The blistering hot exoplanet where it snows
Today's weather on HD 189733 b: It will be hazy with high wispy clouds. The wind will be steady from the east at speeds approaching 6,000 miles per hour (9,656 km/h). Daytime temperatures will average a balmy 800ºC (1,472ºF), while the equatorial hot spot at 30 degrees longitude is expected to top 900ºC (1,652ºF). But, there is a high chance of silicate snow showers, with accumulations expected except in the vicinity of the hot spot.
Nighttime won't come, as our planet is tidally locked. However, if you cross to the dark side, the sunset will be a spectacular show of our orange sun fading away to a red horizon. The dark side will see a chilly 700ºC (1,292ºF), with intermittent snow and steady winds. "Dark side" is something of a misnomer, as the temperature is high enough that the surface gives off a dim reddish glow. The weather is expected to present the same outlook for the foreseeable future.
Just how much can astronomical observations tell us about exoplanets - those worlds orbiting other stars in our galaxy? With patience and cunning, more than you might think.
Two nearly simultaneous reports this year have revealed important new information about HD 189733 b (HDb for short). In the first, a University of Washington team used the Spitzer Space Telescope to examine how light emitted from HDb vanished when eclipsed by its sun - this was observed over several eclipses. This data provided the light intensity of a series of parallel strips on HDb's surface. These strips lay at different angles depending on whether HDb was going behind its star or was emerging from behind. Accordingly, the intensity of spots where the strips overlapped were then woven together in a sort of optical tomography to produce a two-dimensional image of the planet's surface.
In the second report, an international team with members from France, the U.K., and the U.S. used the Hubble Space Telescope to study the temperature of HDb's atmosphere as a function of pressure. They confirmed earlier reports of atmospheric haze in HDb's atmosphere, and characterized it by comparing spectrographs with Rayleigh scattering to determine that the haze particles are sub-micron in size, probably ranging from 0.1 - 0.01 microns in size. Atmospheric chemistry strongly suggests that the particles generating the haze are made of silicate minerals (probably magnesium silicate).
These results have led to a suggestion that HDb could continually experience silicate snow. In the lower atmosphere of HDb, magnesium silicate sublimates, that is, it passes directly from a solid into a gas. But we know there are small silicate particulates in the upper atmosphere. Formation of these particulates requires that the temperature be lowered, and so must have been formed at a temperature inversion in the atmosphere. The generally windy conditions would help some of the tiny particulates grow into respectable snow crystals.
Here are a few quick facts about HDb: it is an extrasolar planet orbiting an 8th magnitude star at a distance of 4.65 million kilometers (2.9 million miles) and an orbital period of 2.2 days. HDb is located some 63 light-years from the Solar System. The planet is classified as a hot Jupiter planet with a diameter and mass slightly larger than Jupiter. The apparent angular size of the planet in the Earth's sky is about 15 nanoseconds of arc, so we won't be able to directly observe surface features in the foreseeable future. HDb does not appear to have any moons which are Earth-sized or larger.
Detailed photometric data of the transits have established that HDb has an atmosphere about 1,000 kilometers (621 miles) thick, with an atmospheric pressure at the optically dense surface about 40 percent of that at Earth's surface.
Spectroscopy shows that the atmosphere contains hydrogen, helium, water vapor, methane, hydrogen, carbon dioxide, sodium, and potassium. HDb would appear blue in visible light, owing to Rayleigh scattering from a haze of submicron-sized silicate particles in the atmosphere.
The skill and delicacy with which astronomers can tease unobservable information out of a few photons of light is truly amazing. They will surely continue to pick away at the mystery of what sorts of planets fill the circumstellar regions of our galaxy, leading us toward a much higher level of comfort when contemplating the possibility of alien life.