Category Archives: WASP planets

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.

NASA’s multimedia presentation on WASP-12b

NASA has been producing presentations for its website: Exoplanet Exploration: Planets Beyond our Solar System. One of these features WASP-12b, chosen because its short-period orbit and large, bloated radius mean that the shape of the planet is distorted by the host-star’s gravity into an egg-shaped Roche lobe.

Meanwhile the Interesting Engineering website has produced a compilation of seven “weird” exoplanets, of which one is the possible ring-system planet found in WASP data, J1407b.

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-134b and WASP-134c: a pair of warm Jupiters

Most of the planets that WASP discovers are “hot Jupiters”, often defined as having an orbital period less than 10 days, though they clump at periods of 3 to 5 days. Occasionally we find “warm Jupiters”, with periods greater than 10 days. There seem to be far fewer of these (and not just because they’re harder to find, which they are, owing to being less likely to transit, because they are further away, and because they produce fewer transits because of the longer periods).

Our latest discovery paper, led by David Anderson, announces the WASP-134 system. An analysis of the radial-velocity observations looks like this:

There are clearly two different cycles from two different planets. Both are warm Jupiters. The inner one (upper panel) has a period just over 10 days while the outer one (lower panel) has a 70-day period. Both orbits are eccentric (the fits are clearly not sinusoids) and both planets have a mass of about one Jupiter.

This is relatively rare. Few systems are known where a shorter-period, Jupiter-mass planet has a Jupiter-mass companion with an orbit as short as 70 days. (Several systems are known where the companion is much further out, with a period of hundreds of days.)

The presence of two such planets makes it unlikely that the inner one got to its present position by the Lidov–Kozai “high eccentricity migration” pathways that are thought to explain many hot Jupiters. Such a pathway for one planet would be disrupted by the presence of the second planet.

This means that it is more likely that the two planets, WASP-134b and WASP-134c, either formed where they are, or moved inwards by “disc migration” mechanisms. Thus the two WASP-134 planets are perhaps a different population, with a different past history, than the majority of the planets found by WASP.

Sapphires and Rubies in the Sky

The Universities of Cambridge and Zurich have put out a press release about a study led by Caroline Dorn. The work discusses how planets form out of proto-planetary discs, and proposes that some planets would form at high temperatures out of condensates rich in Calcium and Aluminium. Their cores could thus effectively be giant rubies or sapphires (different forms of aluminium oxide).

Planets forming at different distances from their star will form at different temperatures, where different minerals will condense out.

Dorn et al suggest that the three planets HD219134 b, 55 Cancri e and WASP-47 e likely to be such objects. “In our calculations we found that these planets have 10-20% lower densities than the Earth”, says Caroline Dorn. The authors suggest that this is because they are rich in Calcium and Aluminium whereas other rocky planets have Iron-rich cores.

A depiction of 55 Cnc e (credit: Thibaut Roger)

“So, we have found three candidates that belong to a new class of super-Earths with this exotic composition,” says Dorn, adding that: “What is exciting is that these objects are completely different from the majority of Earth-like planets, if they actually exist.”

Take up of the press release has included the International Business Times, India Today, Popular Science, First Post, Sputnik News, ZME Science and other websites.

Spectral contamination from starspots on WASP-4

Here’s a topic we’ll be hearing much more about: how the observed spectrum of a transiting exoplanet is affected by transiting across star-spots. In “transmission spectroscopy” the starlight shines through the planet’s atmosphere during transit, and the easiest thing to do is assume that the star itself is a uniform light source.

But as discussed by papers led by Ben Rackham, if the planet passes over a dark region (star spot) or bright region (faculae), this would change the observed spectrum.

A new paper led by Alex Bixel about WASP-4b is the first to attempt to correct for this effect. The authors’ transit observations show a clear crossing of a starspot (the feature is shown in blue, the spot shows as a upward bump since the planet is then removing less light):

And here is the difference it makes. The blue curve is the observed spectrum, presumed to be of the planet’s atmosphere. The orange curve is then the spectrum corrected for the presence of the star spot.

The details of how to do this are complex, and are discussed at length in the above papers. The central message is that “active FGK host stars can produce such features and care is warranted in interpreting transmission spectra from these systems”.

However, there is good news in that: “stellar contamination in transmission spectra of FGK-hosted exoplanets is generally less problematic than for exoplanets orbiting M dwarfs”, and that such signals “are generally minor at wavelengths of planetary atomic and molecular features”. Overall the authors say that their study “bodes well for high-precision observations of these targets”.