Tag Archives: exoplanet transits

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While the WASP data are now superseded by the K2 photometry, it is worth recording the WASP transit ephemeris, which is period = 6.56919 (+/− 0.00036) days, epoch HJD = 2455151.1052 (+/− 0.0084),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.

Gaia detects transits of WASP exoplanets

ESA’s Gaia satellite is a €740-million mission to map a billion stars in our galaxy. By observing repeatedly with unprecedented astrometric precision it is measuring the parallaxes, and thus the distances, of hundreds of millions of stars, and so mapping out the 3-D structure of our galaxy.

Gaia can detect exoplanets in two ways, first by astrometry (measuring the position of a star), so detecting the wobble in the star’s location caused by an orbiting massive planet, and secondly by the transit method, detecting the dip in the light of a star caused by a transiting planet.

The Gaia team have just announced the first detections of exoplanet transits, by looking at the accumulated Gaia data on two already-known WASP planets.

ESA's Gaia satellite detects its first exoplanet transit

The plot shows a year’s worth of Gaia data of the star WASP-19, folded on the 0.79-day orbital period of the planet WASP-19b (the three different panels are the star’s magnitude in three different colours). The coverage is sparse — it is designed for astrometric measurements, not for recording lightcurves — but one observation was made in-transit, demonstrating that Gaia can indeed detect exoplanet transits.

The ESA/Gaia team have also looked at the data on WASP-98, and again detect the transit of WASP-98b.

ESA's Gaia satellite detects exoplanet transit of WASP-98b

Exoplanet cloudiness from transit lightcurves?

An interesting new paper by von Paris et al has explored the effect of the cloudiness of a planet on transit lightcurves. If a planet were cloudy on one limb, but clear on the other limb, then that could make the transit slightly asymmetric. The authors show that, in principle, this effect could be detectable with good-enough quality lightcurves.

An apparent shift in the transit:

Shifted transit

Would then lead to residuals, relative to a “perfect” transit, looking like:

traresids

The authors then claim a possible detection of such an effect in the hot Jupiter HAT-P-7b.

This might open up a new way of exploring the atmospheres of exoplanets. Whether this can ever be done reliably, however, is debatable. A big assumption in the authors’ simulations is that the star being transited is uniform. However, we know that stars are usually magnetically active and so are patchy. Star spots and bright patches on the star are likely to have a greater effect on the transit profile than the cloudiness of the planet’s atmosphere. Still, the effect is worth exploring, particularly for planets transiting magnetically quiet stars.