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A circumbinary planet transiting a WASP binary star

Circumbinary planets are planets that orbit a binary star, rather than a single star, in contrast to most planets known. Possibly they formed out of the same disk of material that formed the two stars. So long as the planet is sufficiently far from the binary, its orbit can be stable.

Several circumbinary planets were discovered by the Kepler mission, but the first such planet found using TESS has recently been announced by Veselin Kostov et al.

Here is the TESS lightcurve of “target of interest” TOI-1338:

Six-percent-deep eclipses recur every 14.6 days, when a small, M-dwarf star eclipses its Sun-like companion. But, when one looks more closely at portions of the lightcurve, one also sees:

Near Day 1483 is a deep eclipse (truncated in this plot). Near Day 1577 is a shallower secondary eclipse, when the Sun-like star eclipses the M-dwarf star. But, in addition, and highlighted by green bands, are two transits where a Saturn-like planet in a wide, 95-day orbit, transits the Sun-like star.

The interest, from the WASP point of view, is that the eclipsing binary had already been found by the WASP survey. The primary eclipses were too deep to be caused by a Jupiter-size planet, and were instead found to be caused by the M-dwarf, but such systems are interesting in their own right, and thus formed the “EBLM” programme offshoot from WASP, led by Amaury Triaud.

Since the eclipsing binary was already known to be an interesting system, this meant it could be observed with 2-min cadence in the TESS survey (instead of the default 30-min cadence data), and that made the detection of the circumbinary-planet transits much easier.

Above is an illustration of how the planet’s orbit precesses around the inner binary over time. The inner binary is illustrated in black and grey. The coloured dots show how the orbit of the planet changes over 2000 days. When the planet passes within the grey ellipses, transits can be seen.

Solar System planet tilts

Planetary scientist James O’Donoghue has put together a neat animation of the tilts and spin rates of the Solar System planets. Click on his Tweet to see the animation, or see the the higher-resolution version here.

The animation shows graphically that the simplistic idea that planets form in an orderly fashion from a proto-planetary disc cannot be entirely right. Uranus is tilted over; Venus spins backwards, rather slowly. Why? There must have been collisions, near collisions, and planets perturbing the orbits of other planets early on in our Solar System’s history. Thus we expect that the same thing will have occurred in the exoplanetary systems that we are now finding.

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

The first transiting exo-comets?

Transits of extra-solar planets are pretty routine these days, but planets are not the only bodies expected to be orbiting nearby stars. How about exo-comets? Unlike planets, comets are fuzzy and changeable, so exocomet transits would vary in shape and depth. A team led by Saul Rappaport have now searched the entire archive of Kepler lightcurves looking for dips that could be exocomet transits. Here’s one in the data for the star KIC 11084727:

The authors reproduce such dips (red line) with a model of a comet about the size of Halley’s comet and having a tail made of dust, hence giving an asymmetric dip. Another Kepler, KIC 3542116, shows six possible comet transits. Here are three:

WASP-20 is a binary star

The host star of the hot Jupiter WASP-20b has been found to be a binary star. A new paper by Daniel Evans et al finds WASP-20 to be a binary separated by 0.26 arcsecs, sufficiently close that the second star had not previously been noticed. Evans et al used the SPHERE instrument on ESO’s Very Large Telescope to find the slightly dimmer companion:

WASP-20 is a binary star

The two stars, WASP-20 A and WASP-20 B seem to be gravitationally bound, and the planet appears to orbit the brighter star. The companion star is 61 astronomical units from the planet-hosting star, close enough that it might have had a gravitational effect on the orbit of the planet.

This is relevant since hot Jupiters are thought to have been created much further from their star than their current close-in orbits, and gravitational perturbations from a third body is one suggested mechanism for causing them to migrate inwards.

Radial velocities of the Sun as an exoplanet host star

The main way of measuring the mass of an extra-solar planet is to record the motion of the host star, caused by the gravitational tug of the planet as it orbits. One can do that by measuring the Doppler shift (radial velocity or RV) of the spectrum of the host star.

However, as a planet gets smaller or further from its star, the tug gets smaller, and so the radial-velocity signal decreases. At some point it gets smaller than the intrinsic variations in spectral lines caused by the magnetic activity of the star. Whether one can account for this will limit our ability to prove the existence of small planets in wide orbits.

Radial velocity of the Sun, bounced off the asteroid Vesta

A team lead by Raphaëlle Haywood, of the University of St. Andrews, and now at Harvard, had the idea of treating our own Sun as a star, by looking at the RV signal in sunlight bounced off the asteroid Vesta. They could then compare the RV signal to images of the magnetic activity on our Sun.

Magnetic activity on the Sun.

Magnetic activity across the Sun’s disc

The spectral lines from each region of the Sun’s disc will depend on the local magnetic activity, but the RV measurement bounced off Vesta would be from light averaged over the whole disc of the Sun, just as we’d record from a star.

The results are shown in the plot below. The top panel shows the variations in the measured RV signal, in metres per second. The second panel shows the magnetic flux aggregated across the Sun’s disc, in Gauss. The third panel shows the fraction of the Sun’s disc filled by magnetic activity (Sun spots).

Radial velocity variations of our Sun

Thus a Sun-like star can show intrinsic RV variability at a level of metres per second, and this will cause a problem for detecting the small RV signals of low-mass planets in wide orbits. For example our Earth produces motion in our Sun of only 0.1 metre per second. Unless there are stars much less magnetically active than our Sun, it is going to be hard to obtain an accuracy sufficient to detect the RV signal of an Earth-like planet in an Earth-like orbit.

The authors note, though, a strong correlation between the RV signal and the total magnetic activity. Thus it might be possible to decorrelate against magnetic activity to provide a way of correcting RV signals for this effect, and so dig out smaller signals caused by planets.

The proposed Twinkle mission to study exoplanet atmospheres

With over a hundred gas-giant exoplanets now known transiting relatively bright stars, thanks to WASP and other projects, scientific attention is being directed to characterising their atmospheres.

The proposed Twinkle spacecraft (Twinkle/Surrey Satellite Technology Ltd)

The proposed Twinkle spacecraft (Twinkle/Surrey Satellite Technology Ltd)

Twinkle is a new proposed satellite, led by a team from University College London, that would be dedicated to studying exoplanet atmospheres. The aim is for a relatively cheap and quick mission, but with a high scientific return.

Twinkle aims to analyse the atmospheres of 100 exoplanets using an infrared spectrograph. By comparing the spectra observed in and out of transit, and spacecraft will detect signatures of molecules in the transiting planet atmospheres.

The project needs £50 million to succeed. WASP planets would be prime targets for Twinkle, and so we hope that Twinkle gets funded and wish it every success.

The Twinkle team invite expressions of support at the Twinkle website.