Physics nerds: Is it possible to have a planet smaller yet denser than our own?

[danzibr]

And by possible I mean reasonable. And by our own I mean Earth.

I was looking up planet stuff (just read about Theia... interesting) and came across the density of celestial bodies. Some of them were quite surprising. I didn't expect (that I necessarily thought it to not be the case) Earth to be the densest. Some people on some other forums suggested this might be because of the collision of Theia and Earth. They also talked about for super huge planets (super-Jupiters), and how, despite potentially being gas giants, they can be very, very dense because of compression at their centers.

But I'm wondering... could you have a planet quite a bit smaller than Earth, but also quite a bit more dense? I guess it'd be the opposite of a super-Earth.

[Fragenstein]

Yes (http://www.nature.com/news/super-dense-celestial-bodies-could-be-a-new-kind-of-planet-1.12599).

Specific mention of a radius similar to Earth is made, so one would have to assume that a smaller one (even if only slightly smaller), is possible. I'm not so sure about your definition of 'quite a bit smaller', though.

[danzibr]

Yes (http://www.nature.com/news/super-dense-celestial-bodies-could-be-a-new-kind-of-planet-1.12599).

Specific mention of a radius similar to Earth is made, so one would have to assume that a smaller one (even if only slightly smaller), is possible. I'm not so sure about your definition of 'quite a bit smaller', though.
Oooh, thanks.

I just mean... someone could say, ``Hey, here's a planet 99% the size of earth, but with 101% the density.'' This I'm not interested in.

[Fragenstein]

Oooh, thanks.

I just mean... someone could say, ``Hey, here's a planet 99% the size of earth, but with 101% the density.'' This I'm not interested in.

Cool. I was hoping that the phrase “If the process is short, you end up with a very compressed super-Earth” qualified.

That article is a year old, though. I wonder where they are now with the research?

[Tirunedeth]

Less exotically, a small planet with a large iron core could be significantly denser than Earth. Mercury is about 1/20 the mass of Earth, yet it is almost as dense. A larger planet with a similar large iron core could easily be denser than Earth. I ran some numbers (http://www.giantitp.com/forums/showsinglepost.php?p=15000987&postcount=1214) on a particularly extreme version of such a planet, and found that you could get a density of about 8800 kg/m3 with a mass of only 40% that of Earth. For a planet that, somewhat more reasonably, consists of about 70% iron and 30% silicates (roughly the composition of Mercury), we get a density of 7600 kg/m3 for a mass of about 1/3 that of Earth.

Note that because of how dense iron is relative to silicates, a high-iron body can be more dense than Earth down to pretty much any size. Consider, for example, 16 Psyche (http://en.wikipedia.org/wiki/16_Psyche), which is about 0.00038% the mass of the Earth, but 20% denser.

[hamishspence]

If you allow for denser metals than iron - you could have an even smaller planet than that.

Problem is that dense metals are rare - it's hard to justify something made of mostly them - unless it was created artificially.

[factotum]

Yeah, that's kind of the big problem there, considering elements heavier than iron only get created in relatively small quantities in supernova explosions. (I don't think there's any element with a lower atomic number than iron which is denser than it, is there?).

[Kato]

Yeah, that's kind of the big problem there, considering elements heavier than iron only get created in relatively small quantities in supernova explosions. (I don't think there's any element with a lower atomic number than iron which is denser than it, is there?).

Short check says no. Iron has a pretty low number, though. My science in the respect is too rusty but I'm not sure if iron in general is a very common element in the universe... so in theory it's not too hard to have a planet which is denser by just being a rather small ball of iron. Or of any other material The universe is huge, there are many things that can happen and little is stoping it from doing so.

[factotum]

Short check says no. Iron has a pretty low number, though. My science in the respect is too rusty but I'm not sure if iron in general is a very common element in the universe...

Iron is most definitely going to be more abundant than anything above it in the periodic table, because it's the end point of fusion reactions. You can't fuse iron into anything heavier without actually putting energy *in* to the system, which is why those heavier elements only form in supernova explosions--iron, on the other hand, will be produced as the final stage of its lifecycle for any sufficiently massive star.

[Max™]

With regards to denser materials forming what might be called a planet.

https://en.wikipedia.org/wiki/PSR_J1719-1438_b

Basically it was a pulsar and a companion which had lost almost all of the outer envelope material leaving behind an extremely dense carbon core (probably with iron further within) which is around 4 times the radius of Earth or less but masses about as much as Jupiter.

Over time it should shrink further as it cools, but as it is mostly carbon left and should be in a crystalline form it's been nicknamed Lucy, as it is a giant diamond in the sky.

[Daishain]

Short answer, yes, but finding one that is significantly so is not likely.

Technically, there are boatloads of asteroids that are smaller but more dense, though I suspect that's not quite what you mean.

Given the available materials, you don't often find small planets with a core much denser than the standard nickel-iron. The catch with larger bodies like planets is that once a forming body gets above a certain mass, you start gravitically attracting every scrap particle around, hydrogen, ice, silica, etc. Increasing mass even further, but reducing overall density.

One cool, if still unlikely, possibility would be a rogue planet mainly composed of diamond and other dense materials. Large gas giant like Jupiter gets ripped to shreds by a near collision with another heavy body, with the end result being its solid core flung out out in a different direction to the rest, and later captured by a different solar system.

[SLOTHRPG95]

Less exotically, a small planet with a large iron core could be significantly denser than Earth. Mercury is about 1/20 the mass of Earth, yet it is almost as dense. A larger planet with a similar large iron core could easily be denser than Earth. I ran some numbers (http://www.giantitp.com/forums/showsinglepost.php?p=15000987&postcount=1214) on a particularly extreme version of such a planet, and found that you could get a density of about 8800 kg/m3 with a mass of only 40% that of Earth. For a planet that, somewhat more reasonably, consists of about 70% iron and 30% silicates (roughly the composition of Mercury), we get a density of 7600 kg/m3 for a mass of about 1/3 that of Earth.

Note that because of how dense iron is relative to silicates, a high-iron body can be more dense than Earth down to pretty much any size. Consider, for example, 16 Psyche (http://en.wikipedia.org/wiki/16_Psyche), which is about 0.00038% the mass of the Earth, but 20% denser.

For an example of a planet made largely out of iron but closer to Earth's size, see KOI 1843.03 (http://iopscience.iop.org/2041-8205/773/1/L15/). The planet in question has a radius roughly 60% the size of Earth's, and the density is roughly 27% greater than that of Earth's. I'm not sure if this fits what you're looking for, but it is apparently less than 30% silicates, and large enough to be a planet instead of an asteroid.