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An Exposed Planetary Core at TOI-849

In exoplanet research, ‘deserts’ are regions where things are not found. Thus the Neptunian Desert, which is a zone close to a star where planets of Neptune size only rarely appear. Deserts like this (there is also a Brown Dwarf Desert that we’ve examined in earlier posts) raise questions because we don’t know why they occur. What is it we don’t understand about planet formation that accounts for the lack of Neptune-mass planets in 2-4 day orbits?

Exceptions tweak our thinking, and do have NGTS-4b, a world 20 percent smaller than Neptune and 20 times as massive as Earth in a 1.3-day orbit around a K-dwarf (see Into the Neptunian Desert for more on this one, which is now joined by an even more puzzling object).

For today we learn of the discovery of a world of roughly Neptune’s mass with an orbital period of a scant 18 hours, and researchers reporting the discovery in Nature suggest that we are actually looking at a ‘failed’ gas giant, an exposed planetary core. We can thank TESS (Transiting Exoplanet Survey Satellite) for the original data on this one, which is labeled TOI 849b. The object orbits a star much like our own about 730 light years from the Sun.

Follow-up observations with the HARPS spectrograph at ESO’s La Silla Observatory in Chile complement the TESS transit data with the radial velocity readings used to determine that the object is two to three times more massive than Neptune but also incredibly dense. It must, then, consist largely of iron, rock and water, with little hydrogen and helium. Physicist David Armstrong (University of Warwick), lead author of the paper on this work, describes the first intact exposed core of a gas giant found around a star:

“While this is an unusually massive planet, it’s a long way from the most massive we know. But it is the most massive we know for its size, and extremely dense for something the size of Neptune, which tells us this planet has a very unusual history. The fact that it’s in a strange location for its mass also helps — we don’t see planets with this mass at these short orbital periods. TOI 849 b is the most massive terrestrial planet — that has an Earth like density — discovered. We would expect a planet this massive to have accreted large quantities of hydrogen and helium when it formed, growing into something similar to Jupiter. The fact that we don’t see those gases lets us know this is an exposed planetary core.”

Image: Artist’s impression showing a Neptune-sized planet in the Neptunian desert. It is extremely rare to find an object of this size and density so close to its star. Credit: University of Warwick/Mark Garlick.

So is this really a ‘failed’ gas giant, or a gas giant that formed normally and then lost its gas envelope? If the latter, we could be looking at tidal disruption from the objects’ tight orbit, or even a collision with another planet. Photoevaporation of the atmosphere due to its position near the star is a contributing factor, but not one that fully accounts for the loss of gas.

If a failed gas giant, then in TOI 849 we would be looking at a world that never formed an atmosphere in the first place, with implications for the original protoplanetary disk. Christoph Mordasini (University of Bern) led the theoretical analysis of the discovery:

“Once the core of the gas giant formed then something very unusual could have happened and it never formed a massive atmosphere as normally. This could have occurred if there was a gap in the disk of dust and gas that the planet formed from due to gravitational interaction with the planet, or if the disk ran out of material right at the very moment when gas accretion normally follows.”

Thus the possible formation and evolution of TOI-849b, as shown below.

Image: The red line shows the evolutionary track of a simulated planet that finally has similar properties as the actual planet TOI-849b, as found in the Bern Model of planet formation and evolution. The track is shown in the plane of semimajor axis in astronomical units (AU), that is the orbital distance from the star, on the x-axis, and the radius of the planet in units of jovian radii on the y-axis. The blue-red points show other planets predicted by the model. The Earth and Jupiter are shown at their positions for comparison. The planet starts to form at the initial time t=0 years as a small planetary embryo at about 6 AU. The protoplanet grows in mass in the following 1 million year which increases its radius. In this phase, the radius of the planet is still very large, as it is embedded in the protoplanetary disk in which it forms. The increasing mass of the protoplanet causes it to migrate inwards, towards the star. This reduces again the size of the planet. After 3.5 million years, the planet has migrated to the inner edge of the disk.

There, it suffers a very energetic giant impact with another protoplanet in its planetary system. The enormous heat liberated in the collision strongly inflates the gaseous envelope of the planet. The envelope is lost via Roche-lobe overflow, and an exposed planetary core comes into existence. In the following billions of years, the exposed core slowly spirals towards its host star because of tidal interactions. The simulate planet now has properties like a mass, radius, and orbital distance which are very similar the observed properties of TOI-849b that is shown by a black-yellow symbol. In the end, after about 9.5 billion years, the planet falls into its host star. Credit: © Universität Bern / Christoph Mordasini / Physikalisches Institut, Weltraumforschung und Planetologie.

Whether a failed gas giant or a disrupted one, TOI-849b represents an exposed core, something we don’t see in our own Solar System, and it may provide insights into the interior of planets like Jupiter. Future measurements may pick up any trace atmosphere replenished from the core, which would give us data on the chemical composition of the intriguing object.

The paper is Armstrong et al., “A Remnant Planetary Core in the Hot-Neptune Desert,” Nature 1 July 2020 (abstract).

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{ 11 comments… add one }
  • Geoffrey Hillend July 1, 2020, 16:41

    There are some flaws in this theory. Why is the core so big or twenty Earth masses? I like the idea that TOI-849 formed in situ. The planet formed after the birth of the star the way rocky planets form through the accretion of pebbles, etc. All the hydrogen and helium was sucked up by the gravity to make the K dwarf star before TOI-849 was made. I never liked the migration idea especially when we consider the gained angular momentum from the star and centrifugal force keeps the gas giants from moving inwards as it gains mass through accretion?

    Gas giants usually have to form at the time of the star for this giant impact theory to be necessary, but if it formed afterwards, then the theory does not apply. If so it would be a new category of giant rocky planet which can exist only near the star where all of it’s original atmosphere simply was boiled away into escape velocity over time, the future of our Earth in the red giant phase of our Sun.

    • Andrei July 2, 2020, 19:00

      I tend to agree, insofar that the alternative to a gas giant core could be considered here. Especially this fact that it’s very hard to explain that all the atmosphere of a gas giant have been baked out and removed. If this is a Mercury on steroids, it could have been migrated a bit – brought closer to the star by the interaction with the stellar wind, remaining gas in the accretion disk and magnetosphere in the intense era when the star was young.

  • henry cordova July 1, 2020, 17:10

    I’ve often wondered what it would be like if an earthlike planet suffered a nearby supernova, one that vaporized and blew away its crust and mantle, leaving only the metal core behind. A solid sphere of nickel-iron, originally a molten metal ocean, but now frozen solid into a featureless, smooth, perhaps even shiny surface.

    The original explosion could conceivably clear the system of all volatile and mineral residues, so the ball-bearing planet would not even have any impact craters.

  • Dan Ibekwe July 2, 2020, 14:54

    If it really is an exposed Gas Giant core, maybe one day it will become a valuable solid metallic Hydrogen extraction site (after we’ve perfected Warp Drive, natch :-) )

  • Geoffrey Hillend July 2, 2020, 17:30

    I don’t think I can invalidate the theory on how TOI-849 formed in this paper. It makes sense. It might not be the only possibly way it formed. though. If the formed a little latter or too close to the K dwarf, then maybe the larger gravity of the K dwarf grabbed most of the H and He.

    The size of the core at 20 Earth masses is unusual since Jupiter is supposed to have a solid core around the size of the Earth. Maybe the cores of gas giants are larger than previously thought?

    Metallic Hydrogen can’t be extracted because it can’t exist by itself like a piece of metal. Metallic Hydrogen only exits under extreme pressures as a highly compressed gas which has pressure ionization of the atoms which allow the gas to be a conductor of electricity. The electrons of a gas become more freer, so they conduct electricity like a metal by that is the only similarity of metallic hydrogen to metal. For example: At at around 15,000 miles deep into Jupiter’s atmosphere the gas pressure is over a million bars or one million times Earth’s surface pressure of one bar, the temperature is 10,000 K at 15,000 km deep. At some point between a pressure of 1 Mbar and 10Mbar, M=million bars, there is pressure ionization of the gas. The gas pressure increases the closer to the core it is.

    • Mike Serfas July 3, 2020, 9:03

      The group that claimed isolation of metallic hydrogen suggested it actually might be metastable, like diamond. https://www.nature.com/news/metallic-hydrogen-hard-pressed-1.10817 Back in 2012 the more “enthusiastic” news sites were talking about making floating cities out of it.

      That said, they had one sample, and they said they didn’t want to release the pressure on the anvils to avoid losing it … years later the anvils broke and the sample was lost. I think another group also claimed making some … any news?

      Conceivably, you could picture people mining vast outcrops for rocket fuel, but that seems exceedingly optimistic. Besides, if you can get there you probably don’t need rockets … and on a planet that heavy and dense, I doubt even metallic hydrogen is enough to get you back off the surface again!

      For our plot let’s refocus on the massive collision that allegedly destroyed the atmosphere, and a chunk blown out into space which could be passing through the Solar system at any time. This leaves all who are capable scrambling to launch missions to mine those resources before they pass us by forever.

  • Bruce D Mayfield July 3, 2020, 14:54

    One of the factors to consider when looking at exoplanet formation is the metal content of the star, and the paper’s writers of course did so. Their sum up on this was this; “In terms of chemical composition TOI-849
    seems to be very similar to the solar neighbourhood stars showing slight enhancement in the iron-peak elements Cr and Ni.” So negatory on the idea that there might be something strange about the star orbited by this strange planet.

    There’s another point though about the proposed evolutionary track for TOI-849b that makes me question their theory: Look at the illustrated track and note how much of the history is located in another even deeper ‘desert’, exoplanets with much larger radii than Jupiter. If this was a real mode of formation for large metal core planets shouldn’t they be finding more young inflated Jupiters?

  • Michael Fidler July 3, 2020, 22:44

    Here is another gem around a G2 V star that is very similar to our sun. This is the densest planet so far with 2 to more then 3 times the density of earth. It orbits the star in 4 earth days, but there may be a reason this has become so dense then just the exposure to the stars heat or bombardment by particles and radiation.

    Characterization of the K2-38 planetary system. ? ??
    Unraveling one of the densest planets known to date.

    https://arxiv.org/abs/2007.01081

    https://exoplanetarchive.ipac.caltech.edu/cgi-bin/DisplayOverview/nph-DisplayOverview?objname=K2-38&type=PLANET_HOST

    What would the huge increase in neutrinos that can penetrate the large dense cross sections of radionuclide of this close in planet do to such elements???

    The strange case of solar flares and radioactive elements.

    https://phys.org/news/2010-08-strange-case-solar-flares-radioactive.html

    New system could predict solar flares, give advance warning.

    https://www.purdue.edu/newsroom/releases/2012/Q3/new-system-could-predict-solar-flares,-give-advance-warning.html

    [Submitted on 6 Feb 2020]
    Solar Flare Detection Method using Rn-222 Radioactive Source.

    “Solar neutrino detection is known to be a very challenging task, due to the minuscule absorption cross-section and mass of the neutrino. One research showed that relative large solar-flares affected the decay-rates of Mn-54 in December 2006. Since most the radiation emitted during a solar flare are blocked before reaching the earth surface, it should be assumed that such decay-rate changes could be due to neutrino flux increase from the sun, in which only neutrinos can penetrate the radionuclide. This study employs the Rn-222 radioactive source for the task of solar flare detection, based on the prediction that it will provide a stable gamma ray counting rate. In order to ascertain counting stability, three counting systems were constructed to track the count-rate changes. The signal processing approach was applied in the raw data analysis. The Rn-222 count-rate measurements showed several radiation counting dips, indicating that the radioactive nuclide can be affected by order of magnitude neutrino flux change from the sun. We conclude that using the cooled Radon source obtained the clearest responses, and therefore this is the preferable system for detecting neutrino emissions from a controlled source.”

    https://arxiv.org/abs/2002.02787

    • Mike Serfas July 8, 2020, 9:48

      This notion is incredibly provocative, and immensely appealing for any far-future scenario in which nuclear weapons are absent, but I’m not sure it holds up. People were skeptical ( https://www.discovermagazine.com/the-sciences/scientist-smackdown-are-solar-neutrinos-messing-with-matter ) and there are some other publications not finding effects ( https://www.sciencedirect.com/science/article/abs/pii/S0927650518301671 ).

      Now at least for purposes of science fiction, you could make an argument that these results might have been sabotaged or suppressed … after all, just think of the implications if this sort of effect were real!

      Most of the Sun’s energy output is light, and nearly all neutrinos pass straight through the Earth. Imagine if the same square of light that lands on your couch contained enough secret ingredient to change the decay rate of radioactive isotopes all the way through to the far side of the world. Then if some clever researcher could design a “neutrino laser” (not really, but something that produces a lot of neutrinos of a desired energy and lepton flavor in a collimated beam), it could be built in a bunker deep below the ground and used to blow up a nuclear weapon on the far side of the world. Easier still, a nuclear power plant, since it is at the brink of criticality already — no matter how safe the design, that design assumed isotope decay rates are constant. Or a pool with waste fuel rods and a wide variety of daughter isotopes to choose from.

      The penetration of neutrinos even makes them valid candidates for “planet buster” weapons. For example, if aggregations of vulnerable isotopes could be found deep within Venus, then a sustained assault of this type might release their energy, convert liquid to gas, and cause a buildup of pressure that ultimately split the crust apart and covered the planet in magma about a billion years ago, presumably just before their rebellious colonists on Earth received their comeuppance. (True, even with mythical technology, this would still take an unreasonably long time to accomplish!)

      • Michael Fidler July 9, 2020, 11:53

        Well, the easy way to see if this effects radioactivity is to send some Radon-222 on one of the solar probes that are going close to the sun and see what happens. This may have much more important ramifications for Stellar evolution (Nucleosynthesis).

        What happens before a star explodes and dies: New research on ‘pre-supernova’ neutrinos.

        https://phys.org/news/2020-06-star-dies-pre-supernova-neutrinos.html

        The sensitivity of presupernova neutrinos to stellar evolution models.
        “We examine the sensitivity of neutrino emissions to stellar evolution models for a 15M⊙ progenitor, paying particular attention to a phase prior to the collapse. We demonstrate that the number luminosities in both electron-type neutrinos (νe) and their anti-partners (ν¯e) differ by more than an order of magnitude by changing spatial resolutions and nuclear network sizes on stellar evolution models. We also develop a phenomenological model to capture the essential trend of the diversity, in which neutrino luminosities are expressed as a function of central density, temperature and electron fraction. In the analysis, we show that neutrino luminosity can be well characterized by these central quantities. This analysis also reveals that the most influential quantity to the time evolution of νe luminosity is matter density, while it is temperature for ν¯e. These qualitative trends will be useful and applicable to constrain the physical state of progenitors at the final stages of stellar evolution from future neutrino observations, although more detailed systematic studies including various mass progenitors are required to assess the applicability.”

        https://arxiv.org/abs/2005.03124

        As for all the trillions of dollars spent on nuclear and weapons research, this should be well know and may be why it is a secret.

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