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SYFY WIRE Bad Astronomy

A whole (galaxy of) new worlds: The A Capella Science of exoplanets

By Phil Plait

[Proxima Centauri b is a "whole new world" to us humans. And there are billions more like it. Credit: Tim Blais]

You know, if you’ve ever sat around thinking, “I really want to know the history of the discovery of alien planets, but it needs to be done in a video featuring multiply layered a cappella variations on songs from Disney’s Aladdin, then boy howdy, do I have a video for you.

Behold, “Whole New Worlds,” written by Tim Blais and featuring Julien Neel, Sam Robson and my friend Gia Mora:

This video floored me; it’s amazingly well-written, directed, edited, and it gets the science right. I suppose that’s not too surprising, given that Blais is a self-confessed “harmony addict working on a master's in theoretical physics.” That’s why he started A Cappella Science (which you can support on Patreon, incidentally), where he creates videos like this one.

As an astronomer and self-confessed lover of the new field of exoplanetary science, I was amazed at not just the accuracy of the video, but the grasp of it; Blais really did his homework. In fact, there were so many inside jokes and puns I thought it would be fun to give you an overview of them. So, below is a time-tagged list and quick explanation of some of the gags in the video.

But first, I think my own Crash Course Astronomy video on exoplanets might bring you up to speed on the basics, including what these beasties are and how we find them:

Got it? Great! Now, if you want to follow along, the lyrics for the song are under the video itself (click “Show More” to see them).

reflex motion animation
00:20: The first exoplanets were found using what’s called reflex velocity, through which the planet’s gravity pulls on the star as it orbits. As the planet makes a big circle around the star, the star makes a little circle, and that can (theoretically) be detected through the Doppler shift by breaking up the colors of the light from the star into a spectrum and measuring the shift of the spectral lines seen in it (I explain how all this works in my Crash Course Astronomy episode on light). By the 1990s, this method had been in use for a while, but hadn’t yet succeeded.

00:36 “They’ll see if I can show them the sines.” I love this joke. As the star moves back and forth due to reflex motion, the spectral lines observed from it shift back and forth. If you graph the motion of the lines what you get is a sine wave. So, yes, I laughed out loud when I heard this line.

[“This line!” Haha! Ha!]

Also, the next part of the video goes into how hard it was for astronomers to get funding to look for exoplanets; some astronomers thought it wasn’t possible to get the precision needed to make the measurements. It’s a vicious circle: You need money to prove you can do it but until you can do it it’s hard to get money. A star will only show a velocity shift of a few meters per second — basically walking speed —which is phenomenally difficult to see.

But astronomers are pretty darn smart, and eventually 51 Pegasi b was found...

01:54 I love that they mention planets around a pulsar; these were found in 1992 and were the first exoplanets seen. Given that a pulsar is the highly energized dead core of a supernova, though, it’s not terribly Sun-like.

But then 51 Peg b was found. It’s a “hot Jupiter”, a massive planet orbiting its very Sun-like star very close in. Its existence was a shock; we didn’t know planets could orbit their host stars so tightly. The conclusion was that they must form farther out and migrate in, but this was controversial for a while before the evidence became overwhelming.

03:09 The “mean anomaly” is a term used to describe the shape of an object’s orbit. This is a terrific pun. Why? If planets in the galaxy were rare, you’d expect the nearest one to be far away (out of hundreds of billions of stars, what are the odds that the only other star in the galaxy with planets would be right next door?). But if they’re very common, then the nearest one would be close by. The fact that a planet orbits around a nearby star therefore implies there must be planets around many or even most stars! So, 51 Peg b, which is relatively close to us, couldn’t be an anomaly. That must mean that planets are everywhere; they’re mundane.

03:23 Stars with different temperatures have different spectra, and so we use different letters to classify them. The Sun is type G. Also, he says, “We know its mass to be high, half Jupiter by sine i”. This is true: the tilt of the planet’s orbit is called the inclination and is represented by a lower case i. If you do the math to figure out the mass of a planet based on its orbit, it turns out the value you get for the mass depends on the (trigonometric function) sine of the inclination. Sine i. Pretty nerdy lyric, there.

3:35 “And its star is in Pegasus; Give it an A and thus; Label the planet as b”. We name the exoplanets found around other stars by labeling them as the star name plus lower case “b”, assuming the star is upper case “A” (if the star is binary we say the brighter star is A and the fainter B; we use lower case letters for planets to distinguish them from stars). The next planet found around that star would be “c”, and so on.

Transit diagram

03:48 Finding planets using spectral shifts is tedious; you can only look at one star at a time. But we found that we saw some planets’ orbits edge-on, and those planets passed directly in front of their stars, blocking a small fraction of their light. The Kepler spacecraft was launched into space to stare at 150,000 stars at the same time to look for these transits, and has found thousands of exoplanets this way.

04:18 Photometry literally means measuring light. Kepler performs photometry on those thousands of stars, looking for those transit dips.

04:21 The plot shown is real; it’s the transit graph for the exoplanet Kepler 93b, a planet only slightly bigger than Earth orbiting a star much like the Sun.

04:40 This is actually a pretty good (if very brief) description of how we use transits to get the size of the exoplanet and the length of its year.

04:47 An echelle spectrograph is a very precise instrument that can be used to follow up Kepler observations and get the mass of the planet found.

04:58 The habitable zone is the distance from a star where a planet, under certain assumptions, could have liquid water on its surface. It’s more of a guideline than a rule; we see moons around Jupiter and Saturn with liquid water under their surfaces even though they are very far from the Sun.

05:03 Kepler 452b is an Earth-sized planet around a Sun-like star. More or less.

05:27 In 2016, astronomers found a planet orbiting Proxima Centauri, the closest star to the Sun. It’s probably a bit bigger than Earth, and a bit cooler. That’s huge news! Proxima is only 4.3 light years away, a stone’s throw in galactic terms (though a bit of a haul in human terms). The star is a red dwarf, and is so faint you need a telescope to see it at all even though it’s so close.

06:45 We haven’t seen an Earth-sized planet directly, yet; they’ve all been seen indirectly, by their influence on their star (blocking its light, or tugging on it). We have seen planets directly, but only because they’re young and still glowing due to the heat of their formation. We hope to soon see an old, mature planet like our own using infrared telescopes like the James Webb Space Telescope and others being planned now.

Image of four planets around HR 8799

07:00 If a planet has life, there may be a chemical imbalance in its atmosphere. For example, on Earth, oxygen is highly reactive and so you’d expect it all to disappear rapidly. But plants make oxygen, so we see it in our air because it’s constantly replenished. So, get this: If a planet passes in front of its star, its atmosphere absorbs some of that starlight. Oxygen absorbs light at a very specific color, so if we can detect it in an exoplanet’s air that would support (though not prove) that there could be life there.

07:08 An idea for a future mission would be to launch a flower-shaped shade into space along with a telescope. They’d be positioned together  so the shade blocks the star but not the planet, and then the planet could be seen even though the star might be literally a billion times brighter. Optical effects make it easier to see the planet if the shade isn’t a circle, but instead has “petals”; light bending around the edge of a circular shade creates effects that could hide the planet. The petals prevent that.

Also, there’s an idea to send a tiny probe to Proxima Centauri’s planet. Called “Star Shot,” it would use a powerful laser to launch it at high speed. I have my doubts about how well this would work, but the idea is being pursued now, and I’ll be curious to see how they overcome some of the technical issues (like communication to send back information over 40 trillion kilometers).

Finally, at 07:56, he mentions TRAPPIST-1, a red dwarf found in 2017 to have a system of seven Earth-sized planets, with perhaps three of them in the star’s habitable zone. We’ve seen stars with more planets, and red dwarfs with planets, but not a red dwarf with this many planets. So, it’s a pretty cool discovery.

artwork depicting exoplanet Gliese 581c

And there you go; that’s it for the science in the video! Clearly, there’s a lot packed into those songs, but if you find something else in there you don’t understand, poke around the web, or check my Crash Course video again. Exoplanetology is a very rich field, and a young one, so a lot of the background information about it is online.

As an aide, besides all the science, it was fun to see my friend Gia in there. She’s a writer, actor, model, singer, and bona fide science dork. She’s part of a trio of actresses who use their standing to promote science; they call themselves the Scirens and they’re doing good work.

Gia told me something astonishing, too: None of the singers were even in the same room to make the video, and in fact didn’t even know each other! Blais sang all the parts and then sent them audio files along with lyrics. Everyone recorded the audio and video separately, and then Blais stitched it all together. He’s actually pretty amazing.

I’ve said this many times: Science and art are two sides of the same coin. In many cases they’re indistinguishable, and this video makes that clear just by existing. But it also gives examples of it; the first exoplanets around Sun-like stars were found because of a leap of imagination, the idea that we could actually look for them using indirect means that were right at the bleeding edge of what our technology could do. And once the proof of concept was delivered, we found new ways to search the heavens for alien worlds; over a dozen methods have been used now, including directly seeing exoplanets in images taken of their stars.

We now know that most stars in the galaxy likely have planets, and that planets may outnumbers stars! Think about that the next time you go outside and look up at a star-spangled sky. And when you do, don’t just wish upon a star: Discover more about it.