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137496 b: A Rare ‘Hot Mercury’

We haven’t had many examples of so-called ‘hot Mercury’ planets to work with, or in this case, what might be termed a ‘hot super-Mercury’ because of its size. For HD 137496 b actually fits the ‘super-Earth’ category, at roughly 30 percent larger in radius than the Earth. What makes it stand out, of course, is the fact that as a ‘Mercury,’ it is primarily made up of iron, with its core carrying over 70 percent of the planet’s mass. It’s also a scorched world, with an orbital radius of 0.027 AU and a period of 1.6 days.

Another planet, non-transiting, turns up at HD 137496 as well. It’s a ‘cold Jupiter’ with a minimum mass calculated at 7.66 Jupiter masses, an eccentric orbit of 480 days, and an orbital distance of 1.21 AU from the host star. HD 137496 c is thus representative of the Jupiter-class worlds we’ll be finding more of as our detection methods are fine-tuned for planets on longer, slower orbits than the ‘hot Jupiters’ that were so useful in the early days of radial velocity exoplanet discovery.

The discoverers of the planetary system at HD 137496, an international group led by Tomas Silva (University of Porto, Portugal), found HD 137496 b, the hot Mercury, in K2 data, its transits apparent in the star’s light curve. The gas giant HD 137496 c was then identified in radial velocity work using the reliable HARPS and CORALIE spectrographs.

The primary is a G-class star a good bit older than the Sun, its age calculated at 8.3 billion years, but with a comparable mass (1.03 solar masses), and a radius of approximately 1.50 solar radii.

Image: HARPS (orange) and CORALIE (blue) radial velocities. In this figure, we present our RV time series. As is clearly seen, the data show a long-term and high-amplitude trend (semiamplitude of ~ 200 m s-1), typical of the signature of a long period giant planet. Credit: Silva et al.

A hot Mercury should turn out to be a useful find in a variety of ways. As the paper notes:

HD 137496 b (K2-364 b) joins the small sample of well characterized dense planets, making it an interesting target for testing planet formation theories, density enhancing mechanisms, and even the possible presence of an extended cometlike mineral rich exosphere. Together with HD 137496 c (K2-364 c), a high-mass (mass ratio…, high-eccentricity planet, this system presents an interesting architecture for planetary evolution studies. Future astrometric observations could also provide significant constraints on the relative inclination of the planetary orbits, unraveling new opportunities to discover the system’s dynamical history.

Keep in mind that most of the planets we now know about have radii somewhere between that of Earth and Neptune. In this range, numerous different system architectures are in play, and a wide variety of possible formation scenarios. As the authors note, high-density planets like HD 137496 b are distinctly under-sampled, which has been a check on theories of planet formation that would accommodate them.

And the theorists are going to have their hands full with this one. HD 137496 b’s parent star shows too little iron to form a planet with this density. I’m going to quote Sasha Warren on this. Working on a PhD at the University of Chicago, Warren focuses on how planetary atmospheres have evolved, particularly those of Mars and Venus. Of HD 137496 b, she has this to say in a recent article on astrobites about how such planets can become more iron-rich:

Firstly, the protoplanetary disks of dust and gas within which planets form around young stars can change in composition as a function of distance from the star. So, it is possible that a combination of high temperatures and magnetic interactions between the host star and the protoplanetary disk concentrated iron-rich materials where HD 137496 b originally formed. This could mean star compositions might not be very useful to help understand what short period rocky planets are made of. Secondly, planets close to their stars like HD 137496 b are so hot that their rocky surfaces can sometimes just evaporate away!

It will be fascinating to see how our theories evolve as we begin to expand the catalog of hot Mercury planets. 137496 b is only the fifth world in this category yet discovered.

The paper is Silva et al., “The HD 137496 system: A dense, hot super-Mercury and a cold Jupiter,” in process at Astronomy & Astrophysics (preprint).


{ 22 comments… add one }
  • Alex Tolley November 30, 2021, 12:26

    With the star about as hot as the sun, but with HD 137496 b far closer to it than Mercury, I assume that HD 137496 b is tidally locked with a very high surface temperature on the sunward side.

    How hot is this face expected to be?

    I note that oxides of iron and silicon evaporate at a lower temperature than the pure element. Is it possible that this planet is self-smelting to remove all oxides and leave the iron, nickel, and silicon as the main elements in increasingly pure form? The oxides would naturally float to the surface, boil off, and leave the pure elements increasingly concentrated.

    After 8 bny, wouldn’t HD 137496 have increased in temperature and heated the b planet even more than in the past (even if its lower mass has allowed the orbit to increase).

    Is it possible that planet b eventually becomes a super-sized planet equivalent of 16 Psyche, but so hot that it is mostly liquid?

  • Ashley Baldwin November 30, 2021, 15:42

    I’m a little confused with HD137496 c as a ‘cold Jupiter’.

    HD 137496 is a slightly more massive star than the Sun. Planet ‘c’ has a period of about 480 days with a highly eccentric orbit (0.48) with a semi major axis of 1.21 AU. Well interior to Mars around the Sun and thanks to its elliptical orbit, frequently interior to 1 AU around the stellar host. Which would qualify it as a ‘sometimes warm Jupiter’ .

    Assuming the host star has a luminosity on a par with the Sun.

    Which it can’t – with its 59% greater radius. This implies that the star, well over 8 billion years old, has begun to evolve off the main sequence into a sub giant . Thus giving it several times the luminosity of the Sun and making planet ‘c’ a ‘very warm to warm Jupiter’ in the process .

    • Paul Gilster November 30, 2021, 17:46

      Good point, Ashley. ‘A Sometimes Warm Jupiter’ sounds about right.

    • Michael Fidler November 30, 2021, 21:54

      Yes, The NASA’s exoplanet catalog shows the orbit of HD137496 c as being only in the habitable zone for the star for 1/4th of its orbit, but the rest of the orbit is well inside of that. A hot to very warm Jupiter would be correct.


    • andy December 2, 2021, 4:32

      Plotting the known exoplanets on a mass–orbital period diagram, there’s a clump of giant planets at orbital periods less than about 7-10 days or so, there’s another clump at orbital periods above ~200 days, between which is a region which is relatively sparsely populated. The terms “hot Jupiter”, “cold Jupiter” and “warm Jupiter” are often used to refer to planets in the short-period clump, the long-period clump and the intermediate-period “valley” respectively. At ~480 days, HD 137496 c is a member of the long-period clump and thus termed a “cold Jupiter”, regardless of insolation.

      • Paul Gilster December 2, 2021, 6:01

        Thanks, andy. That clarifies things greatly.

        • Martin December 3, 2021, 4:59

          So hot and cold don’t mean hot or cold. Okayyyy…
          Intermediate I have no complaints with (surprise!)

          • Ron S. December 3, 2021, 9:48

            Just wait until you hear what astronomers consider “metals”. Astronomy-speak is fun.

      • Ashley Baldwin December 3, 2021, 10:58

        Thanks Andy. Well explained. But..


        Transit photometry and RV spectroscopy favour short period planets. There hasn’t thus been enough longer period planets discovered to link to more remote ‘clumps’.
        Until Gaia DR4 anyway.

        Meanwhile where does that leave our very own Jupiter ?

        The one and only TRULY original, Jupiter link Jupiter from head to pole.

  • Geoffrey Hillend November 30, 2021, 21:54

    I agree with the idea that the host star’s composition might not be indicative of the hot Mercury’s composition. I wonder what the density of the HD 137496 b? A density near Mercury’s 5.43 g/cm³ should support this hypothesis.

  • Antonio December 1, 2021, 4:03

    OT but relevant to Breakthrough Starshot:


  • Dave Moore December 1, 2021, 16:21

    I really don’t think this planet, 137496 b, should be called a hot Mercury, or any sort of Mercury. There has obviously been some gravitational scattering in this system, and the authors of this paper guess, and I would agree with them, that this planet was scattered into a very elliptical orbit that was subsequently tidally circularized. This is obviously the fried remains of a much larger planet.

    How much larger? This planet has a core of 4 Earth mass. Neptune has a core of 1.2 Earth mass (est.). Taking into account the star’s lower metallicity, I would say the original planet was approaching Saturn’s mass.

    This planet’s history and origin are nothing like Mercury’s. A giant’s core planet would be a better name.

    • Alex Tolley December 2, 2021, 14:10

      So you are suggesting it was a hot gas giant of some type that has lost all its atmosphere and volatiles, leaving a [highly metallic] core? Is there any supporting evidence (e.g. haven’t we seen a hot Jupiter losing its atmosphere elsewhere?), or way to distinguish between the “hot Mercury” and the stripped “hot gas giant” hypotheses?

  • Geoffrey Hillend December 2, 2021, 19:09

    I also find it dubious that a core of 4 masses could be the core of a gas giant. On the other hand, and Earth sized core inside a large gas giant has not been seen but only inferred. The problem with me accepting that idea is that it’s not close enough to the star to evaporate a gas giant down to the core. Maybe in the red giant phase.

  • Geoffrey Hillend December 2, 2021, 19:23

    Also 4 masses is not large enough to be a Jupiter core which is now thought to be 14 to 18 times the mass of the Earth. I Googled it.

    • Ashley Baldwin December 3, 2021, 20:04

      And ‘dilute’ too though goodness knows what would happen to it if it’s crushing gas envelope was removed.

  • Dave Moore December 2, 2021, 20:42

    I might have over-egged it a bit suggesting a gas giant. Saturn has an estimated core mass of 22 Earths, but I do think it’s very plausible that it started off as an ice giant in the range of Neptune’s mass for the following reasons:

    1) HD137496 c has an eccentricity of 0.47, which indicates a highly disturbed system, so there is a good chance that b did not start out where it is. This is also backed up by its orbit being well inside the orbit of the inner planet of G stars with multiple super-earths/Neptunian mass planets in circular orbits. (@ 0.08 AU vs HD137496 b @ 0.027 AU ) I regard the 0.08 AU distance as the inner edge of the planetary nebula for a G star.

    2) The planet’s estimated surface temperature is 2130 deg K, and it’s density is 10.5 grams/cc, which suggests that not only has this planet’s volatiles boiled off but a good part of the mantle has too.

    4) With our system having its innermost planet at 0.36 AU and no super-earth / giant planets inside that distance, it would seem to be a different type of system to HD137496, so our formation history would not apply.

  • Geoffrey Hillend December 3, 2021, 17:16

    If Saturn started out as an ice giant, wouldn’t there be more water vapor in it’s atmosphere?

    • Dave Moore December 6, 2021, 16:20

      No necessarily. The amount of water in a giant’s atmosphere is determined by the equilibria between its various phases.

      I’ve not read of a boundary definition between Ice Giants and Gas Giants, but I would say that if a planet gets a mass fraction of Hydrogen sufficient for the base of the atmosphere to have Hydrogen in liquid/solid/metallic phases, then it would be considered a gas giant.

      Hydrogen being less dense than water would form a layer over it, cutting it off from the atmosphere, or the water could go into solution with the Hydrogen layer. (Our understanding of the cores of giant planets is at a fairly speculative phase.) Either of these scenarios could result in a drier atmosphere.

  • Geoffrey Hillend December 7, 2021, 16:52

    If that were correct, then the density of Saturn would be higher than Uranus and Neptune but it is not. Saturn’s density is 687 kg/m³ Uranus density is 1.27 g/cm³ Neptune’s density1.64 g/cm³

  • Geoffrey Hillend December 7, 2021, 17:01

    Saturn should be at least as dense as Uranus and Neptune, but Saturn is less dense than the ice giants. The atmospheres of Jupiter sized gas giants are very resistant to atmospheric loss due to stripping and high temperature because of their huge gravity. I have to conclude 137496 b is a super Mercury.

  • Mike Serfas December 8, 2021, 9:13

    Because moons of HD 137496 either are or were in the habitable zone, it would be interesting to know or guess at how the magnetic fields of Jupiters/brown dwarfs scale with mass. Wikipedia cites figures of 6 kG for some brown dwarfs, versus 10 gauss for Jupiter at the cloud tops… but I’m not sure those figures are comparable and have no idea if you can draw a line between those points. The Hill sphere and lunar orbital periods should be increased almost 2.8-fold relative to Jupiter, leaving room for many interesting worlds. Would the magnetic fields and associated radiation extend out that far and ruin the tourist industry?

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