Tidal interactions between hot-Jupiter exoplanets and the host star should be causing their orbits to decay, such that the planet gradually spirals inwards. For most systems the change would be too small to detect in the decade or so that we’ve been observing them. However, WASP-12b is an exception, showing a clear change in its orbital period.
In a new paper on arXiv, Gracjan Maciejewski et al present the latest data for WASP-12b:
The graph records the change in transit time (“observed minus calculated” times, or O–C), showing that the transits are now occurring eight minutes early owing to a decreasing orbital period.
Such a rate is far faster than observed in other systems, and too large to be explained by the standard theory of tidal interactions.
However, a new paper led by Sarah Millholland suggests an answer. She suggests that the planet is tilted over, so that the axis around which it spins is tilted with respect to the plane of the planet’s orbit.
This means that the star will give rise to strong “obliquity tides” on the planet, and the dissipation of those tides could explain the decay of the orbit. For this to work something must be keeping the planet tilted over. Millholland suggests that a second planet in an outer orbit might be perturbing WASP-12b, keeping it in the high-obliquity state. This scenario requires some fine tuning, but if WASP-12 is the only system known to show this behaviour then the explanation is plausible.
Since hot Jupiter exoplanets are “phase locked” by tidal interactions (that is, the same side always faces the host star, just as the same side of our moon always faces us), there will be a large flow of heat from the highly irradiated “day side” to the cooler “night side”. This is thought to result in very strong winds rushing around the planet’s atmosphere.
Taylor Bell and Nicolas Cowan have pointed out that hydrogen will tend to be ionised on the day-side face. After flowing to the cooler face in a wind, it will then tend to recombine into neutral atoms, and thus will enhance the transport of heat.
The result is that either heat redistribution will be more effective than previously thought, helping to explain some observations of hot Jupiters, or the winds need be less strong than thought.
Bell and Cowan calculate the difference for WASP-12b. The plot shows models of the difference in temperature (x axis) against the offset of the “hot spot” caused by heat flow (y axis). The different colour coding shows the wind speed. The plot then shows the difference between models including hydrogen recombination, versus previous models by Schwartz. For a given wind speed, including hydrogen recombination results in a larger offset angle, and thus more redistribution of heat.
NASA has put out a press release about Hubble Space Telescope observations of WASP-12b. Taylor Bell et al find that WASP-12b “traps at least 94 percent of the visible starlight falling into its atmosphere”, making it “as black as fresh asphalt”.
WASP-12b “as black as asphalt” (Credit: NASA, ESA, and G. Bacon, STScI)
The article explains that WASP-12b, in a very close, 1.2-day orbit, is so irradiated by its host star that “clouds probably cannot form to reflect light back into space. Instead, incoming light penetrates deep into the planet’s atmosphere where it is absorbed by hydrogen atoms and converted to heat energy”. NASA’s press release has led to coverage on several dozen websites.
WASP-12b is one of the more important of the WASP discoveries, with over 30 refereed papers so far focused on understanding it. Most notably, the fierce stellar irradiation means that material is boiling off the planet and forming a cloud surrounding it.
Closely orbiting hot-Jupiter exoplanets are likely to be spiralling inwards towards their host star as a result of tidal interactions with the star. A new paper by Maciejewski et al reports a possible detection of this orbital-period decay in WASP-12b.
The authors have acquired 31 new transit light-curves over four years, and detect a trend under which the latest transits occur about a minute early compared to an unchanging ephemeris.
Transits of WASP-12b. O–C is the observed time compared to that calculated from an unchanging orbital period. The time (x-axis) is given in both a count of days (BJD) and a count of transits.
This is the most convincing claim yet of a changing orbital period in a hot Jupiter. Whether it shows the spiral infall, though, is less clear. As the authors explain, other tidal interactions between the star and the planet, such as that causing apsidal precession, could account for the effect. Further, in close binary stars there are known to be similar period changes on decade-long timescales that are not fully understood, but which might be caused by Solar-like magnetic cycles on the star.
One suggestion that this is not spiral infall comes from the deduced value of the tidal quality factor, Q, which the authors calculate as 2.5 x 105. This is lower than other estimates of Q as nearer 107.
The way to settle the issue will be to accumulate more data over a longer timespan until the case for spiral infall becomes overwhelming. It will thus be important to continue monitoring WASP-12b, and the other short-period hot Jupiters, over the coming decades.
NASA have put out a press release regarding the largest-ever study of hot-Jupiter atmospheres by the Hubble Space Telescope and the Spitzer Space Telescope. Of the ten planets studied, six are WASP discoveries.
The results, published in Nature, report that hot Jupiters are a diverse group that have atmospheres ranging from clear to cloudy. Strong water absorption lines are seen when the planets have a clear atmosphere, but less so when the atmospheres are dominated by clouds and hazes.
Planets such as WASP-17b and WASP-19b have clear atmospheres and show the strongest water features, whereas planets such as WASP-12b and WASP-31b are more cloudy.
The NASA press release has so far resulted in articles on over 110 news websites worldwide. The paper was lead-authored by David Sing of the University of Exeter.
If a hot Jupiter has a magnetic field of a few Gauss it would be surrounded by a magnetosphere that would carve out a hole in the stellar wind of the host star. Since the planet orbits rapidly, this would lead to a “bow shock” where the magnetosphere ploughs through the stellar wind.
In a new paper, Richard Alexander, of the University of Leicester, and co-authors, report computer simulations of this effect for several hot Jupiters, including WASP-12b and WASP-18b.
In the colour-coded figure (see scale on the right) the blue and red show the density of the stellar wind. A low-density (black) magnetosphere surrounds each planet (white dots).
Since these planets orbit edge on to us, the bow shock would absorb ultra-violet light from the star, and so produce a characteristic light-curve with a broad dip preceding the transit.
This magnetospheric bow-shock is a possible alternative explanation for the UV absorption observed in WASP-12, which has previously been attributed to material being lost from the planet owing to Roche-lobe overflow. Alexander et al suggest that WASP-18 is a critical test of these models, since the much higher gravity of the massive planet WASP-18b means that there should not be any Roche-lobe overflow.