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.
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.
Congratulations to the HATSouth project for the discovery of HATS-18b, a hot Jupiter with the very short orbital period of only 0.84 days. The other known hot Jupiters with periods below 1 day are all WASP-South discoveries (WASP-19b at 0.79 d, WASP-43b at 0.81 d, WASP-103b at 0.93 d and WASP-18b at 0.94 d).
Since such short-period systems are the easiest to find in transit surveys (owing to lots of transits!) they must be very rare, presumably because tidal forces are causing the orbits to decay, so that the planets spiral into their stars on relatively short timescales of tens of millions of years.
The HATSouth team note that the rotational periods of the host stars of HATS-18b and WASP-19b are much shorter than expected given the ages of the stars, and suggest that the stars have been spun up by the same tidal interaction that caused the planet’s orbit to decay. By modelling the in-spiral process Penev et al arrive at constraints on the “quality factor” Q‘* of the star. This is a measure of how efficient the star is at dissipating the tidal energy resulting from the planet’s gravitational tug on the star, and this sets the timescale for the tidal decay. Penev et al argue that the log of Q‘* is between 6.5 and 7, one of the tightest constraints yet estimated.
Estimates of the tidal quality factor, from modelling the HATS-18b and WASP-19b systems. The different models use different assumptions and are explained in the text. Figure by Penev et al.
New HATSouth planets gives us at WASP a check on our methods, since we can look for them in our own data (and if we don’t see them we can ask why not). At V = 14.1, HATS-18 is fainter than any of the WASP host stars, and fainter than we would adopt as a candidate (HATSouth is optimised to get better photometry on a slightly fainter magnitude range, whereas WASP-South is optimised for a wider field). Nevertheless, 26 000 data points from WASP-South do detect the transit of HATS-18b, giving a detected signal at the 0.837-day period and its first harmonic (1.67-d) in the period search:
There is then a clear detection of the transit when the data are folded on the transit period:
This is thus the faintest detection of a planet yet by WASP-South and so is reassuring about WASP data quality.