Hot Jupiters are gas giant planets like Jupiter but orbiting so close to their host stars that they can be as hot as small stars. In fact, some suffer such strong irradiation that their atmospheres are being blasted off into space. These objects were among the first exoplanets detected, and their origins and fates remain unclear.
Being so hot, their atmospheres are also unlike any planet’s in our solar system, but we can be certain that, with temperatures hot enough to vaporize rock, the weather on these planets is highly dynamic. The video below shows what happens to one such planet as it gets blast-roasted by its host star – a giant thermal wave screams around the planet.
Weather forecasts on hot Jupiters would to have keep track of super-sonic winds, ruby rain, and warm fronts heated by magnetic fields. And a recent study using observations from the Hubble Space Telescope shows that these dynamic atmospheres remain mysterious.
Led by Jacob Arcangeli of the University of Amsterdam, the study used the Wide-Field Camera onboard Hubble to watch WASP-18 as the hot Jupiter circled its star.
As the planet swings revolves around its star, we see first the cool nightside of the planet and then the blistering dayside. By measuring the diurnal cycle of infrared light emitted from the two sides of the planet, called the phase curve, Arcangeli and colleagues were able to measure their respective temperatures and learn something about the planet’s weather.
The figure above shows WASP-18b’s phase curve as blue points and a model fit to the data as a black line. By applying computer simulations for the planet’s weather, Arcangeli and colleagues estimated what they expected the phase curve to look like various assumptions about the planet’s composition and atmospheric dynamics.
Using their models, they were able to draw a rough map of temperatures in WASP-18b’s atmosphere (shown above), like a weather map for the Earth. Like most hot Jupiters observed show far, the hottest place in WASP-18b’s atmosphere lies a little east of the point that receives the most sunlight, the substellar point. That’s because high windspeeds blow the strongly heated gas away toward the east, a little like the jet stream on Earth.
However, the winds inferred from the Hubble observations weren’t as strong as expected from the model, suggesting there is some form of drag or friction in WASP-18b’s atmosphere.
The likely culprit: the planet’s very own magnetic field. Gas in WASP-18b’s atmosphere is actually so hot, it can become somewhat plasma-fied, and plasma, consisting of hot charged particles, can interact with the hot Jupiter’s magnetic field.
This is totally unlike planets in our own solar system, where the atmospheres are comparatively cool and none of the gas has turned into plasma.
The upshot of this result is that we may be able to use observations of the meteorology on distant worlds to learn something about the planets’ magnetic fields, which originate deep inside the planet. So their weather may allow us to plumb the depths of these distant worlds.
And already some of that promise is being fulfilled. Astronomers have used TESS data to find exocomets, probe the atmospheres of planets we already knew about, and most recently to find possible evidence for imminent planetary destruction.
The possibly doomed planet is WASP-4b, a gas giant planet that passes in front of its host star every 1.5 days. It orbits so close to its host star that tidal interactions between the star and planet can cause the shape of orbit to vary over time, giving rise to transit-timing variations, illustrated in the video below.
Just this week, Luke Bouma, a graduate student in Princeton astronomy, and colleagues analyzed recent TESS observations of WASP-4b to see if the data show any signs of orbital variations. Not only did they find signs of orbital variation, they found that the orbital period seems to be getting shorter, at a rate of about 12 milliseconds per year, or about one jiffy every year. This about 1000 times faster than slowing of the Earth’s day due to tidal interactions with the Moon.
It’s not clear exactly what is causing WASP-4b’s orbit to change, and Bouma explores a couple of options. One idea is that WASP-4b’s orbit is very slightly eccentric and that tidal interactions between the planet and star may be causing apsidal precession, similar to effects experienced by the Moon that complicate the timing of eclipses.
Another, and to my mind more exciting, possibility is that tidal interactions with the host star are drawing the planet inexorably inward. In that case, WASP-4b may eventually be torn apart by its host star’s gravity, a fate that may have befallen many a hot Jupiter.
The great thing about both hypotheses is that they can be tested by additional observations. In fact, amateur astronomers may be able to contribute. Indeed, there is a cottage industry of amateur astronomers observing exoplanet transits, and the hardware, software, and expertise required are pretty minimal for serious amateurs.
Bouma predicts that, over the next several years, WASP-4b’s orbital period might change by several minutes. If the period drops and then increases again (orange curves above), then the variations are likely due to precession, and the planet is probably safe against tidal decay.
However, if the period continues to drop (blue curves above), then the planet is likely doomed to tidal disruption in the next nine million years, short on cosmic timescales.
Hippocamp is about 12 km across, so small and dim that it wasn’t seen when Voyager 2 flew past in 1989, back when the B-52s were heading down the Atlanta Highway. In fact, Showalter and colleagues had to use high-precision Hubble observations and a new data-processing approach to spot the little moon circling Neptune just interior to another moon Proteus.
Hippocamp orbits so close to Proteus that Showalter and colleagues suggest it may have originated from this larger moon in a massive collision. That same collision may have created Proteus’ enormous impact basin Pharos, and Showalter suggests that collision would have liberated debris, some of which later accreted interior to Proteus’ orbit to form Hippocamp.
If Hippocamp really did form from such an impact, it has probably experienced numerous disruptive collisions itself over its billion year history. Based on studies of the frequency of large cometary collisions out near Neptune’s orbit, Showalter and colleagues estimate that Hippocamp may have been disrupted and re-accreted about 9 times in the last 4 billion years.
Having risen from its own ashes so many times, Hippocamp may be less like a mythical sea-horse and more like a cynthian phoenix.