Category Archives: Hot Jupiters

Discovery of the hot Jupiter WASP-167b (KELT-13b)

The websites sci-news.com and phys.org have published articles on our recent discovery of WASP-167b (KELT-13b) — the highest WASP number so far announced — along with an image comparing it to Jupiter:

WASP-167b is notable for two reasons. First, it orbits a hot star with a surface temperature of 7000 Kelvin. Planets transiting hot stars are harder to validate since the star’s spectra shows only broad and weak spectral lines, which makes it harder to get accurate radial-velocity measurements and thus prove that the transiting object has the right mass to be a planet.

The WASP project had tended to put such candidates on the back-burner and go after easier targets, but having succeeded in finding over 100 planets transiting cooler stars we are now focussing on the hot ones.

Secondly, WASP-167b is a joint discovery with the KELT project (hence the additional name of KELT-13b), the first time two of the transit-search teams have combined an announcement. Both projects had put much effort and telescope time into following up this candidate, and a joint paper recognises both of these campaigns.

Hubble finds a stratosphere in WASP-121b

Orbiting a hot F-star in only 1.27 days, WASP-121b is a highly irradiated hot Jupiter found by Laëtitia Delrez et al using the WASP-South survey. A team led by Tom Evans at Exeter has now pointed the Hubble Space Telescope at WASP-121b and found that its atmosphere shows a “stratosphere”. That is, the higher layers of the atmosphere appear to be hotter than the lower layers.

This is possible if molecules high in the atmosphere absorb radiation very efficiently. The “stratosphere” interpretation comes from finding spectral features caused by water, but seeing them in emission (as expected if the atmospheric temperature increases with height) rather than in absorption (expected if the temperature declines with height).

The data show the Hubble spectrum observed during transit using the WFC3 instrument. The red line is a model including a stratosphere. The blue lines are, for comparison, colder “brown dwarfs” which don’t have a stratosphere. The WFC3 data (circles with error bars) clearly favour the stratosphere interpretation.

NASA have put out a press release about the discovery, while the press team at Exeter have produced an illustration of the highly irradiated planet:

The story has been picked up by CNN, The Telegraph, New Scientist, NDTV, phys.org, the Mail Online, the International Business Times, Gizmodo Australia and over 40 other news and science websites.

One we missed: EPIC 228735255b

At WASP we routinely “reverse engineer” transiting exoplanets announced from other surveys to see whether we could have found them. Since the K2 mission has vastly better photometry it will find anything we’ve missed in K2 fields.

An interesting case is EPIC 228735255b, a transiting hot Jupiter in a 6.57-day orbit around a V = 12.5, G5 star, newly announced by a team led by Helen Giles, a PhD student at the University of Geneva.

In principle this planet should be within the reach of the WASP survey. However, at V = 12.5 it is at the faint end of the survey, and with a period of 6.57 days (fairly long for hot Jupiters) fewer transits get covered. Further, the WASP camera use large pixels, in order to get wide-field coverage, and for this object there is another star on the edge of our photometric aperture (see left), which degrades our photometry. Lastly, at a declination of −09 it is just below the sky covered by SuperWASP-North and so we have data only from WASP-South, principally 4600 data points from 2009 and 5700 data points from 2010.

Nevertheless, the transit was detected in WASP data, found by our standard transit-search algorithms (the WASP transit period is 6.5692 days, which compares with the Giles et al period of 6.5693 days, where the match affirms that our detection is real).

The plots show the search periodogram, showing a clear “spike” at the transit period and at twice the transit period, and (below) the WASP data folded on the transit period (transit is at phase 0).

The problem is that there is always a lot of “red noise” in WASP data, and picking candidates always involves a judgement call as to whether the signal is real. This one was just not quite convincing enough for us. The folded light curve looks pretty ratty, and the individual transit lightcurves are not particularly convincing. It had been flagged as a possible candidate, but rated as not secure enough a detection to send to the radial-velocity follow-up teams. Perhaps WASP detections might be more reliable than we thought!

While the WASP data are now superseded by the K2 photometry, it is worth recording the WASP transit ephemeris, which is period = 6.56919 (+/− 0.00036) days, epoch HJD = 2455151.1052 (+/− 0.0084), and transit width 3.56 hrs (which results from transit features spanning HJD 2454914 to 2455348).

Since these observations are from March 2009 to May 2010, they greatly extend the baseline of the Giles et al photometry, which covers 2016 July to 2017 March, and so will help refine the ephemeris to assist future observations.

The imminent TESS mission will find all the hot Jupiters that we’ve missed over the whole sky (whereas K2 is confined to the ecliptic plane), but will observe regions of sky for only a limited period and so give poor ephemerides. The above comparison suggests that WASP data will still be of valuable in being able to greatly improve the ephemerides for many TESS finds.

What happens to short-period hot-Jupiter planets?

Hot-Jupiter planets close to their host star will arouse tides in the host star, and the gravitational pull from tidal bulges will cause the planet to gradually spiral inwards. What happens to them? One possibility is that they end up spiralling into the star and are engulfed.

Another possibility is that strong irradiation from the star blasts off the planet’s atmosphere. Over time, all that would be left might be the small, rocky core of the original Jupiter-size gas giant. Maybe, then, the small, rocky, Earth-size planets seen by Kepler at ultra-short orbital periods are the remnants of hot Jupiters?

The figure shows the planetary radius versus the orbital period for a sample of Kepler planets. The Earth-size, ultra-short-period planets are in red, the hot Jupiters are in orange.

Planet radius versus orbital period for Kepler planets

Now, a team led by Joshua Winn of Princeton has tested this idea. The looked at the host stars of both the small, rocky ultra-short-period planets and of the hot Jupiters, and measured their metallicity (the fraction of elements heavier than hydrogen and helium, for which the iron abundance, denoted [Fe/H], is a good proxy).

They ended up with the following figure, which shows the metallicity distribution for the small, rocky planets (red histogram), for medium, sub-Neptune-size planets (blue histogram) and for hosts of hot Jupiters (orange histogram).

Metallicity distribution for hot Jupiter hosts versus hosts of small, rocky, ultra-short-period planets

The distribution for the rocky-planet hosts is significantly different from that for the hot-Jupiter hosts, showing that they cannot be part of the same population. This means that it is unlikely that hot Jupiters turn into rocky, ultra-short-period planets. Such planets might, however, be descended from hot Neptune-sized planets, for which the host-star metallicity does have the right distribution.

WASP-118 is pulsating

The K2 spacecraft is monitoring a series of fields along the ecliptic and so producing Kepler-quality photometry on some of the exoplanet systems previously discovered by WASP.

WASP-118b is an inflated hot-Jupiter planet (0.5 Jupiter masses but 1.4 Jupiter radii) on a 4-day orbit around a bright F-type star of V = 11. It was observed for 75 days in K2‘s Campaign 8. Teo Močnik et al have now analysed the data and see transits of the planet, as expected:

WASP-118 transits as observed with K2

The upper black curve is the raw data, while the lower red curve has been corrected for artefacts caused by drifts in K2‘s pointing. Nineteen transits are seen, recurring with the 4-day orbital period.

But Teo Močnik noticed that the out-of-transit photometry was not as flat as expected. After further investigation he deduced that the host star is pulsating:

WASP-118 is a pulsating star

The pulsations have a timescale of 1.9 days and a very low amplitude of 2 parts in 10 000, only discernable given a lightcurve with Kepler‘s photometric accuracy. Thus WASP-118 appears to be a γ-Doradus pulsator, possibly the first γ-Dor variable known to host a transiting exoplanet.

WASP-43b has an aligned orbit

WASP-43b is the hot Jupiter that is closest to its parent star, around which it orbits in only 19 hours. At such a close location, tidal interactions between the planet and the star will be intense. That means that we expect the planet to be phase locked (with its rotation period equalling the orbital period, so that the same side always faces the star), and we expect the orbit to be circular (any eccentricity having been damped by tides), and we expect the orbit to be aligned with the rotation axis of the star.

Tidal damping of the alignment of the orbit is the subject of much investigation. It seems to be most efficient if the planet orbits cooler stars, and much less efficient if the planet orbits a hotter star. This might be because cooler stars have large “convective zones” in their outer layers, which can efficiently dissipate tidal energy, whereas hotter stars have only very shallow convective zones with little mass in them.

Since WASP-43b orbits a cool star, a K7 star with a surface temperature of only 4400 Kelvin, that’s another reason for expecting its orbit to be aligned. This has now been confirmed by observations with the Italian Telescopio Nazionale Galileo. The way to measure the orbital alignment of a transiting exoplanet is by the Rossiter–McLaughlin effect. As the planet transits a rotating star, it first obscures one limb and then the other, and since the different limbs will be either blue-shifted or red-shifted, according to how the star is spinning, the effect on the overall light of star will reveal the path of the orbit.

Rossiter-McLaughlin effect

A new paper by Esposito et al reports R–M measurements for three planets including WASP-43b. The data show the classic R–M signature of an aligned planet.

Rossiter-McLaughlin effect for exoplanet WASP-43b

The upper panel shows the change in stellar radial-velocity around the planet’s orbit, caused by the gravitational tug of the planet. The lowest panel highlights the data through transit, showing the expected excursion first to a redder light (when blue-shifted light on the approaching limb is occulted) and then to blue light (when the red-shifted receding limb is occulted).

Long-period brown dwarfs for WASP-53 and WASP-81

The WASP project has just released the discovery paper for the systems WASP-53 and WASP-81, led by Amaury Triaud. We’ve known about close-in hot-Jupiter planets around these two stars for several years, but the paper had been delayed owing to an interesting development: the radial-velocity monitoring showed that the planets both had longer-period brown-dwarf companions. Several years of data have been needed to prove the reality of these brown dwarfs, now dubbed WASP-53c and WASP-81c.

Radial velocity monitoring of WASP-53 and WASP-81

The plot shows the “radial velocities” — how much the star is tugged about by the gravity of orbiting bodies — as a function of time (in BJD, a count of days). WASP-53 is on the left and WASP-81 on the right. The red line is a fit to the data. The close-in hot Jupiters (WASP-53b and WASP-81b, with orbits of 3.3 and 2.7 days respectively) cause short-period variations, so fast that they appear as a solid red swathe.

In addition, though, WASP-53 shows a variation owing to a more-massive brown dwarf with an orbital period of about ten years and a mass of at least 16 Jupiters. Similarly, WASP-81 shows a variation caused by a 57-MJup brown dwarf in a 3.5-yr orbit. Both outer orbits are highly eccentric.

The presence of the brown dwarfs has interesting consequences for ideas about how planets form. It is generally accepted that hot Jupiters form further out, where it is colder, where ices can stick together and form a planetesimal. But the presence of eccentric brown dwarfs, disrupting the proto-planetary disc in that region, would have made that hard. So maybe the planets formed further in? Or maybe the brown dwarfs were originally elsewhere, and moved to their current orbits later on?