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Planets of Iron, Planets of Ice

How large a planet is depends upon its composition and mass. Earth is largely made of silicates, with a diameter of 7,926 miles at the equator. Imagine an Earth mass planet made of iron and you’re looking at a diameter of a scant 3000 miles. Interestingly, the relationship between mass and diameter follows a similar pattern no matter what material makes up the planet. Running the numbers, an Earth mass planet made of pure water will be 9500 miles across.

Sara Seager (Massachusetts Institute of Technology) has been studying these things as part of a project to model the kind of Earth-size planets we’re likely to find around nearby stars. About the mass/diameter pattern, she says this:

“All materials compress in a similar way because of the structure of solids. If you squeeze a rock, nothing much happens until you reach some critical pressure, then it crushes. Planets behave the same way, but they react at different pressures depending on the composition. This is a big step forward in our fundamental understanding of planets.”

Mass to radius relationship

It’s a needed step, too, because we often speak of Earth-size planets as if they were likely to resemble the worlds we see in our own Solar System. The team, made up of scientists from MIT, NASA and the Carnegie Institution of Washington, wants to throw out that assumption, going back to the nature of the protoplanetary disks we’re seeing around young stars. Its speculations have produced fourteen different compositions, among them pure water ice, carbon, iron, silicates, carbon monoxide and silicon carbide. Corresponding sizes can be calculated for each planet.

“We have learned that extrasolar giant planets often differ tremendously from the worlds in our solar system, so we let our imaginations run wild and tried to cover all the bases with our models of smaller planets,” says Marc Kuchner (NASA GSFC). “We can make educated guesses about where these different kinds of planets might be found. For example, carbon planets and carbon-monoxide planets might favor evolved stars such as white dwarfs and pulsars, or they might form in carbon-rich disks like the one around the star Beta Pictoris. But ultimately, we need observations to give us the answers.”

Image: Astronomers have calculated the diameters of various types of planets given certain compositions and masses. This image shows the relative sizes of six different kinds of planets with different compositions, and depending on whether they have the same mass as Earth, or five times the mass of Earth. Note that the 5-Earth-mass planets are larger than their 1-Earth-mass counterparts, but they are not five times larger due to the gravitational compression that occurs when a planet’s mass is increased. The planets are shown silhouetted against the Sun, as if they are transiting planets seen from afar. Credit: Marc Kuchner/NASA GSFC.

Comparing a planet’s size and mass with the help of planetary transits is a first step toward determining its composition. The French COROT satellite should be capable of finding planets not much larger than Earth as they pass across the surface of their star as seen from the spacecraft. One tricky call will be a silicate planet vs. a carbon planet — the two model out to roughly the same size for a given mass. Maybe by the time we need to make such distinctions we’ll have the James Webb Space Telescope around for help.

And this comment in the paper’s conclusion on a definition for ‘super Earths’ is interesting:

Planets above the H2O [mass-radius] curve must have a significant H/He envelope. We can therefore easily distinguish between exoplanets with significant H/He envelopes and those without, as is the case for GJ 436b. We therefore define a “super Earth” to be a solid planet with no significant gas envelope, regardless of its mass.

The paper is Seager, Kuchner et al., “Mass-Radius Relationships for Solid Exoplanets,” now in press at The Astrophysical Journal, with publication due in late October (abstract).

Comments on this entry are closed.

  • andy September 26, 2007, 18:09

    I wonder if life would be possible on some of the more unusual planet types. A fairly “mundane” scenario among the “nonstandard” planet compositions would be an iron planet in the habitable zone: if comets deliver enough water to it, could life begin in the oceans?

  • philw September 26, 2007, 18:09

    If the variety of extra-solar Earth-like planets is similar to the variety of Jovians so far detected as the authors speculate, our solar system and Earth with land masses and oceans may well be rare.

  • Adam September 26, 2007, 19:57

    Hi andy

    Iron planets with lots of carbon monoxide might end up with oceans of iron carbonyl, which could be the basis for metallic lifeforms – it decomposes into metal and gas at about 100 degrees, so weird enzymes might allow decomposition at lower temperatures? Natural “robots” could evolve. Not an original idea, sad to say – Stephen Baxter came up with that one.

    But all sorts of odd planets will make life very interesting.

  • andy September 27, 2007, 4:01

    Adam, was that the Gaijin world in Manifold:Space? I seem to remember there were all sorts of weird and wonderful habitats postulated in that book.

  • Adam September 27, 2007, 5:03

    Hi andy

    Yes it was 00:00:00 or whatever they called it. The iron crab crawling out of the iron carbonyl sea in a light shower of iron fillings has stuck in my mind ever since.

    I’m glad someone else around here has read that book. I really love Baxter but a lot of people hated the “Manifold” series. “Space” was the more interesting of the three, but all were worthwhile in their exploration of different features of what being human is about, what the Multiverse might mean, what Fermi’s Paradox is really telling us.

  • andy September 27, 2007, 13:27

    Problem with Manifold:Space is that it is a fixup of (at least?) two different series of short stories which aren’t particularly consistent with each other (how exactly do the saddlepoint transporters survive the extinction events that wipe out all life and advanced technology in the galaxy?). It got a bit confusing, though in a sense it was the most optimistic of the trilogy. Manifold:Time was quite depressing and Manifold:Origin was… blegh.

    Still, none of them pulled a Titan-style ending, which can only be a good thing.

  • Adam September 27, 2007, 17:23

    Hi andy

    I liked the end of “Titan”. Why not? In five billion years anything could happen. One of Baxter’s other short stories has the Titanians emulating long dead humans, but actual physical “survival” is perhaps more likely.

    One thing that annoyed me was that Baxter’s trip calendar for the “Discovery” was just Cassini’s original timetable. He has enough contacts at NASA to get an alternative orbital plan – JPL studies plenty – so it does suggest some contempt for the readers who actually bother about accuracy.

    It’s a flawed novel, that drags in patches, but why is the ending so hard to stomach?

  • andy September 27, 2007, 19:51

    The ending of Titan is totally incongruous and doesn’t fit at all well with the rest of the story. It’s as if the editor complained that the end of the novel was too grim so Stephen Baxter tacked on something happier… but for me it blasted suspension of disbelief out of the water (tholin?)

  • Adam September 27, 2007, 21:12

    Hi andy

    Not an unfair review, but it is consistent with Baxter as it is. He has several short stories with present day protagonists being “revived” or recreated in the deep future, and almost no explanation given as to how, why or by who.

    It does require the Ammonos to have found and preserved their remains for some billennia as Titan thawed before their “reanimation” which is more than a little implausible. But then the Ammonos are a rather delightful alien creation, so I can forgive Baxter that gaffe.

    Not my favourite Baxter novel. I only bother re-reading the end.

  • hiro September 28, 2007, 13:42

    Ring is my favorite Baxter’s novel so far, especially the part about journey from Sol to Great Attractor. The ending of Titan is a little bit off. Otherwise, it’s a good novel.

  • Adam September 28, 2007, 19:17

    Hi All

    Perhaps a bit more germane to the original topic, but Titan is a good example of a 50:50 ice/rock planet, as are Ganymede and Callisto. Oddly enough Pluto and Triton are more rock than the bigger moons, and that’s worth investigating. We know Pluto lost its excess ices as Charon and the other moons, but what happened with Triton? It has one of the youngest, most thermally altered surfaces of all the outer planet moons – what happened?

  • ljk October 14, 2007, 20:14

    New Worlds on the Horizon: Earth-Sized Planets Close to Other Stars

    Authors: Eric Gaidos, Nader Haghighipour, Eric Agol, David Latham, Sean Raymond, John Rayner

    (Submitted on 11 Oct 2007)

    Abstract: The search for habitable planets like Earth around other stars fulfils an ancient imperative to understand our origins and place in the cosmos. The past decade has seen the discovery of hundreds of planets, but nearly all are gas giants like Jupiter and Saturn. Recent advances in instrumentation and new missions are extending searches to planets the size of the Earth, but closer to their host stars. There are several possible ways such planets could form, and future observations will soon test those theories. Many of these planets we discover may be quite unlike Earth in their surface temperature and composition, but their study will nonetheless inform us about the process of planet formation and the frequency of Earth-like planets around other stars.

    Comments: to appear in Science, October 12, 2007

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0710.2366v1 [astro-ph]

    Submission history

    From: Eric J. Gaidos [view email]

    [v1] Thu, 11 Oct 2007 23:58:24 GMT (271kb)


  • ljk December 3, 2007, 14:37

    Habitable Climates

    Authors: David S. Spiegel, Kristen Menou, Caleb A. Scharf (Columbia University)

    (Submitted on 30 Nov 2007 (v1), last revised 3 Dec 2007 (this version, v2))

    Abstract: The Earth is only partially habitable according to the standard liquid-water definition. We reconsider planetary habitability in the framework of energy-balance models, the simplest seasonal models in physical climatology, to assess the spatial and temporal habitability of Earth-like planets. In order to quantify the degree of climatic habitability of our models, we define several metrics of fractional habitability.

    Previous evaluations of habitable zones may have omitted important climatic conditions by focusing on close Solar System analogies. For example, we find that model pseudo-Earths with different rotation rates or different land-ocean fractions generally have fractional habitabilities that differ significantly from that of the Earth itself. Furthermore, the stability of a planet’s climate against albedo-feedback snowball events strongly impacts its habitability. Therefore, issues of climate dynamics may be central in assessing the habitability of discovered terrestrial exoplanets, especially if astronomical forcing conditions generally differ from the moderate Solar System cases.

    Comments: 39 pages, 11 figures, 2 tables, submitted to ApJ. 1 typo corrected

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0711.4856v2 [astro-ph]

    Submission history

    From: David Spiegel [view email]

    [v1] Fri, 30 Nov 2007 03:19:07 GMT (560kb,D)

    [v2] Mon, 3 Dec 2007 06:48:28 GMT (560kb,D)


  • ljk January 10, 2008, 11:23

    Earth: A Borderline Planet for Life?

    Our planet is changing before our eyes, and as a result,
    many species are living on the edge. Yet Earth has been
    on the edge of habitability from the beginning.

    New work by astronomers at the Harvard-Smithsonian
    Center for Astrophysics shows that if Earth had been slightly
    smaller and less massive, it would not have plate tectonics –
    the forces that move continents and build mountains. And
    without plate tectonics, life might never have gained a
    foothold on our world.

    Full article here: