Once Around Iron Planets

[00:00:02]Once around iron planets.

[00:00:05]If you've learned anything from watching any of my other videos, you'll know that I'm very interested in exoplanets of all types, and this particular group really intrigued me.

[00:00:17]So, the idea behind iron planets is that these are a type of terrestrial planets, except they have a particularly large amount of iron and not very much rock.

[00:00:30]Not Not much in the way of a silicate mantle rock. It's basically various metals combined with silicon oxides to make silicate.

[00:00:41]And as such, they will be much denser than the likes of Earth or Venus, which are around 5 5 and 1/2 5.8 sort of grams per cubic centimeter.

[00:00:53]These guys are going to be heavyweights.

[00:00:55]And so, we've got a few sort of examples of the different types of exoplanets shown there, and pure iron planets are probably at the small end of things, and we will see why that is.

[00:01:11]So, if we have a look in our solar system, our planets do have iron cores.

[00:01:17]Certainly, Venus, Earth, and Mars have iron cores, but aren't really iron planets.

[00:01:23]However, Mercury probably would count as an iron planet almost. It doesn't have the same degree of thick rocky silicate mantle covering those iron cores. You can see even our moon has a little iron core, but relatively small, much more rock per unit of metal in the center there. But Mercury dominated by its iron core.

[00:01:53]So, there are two ways in which we think this might happen. One is that the planet could be close to its star in the hot inner region when it's formed and really so hot during the formation that only the very refractory, the non-volatile materials such as metals and silicates can condense to form a planet at all.

[00:02:23]Everything else is far too volatile and gets blown away. A lot of the light metals, sodium, potassium, lithium, things like that will get blasted free taking various materials with them.

[00:02:37]Quite a lot of the carbon and oxygen will disappear as carbon dioxide. So, this would leave metal and rock as the real only two components.

[00:02:48]And the other way that it can happen is as the result of a giant impact with the rocky layer, the mantle, being stripped free by an impacting collider blasting the outer layers into space.

[00:03:04]So, the process of planet formation is of course that we start with a solar nebula, the sun forming in the center and a cloud of material around it.

[00:03:17]And that cloud gets fractionated by the heat from the star leaving the refractory, dense, non-volatile materials in the center and progressively blowing the more and more volatile things further and further away.

[00:03:34]So, if there is a system that would form with a very high proportion of metals, then it can form these metal-rich planets right in the center.

[00:03:48]Now, there's even an idea that if these planets became massive enough, perhaps in a very rich nebula, then you could have a process whereby the gravity of the iron core would collapse the object into an iron white dwarf or even directly down to a neutron star if it was massive enough. And so, the traditional story that white dwarfs and neutron stars are only ever formed in the death of either sun-like stars or more massive ones may not be the whole story, and it may account for some of the weirder white dwarf stars that we think we seem to be finding.

[00:04:34]But, I've strayed rather away from planets. So, let's have a look at Mercury. This is a false color image of the surface of Mercury.

[00:04:44]The color is based on the minerals and the metals that are present in the surface, with blue being titanium and the orangey color down to iron.

[00:04:57]And you can see that the iron seems to have burst forth from inside, where there have either been giant impact scars left by collisions or by volcanoes. And we do know that Mercury was volcanically active until fairly recent times and may indeed occasionally erupt today.

[00:05:19]Now, the core of Mercury is partly liquid iron and takes up about 70% of the overall mass of the planet, and it's probably the result of a giant impact.

[00:05:33]Ripped the surface material away, and rather than that material then forming a moon around Mercury, it was lost out into space.

[00:05:46]Another suggestion is that there might be a thick layer of diamonds underneath a thin crust between there and the metal core, perhaps 10 miles thick.

[00:05:58]Um and maybe that's where a lot of Mercury's carbon is locked up.

[00:06:03]Well, that would be interesting. And again, this is somewhat speculative, but uh uh more research needed to try and prove that one, I think.

[00:06:12]If we look further afield to planets around other stars, we can start to look for more iron planets, more super-Mercurys out there or or exo-Mercurys orbiting around their parent stars. And this is perhaps a case in point.

[00:06:29]The star system Gliese 376, around about 30 light-years away, visible to telescopes in the southern constellation of Vela there. Um and this was determined to be a red dwarf with about half the Sun's radius, 0.457.

[00:06:50]Cool at 3,522 K, and and very dim. Result of being uh half the radius is that you've got 1/8 of the volume and the 1/4 of the surface area. And the result of this is a very low power output indeed, just uh under 3% of the energy that the Sun generates.

[00:07:16]Now, normally we can date red dwarfs if they're a bit smaller than this. This one's a little bit on the large side for red dwarfs, which makes things difficult. It makes for the core partly being isolated from the surface. So, it's a little difficult to see just how much of the uh nuclear fusion has gone on in the core and created helium. Um if that's not brought to the surface, um then the surface looks younger than the star actually is.

[00:07:50]And in this case, the age estimates range from 60 million years to 8 billion. So, absolutely enormous factors. So, really nobody's got the faintest idea would be a better way of saying it.

[00:08:04]Now, the terrestrial planet finder TESS had a look at [clears throat] this one and discovered a planet b in 2019 and it was orbiting around the star in just 8 hours. Very, very close indeed. 0.007 astronomical units. That's very, very tight as an orbit.

[00:08:29]And this one's been given a name.

[00:08:33]Tau Hei The host star has a name An Anu Ka based on the names for some Chilean wildflowers. I'm not going to try and pronounce the two Latin names. You can read them on the screen there.

[00:08:50]And these only bloom between 7 and 8 hours each year, which is an echo really and a nod towards the planet's orbital period of 7.7 hours hurtling around its star.

[00:09:05]>> [snorts] >> It's 3/4 the diameter of Earth and 50% as massive. Now, 3/4 of the diameter, you cube that and you find you can compute the density and it seems to be nearly double the density of Earth, 10.2 g per cubic centimeter.

[00:09:26]So, that suggests a metal core taking up 91% of the mass of the planet. A true iron planet if ever there was one.

[00:09:38]And hot, 1,500° C, enough to melt the outer layers, enough to melt any iron that made it to the surface. So, truly extreme indeed.

[00:09:55]Now, there are two other planets in this system, a four Earth-mass super-Earth planet that orbits in 11 days, uh and a six Earth-mass in a 34-day orbit whizzing around out there as well. And so, quite an interesting system, and it'd be great to be able to follow up on some of these planets and look at them in more detail as we are able to uh get direct imaging of them perhaps, or even turn good old James Webb Space Telescope to having a look.

[00:10:32]Another one is Kepler-974.

[00:10:35]Now, this is a bit further away, 400 light-years, not really in our cosmic backyard.

[00:10:41]Another red dwarf M-type star, just under half the size of our sun Sol, and hosts a couple of planets.

[00:10:53]974b and c, the first one in a 4.2-day orbit, very close to the star, 1.6 Earth diameters, so a super-Earth-sized object, but three times the mass, and that's uh fairly dense if you work it out.

[00:11:15]And the second one, smaller than the Earth, 0.6 times the diameter, and that makes it one of the smallest confirmed exoplanets to date.

[00:11:27]Again, more of more of these examples seem to be being located. There's a star HD the Henry Draper catalog with the designation number 23472.

[00:11:40]That has two super Mercury's. HD137496B is another dense hot super Mercury and LHS 3844B another iron rich super Mercury. So these these super Mercury iron planets do seem to be turning up as really quite common features in close to their stars and maybe this is pointing to either that that is very dangerous place to be in terms of collisions in the planet formation process or that the material that can condense in those regions is really only the very high melting point material such as iron. So we get a lot of these super Mercury or Mercury-like planets with huge iron cores close into their stars.

[00:12:37]So thanks very much for listening and I'll see you on the next one.