Blowing hot and cold

Understanding the structure, dynamics, and chemistry of planetary atmospheres is key to exoplanetary science. It's sobering then that as of now it is still an enormous challenge to even model the atmospheres of planets in our own solar system. Despite great advances a variety of trickery has to be employed to simulate something like the Jovian atmosphere, such as pretending that it has a very different soupiness and energy transport in order to overcome computational demands. Modeling the atmospheres of gas giant exoplanets is even more in its infancy. An intriguing result in the past week has come from Crossfield et al. and their study of how we see the infrared light varying in the planetary system of Upsilon Andromedae. Their Spitzer space telescope phase photometry on Ups And reveals the glow emitted by the innermost, roughly Jupiter sized, planet around this F dwarf star (about 1.3 times the mass of the Sun).

The planet orbits very tightly, every 4.6 days, and is expected to have been evolved by tidal interaction with the star to a state of spin-orbit-synchronicity - in other words, in the simplest case, its day will equal its year and there will be permanent day/night sides. This sets the planet up for an extreme case of thermal disparity. We'd expect hot atmosphere from the dayside to flow to the cold night half of the planet - in doing so there might be great jet-stream like structures, and the hottest point of the planet might get shifted along in the direction of these winds. Something like this seems to be happening on Ups And b, but to an extent that is truly puzzling. As it zips around in its orbit the glow of its hot atmosphere betrays the temperature distribution and is seen in the varying number of infrared photons collected by Spitzer. It's not in synch with the planet orbit - or more specifically it is systematically offset or phase shifted by almost 90 degrees. In other words the hottest side of the planet is almost at right angles to the direction of the star. On the Earth this would be a bit like saying the hottest time of day is at sunset instead of noon.

It's a puzzle. Some amount of offset might be expected, driven by the strong hot to cold winds, but this is extreme. There are various possibilities - maybe the stellar heating is reaching to greater depths in the planetary atmosphere than expected and altering the fundamental dynamics. Perhaps the winds are so strong that they are going supersonic, forming great shock waves that pile energy up on this side of the planet. It's a tough call - even models of these hot Jupiter-like planets disagree on such things, and none of them predict exactly what we see on Ups And b. The good thing about this result is that it challenges the modelers to really sort out what works and what doesn't - advances will be made.

Crossfield et al. also end their paper with an interesting fact. This system of Ups And is actually too bright for the upcoming James Webb Space Telescope (JWST) to observe at shorter wavelengths - its sensitive instruments would simply be saturated with photons. They further point out that a small space telescope dedicated to studying the phase curves of nearby hot-Jupiter systems might just provide the data needed to crack the problems of these extraordinary regimes of planetary atmospherics. This is a sentiment that could also apply to the hunt for terrestrial-type exoplanets - especially those that transit stars much closer than the distant Kepler objects - we need a dedicated all-sky survey to find the targets for powerhouse instruments like JWST.