A Nightmare Planet for Fictional Terrans & Real Astrophysicists Alike

I read a few anthology comics for my blog Bad Background ScienceJourney Into MysteryStrange TalesTales of SuspenseTales to Astonish (lots of tales…). This is because a lot of Marvel’s superheroes didn’t start with their own titles. However, I’ve only ever written a blog post about one non-superhero story, from Amazing Fantasy #15 (because I couldn’t find anything to write about science-wise in the first appearance of Spider-Man…), and I’m pretty sure it was the second post I ever made. The reason why should be fairly obvious – no one cares about those.

But every once in a while I see one that could use some scientific commentary. They’re usually space-related, because that’s my jam, but not so different from content that’s either popped up in one of the superhero stories already or I’ve a feeling they’re bound to show up sooner or later.

I don’t know if a superhero will have to deal with a planet 2000 times bigger than the Sun, inhabited by a population of humanoids 1000 times bigger than your average man.

STRANGE TALES ANNUAL 02 063

The Deets (Strange Tales Annual #2)

The year is 2500; mankind has been at peace for a while, relies on solar energy, can cure most diseases, and the few soldiers left in existence are seen as a complete waste of taxpayer money because they do absolutely nothing. But when this nightmare of a planet passes the solar system, 12 of them are sent to the surface (an 18-hour trip) to find out what the giants want with the Earth. Turns out everything on the planet runs on at a much slower time, so everything (except the humans) appears frozen in place; it’ll take a month for the planet to pass the solar system from the perspective of Earth, but a “twinkling of an eye” to the planet’s inhabitants. This recon mission somehow makes everyone back home love the military (…but…but they didn’t do anything…).

 

How Big is Too Big?

Our Sun has an approximate radius of 700,000 km. I say approximate because

  • it’s not a perfect sphere; since the giant ball of plasma rotates, mass that would normally prefer to hang out wherever it darn well pleases gets drawn toward the equator – just like the Earth, or a human being who spins around really fast and watches their limp arms rise up. That being said, its oblateness it actually pretty tiny – the difference between the equatorial and polar diameters is only about 10 km.
  • the Sun doesn’t have an actual physical ‘surface’. Astronomers define the Sun’s ‘surface’ as the inner edge of a specific layer called the photosphere. That’s because below this layer, the Sun is opaque in the visible part of the electromagnetic spectrum; it’s the ‘surface’ where it appears the Sun’s light is being emitted. But again, it’s not a definitive opaque/transparent line.

While the largest object in the neighborhood,1 the Sun is by no means the largest star in the known universe. You might have come across one of many videos or gifs that start out comparing the sizes of small rocky bodies and end up showing the largest known star. The current record holder is UY Scuti, with an estimated radius about 1,700 times bigger than the Sun (If you plopped it down in place of our own star, its photosphere would engulf Jupiter). Measurements for stars this large are uncertain and are regularly updated – the old record holder was VY Canis Majoris with an estimated 1,800-2,100 solar radii (aka R), but it’s now down to 1,420 R(plus or minus 120), and may be as low as only 600 R.

However, there are astronomical objects out there that can reach and surpass that 2000 Rsize. They’re supermassive black holes (SMBHs). The closest quasar,2  named H1821+643, has a SMBH with an event horizon about 89 billion km in radius. The furthest Pluto gets from the Sun is 7.4 billion km (i.e. the EH is 12.7 times further than that). UY Scuti’s radius? 1.2 billion km.

Granted, in 1963 black holes were considered theoretical at best. In fact, the term ‘black hole‘ didn’t show up in print until early 1964, in Ann Ewing’s article “‘Black Holes’ in Space”, but didn’t really hit the ‘mainstream’ until John Wheeler started using it three years later.

It’s been suggested the largest a SMBH can get (roughly speaking) is 50 billion times more massive than our sun, which would have an event horizon 150 billion km in radius. If we’re going to talk about dwarfing everything in the universe, supermassive black holes are the way to go.

At least assuming you can’t use entire galaxies. Those are way bigger…

 

What kind of Nightmare is this?

Lets’s assume the planet’s as big as the comic says it is. As I mentioned before, certain celestial objects can be that size. But with size, comes mass.

With stars, mass and radius don’t have the same relationship that more down-to-earth objects might (i.e. that mass is proportional to the radius cubed), and it varies depending on the star’s initial mass and where it is in its stage of life. For example, consider two stars in Orion: Betelgeuse is about 888 Rand 12 M3 , but Rigel A is 80 Rand 23 M. UY Scuti has an estimated mass only around 10 M. Astronomers have struggled to find any star over 150 M. So, if we haven’t found stars 2000 times more massive than our own, we certainly haven’t found a planet that size.

The largest planets we have found are not Earth-like (i.e. rocky bodies); they’re larger versions of Jupiter. Astronomers are finding new exoplanets as I type, but the largest planets with similar densities to Earth are only about twice (maybe 2.5) as big as Earth. There are a lot of exoplanets with masses in-between Earth and Neptune (the next largest planet in the solar system) with unknown compositions; they might be tiny ‘gas giants’, or large ‘terrestrial planets’.

But the writer clearly intends this planet to be an Earth-analog, so let’s calculate its mass as such. To be made of the same materials as Earth, and be that big, it would need to have a mass of 6.2×1040 kg.

That’s about 31 billion times more massive than the Sun, roughly equivalent to that supermassive black hole in the center of quasar H1821+643. (Aside: Even if we only wanted the planet to be 2000 M, black holes are still the only astronomical objects that we know can be that massive.)

With a mass of 31 billion M, and a radius 2000 R, gravity is so strong that even 92 billion km away from the surface light can’t escape. (With a mass akin to a SMBH, what would you expect?) Not only should people on Earth not be able to see the planet and report it’s “inhabited by soldiers of enormous size”, if that thing came anywhere near our planet (i.e. “millions of miles away” as mentioned above), the entire kit n’ kaboodle would pass the event horizon and never be able to come out. The Earth. The Sun. All the planets no matter where they are in their orbits…

…If it passed near but not event horizon-crossing close, its gravity would still be so strong it would ruin all of the planets’ orbits and probably fling all of them out into the void of space.

Meanwhile, gravity on the surface of the planet would be about 2 million meters per second per second, compared to 9.8 for us. Or, put in terms of g‘s, 217,000g. Any humans would be crushed into a jelly under that force. Those dozen military guys the Earth sent to check things out?

Dead.

Everybody dead.

[Side note: If the mass was only 2000 M☉, the event horizon radius would be 5900 km, which is smaller than the Earth’s radius by a few hundred km. If both mass a radius were 2000 times the Sun’s values, the gravitational acceleration at the surface would be 0.14 m/s2. Tiny.]

 

The wibbly-wobbly timey-wimey

It is true that in the presence of a larger gravitational force, time slows down. This was explained by Albert Einstein in his General Theory of Relativity, and is where all (or most) of the drama in Interstellar comes from. It works even with small differences in gravity – GPS satellites orbiting the Earth are further away from the planet’s center of mass than we are on the surface, and therefore experience less of a gravitational pull than we do. And they have to compensate for the time dilation that happens down on the surface relative to them.

You can’t calculate time dilation for people inside the event horizon of a black hole (or a planet, in our case). It’s infinity. But even if the planet were only so massive that the event horizon were just under the surface (about half a billion solar masses), as soon as the away team lands on the planet, they’re under the same gravitational force as the giant bugs and the 6000-foot tall humanoids (Note: based on the drawings, they’re clearly not 1000x taller than a man, as the news anchor claimed. They might be 100ft tall, tops…). They’d experience the same slowdown of their internal clocks.

[Side note: If both mass and radius were 2000 times the Sun’s values, time dilation on the surface would be basically nonexistent.]

The recon team would be stuck on the planet as it flies through our solar system – everyone back on Earth long dead by the time they make it back to report their findings.

 

Happy endings, all around…

 


1. “Neighborhood” meaning within 4.3 lightyears. The closest star system to ours, Alpha Centauri, has three stars – one of which (Alpha Centauri A) is slightly bigger than our Sun in both mass and radius.
2. a galactic center powered by a SMBH ‘eating’ matter to such a degree that the light released from the process outshines all the rest of the light in the galaxy
3. 1 M– i.e. the mass of the Sun – is 2×1030 kg. Also, Betelgeuse’s size has some pretty big uncertainty – we’re talking plus or minus 200 solar radii. 

 

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