Tag Archives: WASP-107b

A second planet for WASP-107

WASP-107b is a hugely bloated planet, with a mass of only two Neptunes, but a radius near that of Jupiter, making it one of the puffiest planets known. As such it has been heavily studied, and indeed was the exoplanet showing the first detection of helium.

Long-term monitoring of the WASP-107 system with the Keck telescope has now revealed a companion planet, WASP-107c, as announced by Caroline Piaulet et al.

The new planet is in a much wider orbit, with a period of 1088 days and a high eccentricity of e = 0.26. It likely does not transit, and has a mass of perhaps a third that of Jupiter.

The discovery of a second planet is important for understanding the nature of WASP-107b itself. The tight, 5.7-d orbit, and the fact that the orbit is mis-aligned with the star’s rotation, might be explained by gravitational interactions with the second planet. The bloated size could then result from tidal interactions with the host star, as the planet circularised in its orbit, after the interactions with WASP-107c.

The authors conclude that, “Looking ahead, WASP-107b will be a keystone planet to understand the physics of gas envelope accretion”, starting with a planned observation with the soon-to-be-launched JWST.

Helium reveals the extended atmosphere of WASP-107b

Here’s a plot from a new paper on WASP-107b by James Kirk et al. It shows data taken with a near-infra-red spectrograph on the 10-m Keck II telescope on Mauna Kea, and is focused on the Helium line at 10833 Å. The plot shows the spectra as a function of time (y-axis), though a transit. When the planet passes in front of its host star (white horizontal lines are times of ingress and egress) the helium line shows excess absorption. This helium is in the atmosphere of the planet and is absorbing some of the starlight. There is a slight change in the wavelength of the absorption owing to the orbital motion of the planet (denoted by the dashed white lines).

The paper shows, firstly, that ground-based telescopes such as Keck can do a fine job of discerning the compositions of exoplanet atmospheres. Secondly, the fact that the absorption extends beyond transit-egress indicates that the atmosphere is boiling off the surface of WASP-107b, under the fierce irradiation of the star, and is forming a comet-like tail.

Helium in WASP-69b, HAT-P-11b and HD 189733b

Earlier this year helium was found in the outer atmosphere of WASP-107b, the first detection of helium in an exoplanet. Several teams have now used similar techniques to find helium in WASP-69b, HAT-P-11b and HD 189733b, leading to a slew of papers and accompanying press releases from the Instituto de Astrofísica de Andalucía, the University of Exeter and others (see [1], [2], [3] and [4]).

Artist’s impression of an escaping envelope of helium surrounding WASP-69b. (Credit: Gabriel Perez Diaz, IAC)

Lisa Nortmann, lead author of the WASP-69b paper, explains that the helium is escaping from the atmosphere, forming a comet-like tail: “We observed a stronger and longer-lasting dimming of the starlight in a region of the spectrum where helium gas absorbs light. The longer duration of this absorption allows us to infer the presence of a tail.”

The press releases have led to extensive coverage including by CNN, the Daily Mail and Tech Times.

The IAA press release includes a video illustration of WASP-69b, created by Gabriel Perez Diaz of the IAC:

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.

First results on the atmosphere of WASP-107b

Being a Neptune-mass planet (0.12 MJ) bloated to a near-Jupiter radius (0.94 RJ) makes WASP-107b’s atmosphere very fluffy, and that, coupled with it transiting a moderately bright K star (V = 11.6) makes it a superb target for atmospheric characterisation.

Laura Kreidberg et al have pointed the Hubble Space Telescope at WASP-107b to make the first atmospheric study. Here’s the WFC3 spectrum:

Hubble Space Telescope spectrum of WASP-107b

The broad features at 1.15 and 1.4 microns are due to water absorption in WASP-107b’s atmosphere. Kreidberg et al model the features, finding that they are compatible with expectations given solar abundances. They are not deep enough, though, to be produced by fully clear skies, and a layer of high-altitude cloud is also required.

WASP-107b is one of the prime exoplanets already chosen for early observations with the imminent James Webb Space Telescope, so it is exciting to know that its atmosphere does show prominent molecular features.

Super-Neptune WASP-107b has an oblique orbit

WASP-107b is only twice the mass of Neptune but nearly the radius of Jupiter. It is thus a hugely bloated and fluffy exoplanet and one of the more important of the recent WASP discoveries, being a prime target for atmospheric characterisation (see the discovery paper by Anderson et al 2017).

WASP-107b was also in the Campaign-10 field of the K2 mission, leading to a Kepler-quality photometric lightcurve. Recent papers by two teams, led by Teo Močnik and Fei Dai, have arrived at a similar conclusion: WASP-107b seems to be in an oblique orbit, rather than in an orbit aligned with the rotation axis of the host star.


The conclusion comes from star spots. If the orbit is aligned, consecutive transits will repeatedly cross the same star spot, producing a “bump” in the lightcurve each time, whereas if the orbit is oblique this will not happen.

Thus one can play the game of looking for transit bumps and seeing if they repeat. But spots can change, by growing or shrinking, so is a smaller bump in the next transit the same spot, or a different one? Also, if there is some uncertainty in the rotational period of the star, then we’re not fully sure exactly where in the next transit the spot will recur.

Star spots in transits of exoplanet WASP-107b

In the figure at left (in which the transit itself, between the dashed lines, has been removed, leaving only the starspot bumps), obvious spots are circled in red, while possible spots are marked with a lighter red. The rotational period of the star is nearly three times the orbital period of the planet, and so, if the spots recurred, they would be seen every three transits. (The gap, and thus the missing of transits 3, 4 and 5, arose from a spacecraft malfunction.)

The conclusion is that the star spots do not seem to recur and thus that WASP-107b is in an oblique orbit.