How Big is the Biggest Possible Planet?

By Corey S. Powell | August 4, 2017 5:37 pm
KELT-11b, one of the physically largest objects known, is 40 percent wider than Jupiter and has the density of styrofoam. (Credt: Walter Robinson/Lehigh University)

KELT-11b, one of the physically largest objects known, is 40 percent wider than Jupiter and has the density of styrofoam. (Credit: Walter Robinson/Lehigh University)

Last week, a team of astronomers reported the first potential discovery of an exomoon–a satellite orbiting a planet around another star. Part of what is so striking about the report is the scale of this possible planet-moon system. In this case, the “moon” appears to be about the size of Neptune; the planet it orbits is some 10 times the mass of Jupiter, or about 3,000 times the mass of Earth!

The system pushes at the limits of how we normally categorize objects in space and invites questions about where we stand in the scale of things. What is the biggest possible planet? Viewed through the full range of possibilities, is Earth a big planet or a small one?

There are two ways to come at the question, depending on what you mean by “big.” If you think of the size of a planet in terms of mass, then there is a specific but rather technical answer. Planets are defined as bodies that do not generate their own energy from nuclear fusion. Any planet more than about 13 times the mass of Jupiter (4,o00 Earth masses, roughly) generates enough heat and pressure in its core to trigger limited fusion reactions of deuterium, a heavy isotope of hydrogen. At that point, the object is considered a brown dwarf instead of a planet.

The nuclear-ignition boundary between planet and brown dwarf is based on hidden interior processes, however, and it is not at all obvious from the outside. The critical mass for fusion also depends on on the mix of elements inside the object. For a plausible range of compositions, the cutoff point could be anywhere from 11 to 16 times the mass of Jupiter. If you want the full hairy details, they are here.

Outside of that gray zone, though, things turn fairly black-and-white. Anything well below the lower limit of 11 Jupiter masses (3,500 Earth masses, give or take) is indisputably planetary. Anything well above that high end, on the other hand, is definitely capable of creating some energy of its own and no longer fits the standard astronomical definition of a planet.

The strange constancy of planetary sizes: Objects more massive than Jupiter get denser but not larger, all the way up to red dwarf stars that are 80 times Jupiter’s mass. (Credit: Robert Hurt/IPAC-Caltech, with new annotations)

But there’s also a more literal take on the question: Is there a limit on how physically large a planet can be? Here there is a definite and rather surprising answer. Jupiter is 11 times the diameter of Earth, and it turns out that is about as large as any planet can be! If you kept dumping more matter on Jupiter, it would not get any larger. Instead, gravity would crush its mass together more tightly and efficiently.

Through the whole range from a Jupiter-mass planet to the brown dwarf boundary, all the way up to the lowest-mass dwarf stars (about 70 times the mass of Jupiter, the point at which sustained lithium and hydrogen fusion occurs), the size barely budges. All of these objects are within about 15 percent of the same diameter. That constancy has some odd consequences.

Take, for example, the star Trappist-1A, which was in the news recently because it has seven Earth-size planets orbiting it. Trappist-1A is a red dwarf, just 1/2000th as bright as the sun, but it’s genuine star, no question. It is powered by steady, sustained nuclear reactions that will burn for a trillion years or more. It is 80 times as massive as Jupiter.

On the other hand, Trappist-1A is less than 10 percent larger in diameter than Jupiter. Put those two details together, and you quickly realize that this little star must be extremely dense–as indeed are all extremely dim, cool red dwarf stars. Trappist-1A is about 60 times as dense as Jupiter. Translated into more familiar terms, this little ball of glowing hydrogen plasma is 25 times as dense as granite, and more than six times as dense as lead.

Although Trappist-1A is sustaining fusion reactions, it is doing so at such a low rate that the outpouring of energy barely props up the star’s bulk against the pull of gravity. Even more extreme is the red dwarf star EBLM J0555-57Ab, recently measured to be 15 percent smaller than Jupiter, about the size of Saturn. It is the tiniest known mature star (as opposed stellar cinders like white dwarfs or neutron stars), and it is 17 times the density of lead–188 times the density of water!

There are some notable exceptions to this pattern. Some planets that orbit extremely close to their stars get overheated and puffed up to abnormally large diameters. The ‘styrofoam’ exoplanet KELT-11b is 40 percent larger than Jupiter despite having just 1/5th the mass. HD 100546bn is about 7 times the diameter of Jupiter, which would make it the largest-size planet known, but with some caveats: It appears to still be forming, and the current observations leave a lot of uncertainty about its nature. It may actually be an infant brown dwarf.

Outside of such outliers, the pattern is ironclad. As planets get more massive, they don’t get physically larger. They just get much, much more tightly packed, until they ignite and are no longer considered planets at all.

Follow me on Twitter for more science news: @coreyspowell


  • Ugo

    great article.
    thanks 🙂

  • jonathanpulliam

    Trummpist-45 is the densest presidentoid yet observed.

    • Henrique Luiz

      Make this planet great again

      • Uma pizoa

        This planet is great! The only problem?! People suck choosing Presidents!! Você deve saber bem disso por esses lados…né Henrique!!

        • Henrich

          I’m anarchist. The problem isn’t the presidents, is the whole State.

    • Al Yasit

      If that is the case, the previous one would obviously be the densest NON-presidentoid yet observed.

    • JamieMcG

      I think Prof Trump is wiser in the ways of astronomy than we give him credit for – I believe he would categorize these planets as “bigly”

  • Bob Schmellen

    What about rocky planets? Assuming that their material isn’t fusible, is there an upper limit on their size?

    • Raphael Souza de Melo

      Good question.

    • Whatevs

      This article suggests it’s about 10 earth masses before the planet starts pulling in surrounding gasses.

      • MarkinGermany


      • Bob Schmellen

        Thanks for sharing that link! This suggests how Jupiter may have formed. It would be interesting to see if there’s a theoretical upper limit on rocky planet mass, assuming accretion of local solid matter sans gases. Largest possible ball of iron, that kind of thing. Google, here I come!

  • Billybob9

    Since childhood I’ve loved these “how big is big” and “how high is up” stories.

    This one points to a constant we need to know about ~ that at a certain point the combination of size and mass results in a denser planet. That, by itself, has implications for the possible sizes of living beings on planets ~

    • OWilson

      Size is relative, not absolute :)

      If everything was to suddenly double in size, including baryions and leptons and their fields, we would have no evidence by which to measure the change.

      On a different level, our “Expanding Universe”, the expansion of space, could also be explained by the shrinking of matter itself?

      It may be meaningless to point out, but it is good to keep in mind that we really have no absolute benchmark for any physical property, size, time, space, distance, or motion!

      • Ty Harris

        That’s what she said.

      • KieSeyHow

        Excellent thought exercise indeed. The absolute measurement of anything, requires comparative reference. If all references scale, then the relative measurement would remain the same. This is why I always said, if we really did colonize other planets, such things as weights and time would be a very confusing and messy process indeed! We would not only need currency conversion charts, but charts for most other popular measurements as well.

        • OWilson

          That’s one of the reasons travel is such a great education.

          I would only grant a post grad degree to one who has traveled the entire Earth in a year, on a very limited budget!

          Here you can get an MBA, without ever leaving the classroom, much less having had a real job! :)

          • KieSeyHow

            Well said!

  • Mike Richardson

    Fascinating stuff. It makes sense that red dwarves would be more dense than their larger brethren, but I had no idea they could be denser than rocks or even lead. Not nearly as dense as white dwarves or the most extreme stellar remnants short of black holes — neutron stars — but they definitely rank as the most massive per volume main sequence stars.

    I also was excited about the potential exomoon around Kepler 1625B, as the discovery of a moon that size would raise the likelihood of earth -sized moons around large gas giants. Since we’ve already found gas giants orbiting in the habitable zones of other stars, this in turn means potentially habitable moons — something often portrayed in science fiction films from Star Wars to Avatar, but considered speculative at best, and fantasy from more conservative members of the astronomical community, based on the examples of gas giants and their moons found in our own star system. Previously, similar arguments were made against planets existing in binary star systems, but sure enough, the observations of the past decade or so have proven that such planets exist. Makes you wonder what else science fiction may have gotten right in more imaginative portrayals of alien star systems that we haven’t yet discovered.

    • Laurie

      If the gas giant is “fusionable” and radiating some heat around it then the “moons” that orbiting it at correct distance ,could develop liquid water if had enough gravity to keep water from escaping…

      And that system already exist, such article mention Trappist-1
      And though it will be called “mini-star” not gas giant, just as article described it.

  • KieSeyHow

    Absolutely fascinating!!!!

  • Craig

    Sounds like we should be dumping all our trash from the planet into Jupiter and we would eventually get another heat source for the solar system and likely make many new worlds much more habital than they are currently.

    • Mikey Clarke

      It’s a nice thought, but the fuel expense of freighting our trash up and out of Earth’s gravity would negate that.

      • Craig

        Sorry I should have explained more clearly. We will be using a giant railgun built in the midwest and supplied with electrical energy from a massive field of solar arrays and wind farms combined with battery storage in order to “launch” the trash capsules into earth orbit where they will be retrieved by a “space truck” to give them a boost on their path to Jupiter. C’mon man, easy peasy.

        • KieSeyHow

          I was going to post a rail-gun comment, but you beat me to it. Yes, that is the most practical solution. However, our technology is not quite there yet. Perhaps this is another project the private sector can take on effectively.

          • Craig

            Yeah, you haven’t seen the things I have likely. Working at INEL twenty years ago we could easily achieve this goal with a nuclear reactor DESIGNED to charge and discharge energy rapidly as would be required for a laser or a rail gun. Technology is not the issue, resistance to change and need to recoup the massive investment in rocket science is the only thing preventing us from fully implementing this tech. The scale WOULD be something previously unseen in orbital delivery services, even compared to the gargantuan Space Shuttle and latest heavy payload rocket engines.

          • KieSeyHow

            Yes, exactly. With a precisely calibrated system, it would be possible to fire a payload and have it arrive in any orbit gently or captured at nearly zero velocity by a craft that is waiting to receive it. It could then be loaded into another rail-gun, and fired at ANY destination in the solar system through a basic application of orbital mechanics. A simple cost-effective two-stage waste management system.

  • Uma pizoa

    Very impressive!! How can we be so small and so arrogant in this universe!!

    • Mikey Clarke

      What’s the point in being small if you don’t get to be arrogant?

  • Daroga Jee

    When we can start extraction and exploitation of these planets as raw material to form out own planet…


Out There

Notes from the far edge of space, astronomy, and physics.

About Corey S. Powell

Corey S. Powell is DISCOVER's Editor at Large and former Editor in Chief. Previously he has sat on the board of editors of Scientific American, taught science journalism at NYU, and been fired from NASA. Corey is the author of "20 Ways the World Could End," one of the first doomsday manuals, and "God in the Equation," an examination of the spiritual impulse in modern cosmology. He lives in Brooklyn, under nearly starless skies.


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