Category Archives: KELT planets

Comparing WASP-173 to KELT-22

WASP-173 and KELT-22 are the same object. The WASP and KELT teams are both trying to find transiting exoplanets around relatively bright stars, and this means that sometimes our discoveries overlap. We announced that WASP-173 hosts a hot Jupiter in a paper on arXiv on the 7th March, and then on the 21st March KELT reported an entirely independent discovery of the same planet.

Since the two teams use different facilities, techniques and software, comparing the two sets of system parameters provides an interesting check on the methods. So let’s see how similar the reports are.

WASP-173Ab discovery photometry

The biggest difference is a somewhat different transit depth. We (WASP) report a depth of 0.0123 ± 0.0002 whereas KELT report 0.0145 ± 0.0008, where the difference is greater than the error bars quoted. Now this system is a double star, with a companion star 6 arcsecs away and 0.8 magnitudes fainter. That makes it hard to measure the depth. One either uses a much smaller photometric aperture than normal, excluding the nearby star, or one uses a much wider aperture, containing both stars, and makes a correction for the dilution of the companion. Either approach could introduce systematic errors more than normal. Then, of course, there could be red noise in the light-curves owing to observing conditions or stellar activity.

KELT-22Ab transit photometry

The greater depth in the KELT paper means they arrive at a slightly larger planet radius (1.29 ± 0.10 Jupiter radii) than we do (1.20 ± 0.06) but here the error ranges overlap. The planet mass (derived mostly from the radial velocity data) is comparable, 3.47 ± 0.15 Jupiter masses in the KELT paper, and 3.69 ± 0.18 in ours.

WASP-173Ab radial velocities (from CORALIE)

The differences in the parameters of the host star are all within the error ranges. KELT report a G2 star with an effective temperature of 5770 ± 50 K, a surface gravity (log g) of 4.39 ± 0.05, and a mass and radius of 1.09 ± 0.05 and 1.10 ± 0.08 in solar units, whereas WASP report a G3 star with effective temperature of 5700 ± 150 K, a surface gravity of 4.5 ± 0.2, and a mass and radius of 1.05 ± 0.08 and 1.11 ± 0.05.

KELT-22Ab radial velocities (from TrES)

Another comparison is the “impact factor” (how near the center-line the transit chord is), which we have as 0.40 ± 0.08 while KELT report 0.31 ± 0.18. Our higher value results from our having a higher transit width, 0.0957 ± 0.0007 days, compared to KELT’s 0.0981 ± 0.0025. Again, the differences point to red noise in the transit lightcurves, which is likely to produce uncertainties greater than the formal error bars.

Overall, the values are sufficiently similar that we can have broad confidence in the values, but the presence of systematic noise does need to be borne in mind.


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

The websites and 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.

KELT-16b and sub-1-day hot-Jupiter exoplanets

Until recently the only hot-Jupiter exoplanets known with orbital periods below one day were the four discovered by WASP-South (WASP-18b, WASP-19b, WASP-43b and WASP-103b). But last month HATSouth reported that HATS-18b has a 0.84-day period and now KELT have announced KELT-16b at 0.97 days.

The KELT team, lead by Thomas Oberst, have produced this figure showing planetary masses against orbital separation (semi-major axis):

Short-period hot Jupiter exoplanets

One can see that all the planets just mentioned are Jupiter-mass or heavier. There are relatively few planets in the blue-shaded region, where they would have both Neptune-like masses and very short orbital periods. There are, though, Earth-mass planets known at these orbital periods. The paucity of short-period Neptunes cannot just be a selection effect, since they would have been readily found in the Kepler mission.

Instead, the currently favoured explanation is that planets in the blue-shaded region would rapidly be evaporated and be stripped down to their cores. At such short separations from their stars planets are subject to high irradiation and tidal forces. The combination can inflate the planets to the point that their atmospheres “boil off” and overflow the planet’s Roche lobe.

They avoid this fate only if the planet has enough mass, and thus gravity, to hold on to its atmosphere. Thus, at these very short orbital periods, we see either large, Jupiter-mass planets, or small, dense, rocky planets (possibly remnant cores of evaporated larger planets) — but not any in-between planets the size of Neptune.