiting Eps Ind A to explain the old observations? Actually, probably not. After cranking through the equations, the astronomers find that the newly calculated properties adequately fit the old observations. It’s clearly the dominant source of gravity tugging on the star. This doesn’t rule out another planet, to be clear! But any other planet in the system is likely to be less massive. From what we understand about planetary formation, it’s unlikely for a star to have a single lone planet, so there probably are more, but they’re not detectable with the current observations.

Eps Ind Ab is the coldest exoplanet ever directly imaged. Judging from the colors, the temperature is about 0° C. Interestingly, it’s not as bright at shorter infrared wavelengths as expected. That could be due to clouds in the atmosphere, but the best models of exoplanetary atmospheres don’t show enough clouds to account for that. More likely there’s carbon monoxide, carbon dioxide, and methane in the air there, which are all good at absorbing light at those colors. That’s surprising for a super-Jupiter, which typically don’t have as much of those molecules.

It’s also the oldest exoplanet ever imaged, too! All the ones we’ve seen so far (several dozen) are young and hot, so they glow brightly in infrared, making them visible to telescopes on the ground. JWST, being in space, is more sensitive to fainter objects (especially ones near bright stars), so it can see this planet easily. Up until this one, all the exoplanets directly seen are less than 500 million years old, and most are younger than 100. Eps Ind Ab is more like 3-6 billion years old, comparable to the age of our own solar system.

That’s a big deal, because this means we can study an older, colder gas giant in some detail, which was previously not possible (besides the ones in our own solar system). We now have lots of physical models of how planets should look over time given things like their age, composition, and so on. However, it’s hard to test those models when all the samples you have are closer together in some parameter, like age. With this planet (and hopefully many more like it to come) we’ll be able to build our models better and have more confidence understanding what planets around other stars look like.

Just as a reminder, the first exoplanet was only discovered in the early 1990s! We’ve come a long, long way since then. We’ve found over 5,000 now, but there are likely hundreds of billions if not trillions in our Milky Way alone, so we also have a long, long way to go.

You can email me at [email protected] (though replies can take a while), and all my social media outlets are gathered together at about.me. Also, if you don’t already, please subscribe to this newsletter! And feel free to tell a friend or nine, too. Thanks!