Exo-itement! What’s in an exoplanet name?

Since starting as the Deputy Manager of NASA’s Exoplanet Exploration program this April (that’s the exo-itement of the title), I’ve been dusting off my astrophysics degree and spinning up on a new (to me) field of science: Exoplanets, planets outside of our solar system.

I’ve bulked up on textbooks, audiobooks, popular science books, and exoplanet science communicators in my social feed. Shoutout to Dr. Jessie Christiansen and NASA Exoplanets who are two must follows; I also follow the list Exoplanets. I found a fantastic Great Courses audio series, really enjoy the Exocast podcast, and have been reading The Planet Factory.

My bedtime exoplanet readings: exhibit Textbook a Textbook b and Textbook c. These have some serious gravitational pull

One of the first new things I learned to appreciate about exoplanets was how to start to decipher their fantastic names. The IAU has a nice writeup about exoplanet nomenclature if you want to do a deep dive. The first part of an exoplanet’s name is noun or noun/number combination that designates either (a) chosen name(s) of the star(s) the exoplanet circles, or, as is the case with many exoplanets, the name of the instrument that originally observed the star/exoplanet.

Stars are then labelled with capital letters: A if it’s a single star, and A, B, C, etc. for multiple stars. Planets orbiting said star(s) are labelled with lowercase letters, in order of discovery, this time starting with the letter b. So that’s b for the first planet to be discovered around the star(s), c for the second, and so on. If there’s just one star, then we can drop the capital A as part of the name. If there’s more than one star, then we can use parentheses around the capital letters to indicate which of the stars the planet orbits.

Let’s decipher a few names.

Here we have 51 Pegasi b, the first exoplanet to be discovered orbiting a sun-like star. 51 Pegasi is the name of the star, b is the name of the planet.

Greetings from your First Exoplanet
51 Pegasi b, illustrated poster.

Next we have Kepler-186f, the first Earth-size planet discovered in the temperate (potential for liquid water) zone around another star. f means this was the 5th planet to be discovered around the star Kepler-186, so named as it was discovered by NASA’s Kepler Space Telescope.

Kepler 186f
Kepler-186f, illustrated poster

Next we have another Kepler discovered planet, Kepler-16b. As you can see by the poster, it’s a world that orbits two stars, and was in fact the first confirmed exoplanet that orbits two stars. This means that Kepler-16b is a shorthand for its formal name: Kepler-16(AB)-b indicating that it orbits both the A star and the B star.

Kepler-16b illustrated poster

Whew! There’s a lot of options for naming. In astronomy there are a variety of sources of names, from constellations, to lists (catalogs), people who discovered the object, collaborations, instruments, and more. We have numbers, letters, shorthand, and jargon coming into play as well. 51 Pegasi b is sometimes known as “51 Peg b” for example, or as I like to call her “Peg” for short. You can come up with your own names as well! In the words of the International Astronomical union:

The IAU does not consider itself as having a monopoly on the naming of celestial objects — anyone can in theory adopt names the way they choose. 

But since the IAU choices do carry a lot of weight, since 2015 they have accept suggestions for formal, more approachable, names to adopt.

The IAU fully supports the involvement of the general public in the naming of astronomical objects, whether directly or through an independent organised vote, in the naming of planetary satellites, newly discovered exoplanets, and their host stars.

For example, the planet I affectionately referred to as Peg above, 51 Pegasi b, is formally named by the IAU as Dimidium, a name which was originally proposed by the Astronomical Society of Luzern, Switzerland.

I’ve now added it to one of my life goals: name an exoplanet! If you could name a planet, what would you call it?

Leave a Reply

Your email address will not be published. Required fields are marked *