Category Archives: Kepler planets

Two K2 planets transiting bright stars

With the launch of the James Webb Space Telescope only a year away the exoplanet community is gearing up to exploit its capability for characterising exoplanet atmospheres. A new paper by Yu et al contains a plot of the best targets, giving the expected “signal to noise” for each planet as a function of the planet’s mass. The higher the S/N the better, enabling more atmospheric features to be discerned.

It is notable that most of the best targets do not come from Kepler (which had a relatively small field of view, and so looked at mainly fainter stars), but instead from the ground-based transit surveys (which focus mainly on brighter stars, which are thus better targets for follow-up). WASP features strongly, supplying half of the best targets.

The focus of the Yu et al paper, however, is the discovery of two very good targets from the K2 phase of Kepler‘s mission. K2 is observing more fields for less time than the original Kepler, and so covers more bright stars.

HD 89345b (labelled in red above) is only 10% of Jupiter’s mass but is bloated to 0.6 Jupiter radii. Transiting a bright star of V = 9.4 makes it a prime target.

The transit depth of only 0.15% means that it is too shallow to have been detected by WASP (which can do 0.2–0.3% at best), especially given the 11.8-day orbit, which means that it produces fewer transits than shorter-period planets.

The other new discovery, HD 286123b (which had also been independently found by Brahm et al), is a larger and more massive planet producing a 0.8% dip. This one should have been within the reach of the WASP survey, but happens to lie in a region of the Northern sky where SuperWASP-North has only limited data.

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.

Inhabitants of WASP-47 could see an Earth transit!

The planetary system WASP-47 is highly popular at the moment, following the K2 discovery of two more transiting exoplanets, and the radial-velocity detection of a longer-period outer planet, in addition to the orginal hot Jupiter, WASP-47b.

But here’s an additional curiosity: WASP-47 is so close to the plane of our own solar system, aligned to better than 0.26 degrees, that Earth would be seen to transit from WASP-47!

Christopher Burke, of the SETI Institute, has produced this graphic of an Earth transit as seen from the location of WASP-47.:

Earth transit seen from WASP-47

And, supposing that the inhabitants of the WASP-47 system had a spacecraft like Kepler, this is what the transit they might record would look like:

Earth transit seen from WASP-47

Of course it is only speculation that there are inhabitants of WASP-47, though, with four planets known so far in the system, there might be some planet or moon that is inhabitable. If they had detected Earth, they might then point their biggest telescope at the next transit, and perhaps find free oxygen in Earth’s atmosphere, and so deduce that Earth has life.

Chris Burke suggests that WASP-47 is a very good SETI target; they might already know about us!

Four planets around WASP-47!

As NASA’s Kepler mission covers fields in the ecliptic previously surveyed by WASP, it is obtaining photometry of unprecedented quality on some WASP planets. The big news this week is the discovery of two more transiting planets in the WASP-47 system.

WASP-47 had seemed to be a relatively routine hot-Jupiter system with the discovery of a Jupiter-sized planet in a 4-day orbit, reported in a batch of transiting planets from WASP-South by Hellier et al 2012.

But WASP-47 is anything but routine. Now Becker et al have announced that the Kepler K2 lightcurves show two more transiting planets: a super-Earth planet in an orbit of only 0.79 days, and a Neptune-sized planet in an orbit of 9.0 days. Being much smaller, these planets cause transits that are too shallow to have been seen in the original WASP data.

WASP-47 transits with Kepler K2

The super-Earth, labelled WASP-47c, has a radius of 1.8 Earths while the Neptune, labelled WASP-47d, has a radius of 3.6 Earths. The triple-planet system is dynamically stable, but the gravitational interaction causes perturbations in the orbits, leading to variations in the times of the transits.

Such “transit-timing variations” or TTVs lead to estimates of the planetary masses. Becker et al find that the hot Jupiter has a mass of 340 Earths (consistent with the mass of 360 Earths originally reported by Hellier et al from radial-velocity measurements), while the Neptune has a mass of 9 Earths. The super-Earth must be less massive than that, but current timing measurements are not sensitive enough to say more.

WASP-47 TTVs Transit timing variations

As if three planets were not enough, there is a probable fourth planet orbiting WASP-47. The Geneva Observatory group routinely monitor known WASP systems, taking radial-velocity measurements over years, to look for longer-period planets. Marion Neveu-VanMalle and colleagues have recently reported the detection of another Jupiter-mass planet orbiting WASP-47, this time in a much wider orbit of 571 days.

The WASP-47 system has now become hugely interesting for understanding exoplanets, and will trigger many additional observations of the system. For example, being bright enough to allow good radial-velocity data, it will provide a much-needed check that the mass estimates from TTVs match those from the more traditional radial-velocity technique.

The system will also be of strong interest to theorists, who will want to understand the formation and origin of a planetary system with this architecture. One immediate consequence is that it shows that a hot Jupiter can arise by inward migration through the proto-planetary disk, without destroying all other planets in its path.

A tribute to supreme planet-hunter Bill Borucki

Bill Borucki Kepler mission We pay tribute to supreme planet hunter William Borucki, who retires this week from NASA. Bill Borucki spent decades first advocating for a transit-search satellite and then leading the Kepler mission to outstanding success. Kepler has now found over 1000 transiting exoplanets, and in particular has opened up whole new fields of research on small planets and on multiple-planet systems.

Planet detection history in 60 seconds

Hugh Osborn, a PhD student on the WASP project at Warwick University, has produced a graphic illustrating the “gold rush” of exoplanet detection in recent times.

The animation shows the planet masses and orbital periods against year of discovery.

Exoplanet gold rush

The symbols are colour-coded according to detection method. The WASP project is responsible for a large fraction of the transiting (green symbol) “hot Jupiters” — massive, short-period planets at upper-left. Kepler has found most of the other transiting exoplanets. For more explanation see Hugh’s blog.

WASP and Kepler K2

WASP-85 is a binary star, with the hot Jupiter WASP-85Ab orbiting the brighter star of the pair. It was in the Campaign 1 field of the revamped Kepler K2 mission, and thus we have the first extensive Kepler-quality lightcurve of a WASP planetary system.

K2 light curve of WASP-85

The WASP discovery paper by David Brown et al presents an initial look at the long-cadence K2 data. The upper plot shows the entire light curve, with obvious variability of the star (presumably because it is magnetically active) and narrow dips caused by the transits. The lower plot shows the data folded on the transit.

K2 light curve of WASP-85 transit

The higher-time-resolution “short cadence” data will be available soon, and should allow a high-quality analysis of this system. The WASP planets WASP-47b and WASP-75b are being observed in the current K2 Campaign 3, which should lead to more space-quality light curves of WASP systems.

In other news, WASP played a minor role in the discovery of the first K2 planet, a super-Earth-sized planet orbiting the bright K-dwarf star HIP 116454. There is extensive WASP data on this star, and while the transits (only 0.1% deep) are too shallow to see in WASP data, the WASP data contribute by showing a possible 16-day rotation period of the host star. The discovery paper by Andrew Vanderburg et al featured in a NASA press release.

Kepler-91b: a hot-Jupiter planet orbiting a red-giant star

When we at WASP search for candidate transit dips we usually ignore any star that we think is a red giant star. Such stars have evolved off the Main Sequence and the outer layers are becoming bloated and puffed-up to many times their original size. Because of this there is no room for a hot-Jupiter planet in a typical orbit of 2 to 4 days, it would be inside the star. Thus if we see a short-period dip apparently on a red giant, it usually means that the system is blended, and that the dip is caused by a fainter, unseen binary.

However, a team led by Jorge Lillo-Box at the Centro de AstrobiologĂ­a in Madrid have reported that the Kepler “object of interest” KOI-2133 — now promoted to being called Kepler-91 — is a red giant orbited by a Jupiter-mass planet on an orbit of 6.2 days.

This star is the biggest known to host a planet, with a bloated radius of 6.3 times that of the Sun. The planet orbits just outside, and, since the red giant star is still expanding, the authors estimate that it will be engulfed within 50 million years, a very short time compared to usual planet lifetimes. Viewed from the planet the star would fill nearly half the sky.

Artists conception of Kepler-91 and its planet

So puffed up and tenuous are the star’s outer layers, and so close are they to the planet, that the planet’s presence distorts them into an ellipsoidal shape. This produces a modulation of the star’s light at the planet’s orbital period, a modulation that had previously led to Kepler-91 being considered to be a binary star, not a planet host.

Lightcurve of Kepler-91

The Kepler lightcurve of Kepler-91b showing the transits and the “ellipsoidal modulation” caused by the planet distorting the star’s outer layers

The WASP cameras have also been watching Kepler-91 over the years, accumulating 20,000 data points, but could never detect the dip caused by Kepler-91b. Not only is Kepler-91 relatively faint, at V = 12.9, but the transit dip is very shallow.

A typical hot-Jupiter planet transiting a typical star produces a dip of about 1% in the light, which is detectable by WASP. However with Kepler-91 being bloated to 6.3 solar radii its surface area will be 40 times bigger, and hence the planet occults only 1/40th as much of the light, producing a vastly shallower dip. The transit depth in Kepler-91 is only 0.04%, detectable by the superb Kepler photometry from the stability of space, but not detectable from the ground, another reason why WASP ignores stars that are giants.