The story of Superman is so well known that I hardly need go into detail. But in case you’re some sort of commie, the idea is that he was born on the planet Krypton orbiting a far away red star, and sent to Earth while still a baby by his parents as their home planet exploded around them. Our yellow Sun somehow gives Kal-El superpowers, and he goes on to star in a series of increasingly poorly-made movies*.
I’ve often wondered exactly what kind of star Krypton orbited and where it was. Up until now all we’ve known is that it was red, and red stars come in many flavors, from dinky red dwarfs with a tenth the mass of the Sun up to massive supergiants like Betelgeuse which outweigh the Sun by dozens of times (I’ll note that a deleted scene in "Superman Returns" indicates it’s a red supergiant).
Well, that’s about to change. DC comics is releasing a new book this week – Action Comics Superman #14 – that finally reveals the answer to this stellar question. And they picked a special guest to reveal it: my old friend Neil Tyson.
Actually, Neil did more than just appear in the comic: he was approached by DC to find a good star to fit the story. Red supergiants don’t work; they explode as supernovae when they are too young to have an advanced civilization rise on any orbiting planets. Red giants aren’t a great fit either; they can be old, but none is at the right distance to match the storyline. It would have to be a red dwarf: there are lots of them, they can be very old, and some are close enough to fit the plot.
I won’t keep you in suspense: the star is LHS 2520, a red dwarf in the southern constellation of Corvus (at the center of the picture here). It’s an M3.5 dwarf, meaning it has about a quarter of the Sun’s mass, a third its diameter, roughly half the Sun’s temperature, and a luminosity of a mere 1% of our Sun’s. It’s only 27 light years away – very close on the scale of the galaxy – but such a dim bulb you need a telescope to see it at all (for any astronomers out there, the coordinates are RA: 12h 10m 5.77s, Dec: -15° 4m 17.9 s).
Which brings us back to the Superman story. I was sent an advance copy, and it’s actually a clever tale, with some relatively solid science in it. I won’t spoil it, but apparently Superman comes to visit the Hayden Planetarium in New York City (where Neil is the director) every 382 days, which happens to be the period of Krypton around the star (known as Rao in the comic canon). Although it’s not said explicitly in the story, it sounds like they try to observe Krypton when it’s at the point in its orbit where it appears farthest from its star, reducing the glare and making it easier to spot†.
As for the major plot point of the story, I won’t reveal it. But I’ll give you a hint: Superman is about 27 years old. PLEASE don’t leave any guesses in the comments below until a few days after the issue is out. I want to avoid spoiling it for any other readers.
Being a dork, I have to comment on some of the science in the story, though. Given the mass of a star and the period of a planet orbiting it, you can find the distance between the two. Doing the math (I’m a dork, remember?) I find the distance of Krypton to its Sun is about 100 million kilometers, somewhat closer than Earth is to the Sun (150 million kilometers).
But remember, Rao is a dim red dwarf! It’s so cool and faint that even at that closer orbital distance, Krypton would be a chilly world. Even if the planet is black as soot (and thereby absorbing all the heat falling on it from Rao) its temperature is still something like -170° Celsius – about -270° F! [If you're curious, I outline how to calculate this on the Bad Astronomy website.] At that temperature oxygen and nitrogen are still gases – barely – but it’s way below the freezing point of water. And if it’s not black, but instead snowy and white, the temperature will be even lower.
So Krypton maybe isn’t the best place for life to arise… still, there are ways out of this. Maybe either the Kryptonians migrated there (they couldn’t find a warmer planet?) or there’s something else going on. If it’s really volcanic then greenhouse gases could be prevalent, raising the temperature. Possibly the planet’s interior is still warm from heat leftover from its formation… or maybe whatever made it warm enough to be habitable also led to its destruction. Comic book science can be pretty ironic.
[DC comics: call me! I have ideas.]
I also feel obligated to note that in the comic, they made the planet look much larger than the star. That doesn’t work; the two are so far away it doesn’t matter if Krypton was on Rao’s near or far side; it would have to appear smaller than the star. We know Krypton is not a gas giant, so it can’t be much more than a few times Earth’s size. Even compared to a red dwarf that’s pretty small.
Still, it does make for a dramatic series of panels, and I’m always willing to let art trump science if need be. And this really is a pretty nifty story.
The issue comes out on November 7, and I’ll be heading over to my local comic store (Time Warp) to pick up a copy. Next time I see Neil maybe I’ll get him to sign it. It’s not too often I get to do that with someone who knows Superman.
Image credits: DC Comics; Digitized Sky Survey/NASA/Skyview
* I love – LOVE – the 1978 Superman movie, and I still to this day listen to the soundtrack, so you can argue with me over this, but you will be wrong.
† This actually happens twice per orbit, when it’s on either side of its star. That means the orbital period is actually twice 382 days, or well over two years… and as you’ll see, that puts it farther from its star, making things worse.
There’s been so much exoplanet news this week! I was in Texas the past couple of days giving a bunch of talks, so I’m trying to catch up. All the exoplanet news is way cool, but too much for one post, so I’ve split them up. I’ll post the other parts shortly.
Part 1: A trio of hot little rocks
First up? The three smallest exoplanets found so far. I usually don’t like to write about incremental discoveries, but this one is truly cool: all three orbit the same star, and all three are smaller than Earth! Any one of these would be a record breaker, but to find all three at once, in the same place? Amazing.
They orbit the star KOI-961 (short for Kepler Object of Interest), and were observed by the Kepler Observatory (details on how that all works can be found here). They all orbit the star extremely close in: the farthest one is a mere 2.3 million km (1.5 million miles) from the star! They’re so close they all take less than two days to circle it once. And even though the star is a red dwarf, and therefore relatively cool, they are so close to it that they probably resemble airless, heat-blasted Mercury more than Earth. They are almost certainly rocky/metallic bodies, since they are so small: 0.78, 0.73 and 0.57 times the diameter of the Earth. Although we’ve been surprised before, it’s hard to imagine anything that small could hold onto much atmosphere when they are so hot.
Funny, too: the star is tiny, only a bit bigger than Jupiter. And the planets are so close in the KOI-961 system looks more like Jupiter and its moons than our own solar system! The artwork above drives that point home. Everything there is to scale: the relative size of the star, the planets, Jupiter, and its moons. [Edited to add: Note that the distances are not to scale!]
Why is this news important? Well first, it adds more weight to the idea that planets smaller than Earth exist and can be found around other stars. Second, it shows that red dwarf stars can form and hold onto planets… which itself is important because red dwarfs are by far the most common kind of star in the Universe. They make up roughly 80% of the total number of stars! So finding multiple planets around one means, once again, planets are almost certainly common in the galaxy.
And third, it just shows once again that the Universe is a surprising place. This mini-solar system proves that nature is diverse, and will fill any niche it can. It also implies, very strongly, that we need to broaden our concepts of how solar systems form, what they look like, and how they behave.
Image credit: NASA/JPL-Caltech
- Kepler finds a mini solar system!
- Another Kepler milestone: Astronomers find two Earth-sized planets orbiting the same star!
- No, it’s *not* the smallest exoplanet found!
- A boiling superEarth joins the exoplanet roster
It’s kind of amazing that with nearly 500 planets discovered orbiting other stars, we’re still finding ones that are really weird. Massive planets orbiting so close to their stars they are practically plowing through the stellar atmosphere; hot spots on the planet not aligned with their stars; planets orbiting so far out it’s a struggle to understand how they got there.
And now we can add the planets NN Serpentis c and d to that list.
Lying about 1500 light years from Earth, NN Ser is a binary star — most stars in the sky are part of multiple systems, so that in itself isn’t all that odd. But NN Ser is weird: it’s a very dinky red dwarf orbiting very close to a white dwarf. And by very close, I mean really close: they’re separated by only 600,000 km (360,000 miles), which isn’t much farther apart than the Earth and the Moon!
I’ll get back to the stars in a sec. The planets found (named c and d because the two stars are a and b, according to the naming conventions) are Jupiter-scale beasts, with masses of about 6 and 2 times Jupiter’s, orbiting the binary stars at a distance of roughly 825 and 450 million km (500 million and 270 million miles).
Those numbers don’t seem too odd; lots of planets have been found with similar characteristics. But when you take a closer look at the system…