Strong day-night temperature differences on a tidally locked planet.
Since the discovery of the very first exoplanets, it's been clear
that there are lots of worlds out there that are markedly different
from our solar system: hot Jupiters nearly skimming their host stars'
surface, super-Earths, mini-Neptunes.But we don't know exactly what these worlds look like. For the most part, we've been left to infer their properties using indirect measurements.
This week's edition of Science contains a description of one of the exceptions. The Hubble Space Telescope has imaged light from a hot Jupiter called WASP-43b, detecting temperature differences between the planet's day and night sides. The results suggest that the planet has an eastward jet stream that redistributes some of the heat from its host star, but otherwise there's very little circulation of heat.
WASP-43b is a Jupiter-sized planet that's hot because it orbits so close to its host star—its year is only 19.5 hours. Its host star is relatively small, so WASP-43 isn't in danger of being swallowed by it in the near future. But it is close enough to be tidally locked, meaning one side perpetually faces the host star, while the other resides in permanent darkness.
The planet's orbit takes it between Earth and its host star, creating a small eclipse with each orbit. But it's the rest of the orbit that Hubble imaged. That's because, at the wavelengths used (in the near-infrared), the planet itself radiates and reflects light back into space. And, given the quality of the Hubble's instruments, it's possible to detect this light as an addition to the star's normal output.
So, as WASP-43b moves out from in front of the star, its day side gradually comes into view as a thin crescent that grows ever larger. As it moves towards a position behind the star, nearly the full daylight side comes into view, providing its maximum contribution. When the planet is behind the star (called a secondary eclipse), there's a sharp cutoff back to the levels of light normally output by the star. When the planet emerges from behind it again, the whole process repeats in reverse.
Not only was the Hubble able to image all of this, it was able to see that the pattern wasn't perfectly symmetric. Instead, the light from the planet peaked just before it slipped behind the star.
The authors use this to infer that there's a jet stream at the planet's equator that flows eastward. This brings material from the hot region of the atmosphere—the part facing the star—over to the dark side of the planet, making it somewhat warmer. The directionality of the jet stream accounts for the asymmetry seen in the Hubble images.
The size of this effect provides some indication of the rate at which heat is transferred within the atmosphere from the lit side to the one facing away from the nearby star. And all indications are that the answer is "not much." Although the planet seems to absorb a lot of the light it receives from the star—primarily due to water vapor in the atmosphere, it's mostly radiated back into space rather than circulated within the planet. As a result, there's an enormous day/night side temperature difference.
This is not to say that the authors could explain everything they observed with the Hubble. The coldest spot on the planet, which you'd expect to be opposite the side that faces the star, isn't. None of the models that the researchers built of the planet can really explain why this shift has occurred.
Still, it's impressive how much information the researcher team was able to gather just by observing a handful of orbits. It certainly doesn't give us the complete picture of WASP-43b, but it does at least provide a rough outline of the dynamics of the world. And once again, the world is like nothing we've ever seen in our solar system, where the gas giants are all evenly heated, primarily by the energy left over from their gravitational collapse.
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