The atmosphere of gas planet WASP-107b is a bit lopsided, and that could teach astronomers something new about alien weather.
WASP-107b is a strikingly fluffy planet, only about a tenth as dense as Jupiter. It also turns out to be slightly lopsided; the planet’s atmosphere is a few hundred miles taller on one side than the other. Astronomers have seen this slight unevenness on hot Jupiters (gas giants that orbit very close to their stars, basking in the scorching heat) before, but it shouldn’t be possible for WASP-107b — technically, the planet is too cool for that.
“Further observations of other cooler exoplanets are needed to determine whether WASP-107b is an outlier or whether the models have underestimated the presence of limb asymmetry in exoplanets,” write University of Arizona astronomer Matthew Murphy and his colleagues in their recent paper. The astronomers published their work in the journal Nature Astronomy.
A Slightly Lopsided Planet
Murphy and his colleagues recently used the James Webb Space Telescope (JWST) to watch as fluffy gas giant WASP-107b passed between Earth and its star. WASP-107b (like most planets that orbit extremely close to their stars) is tidally locked, which means it makes a full lap around its star in the same time it takes to rotate once. That creates the illusion that the planet isn’t rotating at all, because it keeps the same side of the planet always facing the star. Watching a planet like this transit gives astronomers a rare glimpse of the terminator, which is the line between the endless daylight on one side of the world and perpetual night on the other.
WASP-107b is a truly alien world: a planet unlike any of our neighbors here in the Solar System. It orbits its star backward, at a rakish tilt. And it’s only about a tenth as massive as Jupiter, but it orbits extremely close to its smallish orange star — so close that heat from the star has inflated the planet to about the size of Jupiter. That makes it a remarkably light, fluffy planet.
The terminator completely encircles the planet, dividing it in half. But because the planet is still rotating, and because wind and heat flow around the planet, we can also divide the terminator in half: If you’re following the direction that heat flows through the atmosphere, you’ll cross the terminator once as you move from day to night (the evening limb) and again moving from night to day (the morning limb).
At the evening limb, WASP-107b’s atmosphere is about 560 miles taller than on the morning side.
“Atmospheres are just a bunch of gas, so there’s no real physical ‘end’ like there is for a building,” Murphy tells Inverse. So when astronomers talk about the height or radius of a gas planet’s atmosphere, they’re talking about the layers where gas is so deep and thick that it blocks all the light from its host star. On WASP-107b, that point is about 560 miles higher on the evening limb than the morning limb.
In other words, the planet is just a little bit lopsided.
“It’s primarily because the temperatures are different on each limb, with the morning limb being cooler,” says Murphy.
At the morning limb, relatively cool atmospheric gas is flowing from the night side toward the day side, so the local temperature is much cooler than at the evening limb on the other side of the world. And cooler air is more compact than hotter air, so the morning limb’s atmosphere is a few hundred miles shorter.
A New Puzzle to Solve
The lopsided planet caught Murphy and his colleagues by surprise.
“Some of our models told us that a planet like WASP-107b shouldn’t have this asymmetry at all,” says Murphy in a recent statement.
WASP-107b has an average temperature of about 890 degrees Fahrenheit; it’s a hot Jupiter, but not a hot hot Jupiter. Most models say that planets with an average temperature below about 1700 degrees Fahrenheit shouldn’t have noticeably lopsided atmospheres. The difference in temperature between the dayside and the nightside should be too small to affect the height of the atmosphere.
“So we’re already learning something new,” says Murphy.
