Hot Jupiters: Cloudy or Clear?

New analysis of Kepler observations shows that hot Jupiters' temperatures might predict their weather report.

Hot Jupiter, reflected light and thermal emission

Astronomers can track the weather on alien worlds by measuring their reflected light and thermal emission.
Lisa Esteves / University of Toronto

“Clouds before seven, clear by eleven.” An old weather adage that’s only sometimes true on Earth might fare better on some alien worlds.

Lisa Esteves (University of Toronto, Canada) and colleagues sifted through 4½ years of Kepler data to pick out 14 exoplanets that show clear brightness changes as they orbit their host stars. The data the team collected, published in the Astrophysical Journal, show a trend that could predict the weather on certain exoplanets based on their temperatures.

Seeing Exoplanets For Themselves

Ordinarily, Kepler-detected exoplanets make themselves known indirectly, by the slight blips they cause in their host star’s light as they momentarily (but repeatedly) pass in front of it.

But the planet is a light source too and effects its own changes. The planet’s reflected light (that is, light scattered off of clouds in its atmosphere) is hidden from view during transit, even as its glow, the thermal emission generated from the planet’s internal heat, still adds to the starlight. And when a planet passes behind its star, both reflected and thermal radiations are blocked.

The reflected light and thermal glow are too faint to measure in most exoplanets. But in 14 cases, most of them hot Jupiters on tight orbits no longer than a few days, Esteves’s team was able to detect brightness changes as the planets passed not only in front of, but also behind and next to their host stars.

By observing these exoplanets shortly before, during, and after eclipse, as well as before, during, and after transit, the astronomers were able to work out how much of each planet’s light is reflected and how much of it comes from the planet’s own heat.

Cloud cover on a hot Juptier

Hot Jupiters always show the same face to the host star, the planet's dayside. But the dayside's brightest point might not be right at its center, depending on the planet's cloud cover and internal heat.
Lisa Esteves / University of Toronto

In doing so, the team also measured each planet’s bright spot. Almost all of the 14 planets are hot Jupiters, gas giants that circle their stars on close-in orbits, so they’re probably tidally locked and show the same face to their star at all times. (One planet is much smaller and probably rocky, but is nevertheless on a tight, tidally locked orbit.) So you’d expect the brightest part of the planet’s atmosphere to be smack in the middle of the side facing the star, the planet’s dayside.

But, in six cases, it’s not. In four hot Jupiters, where the average temperature hovers around “only” 2000 Kelvin (3100°F), the bright spot is offset so that the planet’s brightness peaks after its eclipse behind the star. In these relatively cooler planets, Kepler doesn't see the planet's own heat, which radiates infrared outside of Kepler's range. Instead, Kepler is seeing the planet's reflected light. If the pre-eclipse side is clear while clouds permanently cover the post-eclipse side, then the additional reflected starlight could shift the bright spot to the post-eclipse side.

Meanwhile, on two scorching-hot Jupiters where average temperatures approach 3000 Kelvin (4900°F), the hotspot is offset in the opposite direction, peaking before the eclipse. The results suggest completely clear skies in the hottest Jupiters, so that the planet’s own heat (radiated within Kepler's optical range for these hotter planets) is what offsets the hotspot toward the pre-eclipse side.

More Data Needed

“It is a very interesting trend that does suggest that clouds are playing a role,” says Jonathan Fortney (University of California, Santa Cruz). “My only worry is that a number of the planets show no phase offset at all.”

Indeed, while six of the planets have a hot spot offset from center, the remaining eight planets do not. That means there are as many outliers as there are data points that follow the trend.

“What’s interesting is that [the trend] is not seen for all planets of similar temperatures,” Fortney adds, which he suggests could mean another component, such as the planet’s surface gravity or some property of the host star, is at play.

Webb telescop in orbit

Artist's conception of the James Webb Space Telescope in orbit.

Studies such as this one show that astronomers are still only beginning to understand exotic worlds on a detailed level. (And as difficult as hot Jupiter weather reports are to understand, Earth-like analogs are still further out of reach.) But for the larger variety of alien worlds at least, the launch of the highly anticipated (and much-debated) James Webb Space Telescope, scheduled for 2018, should reveal these worlds in far more detail.

Exoplanets, News
Monica Young

About Monica Young

Monica Young, a professional astronomer by training, is web editor of Sky & Telescope, where she creates, manages, and maintains website content, and contributes to the magazine.
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