Who Gives a Damn About Planet HD 131399Ab?

The way we name exoplanets is bizarre and it might be diminishing public support for important scientific missions.


Recently, a bizarre exoplanet orbiting three suns was announced. Its name? HD 131399Ab. Really rolls off the tongue, doesn’t it?

You’d think that exoplanets — the term refers to any planet orbiting a star outside our humble solar system here — would somehow inspire some fabulous nomenclature. After all, our solar system’s planet names are based in mythology; shouldn’t the other 3,368 confirmed planets in space be afforded the same dignity?

Not quite. The current system is mechanical, in which newly discovered exoplanets get minted with what is essentially an astronomical social security number. Official names that don’t include a serial code-looking label, like Arion or Dagon, are few and far between. This is partially because good names don’t matter to astronomers, but really, it’s because giving unique names to thousands of objects is a gargantuan task. And the first attempt to determine official public names for exoplanets exposed a major flaw in the international effort to name things in space.

“The discovery of an exoplanet starts with the discovery of a star,” says Thierry Montmerle, an astronomer at the Institut d’Astrophysique in Paris and chair of the International Astronomical Unions exoplanet naming group. The current unpronounceable names of most exoplanets point back to their stars, first taking the astronomical index for the star — uppercase letters followed by a string of numbers — followed by a lowercase letter. Exoplanets are given letters in order of discovery, not distance from a star, so they don’t have to be renamed if a closer exoplanet is found later. To make this more confusing, because of an old convention with binary stars, the lettering starts with ‘b’ and not ‘a’ as might be assumed. For the example that kicked off this story, HD131399Ab is the first exoplanet discovered around the star HD 131399 A.

And in case this system wasn’t confusing enough already, different research teams also use different designation methods; some name exoplanets after a mission, others tip their hat to the individual who discovered them. This means you not only get HD131399Ab but also Kepler-132e and a HAT-P-11b.

The International Astronomical Union is a volunteer organization of astronomers around the world, so there are many perspectives here: some astronomers wanted the more numerical system of naming exoplanets, others wanted a more traditional approach. And astronomers have been hesitant to let go of their favorite names, so the IAU working group hasn’t suggested standardizing designations more than they currently are, which means there’s a stalemate that affects not just the identification of exoplanets but also makes it hard for the public to care about new discoveries.

“It’s still very dry nomenclature and when you see it in press releases the public just yawns,” says Eric Mamajek, an astronomer at the University of Rochester and a member of the working group to name exoplanets.

An artist's concept of Kepler-132 e, an Earth sized exoplanet that astronomers still don't know much about. 


The future of exoplanet naming is a muddled one. One of the most important things to the IAU in naming the next exoplanets was cultural representation; having planets that revered Roman gods is passe. In the first NameExoworlds contest, which concluded in December 2015, the working group had asked the public to submit entries to be part of a public vote to name 15 stars and 32 exoplanets. Before names could be voted on, the working group had to make sure the suggested names weren’t already being used for another celestial object. “It became clear that they didnt have an official catalog of IAU recognized [star] names,” says Mamajek.

“People from different cultures have given names to these stars,” says Montmerle, “but there’s no official recognition of these names, and this is not trivial.” In the process of trying to honor world cultures, they had discovered that they had ignored the fact that other cultures had already named many stars.

The astronomers decided this had to change. “We shouldn’t be using new names, paving over an old name in a different culture,” says Mamajek. So in May they created the IAU Working Group on Star Names to catalog all of the different names given to stars by cultures across the world, which Mamajek is chairing. “A lot of these names have really interesting histories behind them,” Mamajek says. “That’s going to be sort of a long term goal, recording this sort of long-term astronomical culture.” It’s not an easy task, but Montmerle says the group is actively starting the effort now, beginning with the names of the brightest stars.

When they started trying to name exoplanets, “We did not really suspect that we would end up with this question of ‘What is the name of the star?’” Montmerle says. But recording all the star names and preserving this heritage is important for naming future exoplanets — especially since the approved exoplanet names often share a theme between the star and its exoplanets. As a result of the first contest, the star 47 Ursae Majoris was officially named Chalawan, after a mythological crocodile king in a Thai folktale. Chalawan’s two exoplanets are now named Taphao Thong and Taphao Kaew, the two sisters in the folktale.

The names selected in the first NameExoWorlds contest were from all over the world.


Along with recognizing the connection between stars and world culture, a catalog of all the star names will help the exoplanet naming committee for what Montmerle calls “the big fight.” When life is discovered, the IAU wants to make sure the public can vote on names that havent been used for any other object, in any culture, across the world. “We know that the IAU and other groups will be flooded with proposals,” Montmerle says. “This is why it’s so important that we don’t make any errors at this stage.”

Sure, we have to deal with terrible exoplanet names for now, and because there are potentially billions of exoplanets, the unpronounceable designations are going to keep on coming. Only the exoplanets of particular interest to the public are likely to get real names. But maybe, just maybe, HD 131399 Ab will get a long-awaited identity makeover.

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