Scientific American Blogs has picked up on our recent announcement of WASP-189b, an ultra-hot Jupiter transiting the bright A star HR 5599 in a polar orbit.
The host star, HR 5599, has a visual magnitude of V = 6.6, making it the brightest host star of a transiting hot Jupiter. The Scientific American piece, written by Caleb Scharf, focuses on the fact that the planet is in near-perfectly aligned polar orbit, saying:
“Like with other mis-aligned hot-Jupiter worlds, the big question is how does this situation arise? We don’t know for sure. One idea is that these planets have to form at larger distances from their stars and then migrate inwards — due to interactions either with a proto-planetary disk or other worlds, or both. Those interactions can also pump up the ellipticity of the orbit and its inclination. Later on the tidal forces between the planet and the star can pull it in close, but preserve a high orbital inclination…maybe.”
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.
NASA JPL have put out a press release about ultra-hot Jupiters including WASP-18b, WASP-103b and WASP-121b.
The work, led by Vivien Parmentier, used the Spitzer and Hubble space telescopes to study how the planets’ atmospheres change from the irradiated day side to the cooler night side.
“Due to strong irradiation on the planet’s daysides, temperatures there get so intense that water molecules are completely torn apart. […] fierce winds may blow the sundered water molecules into the planets’ nightside hemispheres. On the cooler, dark side of the planet, the atoms can recombine into molecules and condense into clouds, all before drifting back into the dayside to be splintered again.”
Simulated views of the ultrahot Jupiter WASP-121b show what the planet might look like to the human eye from five different vantage points, illuminated to different degrees by its parent star. (Credit: NASA/JPL-Caltech/Vivien Parmentier/Aix-Marseille University)
“With these studies, we are bringing some of the century-old knowledge gained from studying the astrophysics of stars, to the new field of investigating exoplanetary atmospheres,” said Parmentier.
Harvard’s CfA have also produced a press release on the work, focusing on the analysis of WASP-103b led by Laura Kreidberg.
“A crucial observational advance by Kreidberg and her team was that they observed the planet for an entire orbit, enabling them to map the climate at every longitude and derive detailed information about the temperatures on the planet’s dayside and nightside. This is only the second time that such a complete exoplanet observation has been performed with HST.”
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.
Brown dwarfs are intermediate between planets and stars. They are not massive enough to undergo hydrogen fusion in their cores, as required to be a star, but are too massive to be planets, and can fuse deuterium. Those conditions produce a range from about 13 Jupiter masses to about 80. Some people, however, argue that the distinction between a planet and a brown dwarf should not be about their mass, but about whether they formed in a star-like way, by gravitational collapse, or in a planet-like way, by accumulation of planetesimals in a proto-stellar disc.
Comparative sizes. Credit: NASA Goddard Space Flight Center
WASP was designed to look for transiting Jupiter-sized planets, but brown-dwarf stars are much the same size as Jupiter and so produce planet-like transits. That means we only discover which is which by measuring the mass of the transiting body by radial-velocity techniques.
So we should find brown dwarfs as readily as planets. But we’ve found only two, WASP-30b and now WASP-128b, compared to over 150 planets. That means that closely orbiting brown dwarfs must be much rarer than planets. It seems that star-like, gravitational-collapse formation rarely produces objects with a mass as low as 30 to 50 Jupiters (that’s not enough mass to collapse easily), while planet-like accumulation of planetesimals rarely builds up to mass that high (there aren’t enough planetesimals).
Masses and radii of known brown dwarfs. WASP-128b is the object with a mass of 37 Jupiters, while WASP-30b has a mass of 61 Jupiters. The coloured regions denote theoretical models for the mass–radius relation at different ages.
Which means that WASP-128b, newly announced on arXiv today in a paper by Vedad Hodžić etal, is a very rare object, being a brown dwarf with a mass of 37 Jupiters in a 2-day orbit around a G-type star. The nearest comparable object is KOI-205b, at 40 Jupiter masses, though that transits a star that is 2 magnitudes fainter and so is harder to study.
The Kepler space telescope, shown here in an artist’s rendering, helped detect a carbon-black planet. Credit: NASA
“Astronomers led by Teo Močnik at Keele University, UK, used NASA’s Kepler telescope to study a star called WASP-104, which lies roughly 144 parsecs from Earth in the constellation Leo. Earlier observations had documented a dimming of WASP-104’s light every 1.76 days, indicating that a planet was regularly crossing the star’s face. But Močnik’s team could not detect starlight reflecting off the planet, as scientists usually expect after discovering a new world. That led the researchers to conclude that the planet is nearly pure black in colour. The planet’s darkness could help scientists to test their ideas about exoplanetary atmospheres, such as how clouds might form on a world that reflects so little light.”
The paper, published in Astronomical Journal, is here.
The robotic photometer TRAPPIST-South (best known for the discovery of the TRAPPIST-1 planetary system) has long been a part of the WASP-South discovery process, along with WASP-South itself and the Euler/CORALIE spectrograph.
Khalid Barkaoui, lead author of the WASP-161, WASP-163 and WASP-170 discovery paper, alongside TRAPPIST-North.
A new paper announcing WASP-161b, WASP-163b and WASP-170b now marks the first contributions to WASP discovery from TRAPPIST-North. Situated in Morocco, TRAPPIST-North is also a robotic 0.6-m photometric telescope, similar to the TRAPPIST-South in Chile.
Transit lightcurves of WASP-161b from TRAPPIST-North, TRAPPIST-South and the SPECULOOS Europa telescope.
TRAPPIST-North, at the Oukaïmden Observatory in the Atlas Mountains of Morocco