The thermal phase curve of WASP-103b

I was wondering about writing a post on Laura Kreidberg’s new paper on WASP-103b when I noticed that she’d already done it. So let’s just borrow that:

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Hubble detects helium in the atmosphere of an exoplanet for the first time

Press Release (in coordination with Nature and Hubble/ESA): Astronomers using the NASA/ESA Hubble Space Telescope have detected helium in the atmosphere of the exoplanet WASP-107b. This is the first time that this element has been detected in the atmosphere of a planet outside the Solar System. The discovery demonstrates a new method for studying exoplanet atmospheres.

An international team, led by Jessica Spake of the University of Exeter, has discovered helium in the atmosphere of the exoplanet WASP-107b. The discovery was made with the Wide Field Camera 3 on the Hubble Space Telescope.

“Helium is the second-most common element in the Universe after hydrogen”, explains Jessica Spake. “It is also one of the main constituents of the planets Jupiter and Saturn in our Solar System. However, until now helium has never been detected in an exoplanet.”

WASP-107b (the 107th exoplanet discovered by the UK-led Wide Angle Search for Planets, “WASP”) was discovered in 2017 by a team led by Professor Coel Hellier of Keele University.

The team found that WASP-107b is a very low-density planet, being so puffed up and bloated that the atmosphere might be boiling off the planet under the irradiation of its host star.

“As soon as we found WASP-107b we realised it was ideal for studying the atmosphere of an exoplanet” remarks Keele astronomer David Anderson, who wrote the paper announcing WASP-107b.

Artist’s impression of the exoplanet WASP-107b showing the atmosphere boiling off under the fierce irradiation of its star. Image credit: EngineHouseVFX

Jessica Spake decided to point Hubble at WASP-107b, and, by detecting the spectral signature of irradiated helium atoms, proved that the atmosphere is indeed boiling off into space. While it had long been thought that helium would be abundant in exoplanet atmospheres, searches for it had previously been unsuccessful.

David Sing, who leads the Exeter team, says that: “Our new method, along with future telescopes, such as the James Webb Space Telescope, will allow us to analyse atmospheres of exoplanets in far greater detail than ever before.”

Jessica Spake continues. “We know that there is helium in the Earth’s upper atmosphere and this new technique may help us to detect atmospheres around Earth-sized exoplanets.”

The study was published in the paper “Helium in the eroding atmosphere of an exoplanet”, published in Nature.

Note: Dozens of websites have covered the story, including Newsweek, The Independent, the International Business Times, and others.

WASP-104b is Darker than Charcoal

Hot Jupiter WASP-104b was observed in Campaign 14 of the Kepler K2 mission, leading to superb-quality photometry covering 45 orbital cycles of the planet.

Keele graduate student Teo Močnik has analysed the data and concluded that WASP-104b is one of the darkest exoplanets known, reflecting less than 3% of the light from its star.

The conclusion comes from interpreting the “phase curve” produced when the photometry is folded on the planet’s orbital period. Variations in the light are expected to come from the transit and occultation (when the planet passes in front of and behind the star, respectively), from the gravitational distortion of the host star caused by the close-in planet, and from the reflection of starlight.

The phase curve of WASP-104b, as observed by K2, fitted with a red-line model.

The low albedo of the planet is a surprise, but might indicate the absence of clouds (which can be highly reflective) or the presence of ions such as sodium and potassium that absorb light.

The story of WASP-104b was reported by New Scientist, and that then led to articles in Science Alert, Metro, the Daily Mail, Newsweek, the International Business Times, Tech Times and other locations.

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.

Comparing WASP-173 to KELT-22

WASP-173 and KELT-22 are the same object. The WASP and KELT teams are both trying to find transiting exoplanets around relatively bright stars, and this means that sometimes our discoveries overlap. We announced that WASP-173 hosts a hot Jupiter in a paper on arXiv on the 7th March, and then on the 21st March KELT reported an entirely independent discovery of the same planet.

Since the two teams use different facilities, techniques and software, comparing the two sets of system parameters provides an interesting check on the methods. So let’s see how similar the reports are.

WASP-173Ab discovery photometry

The biggest difference is a somewhat different transit depth. We (WASP) report a depth of 0.0123 ± 0.0002 whereas KELT report 0.0145 ± 0.0008, where the difference is greater than the error bars quoted. Now this system is a double star, with a companion star 6 arcsecs away and 0.8 magnitudes fainter. That makes it hard to measure the depth. One either uses a much smaller photometric aperture than normal, excluding the nearby star, or one uses a much wider aperture, containing both stars, and makes a correction for the dilution of the companion. Either approach could introduce systematic errors more than normal. Then, of course, there could be red noise in the light-curves owing to observing conditions or stellar activity.

KELT-22Ab transit photometry

The greater depth in the KELT paper means they arrive at a slightly larger planet radius (1.29 ± 0.10 Jupiter radii) than we do (1.20 ± 0.06) but here the error ranges overlap. The planet mass (derived mostly from the radial velocity data) is comparable, 3.47 ± 0.15 Jupiter masses in the KELT paper, and 3.69 ± 0.18 in ours.

WASP-173Ab radial velocities (from CORALIE)

The differences in the parameters of the host star are all within the error ranges. KELT report a G2 star with an effective temperature of 5770 ± 50 K, a surface gravity (log g) of 4.39 ± 0.05, and a mass and radius of 1.09 ± 0.05 and 1.10 ± 0.08 in solar units, whereas WASP report a G3 star with effective temperature of 5700 ± 150 K, a surface gravity of 4.5 ± 0.2, and a mass and radius of 1.05 ± 0.08 and 1.11 ± 0.05.

KELT-22Ab radial velocities (from TrES)

Another comparison is the “impact factor” (how near the center-line the transit chord is), which we have as 0.40 ± 0.08 while KELT report 0.31 ± 0.18. Our higher value results from our having a higher transit width, 0.0957 ± 0.0007 days, compared to KELT’s 0.0981 ± 0.0025. Again, the differences point to red noise in the transit lightcurves, which is likely to produce uncertainties greater than the formal error bars.

Overall, the values are sufficiently similar that we can have broad confidence in the values, but the presence of systematic noise does need to be borne in mind.

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.

Comprehensive Spectrum of WASP-39b

NASA, ESA and JPL have put out press releases on the atmospheric spectrum of WASP-39b. The paper by Hannah Wakeford et al combined Hubble and Spitzer data to produce a comprehensive spectrum with broad spectral coverage.

“Using Hubble and Spitzer, the team has captured the most complete spectrum of an exoplanet’s atmosphere possible with present-day technology. “This spectrum is thus far the most beautiful example we have of what a clear exoplanet atmosphere looks like,” said Wakeford.”

“WASP-39b shows exoplanets can have much different compositions than those of our solar system,” said co-author David Sing of the University of Exeter. “Hopefully, this diversity we see in exoplanets will give us clues in figuring out all the different ways a planet can form and evolve.”

The strongest features in the spectrum are caused by water:

“Although the researchers predicted they’d see water, they were surprised by how much water they found in this “hot Saturn.” Because WASP-39b has so much more water than our famously ringed neighbor, it must have formed differently. The amount of water suggests that the planet actually developed far away from the star, where it was bombarded by a lot of icy material. WASP-39b likely had an interesting evolutionary history as it migrated in, taking an epic journey across its planetary system and perhaps obliterating planetary objects in its path.”

Coverage of the press release includes that by Newsweek, the International Business Times, the Daily Mail and about 30 other websites.