Planets with atmospheres have their own weather patterns, much like Earth, but in a variety of patterns. JWST gives astromoners the ability to look beyond our solar system to get precise measurements on exoplanets, such as WASP-121b — an ultra-hot Jupiter orbiting close to it's star. Dr. Robert Frazier and collaborators at the University of Michigan studied the planet's atmospheric changes as it orbited, making some surprising discoveries.
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Astro Brief is a podcast collaboration between KSMU, the Missouri Space Grant, and MSU's Department of Physics, Astronomy and Materials Science. Hosted by Dr. Mike Reed, Astro Brief focuses on astronomical events, the field of astronomy, and astronomy-related guests. It airs Thursdays at 9:45 am on KSMU.
Transcript
In this episode, we're going to talk about how atmospheres move.
We're pretty familiar with Earth's jet stream traveling west to east. Sometimes in the winter it will bring down arctic air to make us cold, but for us in southern Missouri, we're usually affected by the subtropic jet stream which brings us warm humid air from the southwest. But other planets also have winds that circulate their atmospheres, and we expect that.
Both Venus and Mars have complex wind patterns that are not quite like ours.
On Venus there is a super rotation wind going east to west, and above that during the day, the winds blow towards the poles, and at night it reverses and flows towards the equator. Mars is a little more like our Earth in that in the north and south there is a west to east jet stream, but in between — above the equatorial regions — there are circular patterns of high and low pressure. Jupiter has a really interesting wind pattern with strong winds in narrow east-west strips — where one will go east to west, while its neighbor will go west to east. Jupiter has nine of these opposite wind pairs between its north and south poles. That's part of what gives it the lovely bands and belts we see in its atmosphere.
Such winds in our own solar system are interesting — and by and large we have the capacity to measure them — but planets around other stars should have similar winds, and JWST has been looking for them on a very interesting planet.
The planet is called WASP-121b, and what makes it interesting is that it's incredibly close to its star. WASP-121b is what's known as an ultra-hot Jupiter. It orbits its star every 30 hours and 30 minutes. Its year is 6 hours longer than our day, and that proximity to its star makes it extremely hot with an average temperature of nearly 4,000 degrees Fahrenheit. And it's Jupiter-like — just 20% more massive and 70% larger in size. Some of that extra size is caused by the extreme heat inflating its atmosphere, but also it's so close to its star that the atmosphere is distorted — sort of pointing in the direction of the star — and it's almost certainly tidally locked with one side always facing the star and the other side never seeing daylight. That should make the day side even hotter, and the night side even cooler.
For WASP-121b, that difference should be over 1,300 degrees. That should make some really interesting effects.
On the day side, water molecules are dissociated — and even the H2 is separated on the day side — absorbing energy that would normally go into heating the atmosphere, while on the night side, the H2 would recombine — releasing energy for heat. Also on the day side, the star's radiation would ionize some atoms — making them charged, and then there would be magnetic fields that would either promote or impede their motion.
So lots of interesting features to sort out.
In an article published in the Astrophysical Journal by Robert Frazier of the University of Michigan and collaborators, they used JWST to observe WASP-121b over an entire orbit to look at how the atmosphere changes in temperature from which they could determine its motion. Their observations used JWST's infrared spectrograph so they could examine temperatures at several different wavelengths. To interpret those data, Dr. Frazier and collaborators produced detailed general circulation atmospheric models, which included ionizing and recombining atoms — accounting for the large tidal bump in the atmosphere, and making and diffusing clouds. And those clouds could be quite exotic, including potassium chloride, silicon dioxide, chromium, vanadium oxide, and even aluminum oxide. So they could get some really heavy material into their atmosphere, which is pretty cool.
Matching those models to the observations provides a detailed examination of WASP-121b's atmosphere, and what they found was a little surprising.
Their models predict the hot side to be 12-15% brighter than what's actually observed — though this could be attributed to the tidal bump, which makes determining the actual radius a bit difficult — but the hot spot also gave them a surprise. For such planets, the hottest spot is typically not directly facing the star, but is pretty pulled to one side because of strong winds. However, on WASP-121b, the hottest spot is very nearly pointing right at the star. This was surprising and must mean that the winds are being slowed or even broken up by some type of friction — and the effect is stronger at shorter wavelengths, which is not predicted by any models.
Such precision measurements of a planet 900 light-years away are showing us unexpected and exciting features.