by Public Service Associate Autumn

Unless you have been living under a rock (or outside of North America), you probably noticed the eclipse that occurred on April 8, 2024. The media took to calling it “The Great American Eclipse”, as it covered only Mexico, the mainland United States and a small swath of Canada. Maybe you traveled to see the totality personally, maybe you just noticed the streetlights coming on at two in the afternoon and maybe you decided it wasn’t worth the hassle and stayed inside, away from the madness.

Now, as you may know, eclipses occur when one celestial body passes between another one and a source of light (traditionally a star).¹ The star doesn’t have to be completely blocked out for the event to be considered an eclipse. Eclipses interrupt the view of the light from the star and because eclipses can occur whenever any planet or moon passes in front of its star, this fluctuation is one way that scientists are looking for life around the universe.² When these eclipses do happen, enough of the glare of the star is blocked that we can now see these extrasolar planets orbiting them. In celebration of the wild world of extrasolar eclipses and exoplanets, let’s take a look at the weird and fascinating extrasolar worlds out there and how scientists find them. 

Exoplanets are planets that orbit stars outside our solar system.³ Astronomers theorized for a long time that such planets existed, though without a way of proving it.⁴ It’s not as though most exoplanets give off light that we can detect, after all.⁵ Currently, we know of more than five thousand exoplanets, not including the more than ten thousand potential planets that are still being verified.⁶ That’s about one exoplanet for every other day since we first began discovering them.  But remember,  it’s how we find them that relates back to eclipses. How do you find something that doesn’t give off or reflect visible light? Well, you look at where the light should be, but isn’t (the transit method), or you hunt for invisible light (the radical velocity method).⁷

Let’s take a look at the transit method first, as it’s the method that involves an eclipse. Here, scientists watch selected stars closely and track if their light ever dims. If it does, that’s most likely because a planet passed between the star and the earth. From our point of view on Earth, this doesn’t form a complete eclipse like the moon and our sun do,⁸ and, in fact, the light from the star generally only dips about 1% or less.⁹ That 1% is for big planets, by the way, orbiting close to their stars; smaller planets are harder, if not impossible to detect¹⁰ like this.

Visually, the transit method looks something like this:¹¹

There are a few downsides to this method, however. Outside of having a bias toward discovering larger planets, all planets found this way have to be orbiting parallel to the Earth. If the planet never passes between its star and the Earth, scientists can’t observe the dip in the star’s light.¹² Even more frustratingly, planets eclipse their stars for very brief periods of time. It might take a planet four years to circle its star and only transit between its star and our view for a few hours. It can be hard to catch a dip that narrow in four or more years of data. In addition, the dip in light can be so small that scientists can have trouble determining if it is a planet causing the variation in light or some sort of bug in the data collection. Despite these problems, this method has still been the most successful means scientists have for finding exoplanets. According to NASA, scientists have located 4,216 planets by looking for dips in starlight, and only¹³ 1,089 planets via radical velocity.¹⁴ 

Unfortunately, the radical velocity method is slightly more complicated to explain, and it involves a lot of physics, so bear with me. Planets, like stars themselves, exert a gravitational pull on everything in their vicinity. We normally think of this in connection to the gravity that keeps us firmly planted on the ground or how the Earth keeps the moon in orbit. But planets also pull slightly at their stars, too. This causes the star to move ever so slightly back and forth as the planet rotates around it.¹⁵ Now, this is cool to know, but how can scientists tell that from so far away? 

Amazingly, they can discover this data via the Dopler effect and invisible light.¹⁶ For those who might not remember all the way back to middle school science, the Dopler effect is “an increase (or decrease) in the frequency of sound, light, or other waves as the source and observer move toward (or away from) each other. The effect causes the sudden change in pitch noticeable in a passing siren, as well as the redshift seen by astronomers.”¹⁷ In scientific terms, the waves of light or sound lengthen and shorten as the object making them moves, so as the observed stars wobble, they give off a pattern of redder light and then bluer light on repeat.¹⁸ The very first exoplanet discovered, named 51 Pegasi b,¹⁹ was discovered this way. 

Radical Velocity looks something like this:²⁰ 

To pile on top of all this coolness, scientists can also determine the type of planets they find (at least sometimes) by combining these two methods. Originally, scientists relied on both methods just to verify that the planets existed. Having a star that wobbles and dims at the same time is proof that it has at least one planet orbiting it. But looking at how much the star dims versus how much it wobbles also allows astronomers to see what type of planet they have. A star that dims only slightly, but wobbles a lot more indicates a dense, rocky, Earth-like world orbits it. A star that dims a lot, but wobbles much less, is likely to have a large gas giant floating around it. A big, but not very dense, planet.²¹ Something along the lines of Jupiter or Saturn.

That is not to say either of these methods are infallible. Recently, scientists took another look at the super-Earth HD 26965 b, known colloquially as Vulcan.  As HD 26965 b orbited orange star 40 Eridani A, the star that hosts the home planet of Star Trek’s Spock, the name was perhaps inevitable. This planet was discovered using the radical velocity method in 2018. However, even when they announced the discovery, the scientists involved in the original study warned that the planet might be a false positive from the data, as they could not determine Eridani A’s rotation rate.²² Four other groups conducted studies looking for this planet, with the latest one using new technology to measure the radical velocity of Eridani A, and have concluded that the wobble is an effect of the star itself and not because a planet is tugging on it. Sadly, as in Star Trek itself, Vulcan is no more.

Now we come to my favorite and least favorite question: why do we care? First, because this is just epic and why wouldn’t you want to know and secondly, because finding exoplanets, and determining how they form, expands our knowledge of the universe and how it works. For example, when 51 Pegasi b was found in the 1990s, it blew astronomers’ minds and forced them to rethink how solar systems were formed. 51 Pegasi b is a gas giant larger than Jupiter that orbits ten times closer to its star than Mercury does to our sun.²³ It also circles its star every four days. But basic physics says this should be impossible. A developing star would not leave behind enough matter for such a large planet to form. Then how did it get there? It turns out that planets are capable of moving within their solar systems after being formed, but before the star ignites nuclear fusion. Without this planet, no one would have thought that was possible. Who knows what else there is out there to learn. And we won’t ever know, unless we go looking. 

This has just been a brief sampling of the amazingness of space and eclipses and light. So, if you want to learn more, I recommend looking at the books below, or take a look at some of the links hidden in my footnotes. I also want to highlight this website which talks about detecting exoplanets from your own yard and this website where you can submit your own ideas for what to name exoplanets.

Non-Fiction Books

• Impact by Greg Brennecka | Book 
• Black Holes by Brian Cox and Jeff Forshaw | Book 
• Alien Earths by Lisa Kaltenegger | Book 
• Asteroid Hunter by Dante S. Lauretta | Book 
• How to Read the Solar System by Chris North and Paul Abel | Book 
• The Planet Factory by Elizabeth Tasker | Book 

Fiction Books

• Foundation by Isaac Asimov Book | Libby
• The Expanse Series by James Corey Book | Libby
• The Peripheral by William Gibson Book | Libby
• Luna: New Moon by Ian McDonald Book | Libby
• Red Mars by Kim Stanley Robinson Book | Libby
• Project Hail Mary by Andy Weir Book | Libby

Footnotes

¹I learned while researching this that there is another definition of eclipse: “a phase during which the distinctive markings of a bird (especially a male duck) are obscured by molting of the breeding plumage.” Weird.

²That’s right, I tricked you, this blog is about exoplanets.

³So if it isn’t Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, or Neptune, but it’s still a planet, it’s an exoplanet. I’m sorry, Pluto. 

⁴The first evidence of an exoplanet was actually found in 1917, though the astronomers looking at it did not realize what they had discovered. In fact no one would work it out for another hundred years. For more information, take a look at this article: https://www.jpl.nasa.gov/news/overlooked-treasure-the-first-evidence-of-exoplanets 

⁵ I say most because some very young planets are hot enough to give off detectable light, as proven by astronomer Christian Marois, who managed to take a picture of the four young planets around star HR 8799.

⁶Alien Earths by Lisa Kaltenegger, pg. 172

⁷And no, I am not joking.

⁸The planet is much too close to its sun to eclipse it for us earthlings. 

⁹https://lco.global/spacebook/exoplanets/transit-method/

¹⁰Though scientists are having an easier time with smaller planets since the James Webb telescope launched.

¹¹https://www.skyatnightmagazine.com/advice/skills/how-to-detect-exoplanets-from-your-back-garden

¹²To put that into perspective, for a planet the size of the earth, orbiting its star at about the same distance as we orbit the sun, it’s estimated there is a mere .47% chance it is lined up properly for us to see the dimming effect. 

¹³I feel like only should be in quotations here. As though finding more than a thousand exoplanets wasn’t an astonishingly impressive feat.  

¹⁴https://exoplanets.nasa.gov/alien-worlds/ways-to-find-a-planet/?intent=021

¹⁵The same way that the moon causes tides on Earth.

¹⁶ Invisible to the human eye, anyway.

¹⁷Oxford English Dictionary

¹⁸If you’re wondering why you cannot see such a shift in stars, that’s because the change in light frequency when a star does this is outside the range of human visible light, and scientists have to use spectrometers to measure it.

¹⁹I understand why planets are named like that, but it’s not very catchy.

²⁰Radial Velocity: NEID Spectrograph Goes to Work | Centauri Dreams (centauri-dreams.org)

²¹I read Alien Earths for this blog, and the author compared one such planet’s density to that of a marshmallow, as it is larger than Jupiter, but half as dense as Saturn.

²²https://www.sciencealert.com/scientists-confirm-the-planet-vulcan-doesnt-actually-exist-after-all

²³Planets like this are now known as ‘hot Jupiters’ and are quite common. Or, at least, they’re the perfectly sized and positioned to be found by scientists.