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Light, Dry Atmosphere of a ‘Super-Earth’

We’re probing the atmospheres of exoplanets both from the Earth and from space. Transmission spectroscopy allows us to look at the spectra of starlight at various wavelengths as a transiting planet passes first in front of its host star, and then moves behind it. Now we have news of a successful detection of gases in the atmosphere of a super-Earth, using data from the Hubble Space Telescope. The team, made up of researchers at University College London and Catholic University of Leuven (Belgium) calls this a significant first.

“This is a very exciting result because it’s the first time that we have been able to find the spectral fingerprints that show the gases present in the atmosphere of a super-Earth,” said Angelos Tsiaras, a PhD student at UCL, who developed the analysis technique along with colleagues Dr. Ingo Waldmann and Marco Rocchetto in UCL Physics & Astronomy. “Our analysis of 55 Cancri e’s atmosphere suggests that the planet has managed to cling on to a significant amount of hydrogen and helium from the nebula from which it formed.”


Image: This artist’s impression shows the super-Earth 55 Cancri e in front of its parent star. Using observations made with the NASA/ESA Hubble Space Telescope and new analytic software scientists were able to analyze the composition of its atmosphere. It was the first time this was possible for a super-Earth. 55 Cancri e is about 40 light-years away and orbits a star slightly smaller, cooler and less bright than our Sun. As the planet is so close to its parent star, one year lasts only 18 hours and temperatures on the surface are thought to reach around 2000 degrees Celsius. Credit: ESA/Hubble, M. Kornmesser.

55 Cancri, about 41 light years from the Sun, is a binary consisting of a G-class star (55 Cancri A) and an M-class dwarf (55 Cancri B). We know of five planets around 55 Cancri A, the innermost being 55 Cancri e. If you’re worried about the nomenclature, that’s because we could more formally call the planet 55 Cancri Ae, in order to clarify which star the planet orbited in a binary system. All five known planets orbit 55 Cancri A, with 55 Cancri B some 1065 AU out.

Although it is a transiting world, 55 Cancri e and all the other planets in the system were discovered by radial velocity measurements. Temperatures on 55 Cancri e are thought to reach 2000 degrees Celsius on a world whose year is a scant eighteen hours. With a minimum mass of 8.3 M, the planet has a radius about twice that of the Earth.

Tsiaras and team used the Wide Field Camera 3 (WFC3) on Hubble, retrieving 55 Cancri e’s spectral signature by capturing a number of spectra made with quick scans across the target. The method allows the telescope to slew during the exposure, preventing saturation of the detector. The results were then fed through pipeline software that could remove systematic distortions caused by the scanning method. The paper explains the nature of the correction:

In the case of very long scans, we have to take into account the geometric distortions (dispersion variations across the scanning direction and inclined spectrum) and the positional shifts (horizontal and vertical), as their effect on the structure of the spatially scanned spectrum becomes significant. Especially for fast scans, we found that the vertical shifts are as important as the horizontal ones, because they are coupled with the reading process of the detector (up-stream/down-stream effect), causing exposure time variations. We also found that the time-dependent, long-term systematics, appear to have a different behavior per wavelength channel.

The resulting data show hydrogen and helium in the atmosphere of 55 Cancri e, but no water vapor. There is also an intriguing hint of hydrogen cyanide (HCN) with a possible additional
contribution of other molecules, such as carbon monoxide (CO) , carbon dioxide (CO2) and acetylene (C2H2). HCN is considered to be a marker for carbon-rich atmospheres, which weaves in usefully with what was previously known of the planet. 55 Cancri e has sometimes been called the ‘diamond planet’ because of the possibility that it might have a carbon-rich interior. The current work points in the same direction, with its indication that the planet’s atmosphere has a high ratio of carbon to oxygen.

“If the presence of hydrogen cyanide and other molecules is confirmed in a few years time by the next generation of infrared telescopes, it would support the theory that this planet is indeed carbon rich and a very exotic place,” said Professor Jonathan Tennyson, UCL. “Although, hydrogen cyanide or prussic acid is highly poisonous, so it is perhaps not a planet I would like to live on!”

The paper is Tsiaras et al., “Detection of an Atmosphere Around the Super-Earth 55 Cancri e,” accepted at The Astrophysical Journal (preprint).


Comments on this entry are closed.

  • Andrew LePage February 16, 2016, 13:02

    This analysis of 55 Cnc e (which has a mass of ~8.3 times that of Earth and a radius 2.0 times) which suggests the presence of substantial amounts of hydrogen and helium in its atmosphere lends further support to the emerging view that planets transition from being rocky to volatile-rich mini-Neptunes at radii no larger than 1.6 times that of the Earth (corresponding to a mass of ~6 times).


    • Ashley Baldwin February 16, 2016, 23:50

      All the more reason to hopefully find similarly sized planet 9 on our doorstep and study it in detail. In the absence of a dedicated telescope , JWST will hopefully be able to spare some of its valuable time to look at such sized transiting planets too irrespective of habitability. TESS is bound to turn up a few good targets.

  • Ashley Baldwin February 16, 2016, 14:51

    Hell of an achievement by the team and the glorious old HST . Still strutting its stuff ! Oh for a dedicated transit spectroscopy telescope though. Ready to go .What it could achieve while we perfect the coronagraphs , star shades , wavefront control ,entry pupils and thermomechanical stability necessary for the future.

  • Harry R Ray February 16, 2016, 15:07

    I would very much like Nikku Madhasudhan’ take on this. He must be DELIGHTED with the “strong” hint that the atmospheric composition pretty much confirms his hypothesis that the planet(and probably the STAR, TOO)has a composition with a lot more carbon than oxygen. However, this atmosphere’ s apparrent ABUNDANCE may CONFLICT with his assertion that the planet varies in SIZE as well as TEMPERATURE, due to extreme volcanic activity. In THIS paper, the height of the atmosphere was MODELED to be 240km and the surface pressure(OR pressure at a cloud deck) was MODELED to be 0.1 bar. FINALLY, could this atmosphere be CONSTANTLY escaping, while SIMULTANEOUSLY being REPLENISHED by OUTGASSING?

  • FrankH February 16, 2016, 16:58

    I wonder if this mini-Neptune is also capturing hydrogen from the primary star. It is orbiting very close in, so I would expect some sort of interactions.

    • Harry R Ray February 17, 2016, 10:23

      A very interesting conjecture, but; something EVEN MORE EXOTIC may be going on. A transet of 55 Cancri b has NEVER BEEN DETECTED, but. HST DID detect a “dip” that was CONSISTANT with an escaping ATMOSPHERE! Using the “behemouth” model of Gliese 436 for 55 Cancri b, radiation pressure and solar wind pressure CANNOT DISSIPATE the escaping atmosphere, so it MAY start to fall INWARD. As this infall gains MOMENTUM, it MAY be able to CROSS THE PATH of 55 Cancri e’s orbit, and be captured. ALSO: Even though 55 Cancri b is NOT transiting NOW, it may be SO CLOSE to transit mode, that, in the NEAR FUTURE, it MAY, due to precession and libration of the ENTIRE SYSTEM! Constant monitoring is REQUIRED!

  • Alex Tolley February 16, 2016, 18:03

    Isn’t the expected escape velocity and temperature marginal for retaining hydrogen?

    • Michael February 17, 2016, 0:50

      If it is just at the escape velocity of hydrogen and helium then I would think the atmosphere would be retaining the heavier gases such as CO2 as well, the atmosphere must be very thick with high temperature and pressures.

  • Black Sci-Fi February 17, 2016, 16:43

    Is this planet orbiting in tidal lock with it’s parent star?
    If so, that might explain the detection of gasses that might otherwise never form or survive the day-side temperature of its parent stars.

    • Harry R Ray February 18, 2016, 10:26

      Yes, it IS tidally locked! One mechanism for the slow,CONSTANT replenishment of the atmosphere COULD BE 55 Cancri b, BOTH through tidal interactions AND heat transfer via REFLECTED LIGHT when both planets pass CLOSE to each other.

  • Erik Landahl February 17, 2016, 18:11

    The scientific community should stop using the term Super-Earth. It thorougly misleads the general public. Friends, co-workers and even my children hear the term Super-Earth, and they think another Earth has been discovered. I mean it’s just like Earth, only bigger, right? It’s a giant version of Earth, right? No one can blame anyone who doesn’t follow astronomy for thinking that Super-Earth means a great big version of Earth.

    I don’t know where to direct my plea. If a person with connections in the astronomical nomenclature world reads this, please consider adopting another term for describing these types of planets, a term that is actually accurate and not so very misleading. I agree with the general public’s common sense interpretation of Super-Earth. The planets labeled this way are most certainly not Super-Earths at all. They’re gigantic super-hot rocks with thick super-hot poisonous atmospheres. Super-Venus?

    • Eniac February 19, 2016, 11:37

      They are hot only when too close to their sun. There are bound to be many super-Earths far enough away to be cold or “just right”. We do not know all the effects on atmospheric density, so many of them may well have thin atmospheres. Even if not, a dense atmosphere is not necessarily hot, nor poisonous, nor even much of a detriment to human activity. As a contrary example: A thicker atmosphere will make flying much easier. When it comes to indigenous life rather than human habitation, there is no limit how thick an atmosphere can be and still permit life. Note that there is life in the deep seas, at immense pressure.

      So, while super-Earth might not be the best term, let’s not inappropriately malign this type of planet, either. There could be some fine, livable specimens out there.

      • Michael February 19, 2016, 14:51

        I doubt very much heavy planets have thin atmospheres, but you are correct in that there could be an equal number of heavier worlds with much denser atmospheres further out that can hold on to warmth to make them habitable even if outside of the stars HZ.

        The HZ in my opinion is getting blurrier by the day!

        • Alex Tolley February 19, 2016, 16:47

          I recall that there was a Science joural article/paper by Keating[?] that posited a warmer earlier Earth if there was an early hydrogen atmosphere.

          In any event, thick atmospheres should push the HZ zone outwards from the star. More internal heat will also help in this regard.

          Doesn’t Jupiter have liquid water at some depth in its atmosphere?
          If such water layer was at a super-earth surface, then perhaps we have an interesting extension to the HZ.

      • Alex Tolley February 19, 2016, 14:53

        Even more relevant is that humans can breathe pressurized gas mixtures at depths of up to 200 bar. So we can imagine a world that has retained its He, H gases, but also has oxygen, and can be made breathable for humans, and certainly for life adapted to such a world.

        • Michael February 20, 2016, 6:10

          A planet with a helium and oxygen atmosphere at 200 bar is ok, one with hydrogen and oxygen at 200 bar not, one stroke of lightning and BANG.

          Cancri 55 in UV, plenty of it to destroy water.


          • Alex Tolley February 20, 2016, 12:53

            Oops. Scrub the retaining H2! As 55 Cancri e has H2, there is no possibility of volumes of O2 in the atmosphere. On such a world, any terrestrial=like photosynthesis releasing O2 would be quickly reduced to water. This is not unlike the conditions on Earth where Fe++ was a sink for O2.

            So this world could be in that early state, with H2 acting as a sink, except that the spectrometry indicates it is a dry world. If there is life, photosynthesis would have to release a less reactive product, e.g. sulphur. The lack of water is still a problem. It is just possible water exists below the surface, but if so, life if it exists would be confined to micro-organisms living in the rocks, which on this world are more carbon than silicon.

            • Eniac February 20, 2016, 23:39

              I think it is not likely there will be a breathable atmosphere on any planet, but you are right that high pressure is not a direct obstacle.

              Indirectly, though, a larger planet will retain more hydrogen and methane which will make for a more reducing atmosphere that it would take much longer to oxidize, should photosynthesis arise (or be planted). A thicker atmosphere could also reduce surface light levels enough to pose an additional problem.

              In terms of being able to walk around with just an oxygen mask, though, a super-Earth looks just as good as an Earth-sized planet. Better, actually, if you prefer to fly rather than walk …

        • Volucris February 22, 2016, 6:36

          Do you have some reference for the 200 bar number? It sounded quite high so I went googling, most pressure I can find is on a table in [1], at around 70 bar (700m! that’s still sounds completely crazy depth for a surface dwellers physiology!!).

          [1] https://en.wikipedia.org/wiki/Deep_diving

          • Alex Tolley February 22, 2016, 12:26

            “…often involve the use of booster pumps to achieve typical diving cylinder pressures of 200 bar (2,900 psi) from lower pressure banks of oxygen and helium cylinders.”


            Assume variable accuracy in Wikipedia articles. 200 Bar => 2000 meters depth, halfway to the average ocean depth. far deeper than submarines maintaining 1 Bar, but less that deep sea submersibles.

            However, I suspect the Heliox article is wrong. On reflection, 2000 meters seems far too deep even with Heliox mixtures.

            • Volucris February 23, 2016, 12:36

              That article is referring to the gas cylinder pressures, which usually are in 200-300 bar range, not the pressure the diver is breathing gas in. The gas from the cylinders always goes through a regulator [1] before breathing, which lowers the pressure to ambient (depending on the depth/pressure the diver is in).

              [1] https://en.wikipedia.org/wiki/Diving_regulator

              • Alex Tolley February 23, 2016, 13:21

                Thanks, I didn’t catch that. You are correct.

          • Michael February 22, 2016, 14:20

            Humans would struggle but other mammals have no problem, a 2010 study documented a seal diving to 2,388 meters or 238 bar! No reason to suspect that humans could not acclimatise in time.

          • Harry R Ray February 22, 2016, 14:54

            I agree. The atmospheric pressure at the surface of Venus is ONLY 100 bar, and you see what THAT did to the EARLY Soviet landers!

            • Michael February 22, 2016, 17:03

              The pressure was not the problem, the heat was, it is seriously hot!

      • Stevo Darkly February 24, 2016, 21:02

        As an alternative to “super-Earth” I would suggest calling it a “rocky giant” planet — as contrasted with “gas giant.”

        This could be a generic term that would include planets that are potentially life-bearing as well as those that probably aren’t.

  • ljk February 17, 2016, 18:38

    So now we can detect volcanoes erupting on exoworlds?!


    And I was so impressed with Io erupting a mere 400 million miles away.

  • ljk February 18, 2016, 14:26

    Eying exomoons in the search for E.T.

    February 18, 2016 by Bryan Gaensler, University Of Toronto, The Conversation


  • ljk February 18, 2016, 17:13

    Hubble directly measures rotation of cloudy ‘super-Jupiter’

    February 18, 2016 by Donna Weaver


  • ljk February 19, 2016, 14:13
  • Harry R Ray March 31, 2016, 9:39

    Yet ANOTHER paper on 55 Cancri e is up on the exoplanet.eu website(but NOT on arxive yet). This one deals temperature differences between the daytime and nighttime hemispheres AND at different LOCATIONS on the daytime hemisphere. COMBINING ALL THREE recent(i.e. within the LAST YEAR)papers, I have come to the conclusion that ONE of them MUST BE COMPLETELY WRONG, because the TOTALLITY OF THE THREE leads to an INCOMPATABLE DATA SET! AGAIN, the three papers deal with the VARIABILITY of the planet’s radius OVER TIME, the CHARACTERIZATION of the planet’s ATMOSPHERE, and heat DISTRIBUTION. It is impossible foe me to determine WHICH ONE IS WRONG, because EACH paper IS compatible with EITHER(but NOT both)of the other ones.