Category Archives: WASP project

A first planet for the Next Generation Transit Survey

The latest transit survey to announce their first planet is the Next Generation Transit Survey. While the planet NGTS-1b has a fairly normal mass and radius for a hot Jupiter, it is unusual in being found transiting an M0-type dwarf, a star of only 0.6 solar radii. Thus the planet is nearly a quarter as big as the star, in terms of radius, the highest planet-to-star ratio yet found.

NGTS is an array of twelve 20-cm telescopes sited at Cerro Paranal in Chile, and has been accumulating survey data since 2016.

Next-Generation Transit Survey

It is important to realise that the newer survey NGTS does not supersede WASP, but instead complements it, being designed to do a different task. WASP, and similar surveys such as HATnet and KELT, use camera lenses (typically 200-mm f/1.8 or 85-mm f/1.2) to survey large swathes of sky. The data is good enough to detect transits of Jupiter- and Saturn-sized planets, but not smaller ones.

NGTS was designed to find smaller planets, down to Neptune and possibly super-Earth size. To do that it uses bigger optics, being telescopes rather than camera lenses, with a much better plate scale (more CCD pixels per chunk of sky). This gives much better photometry, but at the price of a much smaller field of view. A smaller field of view means covering many fewer bright stars.

Indeed, NGTS has a field of view comparable to the Kepler field (1% of the sky), though since it will raster several fields it will add up to sky coverage comparable to that of the Kepler K2 mission phase.

Thus WASP, running with 200-mm lenses surveying much of the sky, finds Jupiters and Saturns transiting stars of typically V = 9 to 13. NGTS can find smaller planets, and is aimed at finding Neptunes, but will likely find them transiting fainter stars of typically V = 13 to 14 (and perhaps, as with K2, an occasional brighter one).

Meanwhile, WASP-South has recently been running with wider, 85-mm lenses, which cover the whole Southern sky and target stars of V = 6.5 to 11.5. Hence the two surveys are entirely complementary: WASP aiming for large, Jupiter-sized planets around very bright stars, while NGTS aims for Neptune-sized planets around much fainter stars.

The main competition for WASP is now KELT and MASCARA, whereas the main competition for NGTS is the ongoing K2 mission. Of course NASA’s forthcoming TESS mission, set for launch in 2018, should out-compete all of the ground-based surveys.

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WASP-South finds the smallest known star

In order to qualify as a proper “star” (as opposed to a brown dwarf or a stellar remnant) it needs to be producing nuclear fusion of hydrogen in its central regions. That requires a mass of at least 83 times that of Jupiter in order for gravity to compress the central regions sufficiently.

Interestingly, despite having masses far greater than any planet, the smallest possible stars are expected to have radii very similar to that of Saturn or Jupiter. That means that in a transit survey such as WASP they can look very like planets, it is only the radial-velocity follow-up observations that show that they a stellar mass.

Alexander von Boetticher et al have just announced the discovery of the smallest known star, which with a mass of 85 ± 4 Jupiters is right at the lower limit of what is possible. The mass and radius of the object, dubbed EBLM J0555−57Ab, are shown below compared to other stars and brown dwarfs, along with (red and blue lines) theoretical models for different ages.

The discovery of EBLM J0555 was a by-product of the WASP-South survey, found because of its planet-like size. In the graphic below the star is compared with Saturn and the slightly larger M-dwarf star TRAPPIST-1.

The discovery has been carried by many websites including The Atlantic, CNET, The Daily Mail, The Smithsonian, Astronomy Magazine, gizmodo, astrobites, phys.org and over 30 others.

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.

WASP’s “Super Saturn” feature for kids

The possible discovery of an exoplanet ring system, a “Super Saturn”, has featured in a Frontiers for Young Minds article aimed at scientists aged 8 to 15 years.

The suggestion of an exoplanetary ring system was an interpretation of the multiple dips in the lightcurve of a star, catalogued as 1SWASP J140747.93–394542.6, as observed in WASP-South data in 2007. The article gives a good introduction to the WASP project at an accessible level, complete with this image of the “Super Saturn”:

Illustration of the "Super Saturn" found in WASP data.

Congratulations to KELT-South on KELT-10b

KELT-South is a competitor to WASP-South, and indeed sits near WASP-South on the same plateau at SAAO’s Sutherland observatory site, performing a similar transit search.

KELT-South have just announced their first discovery, KELT-10b, a highly inflated planet that is larger than Jupiter (at 1.4 Jupiter radii) but less massive than Saturn (at 0.68 Jupiter masses). With WASP-South, HATSouth, KELT-South and the imminent NGTS, there are now four ground-based transit searches discovering planets in the Southern skies.

One advantage of the competition is that we can “reverse engineer” other teams’ planets to improve our own procedures. Indeed, trying to work out why we missed HAT and KELT planets has previously revealed bugs in our software.

Since we cover millions of stars the only way to look for transits is by automated search routines, but these throw up so many “false positive” detections that in the end we have to select candidates by eye. Humans are fallible, and it seems we simply overlooked KELT-10b. We have only relatively sparse data on it, fewer than 5000 photometric points, but, still, the presence of a transit dip at the correct period seems obvious enough once one knows that it is there. Here are the WASP-South data folded on the transit period, and the periodogram analysis revealing the periodic dip:

WASP data on KELT-10b

Congratulations to the KELT-South team on getting there first and on a fine discovery!

15-yr-old work-experience schoolboy discovers a new planet

Press release:

A 15-yr-old schoolboy has discovered a new planet orbiting a star 1000 light years away in our galaxy. Tom Wagg was doing work-experience at Keele University when he spotted the planet by finding a tiny dip in the light of a star as a planet passed in front of it.

“I’m hugely excited to have found a new planet, and I’m very impressed that we can find them so far away”, says Tom, now aged 17. It has taken two years of further observations to prove that Tom’s discovery really is a planet.

Tom found the planet by looking at data collected by the WASP project, which surveys the night skies monitoring millions of stars to look for the tell-tale tiny dips (transits) caused by planets passing in front of their host star.

Tom’s planet has been given the catalogue number WASP-142b, being the 142nd discovery by the WASP collaboration. It is in the Southern constellation of Hydra. While astronomers worldwide have now found over 1000 extra-solar planets, Tom is possibly the youngest ever to have done so.

“The WASP software was impressive, enabling me to search through hundreds of different stars, looking for ones that have a planet”, says Tom. The planet is the same size as Jupiter, but orbits its star in only 2 days. With such a short orbital period the transits occur frequently, making such planets much easier to find.

While the planet is much too far away to see directly, an artist’s impression shows how it might look. The hemisphere facing the star is hot, blasted by the irradiation from the star, while the other hemisphere is much cooler.

Tom Wagg at Keele Observatory 3.

Tom Wagg at Keele Observatory    (Click for high-res version; 3MB)

Tom, a pupil at Newcastle-under-Lyme School who has always been keen on science, asked for the work-experience week after learning that Keele University had a research group studying extra-solar planets.

“Tom is keen to learn about science, so it was easy to train him to look for planets”, says Professor Coel Hellier, who leads the WASP project at Keele. Tom has since achieved 12 GCSEs, all at A*, and wants to study physics at university.

The planet is one of a class of “hot Jupiter” planets, which — unlike the planets in our own Solar System — have very tight orbits close to their stars. They are thought to have migrated inwards through interactions with another planet. Thus it is likely that Tom’s planet is not the only planet orbiting that star.

Artist's impression of Tom's planet, WASP-142b, orbiting its star, WASP-142. The  planet is depicted as seen from a hypothetical moon. A second, dimmer star is seen in the background. Being 1000 light years away, the planet is too distant to  obtain a direct image.

An artist’s impression of the planet WASP-142b, depicted as seen from a hypothetical moon.
(Credit: David A. Hardy. http://www.astroart.org/)    Click for high-res version (1.5MB)

For more information email waspplanets@gmail.com

Updates: Coverage on about 300 news websites worldwide, including: BBC News, ITV News, CNN, TIME, Salon, Yahoo News, The Huffington Post, The Washington Post, Toronto Star, The Telegraph, The Guardian, The Independent, Der Spiegel, News Deutschland, India Today, IOL South Africa, France, Chile, Australia, Mexico, China, and Russia.

[Err, Wow!, successful press release! Now on about 650 news websites worldwide, about 205 in English, 59 in German, 48 in Spanish, 106 in Russian, 30 in Chinese, 23 in French, 20 in Italian, 40 in Turkish, 26 in Portuguese, 23 in Indonesian, 14 in Greek, 12 in Bulgarian, 11 in Hungarian, 8 in Polish, 4 in Slovakian, and others including Tamil, Vietnamese, Thai and Malayalam!]

RAS Gold Medal for Professor Michel Mayor

The Royal Astronomical Society has announced the award of a Gold Medal to Professor Michel Mayor of the University of Geneva. Prof. Mayor was, of course, the co-discoverer of the first extrasolar planet around a solar-like star, with the detection of 51 Pegasi b back in 1995. His Observatoire de Genève group developed a succession of planet-finding spectrographs that have led the way to the discovery of many hundreds of extrasolar planets.

Michel Mayor

Prof. Mayor has been an important collaborator for the WASP project, through the CORALIE spectrograph on the 1.2-m Swiss/Euler telescope at La Silla. The CORALIE spectrograph observes all WASP-South planet candidates, and the detection of the radial-velocity signature of a planet — in about 1 in 8 such candidates — is the crucial step that confirms a new planet discovery. Thus Prof. Mayor was a co-author on many of the early WASP planet papers until his retirement.

The WASP project is hugely indebted to Prof. Mayor and is honoured to have collaborated with him on WASP planet discovery. We congratulate him on the well-deserved award of the RAS Gold Medal.