Water Is Destroyed, Then Reborn in Ultrahot Jupiters

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.”


A bright spot on the host star of WASP-19b

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.

Announcing WASP-128b, a transiting brown dwarf

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.

Nature Research Highlight on WASP-104b

Our recent paper on WASP-104b has been picked up as a Research Highlight by Nature. We reproduce it here:

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.

TRAPPIST-North joins the team

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

NASA’s Webb Space Telescope to Inspect Atmospheres of Gas Giant Exoplanets

NASA have written a publicity page on JWST’s plans to study the atmospheres of gas-giant exoplanets, including an animation on how this is done. Since the prime targets for the “Early Release Science” program are three WASP-discovered planets, WASP-18b, WASP-43b and WASP-79b, we “re-blog” the piece here:

“In April 2018, NASA launched the Transiting Exoplanet Survey Satellite (TESS). Its main goal is to locate Earth-sized planets and larger “super-Earths” orbiting nearby stars for further study. One of the most powerful tools that will examine the atmospheres of some planets that TESS discovers will be NASA’s James Webb Space Telescope. Since observing small exoplanets with thin atmospheres like Earth will be challenging for Webb, astronomers will target easier, gas giant exoplanets first.”

Read the full piece here.

Metals and water in the atmosphere of WASP-127b

Puffed-up, low-mass planet WASP-127b is one of the best targets for atmospheric characterisation. Recently Chen etal have announced the results of observations taken with the La Palma telescopes the Gran Telescopio Canarias and the Nordic Optical Telescope.

The result is one of the most detailed and featured spectra of an exoplanet yet.

The spectrum clearly shows the metals sodium (Na), lithium and potassium (K), along with features due to water and haze. A press release from the University of Cambridge has led to coverage of the paper on several dozen websites.

The Instituto de Astrofisica de Canarias have also produced this artist’s depiction of WASP-127b and its host star: