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Helium Detection at HAT-P-11b

You would think that helium, being the second most common element in the universe, would have been detected in exoplanet atmospheres long ago. A major constituent of the atmosphere at both Jupiter and Saturn, helium seems a natural because planets form from dust and gas from previous stellar generations, but it turns out that the first helium detection on an exoplanet occurred only this year, in a study led by Jessica Spake (University of Exeter).

The planet in question, WASP-107b, yielded its helium signature in data gathered by the Hubble Space Telescope, a detection that showed clear signs of a comet-like tail forming as the planet’s atmosphere escaped. Note the space-based detection: It’s significant because Earth’s atmosphere is opaque to the ultraviolet light the atoms in such an eroding atmosphere absorb.

Could we make this kind of fine-grained study from the surface of the Earth? It turns out there’s a way: Helium in its long-lived metastable state (as compared to its ground state) can be traced in the near-infrared as well as the ultraviolet. If you’re interested in the details on how metastable helium emerges under conditions like these, Kevin Anderton does a nice job explaining things at the atomic level in this Forbes article.

The point is that researchers are beginning to make these detections using ground-based instruments. Thus today’s paper, recounting new work from the University of Geneva (UNIGE), a research effort that includes Spake. The paper’s account of a helium detection from the ground appears in the journal Science for December 6.

The planet in this study is HAT-P-11b, a transiting ‘warm Neptune’ in Cygnus some 124 light years away, where it orbits 20 times closer to its star than the Earth does to the Sun. Central to the work is the Carmenes spectrograph installed on the 4-meter telescope at Calar Alto, Spain, which allowed the researchers to identify helium escaping from an exoplanet whose overheated upper atmosphere is streaming the gas into space in the form of an extended cloud.

Moreover, the instrument’s high spectral resolution allowed the scientists to detect the position and speed of the helium atoms in the atmosphere. According to Spake:

“This is a really exciting discovery, particularly as helium was only detected in exoplanet atmospheres for the first time earlier this year. The observations show helium being blasted away from the planet by radiation from its host star. Hopefully we can use this new study to learn what types of planets have large envelopes of hydrogen and helium, and how long they can hold the gases in their atmospheres.”

Image: Artist’s impression of the exoplanet HAT-P- 11b with its extended helium atmosphere blown away by the star, an orange dwarf star smaller, but more active, than the Sun. Credit: Denis Bajram.

Vincent Bourrier is a co-author of the study and member of the European project FOUR ACES, an acronym standing for Future of Upper Atmospheric Characterization of Exoplanets with Spectroscopy. Bourrier’s numerical simulations of the movement of the planet’s helium support the spectroscopic observations:

“Helium is blown away from the dayside of the planet to its nightside at over 10,000 km/h,” Bourrier explains. “Because it is such a light gas, it escapes easily from the attraction of the planet and forms an extended cloud all around it.”

First author Romain Allart (UNIGE) adds that proximity to the host star led the team to suspect a high impact on the atmosphere, including the shedding of helium into nearby space. We should expect, in other words, to find numerous other exoplanet atmospheres in this configuration, worlds inflated like a helium balloon. But we’re only now beginning to analyze them. While losing atmosphere is not uncommon in a giant planet close to its star, the predominant element identified in eroding exoplanet atmospheres to this point has been hydrogen.

The Carmenes work makes it clear that ground-based observations can retrieve data on the most extreme atmospheric conditions in exoplanet atmospheres. This is good news for future atmospheric studies, as is the fact that two new high-resolution spectrographs similar to Carmenes are in the works. The Near Infrared Planet Searcher (NIRPS) is undergoing testing at the University of Geneva and will be installed in Chile at the end of 2019. The other, called SPIRou (SpectroPolarimétre Infra-Rouge), is beginning an observational campaign in Hawaii after achieving first light in April. The advent of next-generation extremely large telescopes will further the field yet again by allowing study of escaping atmospheres at smaller planets.

Will we begin to find escaping atmospheres laden with heavier elements? Carbon and oxygen would be slow to escape while hydrogen is the easiest element lost, meaning a giant planet should see a greater amount of helium relative to hydrogen over time. The relative mix we find as we study these escaping atmospheres will help in the analysis of planetary evolution.

The Allart et al. paper is “Spectrally resolved helium absorption from the extended atmosphere of a warm Neptune-mass exoplanet,” Science 6 December 2019 (abstract). The WASP-107b work with the first detection of exoplanet helium is Spake et al., “Helium in the eroding atmosphere of an exoplanet,” Nature 557 (02 May 2018), 68-70 (abstract).

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{ 9 comments… add one }
  • Harry R Ray December 7, 2018, 10:36

    “Detection of He 10830A absorbtion on HD 189733b with CARMINES high-resolution transmission spectroscopy.” by Salz et al. Paul Gilster: Could you ask Vincent Bourrier, who has participated in observations of the TRAPPIST-1 planetary system with Gillon et al, if it is feasible to attempt to detect helium on TRAPPIST-1b?

    • Paul Gilster December 7, 2018, 12:38

      Have already done so. Will report any response.

    • Paul Gilster December 7, 2018, 19:12

      Harry, this just in from Dr. Bourrier:

      “Nice hearing from you again. I enjoyed your story, and can certainly understand your reader’s interest for TRAPPIST-1. It’s indeed one of the target we have in mind, however it’s a much smaller and fainter star than those that allowed us and our fellow reasearchers to detect helium. With CARMENES and current infrared spectrographs, it would likely require too much observing time to search for helium around TRAPPIST-1 planets. So it will be a target for future instrumentation, either high-resolution near-infrared spectrographs on >8m ground-based telescopes (larger than the 4-m class telescope hosting CARMENES), or the JWST (which cannot resolve spectrally the helium signature and thus brings limited information about the planet atmosphere, but on the other hand will have a much higher sensitivity).”

  • Bruce D. Mayfield December 7, 2018, 16:19

    Just a general comment/question with regard to these giant planets that are so commonly found in these close in “hot” orbits: There has been (or was) a big question as to whether such planets formed in place or migrated in from further out. Perhaps this issue has been settled on the migration side? To me it seems like the fact that HAT-P-11b is loosing He shows that it must have formed father out, otherwise how could it have grown so massive in a region where it is now shedding mass?

    • Karl Pestell December 8, 2018, 11:19

      I’ve done some reading and calculations and I don’t think this does settle the issue. The paper gives a He mass loss of 3*10^5 g/s, the host star is 6.5 b yrs, the planet is 23.4 Me, so plugging all together with an assumed H mass loss of 60 times the He rate, I get a mass loss to date of 0.0026%, which seems ridiculous, but there’s a heck of a lot of mass in a Neptune sized planet I guess. (I know I have made many heroic assumptions but I wanted to see what the ball park figures would come out as.)

      There’s more on hot Jupiter migration in this http://www.astro.caltech.edu/~jwang/Project5.html and the linked arxiv paper. It seems we think hot Jupiters / exoNeptunes have to have migrated as we can’t work out how they could form so close in.

      • Bruce D. Mayfield December 8, 2018, 19:04

        Thanks Karl. So, if I’m following your points and the link your shared correctly, these hot giant planets must have migrated in, but the unknown today is how. See this quote from your link:
        “Hot Jupiters are too massive to form in situ because a lack of building materials close to a star. One possible solution is that hot Jupiters form further out, where building materials are sufficient, then migrate to their current positions. Migration of hot Jupiters can be caused by different mechanisms…”

        • Karl Pestell December 9, 2018, 0:07

          Hi Bruce, I’ve just been reading an amazingly detailed paper from Jan 2018 https://arxiv.org/pdf/1801.06117.pdf .

          There are three proposed mechanisms: in situ, disk migration and tidal migration. Page 18 tabulates the problems with all three! The paper concludes tidal migration is a strong theory; but it’s believed that more than one mechanism is at work in different systems, and in-situ can’t be ruled out at this stage. Fascinating! There is so much to learn and discover.

          • Bruce D. Mayfield December 10, 2018, 23:16

            Thanks very sincerely Karl! That paper answered my question as throughly as possible. Still taking it all in.

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