Category Archives: WASP project

WASP-South detection of transits of HD 219666b

Transiting a bright star, the “Neptune desert” planet HD 219666b was one of the more important early discoveries from the TESS survey. With a depth of only 0.17 per cent, the transits would be a challenge for any ground-based transit survey.

Nevertheless, we think we’ve found them in WASP-South lightcurves dating back to 2010. Here they are:

The orange lines show times of transit, as found by the WASP transit-detection algorithms. The shallow dips seem to be real, since they align both in period and in phase with the dips seen in the TESS lightcurve. The output from the WASP search algorithm is not itself that convincing:

However, the period that it finds (6.03446 days) matches the TESS period to an accuracy of 0.03 per cent, and the WASP ephemeris then predicts the times of the TESS transits bang on (they occur at 420.99999 cycles on the WASP ephemeris), which together mean that the detection must be real. Here are the WASP data folded on a template of the TESS transit:

With a depth of 0.17 per cent, the transits of HD 219666b are the shallowest that WASP has detected.

The benefit of looking for such pre-detections of TESS planets is that we can then produce a transit ephemeris based on data spanning a baseline of 8 years, rather than the 20 days spanned by the TESS transits. This means we can predict future transits to an accuracy of minutes, instead of hours, which is highly useful for future observations. Hence this WASP-South detection of HD 219666b transits is well worth an AAS Research Note.

Nobel Prize for Michel Mayor and Didier Queloz


Naturally, we at WASP are chuffed at the award of the Nobel Prize in Physics to Michel Mayor and Didier Queloz for their discovery of 51 Peg b, the first extra-solar planet found orbiting a sun-like star. Congratulations to them both! The award is a welcome boost and recognition of the burgeoning field of exoplanets.

The WASP-South camera array

When, back in 2006, we built WASP-South in South Africa and started our Southern survey for transiting exoplanets, the Geneva Observatory team led by Mayor and Queloz were the obvious collaborators, given their world-leading track-record in the radial-velocity discovery of exoplanets, and their Euler telescope with its CORALIE spectrograph, situated at La Silla in Chile.

Prof Queloz, and his then-students Michaël Gillon and Amaury Triaud, started the radial-velocity observations of WASP-South transit candidates. Since many transit candidates turn out to be transit mimics, both the transit data and RV data are necessary to prove the discovery of a planet. That collaboration still continues, and has involved the Geneva Observatory team observing 1500 WASP candidates over many hundreds of clear nights with Euler/CORALIE.

Euler telescope

The Euler 1.2-m telescope

The result has been the discovery of over 150 exoplanets transiting bright stars, and many of them are among the most valuable exoplanets for further observation and study. So far the collaboration between Prof Queloz’s team and WASP-South has led to over 100 refereed papers in leading journals, that have so far been cited over 5000 times.

Announcing WASP-128b, a transiting brown dwarf

Brown dwarfs are intermediate between planets and stars. They are not massive enough to undergo hydrogen fusion in their cores, as required to be a star, but are too massive to be planets, and can fuse deuterium. Those conditions produce a range from about 13 Jupiter masses to about 80. Some people, however, argue that the distinction between a planet and a brown dwarf should not be about their mass, but about whether they formed in a star-like way, by gravitational collapse, or in a planet-like way, by accumulation of planetesimals in a proto-stellar disc.

Comparative sizes. Credit: NASA Goddard Space Flight Center

WASP was designed to look for transiting Jupiter-sized planets, but brown-dwarf stars are much the same size as Jupiter and so produce planet-like transits. That means we only discover which is which by measuring the mass of the transiting body by radial-velocity techniques.

So we should find brown dwarfs as readily as planets. But we’ve found only two, WASP-30b and now WASP-128b, compared to over 150 planets. That means that closely orbiting brown dwarfs must be much rarer than planets. It seems that star-like, gravitational-collapse formation rarely produces objects with a mass as low as 30 to 50 Jupiters (that’s not enough mass to collapse easily), while planet-like accumulation of planetesimals rarely builds up to mass that high (there aren’t enough planetesimals).

Masses and radii of known brown dwarfs. WASP-128b is the object with a mass of 37 Jupiters, while WASP-30b has a mass of 61 Jupiters. The coloured regions denote theoretical models for the mass–radius relation at different ages.

Which means that WASP-128b, newly announced on arXiv today in a paper by Vedad Hodžić etal, is a very rare object, being a brown dwarf with a mass of 37 Jupiters in a 2-day orbit around a G-type star. The nearest comparable object is KOI-205b, at 40 Jupiter masses, though that transits a star that is 2 magnitudes fainter and so is harder to study.

TRAPPIST-North joins the team

The robotic photometer TRAPPIST-South (best known for the discovery of the TRAPPIST-1 planetary system) has long been a part of the WASP-South discovery process, along with WASP-South itself and the Euler/CORALIE spectrograph.

Khalid Barkaoui, lead author of the WASP-161, WASP-163 and WASP-170 discovery paper, alongside TRAPPIST-North.

A new paper announcing WASP-161b, WASP-163b and WASP-170b now marks the first contributions to WASP discovery from TRAPPIST-North. Situated in Morocco, TRAPPIST-North is also a robotic 0.6-m photometric telescope, similar to the TRAPPIST-South in Chile.

Transit lightcurves of WASP-161b from TRAPPIST-North, TRAPPIST-South and the SPECULOOS Europa telescope.

TRAPPIST-North, at the Oukaïmden Observatory in the Atlas Mountains of Morocco

HATSouth announce HATS-59 b and c

Our competitor transit survey HATSouth have just announced the discovery of planets HATS-59 b and c.

HATS-59b is a hot Jupiter producing a typical hot-Jupiter transit, as seen in the HATSouth data:

But what makes it interesting is the presence of an outer companion planet, HATS-59c, on a much wider orbit of 1422 days. This has implications for understanding planetary systems that host hot Jupiters, casting light on the question of whether the gravitational perturbations of outer planets move the hot Jupiters into their close-in orbits.

As usual, we “reverse engineer” planets discovered by our competitors as a check on our own methods. One would expect we’d struggle to see the transit of HATS-59b, after all the host star has a magnitude of V = 14, which is faint for us (we struggle at anything below V = 13).

HATSouth uses bigger optics than WASP-South, aiming to thus get better photometry, but that has the penalty that larger optics produce smaller fields of view which then contain fewer bright stars. So larger-optic surveys such as HATSouth and the similar NGTS typically find planets around stars that are fainter than typical WASP or KELT planet hosts.

Nevertheless, this is what our search routines produce for HATS-59b (from 37,000 observations with WASP-South):

Not very impressive is it? The big scatter in data points comes from the star being faint for the WASP lenses. But the search routines have run and tried to find a recurrent transit and have picked out a best period of 5.41595 days. That compares with the true value, from the HATSouth paper, of 5.41608(2) days. That matches to 99.998% accuracy, which tells us that our detection of the HATSouth planet is real! Though of course it is far too marginal for us to have ever adopted this star as a candidate.

One reason we’re looking at this is that it shows that WASP data should be able to add value to TESS observations, finding extra transits from our multiple years of coverage, even when the dips are too marginal for us to have pursued them.

NASA launches satellite ‘TESS’ in hunt for exoplanets

With the TESS launch scheduled for this very day, I wrote the following popular-level piece for The Conversation (which has also been re-published by the BBC Focus Magazine).

Previous generations have looked up at the stars in the night sky and wondered whether they are also orbited by planets. Our generation is the first to find out the answer. We now know that nearly all stars have planets around them, and as our technology improves we keep finding more. NASA’s newest satellite, TESS (the Transiting Exoplanet Survey Satellite), scheduled for launch on April 16, 2018, will extend the hunt for small, rocky planets around nearby, bright stars.

NASA’s TESS planet hunter (artist’s impression)

We want to know how big such planets are, what kind of orbits they have and how they formed and evolved. Do they have atmospheres, are they clear or cloudy, and what are they made of? Over the coming decades, we will find Earth-like planets at the right distance from their star for water to be liquid. It’s conceivable that one will have an atmosphere containing molecules such as free oxygen that indicate biological activity. TESS is a major step towards this long-term goal.

Planets are so faint and tiny compared to their host stars that it is remarkable we can detect them at all, let alone study their atmospheres. Yet planets can, from our viewpoint, appear to travel or “transit” across the face of their star as they orbit, blocking a small fraction of the star’s light. TESS will monitor 200,000 bright stars in the solar neighbourhood, looking for tiny dips in their brightness that reveal a transiting planet.

To understand the atmospheres of exoplanets, we have to examine how they interact with starlight. As a planet transits across a star, the thin smear of its atmosphere is backlit by starlight. Some wavelengths of the starlight will be absorbed by molecules in the atmosphere while other wavelengths will shine straight through. So looking at which wavelengths reach us and which don’t can reveal what the atmosphere is made of.

The spectrum of starlight passing through a planet’s atmosphere can tell us what the atmosphere is made of. Credit: Christine Daniloff/MIT, Julien de Wit

Such observations are right at the limit of current capabilities, requiring the James Webb Space Telescope (JWST), the $8 billion successor to Hubble scheduled for launch in 2020. With a 6.5-metre-wide mirror, collecting much more light than Hubble ever could, and with specially designed instruments, JWST has been built to study exoplanet atmospheres.

In order to use JWST most effectively, we first need to know which stars host the best transiting exoplanets to study, and that’s why we need TESS. Its predecessor spacecraft, Kepler, surveyed 150,000 stars in a patch of sky near the constellation Cygnus, and found over a thousand planets ranging from gaseous giants like Jupiter to rocky planets as small as Mercury. But Kepler covered only a small patch of sky containing few stars bright enough for us to study their planets.

In contrast, ground-based telescopes have searched wider swathes of the sky looking at many more brighter stars for transiting exoplanets. The most successful has been the UK-led Wide Angle Search for Planets (WASP) project, of which I am a member. Using an array of camera lenses, WASP has spent the last decade monitoring a million stars every clear night looking for transit dips, and has found nearly 200 exoplanets, some of which have now been chosen as targets for JWST.

But ground-based transit surveys have one big limitation: they look through Earth’s atmosphere and that severely limits the data quality. They can detect brightness dips as small as 1%, which is sufficient to find giant gaseous planets that are like our own Jupiter and Saturn. But smaller, rocky planets block out far less light. Our Earth would produce a dip of only 0.01% if seen projected against our sun.

The JWST is currently being readied for launch. Credit: NASA/Chris Gunn

TESS will combine the best of both these approaches, observing bright stars over the whole sky with the advantage of doing so from space. It should find the small, rocky planets that Kepler proved are abundant but find them orbiting stars that are bright enough for us to study their atmospheres with JWST.

TESS will typically observe each region of sky for 30 days. This means that it will detect planets that don’t take long to orbit their stars and so will produce several transits while TESS is looking at them. Planets with short orbits are located close to their stars, meaning that most planets TESS finds will be too hot for liquid water. But planets orbiting dimmer, cooler red dwarf stars might be at the right temperature for life even if they are so close. The dwarf star TRAPPIST-1 is 1,000 times dimmer than our sun, and is known to host seven closely orbiting planets.

While TESS looks for planets orbiting dwarf stars from space, the SPECULOOS survey will be looking at even smaller and dimmer stars from the ground. Any planets it finds will be prime targets for JWST.

This exploration is a step towards finding rocky planets in the habitable zone of stars like our sun. In 2026, The European Space Agency is expected to launch PLATO, a satellite with the potential to discover rocky planets in Earth-like orbits with periods of a year. The race will then begin to find biomarker molecules, such as free oxygen, in the atmosphere of an Earth-like exoplanet.

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.