Tag Archives: magnetic activity

WASP-166b, a Neptune-desert planet

WASP-166b, newly announced on arXiv, is a planet we’ve been following for a while. As we routinely do, we measure the mass of the planet from how much its gravity tugs the host star around, and we measure that from the Doppler shift of its spectrum. The “radial velocity” data for WASP-166, however, didn’t neatly fit the expected orbital motion (the fitted line in the figure), showing, in addition, deviations from the model.

It took us a while to understand this. One suggestion was that it was caused by a second planet also tugging the host star around. So we obtained more data, hoping to trace out the orbit of the second planet, only to find that that didn’t properly explain the data. Eventually we attributed the deviations to magnetic activity on the host star. If we plot the deviations as a function of time we obtain:

Faculae on our Sun

The green sinusoidal lines are at the 12-day period at which we think the star rotates, as judged from the width of the spectral lines (which tells us how fast the star rotates). This suggests that the radial velocity varies with the rotational period of the star, and thus that the deviations are caused by “faculae”, magnetically active patches on the surface of the star.

WASP-166b is a low-mass planet, only a tenth that of Jupiter and twice that of Neptune. It has a large radius, however, at 63% of that of Jupiter. Thus it is a bloated planet with a low surface gravity.

Such planets are rare, especially in short-period orbits around fairly hot stars, as is WASP-166b. Indeed the lack of such planets is called the “Neptune desert”.

The explanation is thought to involve irradiation, heat from the host star evaporating off the atmosphere of the planet. Jupiter-mass planets can resist this because they have a lot of mass and thus gravity to keep hold of the atmosphere. Similarly, smaller, compact planets can survive because they are rocky and denser. But, in the middle, gaseous Neptune-mass planets find it hard to survive when subjected to high irradiation.

Another notable fact about WASP-166b is that it transits a host star with a bright visual magnitude of V = 9.4. The combination of a fluffy atmosphere and bright host star make it one of the best targets for studying the atmosphere by “transmission spectroscopy”, as can be obtained when the planet is projected against the stellar disc during transit. The plot compares the expected atmospheric signal of WASP-166b to the other best targets already known.

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Magnetic activity on planet-host stars

One interesting question is whether close-in hot-Jupiter planets have an effect on the magnetic activity of the host star. There have been suggestions that star–planet interactions might increase magnetic activity on the star, or that tidal interactions might decrease it. Further, if mass lost from planets forms an absorbing cloud around the star, then it might reduce observable signs of magnetic activity, even if it doesn’t affect the magnetic activity itself.

A new paper by Daniel Staab et al, led by the Open University, investigates the issue by looking for markers of magnetic activity in spectra of host stars WASP-43, WASP-51, WASP-72 and WASP-103. In the following plot, RHK is a marker of magnetic activity, plotted against the temperature (B–V) of the star. The green dots are a large sample of field stars, while the four WASP host stars are labelled in red.

Chromospheric activity on planet-host stars.

The result is that at least one planet-host, WASP-43, has an abnormally high degree of magnetic activity, while another one, WASP-72, has abnormally low magnetic activity. Staab et al conclude that there is no single factor explaining the differences, and that more than one effect might be in play. As often, a much larger sample of data is needed to investigate the issue.

Exoplanet cloudiness from transit lightcurves?

An interesting new paper by von Paris et al has explored the effect of the cloudiness of a planet on transit lightcurves. If a planet were cloudy on one limb, but clear on the other limb, then that could make the transit slightly asymmetric. The authors show that, in principle, this effect could be detectable with good-enough quality lightcurves.

An apparent shift in the transit:

Shifted transit

Would then lead to residuals, relative to a “perfect” transit, looking like:

traresids

The authors then claim a possible detection of such an effect in the hot Jupiter HAT-P-7b.

This might open up a new way of exploring the atmospheres of exoplanets. Whether this can ever be done reliably, however, is debatable. A big assumption in the authors’ simulations is that the star being transited is uniform. However, we know that stars are usually magnetically active and so are patchy. Star spots and bright patches on the star are likely to have a greater effect on the transit profile than the cloudiness of the planet’s atmosphere. Still, the effect is worth exploring, particularly for planets transiting magnetically quiet stars.

Radial velocities of the Sun as an exoplanet host star

The main way of measuring the mass of an extra-solar planet is to record the motion of the host star, caused by the gravitational tug of the planet as it orbits. One can do that by measuring the Doppler shift (radial velocity or RV) of the spectrum of the host star.

However, as a planet gets smaller or further from its star, the tug gets smaller, and so the radial-velocity signal decreases. At some point it gets smaller than the intrinsic variations in spectral lines caused by the magnetic activity of the star. Whether one can account for this will limit our ability to prove the existence of small planets in wide orbits.

Radial velocity of the Sun, bounced off the asteroid Vesta

A team lead by Raphaëlle Haywood, of the University of St. Andrews, and now at Harvard, had the idea of treating our own Sun as a star, by looking at the RV signal in sunlight bounced off the asteroid Vesta. They could then compare the RV signal to images of the magnetic activity on our Sun.

Magnetic activity on the Sun.

Magnetic activity across the Sun’s disc

The spectral lines from each region of the Sun’s disc will depend on the local magnetic activity, but the RV measurement bounced off Vesta would be from light averaged over the whole disc of the Sun, just as we’d record from a star.

The results are shown in the plot below. The top panel shows the variations in the measured RV signal, in metres per second. The second panel shows the magnetic flux aggregated across the Sun’s disc, in Gauss. The third panel shows the fraction of the Sun’s disc filled by magnetic activity (Sun spots).

Radial velocity variations of our Sun

Thus a Sun-like star can show intrinsic RV variability at a level of metres per second, and this will cause a problem for detecting the small RV signals of low-mass planets in wide orbits. For example our Earth produces motion in our Sun of only 0.1 metre per second. Unless there are stars much less magnetically active than our Sun, it is going to be hard to obtain an accuracy sufficient to detect the RV signal of an Earth-like planet in an Earth-like orbit.

The authors note, though, a strong correlation between the RV signal and the total magnetic activity. Thus it might be possible to decorrelate against magnetic activity to provide a way of correcting RV signals for this effect, and so dig out smaller signals caused by planets.