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.
Here’s the latest update on the changes in the orbital period of WASP-12b, from a new paper by Samuel Yee et al.
The times of transit are getting earlier, which means that the period is decreasing slightly. By also considering the times of occultation (when the planet passes behind the star), and also the radial-velocity measurements of the system, the authors deduce that the changes are not the effect of some other planet, but are a real decay in the orbit of WASP-12b. This is expected to occur as a result of tidal interactions between the planet and its host star.
One notable conclusion is that the rate of period decay in WASP-12b is much faster than that in WASP-19b, which shows no detectable period change yet, despite it being an even shorter-period hot Jupiter, which should increase tidal interactions. Yee et al suggest that the difference could arise if the host star WASP-12 is a sub-giant star, whereas WASP-19 is not.
Update: Following an article on WASP-12b’s orbital decay, supplied by Liz Fuller-Wright of Princeton University, and appearing in phys.org and Science Daily, the work has gained media attention from CNN, Science Times, Universe Today, and the UK’s Metro.
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.
Thomas Beatty et al have an interesting new paper on arXiv today, primarily about the transiting brown dwarf KELT-1b. They’ve used the Spitzer Space Telescope to record the infra-red light as it varies around the 1.3-day orbit.
They end up with the following plots (KELT-1b is on the right, with the plot for the planet WASP-43b on the left):
The x-axis is “colour”, the difference in flux between two infra-red passbands at 3.6 and 4.5 microns. The y-axis is brightness (in the 3.6 micron band). The underlying orange and red squares show where typical M-dwarf stars and L and T brown dwarfs fall on the plot.
The solid-line “loops” are then the change in position of the atmospheres of KELT-1b and WASP-43b around their orbits. At some phases we see their “day” side, heated by the flux of their star, and at others we see their cooler “night” side.
The blue line is the track where something would lie if there were no clouds in its atmosphere. The fact that KELT-1b’s loop doesn’t follow the blue track, but moves significantly right (to cooler colours) implies that the night side of the brown dwarf must be cloudy. The night side of WASP-43b, however, appears to be less cloudy, according to its track.
Here are the same plots for two more planets:
The plot for WASP-19b shows a loop with a marked excursion to the right, suggesting a cloudy night side to the planet. For WASP-18b, however, the loop follows a trajectory nearer the blue “no cloud” track, suggesting a clearer atmosphere.
Star spots are cooler regions of a star’s surface, caused by magnetic activity, and emit less light. If a planet transits across a spot it blocks less light, and so we see a slight rise, a bump, in the transit profile.
On the left (in blue) is a transit from a new paper by Espinoza et al, who have observed transits of WASP-19b with the Magellan telescope. A clear bump is seen, indicating that the planet passed over a cooler spot.
On the right (in red), however, is another transit showing a clear dip compared to the expected transit lightcurve. This implies that during this transit the planet passed over a brighter region on the star. This is the first time such an event has been seen.
The authors deduce that the bright spot must have a size of about a quarter of the stellar radius and must be 100 K hotter than the rest of the star. Such regions are not seen on our own Sun.
The main point of the observations, however, was not studying spots but studying the planet’s atmosphere by recording how the transit depth changes with wavelength. Here is the state-of-play for the spectrum of WASP-19b, covering optical to infra-red wavelengths:
The red data-points are from the Hubble Space Telescope, showing a spectral feature, but the new data by Espinoza et al (white points) are consistent with a flat spectrum within the limits of the data.
The European Southern Observatory have put out press release about observations of WASP-19b with the Very Large Telescope. A team led by ESO Fellow Elyar Sedaghati have found titanium oxide in the atmosphere of an exoplanet for the first time.
ESO’s graphic (credit: ESO/M. Kornmesser) illustrates how observations during transit allow us to analyse an exoplanet’s atmosphere. The star light shines through the atmosphere, where light at particular wavelengths is absorbed by molecules, causing the light that we see to carry a distinctive signature of the atmosphere’s composition.
The team observed three different transits of WASP-19b, each in a different colour, to produce one of the best transmission spectra of an exoplanet so far. The titanium oxide (TiO) features are marked, along with those from water (H2O), sodium (Na) and scattering due to haze.
ESO’s press release has led to coverage on several dozen news- and science-related websites. ESO have also produced an artist’s impression of WASP-19b:
ESA’s Gaia satellite is a €740-million mission to map a billion stars in our galaxy. By observing repeatedly with unprecedented astrometric precision it is measuring the parallaxes, and thus the distances, of hundreds of millions of stars, and so mapping out the 3-D structure of our galaxy.
Gaia can detect exoplanets in two ways, first by astrometry (measuring the position of a star), so detecting the wobble in the star’s location caused by an orbiting massive planet, and secondly by the transit method, detecting the dip in the light of a star caused by a transiting planet.
The Gaia team have just announced the first detections of exoplanet transits, by looking at the accumulated Gaia data on two already-known WASP planets.
The plot shows a year’s worth of Gaia data of the star WASP-19, folded on the 0.79-day orbital period of the planet WASP-19b (the three different panels are the star’s magnitude in three different colours). The coverage is sparse — it is designed for astrometric measurements, not for recording lightcurves — but one observation was made in-transit, demonstrating that Gaia can indeed detect exoplanet transits.
The ESA/Gaia team have also looked at the data on WASP-98, and again detect the transit of WASP-98b.