Author Archives: waspplanets

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.


Nature Research Highlight on WASP-104b

Our recent paper on WASP-104b has been picked up as a Research Highlight by Nature. We reproduce it here:

The Kepler space telescope, shown here in an artist’s rendering, helped detect a carbon-black planet. Credit: NASA

“Astronomers led by Teo Močnik at Keele University, UK, used NASA’s Kepler telescope to study a star called WASP-104, which lies roughly 144 parsecs from Earth in the constellation Leo. Earlier observations had documented a dimming of WASP-104’s light every 1.76 days, indicating that a planet was regularly crossing the star’s face. But Močnik’s team could not detect starlight reflecting off the planet, as scientists usually expect after discovering a new world. That led the researchers to conclude that the planet is nearly pure black in colour. The planet’s darkness could help scientists to test their ideas about exoplanetary atmospheres, such as how clouds might form on a world that reflects so little light.”

The paper, published in Astronomical Journal, is here.

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

NASA’s Webb Space Telescope to Inspect Atmospheres of Gas Giant Exoplanets

NASA have written a publicity page on JWST’s plans to study the atmospheres of gas-giant exoplanets, including an animation on how this is done. Since the prime targets for the “Early Release Science” program are three WASP-discovered planets, WASP-18b, WASP-43b and WASP-79b, we “re-blog” the piece here:

“In April 2018, NASA launched the Transiting Exoplanet Survey Satellite (TESS). Its main goal is to locate Earth-sized planets and larger “super-Earths” orbiting nearby stars for further study. One of the most powerful tools that will examine the atmospheres of some planets that TESS discovers will be NASA’s James Webb Space Telescope. Since observing small exoplanets with thin atmospheres like Earth will be challenging for Webb, astronomers will target easier, gas giant exoplanets first.”

Read the full piece here.

Metals and water in the atmosphere of WASP-127b

Puffed-up, low-mass planet WASP-127b is one of the best targets for atmospheric characterisation. Recently Chen etal have announced the results of observations taken with the La Palma telescopes the Gran Telescopio Canarias and the Nordic Optical Telescope.

The result is one of the most detailed and featured spectra of an exoplanet yet.

The spectrum clearly shows the metals sodium (Na), lithium and potassium (K), along with features due to water and haze. A press release from the University of Cambridge has led to coverage of the paper on several dozen websites.

The Instituto de Astrofisica de Canarias have also produced this artist’s depiction of WASP-127b and its host star:

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.

WASP-96b: an exoplanet free of clouds

Press Release: Scientists have detected an exoplanet atmosphere that is free of clouds, marking a pivotal breakthrough in the quest for greater understanding of the planets beyond our solar system. (Link to Nature paper)

Figure 1 | Exoplanets in orbits close to the line of sight for us on Earth periodically pass in front (transit) and behind (secondary eclipse) of their host stars. Transits and eclipses are a powerful indirect way to study the composition of exoplanet atmospheres. Image credit: N. Nikolov

An international team of astronomers, led by Dr Nikolay Nikolov from the University of Exeter, have found that the atmosphere of the ‘hot Saturn’ WASP-96b is cloud-free. Using Europe’s 8.2m Very Large Telescope in Chile, the team studied the atmosphere of WASP-96b when the planet passed in front of (“transited”) its host-star (Figure 1). This enabled the team to see the starlight shining through the planet’s atmosphere, and so determine its composition.

Just as an individual’s fingerprints are unique, atoms and molecules have a unique spectral characteristic that can be used to detect their presence in celestial objects. The spectrum of WASP-96b shows the complete fingerprint of sodium, which can only be observed for an atmosphere free of clouds (Figure 2). The result appears today in the prestigious research journal Nature.

Figure 2 | Sodium fingerprint in an exoplanet spectrum. Shown is the absorption due to sodium at each wavelength. More absorption means that we are looking higher up in the atmosphere, and the vertical axis therefore a measure of altitude in the atmosphere of the planet. An atmosphere free of clouds produces an intact sodium fingerprint (left panel). A cloud deck blocks part of the sodium in the atmosphere, partially removing its spectral signature (right panel). Image credit: N. Nikolov/E. de Mooij

“We’ve been looking at over twenty exoplanet transit spectra. WASP-96b is the only exoplanet that appears to be entirely cloud-free and shows such a clear sodium signature, making the planet a benchmark for characterization”, explains lead investigator Nikolay Nikolov from the University of Exeter in the United Kingdom.

WASP-96b was discovered recently by a Keele University team led by Professor Coel Hellier. It is the 96th planet announced by the Wide Angle Search for Planets. WASP-96b is a gas giant similar to Saturn in mass and exceeding the size of Jupiter by 20%. The planet periodically transits a sun-like star 980 light years away in the southern constellation Phoenix.

It has long been predicted that sodium exists in the atmospheres of hot gas-giant exoplanets, and in a cloud-free atmosphere it would produce spectra that are similar in shape to the profile of a camping tent.

“Until now, sodium was revealed either as a very narrow peak or found to be completely missing”, continues Nikolay Nikolov. “This is because the characteristic ‘tent-shaped’ profile can only be produced deep in the atmosphere of the planet and for most planets clouds appear to get in the way”.

“It is difficult to predict which of these hot atmospheres will have thick clouds. By seeing the full range of possible atmospheres, from very cloudy to nearly cloud-free like WASP-96b, we’ll gain a better understanding of what these clouds are made of”, explains Prof. Jonathan J. Fortney, study co-author, based at the Other Worlds Laboratory (OWL) at the University of California, Santa Cruz (UCSC).

The sodium signature seen in WASP-96b suggests an atmosphere free of clouds (Figure 3). The observation allowed the team to measure how abundant sodium is in the atmosphere of the planet, finding levels similar to those found in our own Solar System.

Figure 3 | An artist rendition of ‘hot Saturn’ WASP-96b. A distant observer would see WASP-96b blueish in colour, because sodium would absorb the yellow-orange light from the planet’s full spectrum. Image credit: Engine House

“WASP-96b will also provide us with a unique opportunity to determine the abundances of other molecules, such as water, carbon monoxide and carbon dioxide with future observations “, adds co-author Ernst de Mooij from Dublin City University.

Sodium is the seventh most common element in the Universe. On Earth, sodium compounds such as salt give sea water its salty taste and give the white colour of salt pans in deserts. In animal life, sodium is known to regulate heart activity and metabolism. Sodium is also used in technology, e.g. in the sodium-vapour street lights, where it produces yellow-orange light.

The team aims to look at the signature of other atmospheric species, such as water, carbon monoxide and carbon dioxide with the Hubble and James Webb Space Telescopes as well as telescopes on the ground.

Update: The story has been covered on over 50 websites, including Newsweek, Astronomy Magazine, the International Business Times, the Irish Times and others.