Category Archives: Hot Jupiters

Spectral contamination from starspots on WASP-4

Here’s a topic we’ll be hearing much more about: how the observed spectrum of a transiting exoplanet is affected by transiting across star-spots. In “transmission spectroscopy” the starlight shines through the planet’s atmosphere during transit, and the easiest thing to do is assume that the star itself is a uniform light source.

But as discussed by papers led by Ben Rackham, if the planet passes over a dark region (star spot) or bright region (faculae), this would change the observed spectrum.

A new paper led by Alex Bixel about WASP-4b is the first to attempt to correct for this effect. The authors’ transit observations show a clear crossing of a starspot (the feature is shown in blue, the spot shows as a upward bump since the planet is then removing less light):

And here is the difference it makes. The blue curve is the observed spectrum, presumed to be of the planet’s atmosphere. The orange curve is then the spectrum corrected for the presence of the star spot.

The details of how to do this are complex, and are discussed at length in the above papers. The central message is that “active FGK host stars can produce such features and care is warranted in interpreting transmission spectra from these systems”.

However, there is good news in that: “stellar contamination in transmission spectra of FGK-hosted exoplanets is generally less problematic than for exoplanets orbiting M dwarfs”, and that such signals “are generally minor at wavelengths of planetary atomic and molecular features”. Overall the authors say that their study “bodes well for high-precision observations of these targets”.

Helium in WASP-69b, HAT-P-11b and HD 189733b

Earlier this year helium was found in the outer atmosphere of WASP-107b, the first detection of helium in an exoplanet. Several teams have now used similar techniques to find helium in WASP-69b, HAT-P-11b and HD 189733b, leading to a slew of papers and accompanying press releases from the Instituto de Astrofísica de Andalucía, the University of Exeter and others (see [1], [2], [3] and [4]).

Artist’s impression of an escaping envelope of helium surrounding WASP-69b. (Credit: Gabriel Perez Diaz, IAC)

Lisa Nortmann, lead author of the WASP-69b paper, explains that the helium is escaping from the atmosphere, forming a comet-like tail: “We observed a stronger and longer-lasting dimming of the starlight in a region of the spectrum where helium gas absorbs light. The longer duration of this absorption allows us to infer the presence of a tail.”

The press releases have led to extensive coverage including by CNN, the Daily Mail and Tech Times.

The IAA press release includes a video illustration of WASP-69b, created by Gabriel Perez Diaz of the IAC:

Is WASP-12b’s orbital decay driven by obliquity tides?

Tidal interactions between hot-Jupiter exoplanets and the host star should be causing their orbits to decay, such that the planet gradually spirals inwards. For most systems the change would be too small to detect in the decade or so that we’ve been observing them. However, WASP-12b is an exception, showing a clear change in its orbital period.

In a new paper on arXiv, Gracjan Maciejewski et al present the latest data for WASP-12b:

The graph records the change in transit time (“observed minus calculated” times, or O–C), showing that the transits are now occurring eight minutes early owing to a decreasing orbital period.

Such a rate is far faster than observed in other systems, and too large to be explained by the standard theory of tidal interactions.

However, a new paper led by Sarah Millholland suggests an answer. She suggests that the planet is tilted over, so that the axis around which it spins is tilted with respect to the plane of the planet’s orbit.

This means that the star will give rise to strong “obliquity tides” on the planet, and the dissipation of those tides could explain the decay of the orbit. For this to work something must be keeping the planet tilted over. Millholland suggests that a second planet in an outer orbit might be perturbing WASP-12b, keeping it in the high-obliquity state. This scenario requires some fine tuning, but if WASP-12 is the only system known to show this behaviour then the explanation is plausible.

A Hot Polar Planet

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.”

Credit: NASA, JPL, Caltech

Night-side clouds on hot Jupiters

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.

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.”

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.