Metals streaming from the atmosphere of WASP-121b

The Hubble Space Telescope Science Institute have put out a press release about Hubble observations of WASP-121b, as reported in a new paper led by David Sing of Johns Hopkins University.

WASP-121b is one of the hottest WASP planets, since it is fiercely irradiated by being in a very tight orbit of only 1.27 days around a hot F star. The Hubble spectra show clear absorption features caused by metals including Magnesium and Iron:

“Heavy metals have been seen in other hot Jupiters before, but only in the lower atmosphere,” explains David Sing, “So you don’t know if they are escaping or not. With WASP-121b, we see magnesium and iron gas so far away from the planet that they’re not gravitationally bound.”

“The heavy metals are escaping partly because the planet is so big and puffy that its gravity is relatively weak. This is a planet being actively stripped of its atmosphere.”

The Hubble press release continues: “This exoplanet is also a perfect target for NASA’s upcoming James Webb Space Telescope to search in infrared light for water and carbon dioxide, which can be detected at longer, redder wavelengths. The combination of Hubble and Webb observations would give astronomers a more complete inventory of the chemical elements that make up the planet’s atmosphere.”

STSci have produced an artist’s impression of WASP-121b, showing how the planet’s shape is tidally distorted by the gravity of the star that it orbits:

Artwork: NASA, ESA, and J. Olmsted (STScI)

The press release has led to coverage on over 50 news and science websites, including Newsweek, CNN, Fox News, Metro, The Daily Mail, The Express, and countries including Switzerland, Germany, India, and Malaysia.

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.

Exoplanet formation scenarios

Here’s a nice graphic by Sean Raymond illustrating different scenarios for the formation of exoplanetary systems, one leading to “Super-Earths” and the other to gas giants. The work is explained more fully on arXiv.

The paper’s figure caption includes:

Left: Evolution of the “breaking the chains” migration model for the origin of super-Earths. Embryos within the snow line are entirely rocky and much smaller than those that form past the snow line, which also incorporate ice. Presumably ice-rich embryos migrate inward through the rocky material, catalyzing the growth of purely rocky planets interior to the ice-rich ones. Planets migrate into long chains of mean motion resonances, with the innermost planet at the inner edge of the disk. The vast majority (90–95%) of resonant chains become unstable when the gas disk dissipates. The resulting planets match the distributions of known super-Earths.

Right: Evolution of the planet-planet scattering model for the origin of giant exoplanets. Several embryos grow quickly enough to accrete gas and grow into gas giants. They subsequently migrate into a resonant chain without drastically affecting the orbits of nearby growing rocky planets (or outer planetesimal disks). After the disk dissipates, the vast majority (75–90%) of giant planets systems become unstable. The resulting systems match the correlated mass-eccentricity distribution of known giant exoplanets.