Category Archives: TESS

More TESS phase curves of WASP exoplanets

Ian Wong et al have produced a new analysis of the TESS data on previously known WASP exoplanets. Their main interest is the “phase curve”, the variation of the light around the planet’s orbit.

Two examples are the systems WASP-72 and WASP-100:

In addition to the main transit (planet passing in front of the star) the phase curves show secondary eclipses (planet passing behind the star, at phase 0.5) and a sinusoidal variation due to the heated face of the planet. By modelling the phase-curves of these and other similar planets, Wong et al make the tentative suggestion that the hotter the planet (which can be measured from the depth of the secondary eclipse) the more reflective the atmosphere of the planet is.

Here’s a similar plot for WASP-30. Note, though, that the phase-curve variation peaks at phases 0.25 and 0.75, unlike those for WASP-72 and WASP-100. That’s because WASP-30b is not a planet but a brown dwarf, with a mass of 63 Jupiters. That is massive enough for its gravity to distort the host star into an ellipsoidal shape, and so in this system the variation of the light is caused by the varying projection of the distorted star around the orbit.

TESS phase curve of WASP-19b

The space-based photometry from the TESS satellite is producing high-quality light curves of many of the WASP exoplanets. Here is the lightcurve of WASP-19b, from a new paper by Ian Wong et al:

In addition to the transit (phase zero), the lightcurve shows a shallower eclipse of the planet (phase 0.5) and a broad variation caused by the changing aspect of the heated face of the planet. Unlike in some planets, the hottest part of the planet directly faces the star, so there is no offset in the phase of the broad modulation.

Wong et al deduce that the dayside face of the planet is heated to 2240 ± 40 K, that there is no flux detected from the colder night side, and that the planet reflects 16 ± 4 percent of the light that falls on it. The last value is relatively high compared to other planets.

Gravity darkening of planet-host MASCARA-4

MASCARA is one of WASP’s competitor transit-search projects, so let’s celebrate a neat result from TESS data of transits of MASCARA-4b. The host star, MASCARA-4, is a hot, fast-rotating A-type star. As a result of its fast rotation, the equatorial regions are being flung outwards by centrifugal forces, such that the star has a flattened, oblate shape. As a result, the force of gravity will be less at the equator than at the poles of the star, and that means that the equatorial regions will be slightly cooler and so a bit dimmer (in outline, that’s because gravity inward pull is balanced by gas pressure, and so lower gravity means lower pressure, and the temperature of a gas is related to its temperature through the perfect gas law). This effect is called “gravity darkening”.

The star spins around its axis (thick line) while the planet orbits at an oblique angle.

In a new paper, John Ahlers et al have detected the effect of gravity darkening on a transit lightcurve of the hot Jupiter MASCARA-4b. The planet has a misaligned orbit, first coming onto the stellar face near the equator, and then moving towards a pole. That means it moves from slightly cooler regions to slightly hotter regions, and that changes the amount of light occulted by the planet.

If gravity darkening is not taken into account then the model fit is a bit too deep at the start and a bit too shallow at the end of the transit. One of the benefits of detecting this effect of gravity darkening is that it then tells us the true angle between the star’s spin axis and the planet’s orbit (whereas other methods, such as Doppler tomography, only tell us the projection of that angle onto the sky).

WASP-South detection of transits of HD 219666b

Transiting a bright star, the “Neptune desert” planet HD 219666b was one of the more important early discoveries from the TESS survey. With a depth of only 0.17 per cent, the transits would be a challenge for any ground-based transit survey.

Nevertheless, we think we’ve found them in WASP-South lightcurves dating back to 2010. Here they are:

The orange lines show times of transit, as found by the WASP transit-detection algorithms. The shallow dips seem to be real, since they align both in period and in phase with the dips seen in the TESS lightcurve. The output from the WASP search algorithm is not itself that convincing:

However, the period that it finds (6.03446 days) matches the TESS period to an accuracy of 0.03 per cent, and the WASP ephemeris then predicts the times of the TESS transits bang on (they occur at 420.99999 cycles on the WASP ephemeris), which together mean that the detection must be real. Here are the WASP data folded on a template of the TESS transit:

With a depth of 0.17 per cent, the transits of HD 219666b are the shallowest that WASP has detected.

The benefit of looking for such pre-detections of TESS planets is that we can then produce a transit ephemeris based on data spanning a baseline of 8 years, rather than the 20 days spanned by the TESS transits. This means we can predict future transits to an accuracy of minutes, instead of hours, which is highly useful for future observations. Hence this WASP-South detection of HD 219666b transits is well worth an AAS Research Note.

WASP-121b observed by TESS

As is sometimes the way when prime observations are open access, two independent papers (Daylan et al 2019; Bourrier et al 2019) have, on the same day, announced independent analyses of the TESS lightcurve of the ultra-hot Jupiter WASP-121b.

The phase curve shows the transit (time zero), a “phase curve” modulation caused by the varying visibility of the heated face of the planet (illustrated by schematics of the planet), and the eclipse (when the planet passes behind the star, at −15 hr).

Both analyses report similar findings, saying that the heated “hot spot” directly faces the star, rather than being offset in phase, which suggests that any re-circulation of heat by planetary winds is inefficient.

The planet’s atmosphere shows a temperature inversion (it is hotter at higher altitudes), which could result from absorption of heat by molecules of titanium and vanadium oxide, and H-minus ions.

Extra planets in WASP-18 and WASP-126?

With the TESS satellite observing most of the WASP planets as it surveys the sky, one can use the space-based data quality to look for “transit timing variations” in the WASP planet transits. Such TTVs — slight changes in the time of recurrence of a transit — can be caused by the gravitational perturbation of other planets in the system, and thus can reveal the presence of extra planets even when they themselves do not transit.

A new paper by Kyle Pearson, of the University of Arizona, reports evidence for TTVs in the TESS light-curves of WASP-18 and WASP-126.

Here is the TESS light-curve of WASP-18, showing the transits of the known planet WASP-18b:

And here are possible changes in the transit times, varying systematically with a 2.1-day period (red line):

Here now is the TESS light-curve of WASP-126, showing transits of WASP-126b:

And here are possible TTVs varying systematically with a period of 7.7 days (red line):

The evidence is not yet sufficiently water-tight to be sure of the existence of the extra planets, without adding in further data, but this study points to lots of similar work using TESS data as it continues its multi-year survey.

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.

WASP-100 and WASP-126 in TESS Sectors 1 to 4

As TESS continues its all-sky survey it will produce high-quality data containing lots of transits for all the WASP planets. This is especially so for planets near the ecliptic poles, which TESS will observe over many sectors. With TESS Sector 4 data recently released, here are some plots borrowed from David Kipping on Twitter.

The lower plots show the variations in transit timing (O–C is the difference between the observed timing and the timing calculated from an ephemeris).

These plots seem to show something that I’ve suspected for a while, namely that there are correlated deviations in the transit timings, meaning that if one O–C value is slightly early (or late) then the next one is more likely to be the same. Such deviations can also be larger than expected given the errors (the quoted chi-squared value for WASP-100b of 44 for 32 degrees of freedom tells us that the error bars don’t fully account for the variations).

This must be the result of stellar activity, magnetic variations on the surface of the star such as star-spots and faculae. Any deviation from a smooth stellar profile can then alter the transit profile.

Properly accounting for such effects will be important for two sorts of study. The first is looking for “transit-timing variations”, changes in the transit time of a planet caused by variations in its orbit owing to the gravitational perturbations of another planet. The second is looking for long-term changes in the orbital period, such as the inward-spiral decay of the orbit predicted to be caused by tidal interactions of the planet and its host star. The literature contains marginal claims of the latter effect that might be better explained as the effect of magnetic activity of the host star.

WASP-18 is observed by TESS

The TESS mission will survey the entire sky for new transiting exoplanets, and as a by-product will produce space-quality lightcurves of all the WASP exoplanet systems. The first such paper has just appeared on arXiv, where Avi Shporer et al report on the TESS lightcurve of WASP-18.

WASP-18b is the most massive planet found by WASP, a 12-Jupiter-mass planet in a very tight orbit lasting only 0.94 days. This means it has the strongest planet–star tidal interaction of any known planetary system, such that the planet’s gravity gives rise to large tidal bulges on the host star. Here are the TESS data folded on the orbital cycle:

The out-of-transit data are clearly not flat (shown on a larger scale in the middle panel), and show the “ellipsoidal modulation” caused by the tidal bulges on the star. The heated face of the planet is also eclipsed by the star at phase 0.5, producing a secondary eclipse.

By analysing the lightcurve the authors conclude that very little heat is being redistributed from the heated face of the planet. Strong winds could carry heat to the un-irradiated cooler hemisphere, but there is little sign of this in the data.

So far the results of the analysis are in line with theoretical expectations, though the work points to the potential for similar analyses of other previously-known exoplanet systems.

HATSouth announce HATS-59 b and c

Our competitor transit survey HATSouth have just announced the discovery of planets HATS-59 b and c.

HATS-59b is a hot Jupiter producing a typical hot-Jupiter transit, as seen in the HATSouth data:

But what makes it interesting is the presence of an outer companion planet, HATS-59c, on a much wider orbit of 1422 days. This has implications for understanding planetary systems that host hot Jupiters, casting light on the question of whether the gravitational perturbations of outer planets move the hot Jupiters into their close-in orbits.

As usual, we “reverse engineer” planets discovered by our competitors as a check on our own methods. One would expect we’d struggle to see the transit of HATS-59b, after all the host star has a magnitude of V = 14, which is faint for us (we struggle at anything below V = 13).

HATSouth uses bigger optics than WASP-South, aiming to thus get better photometry, but that has the penalty that larger optics produce smaller fields of view which then contain fewer bright stars. So larger-optic surveys such as HATSouth and the similar NGTS typically find planets around stars that are fainter than typical WASP or KELT planet hosts.

Nevertheless, this is what our search routines produce for HATS-59b (from 37,000 observations with WASP-South):

Not very impressive is it? The big scatter in data points comes from the star being faint for the WASP lenses. But the search routines have run and tried to find a recurrent transit and have picked out a best period of 5.41595 days. That compares with the true value, from the HATSouth paper, of 5.41608(2) days. That matches to 99.998% accuracy, which tells us that our detection of the HATSouth planet is real! Though of course it is far too marginal for us to have ever adopted this star as a candidate.

One reason we’re looking at this is that it shows that WASP data should be able to add value to TESS observations, finding extra transits from our multiple years of coverage, even when the dips are too marginal for us to have pursued them.