Too Bright for JWST: Some Exoplanets are Overwhelming

The planet Upsilon Andromedae b in close orbit to its parent star (NASA/JPL-Caltech)

Understanding the structure, dynamics, and chemistry of planetary atmospheres is key to exoplanetary science. It?s sobering to realize that as of now it is still an enormous challenge to model even the atmospheres of planets in our own solar system. Despite great advances, a variety of trickery has to be employed to simulate a swirling maelstrom like the Jovian atmosphere, pretending for example 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. Nonetheless, an intriguing result a couple of years ago came 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 (light seen as time passes) 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 (about 1,400 Celsius in this case). We?d expect hot atmosphere from the day-side 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 the hot atmosphere betrays that temperature distribution, in a fingerprint of infrared photons collected by Spitzer.

The misaligned hotspot of Ups And b (Credits in image)

In a nutshell ? the hottest part of the atmosphere is 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 strongly blowing hot-to-cold winds, but this is extreme. There are various possible explanations ? 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 theoretical 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 James Webb Space Telescope (JWST) to observe at shorter wavelengths ? its sensitive instruments would simply be saturated with photons, blinded by the light. 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 that are much closer to us than the distant Kepler objects. We need a dedicated all-sky survey to find the targets for powerhouse instruments like JWST, especially those that aren?t going to require planetary sunglasses.

(This post was adapted from an older post on Life, Unbounded in October 2010)

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