Tag Archives: orbital decay

Confirmation of the changing orbital period of WASP-12b

A new paper by Jake Turner et al (Cornell University) analyses TESS data on WASP-12b, showing that the transit timings confirm that the orbital period of the planet is getting shorter.

The orbit is changing on a timescale of 3 Myrs — if it continues the planet will spiral into its star on that timescale. The natural interpretation is that the orbital decay is being caused by tides that the gravitational pull of the planet arouses in the host star. We don’t yet properly understand the effect that tides have on the star, or how internal waves created by the tides then dissipate their energy. Thus the observations of WASP-12b point to the need for a better theoretical understanding of stellar interiors.

Of course the period change has only been measured over a decade, and this is vastly shorter than the orbital-decay timescale. Thus it could be that other mechanisms that we don’t know about can cause short-term changes in planet’s orbital periods. Ongoing monitoring of all the WASP hot Jupiters is thus needed to properly understand what is going on. The TESS satellite, which will observe most hot Jupiters every two years or so, on an ongoing basis, is the perfect tool for the task.

Update: Here’s a Twitter thread by the lead author, Jake Turner.

WASP-4 is accelerating toward the Earth

Here is a plot of the timings of the transits of WASP-4b, taken from a new paper led by Luke Bouma:

The curve in the plot shows that the transits are occurring progressively earlier as time passes. One possible explanation is that the planet’s orbit is decaying under the influence of the tidal interaction between the star and planet. This is expected to occur in most hot Jupiters, though how quickly is debated.

However, Bouma have also obtained radial-velocity observations of the system, which show that the star is accelerating towards us. This can result from it being in a wide orbit with another object (the authors suggest a wide-orbiting companion of 10-to-300 Jupiter masses at a distance of 10-to-100 AU). Since the system is accelerating towards us, the light-travel time is decreasing, and this (not orbital decay) means that the transits occur earlier.

Wide companions are expected in hot-Jupiter systems, since, in most theories for the occurrence of hot Jupiters, the gravitational perturbation of a distant companion is needed to shrink the hot-Jupiter orbit down to the current values of only a few days.

Bouma et al recommend continued radial-velocity monitoring of hot Jupiters in order to distinguish orbital decay from accelerations caused by orbiting companions.

No period change for WASP-19b

Since close-orbiting hot Jupiters are expected to be gradually spiralling inwards, under the influence of tidal interactions with their stars, and since, in addition, the influence of extra, unseen planets in the system could cause changes in transit times, many groups worldwide are monitoring timings of transits of WASP planets.

The latest report on timings of WASP-19b has just been announced by Petrucci et al. The result is the following diagram, showing deviations of timings from a constant ephemeris, plotted against cycle number.

The upshot is that there is no indication of any period change, which then puts limits on how efficient the tidal bulges, caused by the gravitational interaction of the planet with the star, are at dissipating energy.

It is notable, however, that there is clear scatter about the constant-period line, beyond that expected from the error bars on the timings. This means either that the error bars are under-estimating the uncertainties (as would occur if “red noise” in the lightcurves is unaccounted for), or that there is astrophysically real scatter in the timings, perhaps caused by magnetic activity (star spots) on the surface of the star being transited. We need to better understand such timing scatter if we are to be able to judge whether claims of period changes are actually real.

Early arrival of WASP-4b transits

As NASA’s TESS satellite surveys the Southern sky is it observing many of the WASP planets. One interesting piece of analysis is to check how the transit timings compare with predictions, to look for changes in the orbital periods.

Here’s a plot from a new paper by Luke Bouma et al.

The orange Gaussians show the error range within which TESS-observed transits would be expected to occur, based on previous data, if there has been no change in the period. The blue Gaussians are the actual TESS measurements.

For most of the planets the two ranges overlap, which means the transit times are as expected. For WASP-4 (top-left), however, the transits arrived early by 80 secs, too much to be accounted for by the expected error in the ephemeris.

This suggests that the period of WASP-4b might be changing rather rapidly.

Since TESS is likely to re-observe the Southern hemisphere in future years, it will be interesting to see what happens next.

Is WASP-12b’s orbital decay driven by obliquity tides?

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.

How fast do the orbits of hot Jupiters decay?

Tidal interactions between close-in, gas-giant exoplanets and their host star should cause the orbits of the planets to decay. The crucial number in determining how fast that happens is the “quality factor”, Q, which tells us the fraction of the tidal energy that is dissipated in each cycle. A high value of Q, say 107, means that only 1 part in 107 of the energy is dissipated, giving a low rate of orbital decay. A smaller value gives a faster decay.

A new study by Kaloyan Penev et al suggests that Q varies a lot depending on the tidal “forcing period” (that is, the period at which a planet would appear to orbit, if viewed when rotating with the spinning star, with an extra factor of a half since there are two tides per orbital cycle).

Penev estimate the value of Q by comparing the observed spin period of the host star to the most likely spin period expected for that sort of star, if it had no planet, and so modelling how much the star has been spun up by the tidal interaction with the planet.

They find that the Q of the star is high, about 107, when the tidal forcing period is low (< 1 d) but much smaller, about 105.5, when the forcing period is longer.

This work might resolve several puzzles. The Q value expected from studying binary stars is near 105.5, but if that were true for all hot Jupiters then they’d be destroyed too readily, and the current observed population could not be explained. This puzzle is resolved if their orbits decay much more slowly when the forcing period is short.

The different Q values also allow the planets to re-align their orbits with the spin of the star (so that the orbital plane is perpendicular to the star’s spin axis) on a timescale shorter than the orbital period decay, thus explaining why there are many “aligned” hot Jupiters.