Tag Archives: TRAPPIST

WASP-121b: another planet close to tidal destruction

WASP is particularly good at finding hot-Jupiter planets in ultra-short orbits of order 1 day, since such planets produce lots and lots of transits. WASP-121b is the latest WASP-South discovery, with an orbital period of only 1.2 days and a bloated radius of 1.9 Jupiter-radii.

Being so large and so near to its host star, the planet is close to being destroyed by tidal forces. Indeed, tides will be causing the planet’s orbit to decay, and the planet will be spiralling inwards to destruction on a time-scale of maybe only a few million years, short by astrophysical standards.

The planet is also orbiting a hot F-type star, with a surface temperature of 6500 K. This means that the side of the planet facing the star will be among the most irradiated known. This raises the possibility to detecting the heat of the planet, by watching for the occultation when it passes behind its star, half an orbit away from the transit.


Laetitia Delrez, of the University of Liège, who leads the WASP-121b discovery paper, has used the TRAPPIST robotic telescope to look for the occultation. On seven occasions the TRAPPIST team observed the star over the expected phases, using a far-red z’-band filter to increase sensitivity to thermal radiation. They then added the lightcurves together:

WASP-121b occultation

And there it is, a dip of only 6 parts in 10,000, an impressive detection for a small 0.60-m telescope, but revealing the heat of the planet and showing that it is heated to 2400 K by the stellar irradiation.

The ready detectibility of the planet’s occultation, coupled with the fact that the host star is relatively bright star at V = 10.4, mean that WASP-121b will be a prime target for studying the make-up of its atmosphere.

WASP-94: “cousin” planets around twin stars

Press release regarding our discovery of planets around binary stars WASP-94A and WASP-94B:

European astronomers have found two new Jupiter-sized extra-solar planets, each orbiting one star of a binary-star system. Most known extra-solar planets orbit stars that are alone, like our Sun. Yet many stars are part of binary systems, twin stars formed from the same gas cloud. Now, for the first time, two stars of a binary system are both found to host a “hot Jupiter” exoplanet.

The discoveries, around the stars WASP-94A and WASP-94B, were made by a team of British, Swiss and Belgian astronomers. The British WASP-South survey, operated by Keele University, found tiny dips in the light of WASP-94A, suggesting that a Jupiter-like planet was transiting the star; Swiss astronomers then showed the existence of planets around both WASP-94A and then its twin WASP-94B.

Marion Neveu-VanMalle (Geneva Observatory), who wrote the announcement paper, explains: “We observed the other star by accident, and then found a planet around that one also!”.

Hot Jupiter planets are much closer to their stars than our own Jupiter, with a “year” lasting only a few days. They are rare, so it would be unlikely to find two Hot Jupiters in the same star system by chance. Perhaps WASP-94 has just the right conditions for producing Hot Jupiters? If so WASP-94 could be an important system for understanding why Hot Jupiters are so close to the star they orbit.

The existence of huge, Jupiter-size planets so near to their stars is a long-standing puzzle, since they cannot form near to the star where it is far too hot. They must form much further out, where it is cool enough for ices to freeze out of the proto-planetary disk circling the young star, hence forming the core of a new planet. Something must then move the planet into a close orbit, and one likely mechanism is an interaction with another planet or star. Finding Hot-Jupiter planets in two stars of a binary pair might allow us to study the processes that move the planets inward.

Professor Coel Hellier, of Keele University, remarks: “WASP-94 could turn into one of the most important discoveries from WASP-South. The two stars are relatively bright, making it easy to study their planets, so WASP-94 could be used to discover the compositions of the atmospheres of exoplanets”.

The WASP survey is the world’s most successful search for hot-Jupiter planets that pass in front of (transit) their star. The WASP-South survey instrument scans the sky every clear night, searching hundreds of thousands of stars for transits. The Belgian team selects the best WASP candidates by obtaining high-quality data of transit lightcurves. Geneva Observatory astronomers then show that the transiting body is a planet by measuring its mass, which they do by detecting the planet’s gravitational tug on the host star.

The collaboration has now found over 100 hot-Jupiter planets, many of them around relatively bright stars that are easy to study, leading to strong interest in WASP planets from astronomers worldwide.

 An illustration of a planet orbiting one star of a binary system.  In WASP-94, the planet would transit the brighter star, causing a dip in the light that can be detected from Earth.  Another planet orbits the second star at lower-left. It does not transit and is not directly visible, but it can be detected by its gravitational tug on the second star.

An illustration of a planet orbiting one star of a binary system. In WASP-94, the planet would transit the brighter star, causing a dip in the light that can be detected from Earth. Another planet orbits the second star at lower-left. It does not transit and is not directly visible, but it can be detected by its gravitational tug on the second star. [ Image Credit: ESO/L. Calçada/Nick Risinger]

Update: This press release has resulted in articles in phys.org, sciencedaily.com, world-science.net, yahoo.news, Science World Report, and breakingnews.ie.

Three more planets from WASP-South, Euler and TRAPPIST

The latest three WASP-South planets appeared on the preprint server arXiv today, announcing WASP-68b, WASP-73b and WASP-88b.

TRAPPIST by night

The paper is led by the University of Liège, who operate the TRAPPIST robotic 0.6-m photometer, sited at La Silla in Chile. TRAPPIST plays a crucial role in WASP-South planet discovery. Since the WASP photometry, from 200mm, f/1.8 lenses, is relatively crude, a TRAPPIST lightcurve of a candidate gives a better idea of whether the candidate is worth pursuing. Further, the large pixels of WASP data mean that apparent transits are often caused by deeply eclipsing fainter stars within the WASP photometry aperture. TRAPPIST photometry shows up such blends, and thus avoids wasting valuable radial-velocity observations.

Blend image

As an example, the candidate 1SWASPJ113725.66–261925.6 showed a shallow 1% dip recurring with a period of 1.33 days, possibly caused by a planet transit. However, close by are two fainter stars (left, the yellow circle is the extraction aperture used for the WASP photometry; WASP pixels are much bigger than in this Sky Survey image, and so the candidate and the two close stars are merged).


TRAPPIST is able to resolve the stars, and followup photometry showed that the dip is caused by the closest faint star, which is an eclipsing binary with a 20% deep eclipse (the image is reversed left–right). In photometry centered on the bright candidate this eclipse is diluted to a shallow planet-like dip.