Category Archives: TESS

WASP-18 is observed by TESS

The TESS mission will survey the entire sky for new transiting exoplanets, and as a by-product will produce space-quality lightcurves of all the WASP exoplanet systems. The first such paper has just appeared on arXiv, where Avi Shporer et al report on the TESS lightcurve of WASP-18.

WASP-18b is the most massive planet found by WASP, a 12-Jupiter-mass planet in a very tight orbit lasting only 0.94 days. This means it has the strongest planet–star tidal interaction of any known planetary system, such that the planet’s gravity gives rise to large tidal bulges on the host star. Here are the TESS data folded on the orbital cycle:

The out-of-transit data are clearly not flat (shown on a larger scale in the middle panel), and show the “ellipsoidal modulation” caused by the tidal bulges on the star. The heated face of the planet is also eclipsed by the star at phase 0.5, producing a secondary eclipse.

By analysing the lightcurve the authors conclude that very little heat is being redistributed from the heated face of the planet. Strong winds could carry heat to the un-irradiated cooler hemisphere, but there is little sign of this in the data.

So far the results of the analysis are in line with theoretical expectations, though the work points to the potential for similar analyses of other previously-known exoplanet systems.

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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.