Vulcan, for the moment, remains a fictional planet

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Astronomy News

It’s a big Universe. Here’s a thing about it.

I am seriously bummed about this: Vulcan doesn’t exist.

Of course, it wasn’t really Vulcan in the first place. I mean, Vulcan is fictional, but then we found it, but then it turns out we didn’t.

OK, yeah, let’s back up a bit.

In 2018, astronomers made a pretty exciting announcement: They had found a planet orbiting 40 Eridani A, which is just 16.4 light-years from us. It’s what’s called a hierarchichal trinary: A star a bit cooler than the Sun (40 Eridani A) orbited by a pair of stars in a binary. One of those is a cool red dwarf and the other a dead white dwarf.

OK, fine. We find planets around stars all the time, so what’s the big deal?

The big deal is that the primary star in the system (also called Keid) is canonically the home star of the planet Vulcan in the Star Trek universe! When this was announced, my nerd heart did a little dance (you’ll find lots of details and more info at that link from The Old Blog).

Now, it turns out the planet found wasn’t a whole lot like Vulcan in the show. For one thing it looked to have a mass over 8 times Earth’s, making it more likely a gas giant than a rocky terrestrial planet like Earth (though we know from various episodes that Vulcan had higher gravity than Earth, so maybe). It orbited closer in to the star than Earth does, and would be on the inner edge of what we call the habitable zone, the distance from the star where a planet is the right temperature to have liquid water on its surface. And hey, in the show Vulcan was hot, so …?

But hold your warp drive. The news gets worse from here.

Background: A high priority for astronomers in the coming decade is to build a large (8-meter) space telescope designed specifically to get direct images of nearby terrestrial (aka rock and metal) worlds. Time on this ’scope will be precious, so a team of astronomers decided to go through archived data of some nearby stars where planets had been detected orbiting them [link to paper]. They examined the observations to see if they could confirm or disprove the existence of those planets. This is a pretty good idea, since it could help narrow down the number of stars this Habitable Worlds Observatory could take a look at.

Yeah, you can see where this is going even without the main viewscreen: They looked at the data for 40 Eridani A and determined the planet seen in the data didn’t exist.

The original observations used the reflex (or radial) velocity method to look for planets. A planet orbits a star due to the star’s gravity, but the planet has gravity too. So while the planet makes a big circle around the star, the star makes a little circle as well. In reality they both orbit their center of mass, what we call the barycenter. As the star moves back and forth, the light it sends our way will get a tiny blue- and redshift (like a Doppler shift), which can be measured. The signal of this is generally pretty small, but we have methods that can tease it out pretty well.

Even in the original paper they note a problem (and I was careful to talk about it in my original blog post about this as well): The orbital period of the planet — its “year” — was about 40 days, and that was suspiciously close to the rotational period of the star; that is, how long it takes to spin. Stars can have all sorts of features that can be confused for a planet, like starspots (sunspots, but on another star), strong magnetic fields, and more. As the star rotates these features move with it, and can mimic the signal a planet would make via the reflex velocity method. When you find a planet that has an orbital period the same as the star’s rotation period you have to be careful.

And, well, here we are. The new paper says yeah, what was thought to possibly be the reflex signal from the planet is actually just the star spinning around. The planet doesn’t exist.

Frak! (oops, wrong franchise)

Mind you, this doesn’t mean 40 Eridani A doesn’t have planets. It just means the one we thought it might have doesn’t exist. There could be planets there we haven’t detected yet. We just need astronomers to be more enterprising and continue to seek out strange new worlds.

Still, this is a bit of a bummer. But hey, I’d rather know the truth then a comforting falsehood, no matter how much the truth would be awesome.

But dang. I had even created a public talk I gave a couple of times based on this (I called it “The Search for Vulcan” and gave it on a Star Trek cruise)!

Ah well. In the end, it doesn’t take red matter to destroy Vulcan. Just more science. And, in the end, that’s a good thing, because we learned something, and learning is always good.

It’s only logical.

Et alia

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