Tag Archives: K2

Precise masses for the WASP-47 exoplanetary system

In the discovery paper the exoplanet WASP-47b was introduced to the world with the description: “With an orbital period of 4.16 d, a mass of 1.14 MJup and a radius of 1.15 RJup, WASP-47b is an entirely typical hot Jupiter”.

And it did appear to be entirely typical until Juliette Becker et al looked at K2 lightcurves and found two more planets, a super-Earth orbiting inside the hot Jupiter (WASP-47e in a 0.79-d orbit) and a Neptune orbiting just outside it (WASP-47d in a 9-d orbit). Around the same time Neveu-VanMalle et al announced long-term monitoring showing another Jupiter-mass planet (WASP-47c), this one in a much wider orbit of 580 days. Thus WASP-47 was shown to host a whole exoplanetary system, one that is so-far unique.

Since then WASP-47 has been observed intensively in order to measure the planet masses and investigate the dynamics of the exoplanetary system. The state of play is now reported by Andrew Vanderburg et al. The planets’ host star is tugged around by the gravitational pull of the orbiting planets, leading to the following cyclical variations in the observed radial velocities:

Combining all the information, Vanderburg et al deduce that the innermost “super-Earth”, WASP-47e, is not dense enough to be made only of rock. Instead it likely has a liquid or gaseous envelope (possibly water or steam) surrounding an Earth-like core. That is unlike other ultra-short-period super-Earths which appear to be fully rocky.

From modelling the dynamical history of the system Vanderburg et al also deduce that the outermost planet, WASP-47c, is likely in an orbit that is in the same plane as those of the inner planets. If this were not the case then the system would not be stable. Thus they conclude that the likelihood that WASP-47c also transits its star, as seen from Earth, is relatively high, which should motivate a campaign to look for those transits.


Super-Neptune WASP-107b has an oblique orbit

WASP-107b is only twice the mass of Neptune but nearly the radius of Jupiter. It is thus a hugely bloated and fluffy exoplanet and one of the more important of the recent WASP discoveries, being a prime target for atmospheric characterisation (see the discovery paper by Anderson et al 2017).

WASP-107b was also in the Campaign-10 field of the K2 mission, leading to a Kepler-quality photometric lightcurve. Recent papers by two teams, led by Teo Močnik and Fei Dai, have arrived at a similar conclusion: WASP-107b seems to be in an oblique orbit, rather than in an orbit aligned with the rotation axis of the host star.


The conclusion comes from star spots. If the orbit is aligned, consecutive transits will repeatedly cross the same star spot, producing a “bump” in the lightcurve each time, whereas if the orbit is oblique this will not happen.

Thus one can play the game of looking for transit bumps and seeing if they repeat. But spots can change, by growing or shrinking, so is a smaller bump in the next transit the same spot, or a different one? Also, if there is some uncertainty in the rotational period of the star, then we’re not fully sure exactly where in the next transit the spot will recur.

Star spots in transits of exoplanet WASP-107b

In the figure at left (in which the transit itself, between the dashed lines, has been removed, leaving only the starspot bumps), obvious spots are circled in red, while possible spots are marked with a lighter red. The rotational period of the star is nearly three times the orbital period of the planet, and so, if the spots recurred, they would be seen every three transits. (The gap, and thus the missing of transits 3, 4 and 5, arose from a spacecraft malfunction.)

The conclusion is that the star spots do not seem to recur and thus that WASP-107b is in an oblique orbit.

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.

Long-lasting starspots on exoplanet-host Qatar-2

Planets transiting their star can cross a starspot, and that — since the spot is dimmer than its surrounding — causes an upward blip in the light-curve of the transit. The same starspot can be occulted in consecutive transits, and so is seen later in phase each time because the star has rotated between the transits.

An illustration of a starspot feature in consecutive transits.  Image by Klaus Felix Huber.

A starspot feature in three consecutive transits. Image by Klaus Felix Huber.

Keele University PhD student Teo Močnik has looked at the Kepler K2 lightcurve of Qatar-2, a star known to host a hot Jupiter in a 1.34-day orbit. The lightcurve records 59 consecutive transits over a 79-day period and Močnik finds that most of the observed transits are affected by starspots (link to paper).

In the plot below each numbered lightcurve is from a transit, which occurs between the vertical dashed lines. The transit profile itself, however, has been subtracted in order to better show the starspot features.

Starspots in transits of exoplanet host Qatar-2

The starspots occur in groups, shown by red ellipses, and each group is the same starspot being seen in consecutive transits. Interestingly, though, the groups of spots themselves recur. Thus the starspots are lasting long enough that they pass behind the limb of the star, and then re-appear to be transited again one stellar rotation cycle later!

One particular starspot first causes the features in transits 20 to 22, then comes round again to produce the features in transits 33 to 36, and then comes round once again to produce the features in transits 46 to 50. Thus the starspot must have lasted for at least 40 days.

We thus have one of the best observations yet of a starspot on a star other than our sun. From this information we can calculate the rotation period of the star, place limits on the size, position and longevity of the spots, and also show that the planet’s orbit is closely aligned with the spin axis of the star.

Starspots on WASP-85 from K2 transits

If, during a transit of its star, an exoplanet crosses a star spot, it will be covering a region that is dimmer than the rest of the star. Since less light will be being occulted, we will see a small increase or “bump” in the transit profile. WASP-85 was recently observed by the K2 mission, getting sufficiently high-quality photometry that it could reveal such starspot `bumps”.

Here is the transit proflle, from a paper by Teo Močnik et al, which contains all the K2 data folded in:

WASP-85b transit profile observed with Kepler K2.

Teo Močnik then subtracted the overall transit profile, thus showing the departures from the average behaviour, and produced a plot of each transit:

WASP-85 starspots observed with K2

The vertical dashed lines show the regions in transit. The lightcurve bumps circled in red are starspots being occulted. (Blue arrows are times when K2 fired its thrusters, which can cause a feature in the lightcurve.)

The interesting question is whether a bump recurs in the next transit, but shifted later in phase, as it would if the same starspot is being occulted again. This would happen if the planet’s orbit is aligned with the stellar rotation. In that case, as the star rotates, the spot moves along the line of transit, to be occulted again next transit.

Aligned orbit star spot occultation

An illustration of a planet occulting a star spot when the planet’s orbit and the star’s rotation are aligned. Graphic by Cristina Sanchis Ojeda

To judge whether the starspot bumps repeat, Teo gave all the co-authors a set of lightcurves and asked them to judge which features in the lightcurve were genuine bumps. But, to avoid human bias, he first scrambled the order of the lightcurves, so that the co-authors didn’t know which lightcurve came next.

The result is that we think that starspots do repeat, shown by the red linking lines in the above figure. This shows that the planet’s orbit is aligned, and it also allows us to estimate the rotational period of the star.

WASP-157b, a transiting Hot Jupiter observed with K2

Hot on to arXiv is our latest discovery paper. WASP-157b marks a jump upwards in WASP numbering, since we’ve somewhat rushed this one out. WASP-157 was flagged as a WASP candidate in 2014 and added to our program for radial-velocity (RV) and photometric follow-up. Meanwhile, being in the field of K2 Campaign 6, it was observed by Kepler from July to September 2015. Since K2 data are public, this meant that other groups would soon be on its trail.

A TRAPPIST observation of the transit, shortly before the K2 data were due to be released, along with prioritising it for CORALIE and HARPS RVs, rapidly accumulated enough observations to prove it was a planet. Keele student Teo Močnik then did a good job of analysing the K2 data and turning all the observations into a paper. A gap of only four days between the last CORALIE radial-velocity observation and the paper appearing on arXiv is efficient work!

Here is the K2 lightcurve showing the transits of WASP-157b:

Lightcurve of WASP-157b observed with Kepler K2

Masses for WASP-47’s planets

Since the discovery by K2 of two more transiting exoplanets orbiting WASP-47, plus a fourth planet at a much longer orbital period, this system has shot to the top of the priority lists. A team led by Fei Dai of MIT have thus pointed the 6.5-m Magellan/Clay telescope at WASP-47 to try to measure the radial-velocity signal, and hence the masses, of the two planets found by K2.

WASP-47 radial velocities

The figure shows the complex radial-velocity motion in a multiple-planet system. The blue curve is the radial-velocity motion caused by the 4.2-day-period hot-Jupiter WASP-47b, as originally found by the WASP project. The yellow curve is caused by the 0.79-day planet WASP-47e, the green curve by the 9-day planet WASP-47d, and the purple by the much-longer-period WASP-47c. The red line is then the sum of all the planets and the black dots are the measurements by Dai et al.

By fitting all of the data, Dai et al show that the innermost planet, WASP-47e, has a mass of 12 +/- 4 Earths, while the 9-day planet WASP-47c has a mass of 10 +/- 8 Earths. Both results are in line with previous mass estimates from transit-timing variations, which helps to validate mass measurements by both the RV and the TTV techniques.